Thursday, April 24, 2014
Friday, April 18, 2014
Lift graph for 4V Desmoquattro cams using strada inlet cam as exhaust cam
I thought I'd post a graph here for a conversation that I've been having with someone about using the 4V Desmoquattro strada inlet cam as an exhaust cam. I haven't tried this personally, but have been told it works well.
In the graph, you can see the exhaust cams on the LH side. The yellow curve is the G, the corse exhaust cam used from 1992 (888 SPS had it too) until the last of them in 2000. They obviously liked it. The 916/999SPS exhaust cam (blue), while having much less duration, has a fairly similar shape and compared to the pink strada exhaust curve, is completely different. The strada is much slower opening, with what is called an asymmetric profile. The point of this graph, the red line, is the strada inlet cam being used as an exhaust. Profile wise it is very similar to the SPS up to peak lift, although it doesn't hold the peak or closing profile as long. But much better than the strada exhaust, and the SPS cam set does work very well. It's not a straight fit, the closing lobe needs to be narrowed and the opening rockers need some grinding at the feet. But well worth the effort given the cost and complete unavailability of the SPS cams.
How much input the SPS inlet has over the strada inlet in comparison I don't know. The extra high lift duration of the SPS inlet is something the specs don't show on their own, and that sort of extra area under the graph can only be good. The orange curve is the 748RS '506' cam, shown because there's some on Ebay at the moment and duration wise they only have 5 degrees more than the SPS and 11 degrees more than the strada. They do have a lot more lift, and are quite aggressive acceleration wise. But if you kept them to around 10,000 rpm, not the 13,000 or so they would have seen trying rather vainly to keep the Japanese 600's in sight, they won't be too destructive I'd think.
Doug Lofgren's cam page has more info on them too, with cam doctor graphs.
In the graph, you can see the exhaust cams on the LH side. The yellow curve is the G, the corse exhaust cam used from 1992 (888 SPS had it too) until the last of them in 2000. They obviously liked it. The 916/999SPS exhaust cam (blue), while having much less duration, has a fairly similar shape and compared to the pink strada exhaust curve, is completely different. The strada is much slower opening, with what is called an asymmetric profile. The point of this graph, the red line, is the strada inlet cam being used as an exhaust. Profile wise it is very similar to the SPS up to peak lift, although it doesn't hold the peak or closing profile as long. But much better than the strada exhaust, and the SPS cam set does work very well. It's not a straight fit, the closing lobe needs to be narrowed and the opening rockers need some grinding at the feet. But well worth the effort given the cost and complete unavailability of the SPS cams.
How much input the SPS inlet has over the strada inlet in comparison I don't know. The extra high lift duration of the SPS inlet is something the specs don't show on their own, and that sort of extra area under the graph can only be good. The orange curve is the 748RS '506' cam, shown because there's some on Ebay at the moment and duration wise they only have 5 degrees more than the SPS and 11 degrees more than the strada. They do have a lot more lift, and are quite aggressive acceleration wise. But if you kept them to around 10,000 rpm, not the 13,000 or so they would have seen trying rather vainly to keep the Japanese 600's in sight, they won't be too destructive I'd think.
Doug Lofgren's cam page has more info on them too, with cam doctor graphs.
Saturday, April 12, 2014
Timing belt pulley positioning on belt drive Ducatis
There's a thread going on one of the forums about replacing belts, and the point of marking the belts has been raised. I made the comment that I found the marking of belts an over complication of the procedure, but some people do find some comfort in it. In particular for locating the vertical camshaft correctly (vertical inlet on a 4V), due to the load the vertical inlet closing spring places on the camshaft when the cam is at the correct position. Coupled to the fact that on most of the 2V models the vertical cam pulley has a surround covering it and making it hard to get a good hold of it. I can understand the issues this brings to those who do this very infrequently.
So, to confuse people further, I thought I'd introduce another method to bypass this. I did teach this method to a previous workmate, who them went and bent some valves, so if you don't understand it, don't use it. But it seems pretty simple to me.
The timing shaft pullies traditionally have 4 holes in them, and the locating dot on the outer edge. See the picture below:
In this instance, the locating dot is lined up with the mark on the primary drive cover. This denotes the engine is at TDC firing on the horizontal cylinder, which is the master position for most setting procedures.
As the timing shaft turns at half crank speed, it rotates 180 degrees for one crankshaft rotation. From the RH side the crankshaft rotates clockwise and the timing shaft rotates counter clockwise. Vertical TDC firing is 270 crankshaft degrees after horizontal TDC firing, or 135 timing shaft degrees. If you look at the above, you can see 4 holes spaced at 90 degrees, and conveniently located at 45 degree spacing either side of the locating dot. This means that the second hole clockwise from the locating dot is 135 timing degrees from horizontal TDC firing, and as such, it's a locating dot for vertical TDC firing when aligned with any of the aligning marks on the primary drive cover or camshaft surrounds. As below:
On the engines with adjustable cam pullies, these holes aren't present. But of the three locking screws that hold the halves together, one is in the correct position.
The locating dot still gives the position anyway. On the 2V pre DS motors, when the timing shaft dot is at 3 o'clock, the vertical camshaft dot will be at 4:30.
On the later 4V Testastretta and 2V DS style engines with the 20 tooth pullies, the small holes are still there. But there are many larger holes, just to confuse the issue.
And, as always, when done, the locating dots on the timing shaft and all cam pullies must align with all the correct marks once set back to horizontal TDC firing.
So, to confuse people further, I thought I'd introduce another method to bypass this. I did teach this method to a previous workmate, who them went and bent some valves, so if you don't understand it, don't use it. But it seems pretty simple to me.
The timing shaft pullies traditionally have 4 holes in them, and the locating dot on the outer edge. See the picture below:
In this instance, the locating dot is lined up with the mark on the primary drive cover. This denotes the engine is at TDC firing on the horizontal cylinder, which is the master position for most setting procedures.
As the timing shaft turns at half crank speed, it rotates 180 degrees for one crankshaft rotation. From the RH side the crankshaft rotates clockwise and the timing shaft rotates counter clockwise. Vertical TDC firing is 270 crankshaft degrees after horizontal TDC firing, or 135 timing shaft degrees. If you look at the above, you can see 4 holes spaced at 90 degrees, and conveniently located at 45 degree spacing either side of the locating dot. This means that the second hole clockwise from the locating dot is 135 timing degrees from horizontal TDC firing, and as such, it's a locating dot for vertical TDC firing when aligned with any of the aligning marks on the primary drive cover or camshaft surrounds. As below:
On the engines with adjustable cam pullies, these holes aren't present. But of the three locking screws that hold the halves together, one is in the correct position.
The locating dot still gives the position anyway. On the 2V pre DS motors, when the timing shaft dot is at 3 o'clock, the vertical camshaft dot will be at 4:30.
On the later 4V Testastretta and 2V DS style engines with the 20 tooth pullies, the small holes are still there. But there are many larger holes, just to confuse the issue.
And, as always, when done, the locating dots on the timing shaft and all cam pullies must align with all the correct marks once set back to horizontal TDC firing.
Sunday, April 6, 2014
Fitting a 2003 onwards 696/796/800 6 speed gearbox into a pre 1998 600/750
One of the motors I have in bits is a 600SS motor, which came from the bike I sold the original engine from Minnie into. I actually sold more of this engine last week, just to make it harder for me to pointlessly waste a heap of money rebuilding it. This was mostly due to a mock up of the bits I had done the week before, trying the 68 mm stroke 900SS crank (a narrow one) to see what happened.
There was contact in a couple of places between the crank webs and the cases, at the inside of the starter drive idler gear boss in the LH case, and in a similar position on the RH, with a casting ridge that I'm guessing would be for an oil gallery. So I can't really cut either out, meaning the crank would have to be relieved at the outer edge, which reduces the most significant section of balance weight (furthest from the centreline).
The same section of crank web also hit the timing shaft on the RH side, where the later model timing shafts don't have a locating ridge. The pre 98 small blocks have a timing shaft that is shimmed between the cases to locate it, unlike the big block or 98 on small blocks where the shaft is held against the LH bearing by the timing gear.
The bottom of the 600 cylinder did just clear the web, with the underside of the piston quite a distance away. And the rods cleared the timing shaft.
And the 620 pistons I was going to use (it was going to have 750 heads, similar chamber to 620 and much larger than the small chamber in the 600 heads, which I had sold anyway) stick out a bit, but a thick base gasket shaped spacer would fix that. The 6.5 mm stroke difference between 620 and 900 (61.5 mm to 68 mm) would suggest the spacer would need to be 3.25 mm thick.
Anyway, I digress. The point of this was trying to put a late model small block 6 speed into pre 98 cases. The complete 6 speed box I bought from Ebay was from an M696. The easiest way to show why it wont drop in is with some photos.
Input shaft:
The input shaft has a larger nut on the clutch end, like all other late model boxes. So you'd need a bigger nut and possibly to do something to the clutch hub to accommodate it. The length of the clutch snout looks to be the same from threaded end to first gear. I didn't notice any difference there, but I didn't measure them. I might do that. 6 speed at the top.
The LH end of the input shaft has an extension that goes through into the recess which the clutch pushrod passes through. As the pre 98 doesn't have the clutch pushrod passing from left to right, there's no need for the extra snout, and no room in the LH case for it anyway. It has to be removed, and I figured the drop saw would take care of that.
Output shaft:
Not a great deal of difference here. The shaft outside of the LH bearing is 10 mm longer on the 6 speed, but you'd just turn the sprocket around to fix that. The only other thing I noticed is that, while both shafts are 25 mm diameter at the sprocket spline, the 6 speed wouldn't slide into my 600 cases like the 5 speed did. Maybe it just didn't want to be there.
Shift drum:
The shift drum is the biggest issue, with quite significant differences.
The RH end of the 6 speed drum has the 6 groove snout (2) for the sprung detente lever the later bikes use. You can see at the bottom left of the snout that one ridge is missing its top, that's neutral. The sprung detente lever at the RH end of the shift drum was introduced with the 916 from memory, and every new model since has adopted it. The sprung ball detente at the LH end of the shift drum is now only for neutral, so there is only one corresponding groove in the LH end.
The neutral light activation has gone from a 2 wire switch that is pushed in by the lump on the 5 speed drum (3) to a single wire switch that is earthed by the drum itself. The light brown covering on the drum, with the space at the (3), is an insulating coating that keeps the earth circuit open at all times except for when the drum is in the neutral position.
The difference in the shift fork groove pattern is obvious, so clearly you can't use a 5 speed drum.
I had various thoughts for overcoming these issues, from cutting and shutting the end of the 5 speed drum onto the 6 speed drum to making a spacer ring and welding on a switch pushing lump and cutting off the detent snout with the drop saw. Attempting the shut once the cuts were made is the only way you would be able to determine if that was a viable option. If not, you have two scrap shift drums.
So I didn't get any further with this fitment.
The other issue I have with the concept is that while a 6 speed is nice, the niceness comes from the ratio spread. The 5 speed ratio spread is 2.59, the 6 speed is 2.67. The big block wide ratio 6 speed is 2.88, a comparatively much bigger change. To overcome this there is the possibility of having another gear made for 6th, either 1 tooth bigger on the input or 1 tooth smaller on the output. The 26/24 ratio gives an 8% gap between 5th and 6th. 27/24 gives an 11% gap, 26/23 12%, with a ratio spread of 2.78. That makes about a 3 tooth on the rear difference between the top gear ratio of the 5 and 6 speeds. The shape of the current teeth determines if the 1 tooth idea will fly. The input shaft gear will be the easiest to make, as the output shaft gear has a shift fork groove on it.
.
There was contact in a couple of places between the crank webs and the cases, at the inside of the starter drive idler gear boss in the LH case, and in a similar position on the RH, with a casting ridge that I'm guessing would be for an oil gallery. So I can't really cut either out, meaning the crank would have to be relieved at the outer edge, which reduces the most significant section of balance weight (furthest from the centreline).
The same section of crank web also hit the timing shaft on the RH side, where the later model timing shafts don't have a locating ridge. The pre 98 small blocks have a timing shaft that is shimmed between the cases to locate it, unlike the big block or 98 on small blocks where the shaft is held against the LH bearing by the timing gear.
The bottom of the 600 cylinder did just clear the web, with the underside of the piston quite a distance away. And the rods cleared the timing shaft.
And the 620 pistons I was going to use (it was going to have 750 heads, similar chamber to 620 and much larger than the small chamber in the 600 heads, which I had sold anyway) stick out a bit, but a thick base gasket shaped spacer would fix that. The 6.5 mm stroke difference between 620 and 900 (61.5 mm to 68 mm) would suggest the spacer would need to be 3.25 mm thick.
Anyway, I digress. The point of this was trying to put a late model small block 6 speed into pre 98 cases. The complete 6 speed box I bought from Ebay was from an M696. The easiest way to show why it wont drop in is with some photos.
Input shaft:
The input shaft has a larger nut on the clutch end, like all other late model boxes. So you'd need a bigger nut and possibly to do something to the clutch hub to accommodate it. The length of the clutch snout looks to be the same from threaded end to first gear. I didn't notice any difference there, but I didn't measure them. I might do that. 6 speed at the top.
The LH end of the input shaft has an extension that goes through into the recess which the clutch pushrod passes through. As the pre 98 doesn't have the clutch pushrod passing from left to right, there's no need for the extra snout, and no room in the LH case for it anyway. It has to be removed, and I figured the drop saw would take care of that.
Output shaft:
Not a great deal of difference here. The shaft outside of the LH bearing is 10 mm longer on the 6 speed, but you'd just turn the sprocket around to fix that. The only other thing I noticed is that, while both shafts are 25 mm diameter at the sprocket spline, the 6 speed wouldn't slide into my 600 cases like the 5 speed did. Maybe it just didn't want to be there.
Shift drum:
The shift drum is the biggest issue, with quite significant differences.
The RH bearing area is much smaller diameter on the 6 speed (1 arrowed).
The RH end of the 6 speed drum has the 6 groove snout (2) for the sprung detente lever the later bikes use. You can see at the bottom left of the snout that one ridge is missing its top, that's neutral. The sprung detente lever at the RH end of the shift drum was introduced with the 916 from memory, and every new model since has adopted it. The sprung ball detente at the LH end of the shift drum is now only for neutral, so there is only one corresponding groove in the LH end.
The difference in the shift fork groove pattern is obvious, so clearly you can't use a 5 speed drum.
I had various thoughts for overcoming these issues, from cutting and shutting the end of the 5 speed drum onto the 6 speed drum to making a spacer ring and welding on a switch pushing lump and cutting off the detent snout with the drop saw. Attempting the shut once the cuts were made is the only way you would be able to determine if that was a viable option. If not, you have two scrap shift drums.
But
you still have the issue of the LH end to deal with. The LH end of the 5
speed shift drum shows the old style detente grooves. You would have to
machine these into the 6 speed shift drum to use it, which for me starts to make the
whole thing much more of a hassle as I don't have a mill. One option around this is to use a non RH
snout detente style shift drum from a pre 916 big block as a starting point,
ie, 851, 888, 900. I'm sure I have one of these around somewhere. I
did compare a 900ie shift drum, which does have the RH snout detente, and the
grooves appeared to be functionally the same. They weren't as smooth
flowing, with a step at each transition point, but all appeared to do the same
thing at the same setting.
So I didn't get any further with this fitment.
The other issue I have with the concept is that while a 6 speed is nice, the niceness comes from the ratio spread. The 5 speed ratio spread is 2.59, the 6 speed is 2.67. The big block wide ratio 6 speed is 2.88, a comparatively much bigger change. To overcome this there is the possibility of having another gear made for 6th, either 1 tooth bigger on the input or 1 tooth smaller on the output. The 26/24 ratio gives an 8% gap between 5th and 6th. 27/24 gives an 11% gap, 26/23 12%, with a ratio spread of 2.78. That makes about a 3 tooth on the rear difference between the top gear ratio of the 5 and 6 speeds. The shape of the current teeth determines if the 1 tooth idea will fly. The input shaft gear will be the easiest to make, as the output shaft gear has a shift fork groove on it.
.
Saturday, February 8, 2014
Valve train article from Kevin Cameron
This atricle by Kevin Cameron is the second in a collection of 3 regarding valve trains. Some of the comments regarding Desmo operation and its limitations may come as a surprise to some.
Sunday, February 2, 2014
Facebook Page!
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I've started a Facebook page for Brad The Bike Boy. My main plan is to use it to get to the local customer base in a short time frame. We'll see how it works.
Of course it could just be another thing I don't get round to spending as much time on as I should.
.
I've started a Facebook page for Brad The Bike Boy. My main plan is to use it to get to the local customer base in a short time frame. We'll see how it works.
Of course it could just be another thing I don't get round to spending as much time on as I should.
.
Monday, January 27, 2014
996R and 998 (and sort of 748R) Non Linear TPS Baseline Adjustment
Summary: How to set up throttle bodies
and idle mixture on the 996R and 998.
And 748R too, which are somewhat the same.
Photos with thanks from Phillip at Two Wheelers in
Griffith.
The 996R and 998
use a unique set of throttle bodies (although the 748R is similar) with shower
injectors. The photos below show the features
of these throttle bodies.
In the photos
you’ll notice I’ve numbered all the parts we need to know about in series
across all the photos. That way, if I
give a number, you know which part I mean.
Items viewed
from the LH side on the horizontal throttle body. Ducati call it the master.
1/ TPS
2/ TPS
adjustment screws (only one visible), usually T20 torx.
The TPS (1)
is the small PF3C Non Linear part used on the 2V models from ST2 onwards in
addition to the 748R, 996R and 998.
Items viewed
from the RH side on the horizontal throttle body.
4/ Throttle
cable wheel
5/ Throttle
cable mount
6/ Throttle
linkage rod
7/ Horizontal
cylinder air bleed (obscured by wiring)
8/ Horizontal
cylinder idle stop screw
Items viewed
from the RH side on the vertical throttle body.
Ducati call it the slave.
6/ Throttle
linkage rod
9/ Vertical
cylinder idle stop screw
10/ Vertical
cylinder air bleed (behind throttle arm)
11/ Throttle
linkage adjustment screw (accessed from below)
12/ Fast idle
arm
13/ Fast idle
adjustment lever screws
14/ Fast idle
cable mount and adjuster
Although it
is hard to see in the above photos, the throttle body balance adjustment set up
is a hex head screw (7mm) with a flat blade slot accessed from underneath. You can see the end of the screw numbered.
748R: The 748R doesn’t have # 11 to
14. The throttle linkage adjustment is
done by loosening the little nuts and rotating the rod, which has a left hand thread
at one end and a right hand thread at the other. It’s as frustrating as it sounds. Fast idle is done with the little button at
the twist grip, like all the other 748 – 996 models.
Fast idle adjustment for 996R and 998
The fast idle
actuating lever is obscured in these photos, but if you look directly from the
side you can see the ends of the slot in the fast idle lever and the boss that
stops it. This means the ends of fast
idle lever (12) travel are constrained, and this needs to be remembered when
you adjust the cable (14). There’s no
point adjusting the cable so the lever doesn’t move, as the adjustment for when
the fast idle arm starts opening the throttle bodies themselves is controlled
by the fast idle lever (13) adjustment, which is done by undoing the two screws
(13) and sliding the assembly forward or back, depending on whether you want
more or less. As such, adjusting the
fast idle lever (13) can be a little tricky, and realistically there’s not much
point, as below.
On the 996R
they went to a fast idle cable, but this cable acts on the vertical throttle
body. The throttle cable acts on the
horizontal, so when you open the throttle cable you open the front throttle, then
take up the free play in the throttle rod, then open the vertical throttle. So when you set the running balance (sync for
the Americans) by holding the throttle open, all the free play is allowed for.
But when you pull
the fast idle lever on, the above action is reversed and so the free play is introduced
as an imbalance. The means the vertical
throttle is open more than the horizontal.
As the TPS is on the horizontal throttle body, the vertical is open more
than the ecu expects and so will run leaner than the horizontal. All of which is not conducive to reliable
cold starting. It’s a completely stupid
design, and it’s why most 996R and 998 will start from cold better without any fast
idle and with the throttle closed or opened a little at the grip.
To set the “Non Linear” TPS
Preparation: you
need to get to the top of the throttle bodies to wind out the stop screws and disconnect
the linkage rod ends, so you really can’t do this unless the air box is removed. Removing the air box on these involves
removing the trumpets with the injectors and fuel hoses built into them. Be very careful not to crimp the plastic fuel
lines when twisting the trumpets. Remove
the wiring to the injectors first, and don’t pull to hard on the connectors, as
you can pull the plastic centre out of the steel body. When you lift the air box out you need to
feed the wiring and it’s grommet down through the hole in the base. You can leave the fuel lines in the air box base,
but one of them usually gets hooked under the vertical throttle body tabs. So when you’re pulling upwards on the air box
and it just won’t come out, remember this.
And the drain hose at the front LH of the base.
TPS
base line setting procedure (as I do it)
Every step is as important as any other, and the predominant
aim of this procedure (apart from being “correct”) is repeatability and consistency. I normally do it with the engine and throttle
body cold. This is simply the
convention, I do it hot if I have to.
Disconnect the throttle cable at the throttle cable wheel (4). Easiest to just take the cable out of the
wheel rather than play with the adjuster.
Disconnect the throttle linkage rod (6). You can pop off the plastic connector at
either end. I use a pair of side cutter
pliers with the taper side against the throttle cable wheel side – it pushes
the plastic connector off nicely. Disconnecting
the throttle linkage rod (6) removes the dumb arse fast idle system from having
any impact.
Connect to the TPS by whichever method you are using to
measure the TPS output voltage – back probing wires, Mathesis, DDS, etc. I think you need to go into the outer two
wires if back probing. You'll either get 5V, nearly 5V or 350 - 450mV. Given the
connectors fitted to the 5.9M ecu, back probing at the ecu can only lead to
much agro.
On the 748R you can back probe the 1.6M ecu quite easily. See the MV TPS for the procedure.
On the 748R you can back probe the 1.6M ecu quite easily. See the MV TPS for the procedure.
With the throttle closed before you start you should be seeing
voltage in the range of 350 – 450 mV, depending on who’s been there before you. I usually record this voltage, just in case I
need to go back to it. Remember, you
need to have the ignition on when doing this, and you don’t want to short out
any wires. Disconnecting the headlight
or removing the headlight fuse can also be a good idea if you don’t have a
"lights on/off” switch.
Back off the horizontal
cylinder idle stop screw (8) to allow the throttle butterfly to
close fully and jam if snapped or pushed.
These screws are 8mm hex.
Open and close the horizontal throttle butterfly gently
until the voltage stabilises at a minimum.
Do not snap the throttle butterfly shut, as this will jam the butterfly
and may give a false lower voltage reading.
Usually the voltage at this point will be in the range of 90 to 170mV,
most likely around 100. Again, I usually
record this voltage out of suspicion.
Once you’ve ensured the horizontal throttle butterfly is
closing properly, reset the TPS to give an output voltage of 150 mV by
loosening the two screws (2) and rotating the TPS as required. These screws are usually T20 torx head. Tighten the two screws (2) and ensure the setting
remains consistent.
Wind in the horizontal throttle body stop screw (8) until the
TPS output voltage is around 390mV. The specified
idle setting for the 748R is 2.2 degrees and for the 996R and 998 is 2.3
degrees, which theoretically equates to 380mV and 390mV respectively. I always use the throttle angle the ECU is
reading to set the “base idle” setting, as what the ECU is reading is more
relevant than the actual voltage, as there is always variation in the
relationship between them. However, you
need some sort of diagnostic tool or software to read the throttle angle, so
for many the voltage is as close as you can get. It’s still quite valid.
Open and close the throttle a few times to check the idle
setting is stable and adjust if required.
The TPS is now set and the horizontal throttle body stop screw (8)
should not be touched again.
Reconnect the throttle linkage rod (6). Remove the vertical cylinder idle stop screw (9)
and turf it. This way, the throttle
linkage rod (6) is loaded by the vertical throttle butterfly all the time and
free play is not an issue.
Reconnect the throttle cable to the cable wheel (4) and set
the free play in the cable. You can do
this at the throttle body (5) or the twist grip, up to you. On the 748R the free play sets the fast idle
speed with the button pressed. Turn the
bars fully left and right while doing this just in case the cables move when
doing that.
Refit the air box and fuel tank. You can get to all the adjusters you need to
from the RH side, though they can be a bit fiddly. Start the engine and get some heat into it.
Wind the air bleeds (7, 10) fully in. Again, I record how far out they were, just
in case I need to go back to where I started.
You will need the fast idle on to get the engine to idle, but remember
it will be unbalanced. Set the running balance
(synchronisation) via the throttle
linkage adjustment screw (11) using mercury sticks, etc at
various RPM. But remember to raise the
rpm by holding the twist grip open, not using the fast idle lever. This is often a case of best compromise, and
remember that cruise at 4,000 RPM has more throttle than free rev at 4,000
RPM. I usually go up to 6,000 or so RPM
free revving to check for variation.
It’s not uncommon to see one cylinder lead then the other lead at
differing RPM. Remember the word
“compromise”.
Next set the idle speed using the air bleeds (7, 10) as
required. Set the idle speed at approx
1100 – 1150 RPM once the engine is hot. There
is no general rule for how far out the air bleeds should be, although I find
the 748R will be nearly closed. At this
point most will set the air bleeds so the cylinders are balanced at idle. I don’t worry about vacuum balance at idle
though – we balance the mixture using the air bleeds and idle trimmer, as set
out below.
That’s the end of the throttle body fiddling. Next is the setting of the idle mixture. I often say to people that if they can’t
carry out this final step then all the previous steps may have just been in
vain. You might get lucky and be
close. Or not lucky and still have a
poorly idling or running bike. The idle
mixture can have a very large impact on general running at low throttle
openings.
For example, if the bike is too lean and idles low, winding
out the air bleeds (7, 10) to try to raise the idle speed will only lean it out
more, so the idle will most likely not increase. You can sometimes make a bike idle better by
winding the air bleeds (7, 10) in and richening the idle mixture if it was too lean
to start with, even though the idle is low.
As an indication based on experience, a full turn on the air bleeds can
change the mixture from 1% CO to 4% CO, so the air bleeds have a big affect.
To
adjust the idle mixture you need to get into the idle trimmer
I use 4% CO as the idle mixture target for the 748R and 5% for the 996R and 998. This tends to
work well and give good low speed running and fuel economy. It's a bit leaner on the 748R to try to stop the off idle hiccup that they often have with Ducati based eproms, although because the problem isn't really fueling related (it's more ignition advance) it's a bit of a badly applied band aid.
Take a sample from both header pipes using the little 1/8 gas threaded bungs. You can buy from Ducati a tube to go into this bung – it’s a piece of copper pipe with the appropriate fitting on the end – that is surprisingly cheap. You could also get some made up by a local hydraulics supplier. I have a few of varying lengths and bends for all occasions.
Take a sample from both header pipes using the little 1/8 gas threaded bungs. You can buy from Ducati a tube to go into this bung – it’s a piece of copper pipe with the appropriate fitting on the end – that is surprisingly cheap. You could also get some made up by a local hydraulics supplier. I have a few of varying lengths and bends for all occasions.
Given there is only one idle trimmer for both cylinders, I
check the mixture and then adjust the trimmer to get the average between both
cylinders where we want it. Then I
adjust the air bleeds to give the same mixture in both headers. This means that the manifold vacuum balance
or synchronisation at idle will often not be equal. That’s just how it is. Equal mixture CO% is more important in my
experience.
748R
with 1.6M ECU
The idle trimmer is a trim pot inside the 1.6M ECU, which is
under the seat. The 1.6M ECU has a rubber access plug
that is hopefully hidden under a “do not remove” sticker of some sort or some
race tape if someone has been in there before.
Once you’ve finished, cover the plug again – they can leak water into
the ECU if you leave the plug uncovered.
Either remove the whole sticker, tape, etc or just use a sharp
knife/blade to cut around the plug and gently pry it out with a little screwdriver. Once removed you can see the eprom directly
under the access hole.
The idle
trimmer is a little metal square just next to the eprom with a plastic rotating
pot inside. This adjusts the idle
mixture, working the same as the electronic idle trimmers in the 1.5M and 5.9M
ECU. It adds or subtracts a given pulse
width from the map fuel number across the whole map, therefore having a much
greater percentage effect at small pulse widths.
This trimmer
is adjusted by rotating it between the ends of its travel. The total travel is 270 degrees (3/4 of a
full turn) as shown in the next photo so if you manage to make it go all the
way round you’ve wrecked it and it’s time for a new ECU. Be very gentle! As with a mixture screw, clockwise is lean,
anti clockwise is rich. The mid point,
where the trimmer slot points directly at the eprom socket, is nominally
zero. Best to use a non metallic
screwdriver when adjusting this especially when you have the engine running and
the ECU circuit board is live. You don’t
want to be shorting anything out.
In the photo
you can see the trimmer is set at about 60 degrees lean (which looks identical
to 120 degrees rich, unless you look really hard and can see the little dots that
act as arrow head edges).
If you don’t
have a gas analyser, as a bit of a rough guide, I’ve found that the best way to
set the idle mixture is to move the trim pot slowly clockwise (leaner) until
the revs begin to drop. Then go richer
until the revs begin to drop. Now you
have these two end points, set the trimmer 1/3 of the way from the lean stumble
point towards the rich stumble point.
This usually gives 4% CO when I’ve tried this method and then checked
with the gas analyser.
There is no default
or ideal setting for the trim pot (except for the ones I specify as guides for
my eproms), just wherever it ends up.
You can manipulate it somewhat with air bleed position if you need to,
and it can have quite an impact on low speed running and fuel economy. And it does act over the whole fuel map, so
can affect WOT running too.
996R
and 998 with 5.9M ECU
The idle trimmer on these models is adjusted electronically
via diagnostic tools or software. There
is no way to get around it. Mathesis, DDS,
Technoresearch Centurion, Navigator, etc.
There is no default
or ideal setting for the idle trimmer, just wherever it ends up. You can manipulate it somewhat with air bleed
position if you need to, and it can have quite an impact on low speed running
and fuel economy. And it does act over
the whole fuel map, so can affect WOT running too.
.
Thursday, January 16, 2014
Marzocchi 43mm fork design and impact of oil level on spring rate, Part 2
.
I tested the theoretical 15 mm more preload result today, and it was pretty good. Green is my theoretical prediction, and red is the measured.
Compared to the ST 0.83 kg/mm spring and 125 mm oil level, the end result is fairly similar. It would be interesting to see how this rode, and what sort of rider it would suit. Overall, the difference between the heavier ST spring and 125 mm oil level and the original spring with more preload is fairly minor. Minor enough to make me re-evaluate my believe that you need to replace the springs. I have always been wary of just adding preload, but maybe that view was wrong. The linear spring certainly gives a more linear result from no load to bottomed, but the linearity is much more dependant on oil level than I would ever have expected prior to having this play. Yellow is the ST spring, red the Monster spring.
Next I took the oil level thing to another extreme, just to see what would happen. With the spring preloaded an extra 15 mm as above, I dropped the oil level to 215 mm. As you can see, it's moving closer to a linear rate overall, but still the rate at full compression is around double that at the beginning.
So to give a visual summary of where I started and where I sort of ended up in an untested guess, red is the start, green the end. I still don't get the desire for such an aggressive progression on effective (assembled) spring rate. Maybe if the initial travel wasn't so soft (giving so much sag) I'd see more sense in it.
.
I tested the theoretical 15 mm more preload result today, and it was pretty good. Green is my theoretical prediction, and red is the measured.
Compared to the ST 0.83 kg/mm spring and 125 mm oil level, the end result is fairly similar. It would be interesting to see how this rode, and what sort of rider it would suit. Overall, the difference between the heavier ST spring and 125 mm oil level and the original spring with more preload is fairly minor. Minor enough to make me re-evaluate my believe that you need to replace the springs. I have always been wary of just adding preload, but maybe that view was wrong. The linear spring certainly gives a more linear result from no load to bottomed, but the linearity is much more dependant on oil level than I would ever have expected prior to having this play. Yellow is the ST spring, red the Monster spring.
Next I took the oil level thing to another extreme, just to see what would happen. With the spring preloaded an extra 15 mm as above, I dropped the oil level to 215 mm. As you can see, it's moving closer to a linear rate overall, but still the rate at full compression is around double that at the beginning.
So to give a visual summary of where I started and where I sort of ended up in an untested guess, red is the start, green the end. I still don't get the desire for such an aggressive progression on effective (assembled) spring rate. Maybe if the initial travel wasn't so soft (giving so much sag) I'd see more sense in it.
.
Saturday, December 28, 2013
Marzocchi 43mm fork design and impact of oil level on spring rate
.
Disclaimer: I'm not a suspension expert, just curious. If any experts would like to comment on my rambling, I'd be more than happy. Plus I keep updating the text, now into its third day of writing and editing based on some more info from Rick. Also with Rick's replies I have realised that this leg is (possibly, maybe it never had it) missing the inner spring guide, which is a plastic slotted sleeve that goes over the damper rod above the cartridge to help support the spring from the inside. See the circled part in the parts catalogue diagram below. Also note that this diagram is shown for all the 43mm non adjustable Monster forks from 03 - 07 that I looked at, and is possibly a Showa from memory with the spring located slotted collet.
There was a post on the Monster Forum about the Marzocchi 43mm non adjustable forks fitted to a M800ie. I have an old M400ie fork leg at work, one with nasty external damage that I expect came from rubbing against something in the container on the trip over from Japan. So I figured I'd strip it and have a look at the valving system.
Typical of the springs Ducati fit to many of the non SBK models, it is a two stage style. I wouldn't call it progressive, as that implies something some see as desirable and useful. Don't see either myself.
In terms of spring rate, you get an overly soft spring until all the tight coils have bound, then you get a fairly sudden change to a much harder rate. Spring rate calculations are based on the wire diameter (bigger = stiffer), coil diameter (bigger = softer) and number of coils (more = softer). So when the tight coils bind, you go from 22 to 14 and the rate changes. The measured graph is as below, mm of compression along the bottom, kg up the LH side:
Adding two constant gradient lines to the graph shows the rate change more clearly, starting at 0.63 kg/mm and changing at 90 mm of compression to 0.91 kg/mm. In a Monster I'd usually fit 0.85 to 0.95 kg/mm springs, depending on rider weight. I don't understand why they choose these springs. But, if you cut off the tight section, you get a 0.9 ish kg/mm rate spring that is quite usable (possibly a bit short though, the Showa ones are better to cut down in my experience).
The springs as fitted have 20mm of preload. With what I find as your average rider on board they'll give 45 to 55 mm of sag. Which means they'll be compressed 65 to 75 mm and have 42 to 49 kg of load on them. More on that later.
On to the internals. The cartridge has heavily swaged ends, so is very much non rebuildable without getting medieval. And destructive. But it's always nice to see what's inside, and the pipe cutter was happy to accommodate. I didn't take a photo of the assembled cartridge, I forgot. The following photos and text are a fairly basic description of the cartridge system.
The cartridge itself is a steel tube with a rod inside it. On the end of the rod is a piston, in this case carrying a rebound shim stack. The bottom of the cartridge is at the RH side, and the valve on it is simply to allow oil to flow into the cartridge through the large round hole you can see. The small round hole is to allow oil to leave the cartridge on compression. There isn't any real compression damping function in the cartridge. When being compressed, apart from the small hole, the oil flows through the piston via large ports that have a flat plate over them. The plate is to stop the oil flowing back through these holes and bypassing the shim stack when the piston is rebounding. While this flat plate is held in place with a soft spring, it's only there to keep it seated.
The four small holes you can see are the rebound ports. The four shims to the right are the rebound shims. They act like a circular leaf spring, and the oil being forced through the ports as the piston moves bends the shims up to allow itself to pass through. On the left is the compression plate. The photo below shows the piston ports. The large ones are the compression transfer ports. The size of the rebound ports can be a restriction in themselves, and is why lighter oils are often used when revalving work is carried out. One of the advantages of the Race Tech Gold Valves, for instance, was the increased flow area. More flow area means you need heavier shims (usually just the same as shown previously, but stacked with more layers) to handle the increased flow, but also increases the ability to widen the range over which you can have effective control of the damping rates.
So, to summarise. The fork cartridge has a basic rebound damping shim stack and no compression damping function. I have heard of the damping being different left to right in these forks, but the parts catalogue shows only one cartridge part # for both legs. Having one compression dedicated leg and one rebound leg is not a new idea, but it is becoming increasingly popular again. For example, the Showa BPF (big piston fork).
I have read posts from Rick at Cogent Dynamics saying that, in his opinion, the valving in these forks is quite good for what they are, and certainly equal to the Showa non adjustable forks also used by Ducati during the era.
I thought I should ask Rick, instead of just misquoting him. His reply was:
"Who knows what I said??? ;-) It is true that while the Marzocchi fork is not serviceable or revalvable, the damping design is better than the crappy Showa, even the adjustable ones from the era. The Showa can be fixed but the list of crap wrong with those cartridges is a long one. The adjustable ones on many of the late 90s to even now for all I know are super crappy in that the rebound adjuster bleeds cartridge pressure into the cap where it effects compression damping as much as rebound. Also, from what I see on the dyno, the damper rod pumps its self up with air and causes lag in VERY hard use.
Moving on, the next stage, and something that I took the chance to spend a couple of hours doing today, was to check the effect of oil height on effective spring rate. In this case, effective spring rate refers to the impact of increased air pressure inside the fork leg as the fork compresses. The air gap inside the fork leg decreases as the leg compresses, leading to a pressure rise. Well, unless the seals are leaking. This pressure acts equally on and perpendicular to all surfaces, so it tries to expand the tubes as well. But what we are concerned with is the pressure acting on the top of the oil (assuming in this instance the top of the outer tube is the fixed portion of the spring system), which pressurises the oil (which is not compressible, unlike air) and thereby transmits that force to the bottom of the fork leg, just as the spring does.
I have an Ohlins chart for this from their manual for the R/T 43 mm forks, as below. In this application, the oil level is specified with the springs fitted. Compared to the Monster settings below, where the oil level is set without the spring fitted, the spring fitted will change (raise) the oil level, probably in the range of 40 to 60 mm from my experience. This system works in the Ohlins R/T 43 because the spring sits at the bottom of the forks and is covered by the oil. In many other forks, the spring sits at the top and protrudes from the oil, both making measuring difficult and introducing another variable (being spring design/configuration).
I did this by reassembling the fork (tack welding the cartridge) as if it were being fitted to a bike and then compressing the leg in the same way as I test springs. This way I measured the overall load on the leg as it was compressed, again in 10 mm steps. I ran 3 different oil heights with the std springs, 105, 125 and 145 mm, as measured without springs, preload tubes or seats. 105 mm is the specified oil height for these forks. Then I replaced the springs with some linear rate ones (oem ST4 from memory) and ran the test again with an oil height of 125 mm. This was as much as time allowed.
There is a small amount of obviously incorrect data in the results (bumps in the graphs), but overall the results were rather interesting.
The first graph shows the overall results, then I'll break it down. The "spring only" curves are for the springs tested out of the leg, and this is the base rate so to speak.
The three oil level settings with the original Monster springs is as below, with the Monster spring only as a comparison. I have offset the spring only curve based on the 20 mm spring preload, to try to be a little more accurate (?). And I have modified the data so all the curves start at 0, just to make it easier to read. I'm not sure if how I have done it is 100% correct, but it gets a bit confusing. The 125 mm curve is a bit high up to 50 mm of compression, so there's a little error there. But the shape of the curve is the main point of all this.
As you can see, even 145 mm oil height increases the spring rate at 120 mm compression from 0.91 kg/mm to around 3 kg/mm. The 105 mm oil height curve ends at 110 mm compression simply because before I got to 120 mm compression the total load went over 200 kg and my scales turned off. At that point the effective rate is over 4 kg/mm.
Next I replaced the original spring with a linear ST series spring measured at 0.83 or so kg/mm. It's shown in comparison to the Monster spring below. As the photo shows, the linear spring at the bottom is longer. In terms of material volume, a basic calculation shows the volume of the linear spring is 41.5 cc and the Monster spring 42 cc, an almost negligible difference. But the black plastic oem spacer at the RH end of the Monster spring is 110 mm long, 38 mm OD and 4 mm thick. Its volume is 47 cc, which is significant. This sort of thick plastic spacer is common in the Marzocchi forks, whereas the Showa have a thin walled steel tube with nylon end supports. The piece of aluminium tube I cut to preload the linear spring is 32mm od and 1.6mm wall thickness. It's what I use for new preload tubes in place of the steel originals when replacing springs in Showa forks. The piece shown has a volume of 4 cc, so the total volume of the Monster spring and spacer combo is around 89 cc, as opposed to 45.5 cc for the linear replacement. That 43.5 cc difference is significant, as the next graph shows. Assuming the ID of the fork tube is 39 mm, it calculates to an oil height difference of around 40 mm.
Both these next curves are for 125 mm oil height, green is the ST series linear spring, purple the oem Monster spring. The 27 cc greater spring/spacer volume of the oem parts relates to maybe 30 mm in oil height at a guess, which is another variable when replacing springs. Dropping the oil level with the oem setup to 160 mm or so might give an equivalent air gap change. Although, as the oem spring volume is more concentrated at the top (you always fit a non linear spring with the tight section at the top to reduce unsprung weight), this may also lead to the air gap volume decreasing at a higher or accelerating rate with increasing compression compared to the linear spring, again compounding the difference. As ever, it's a case of the unknown bringing you undone. Or just confusing you.
Adding the Monster spring with 145 mm oil height curve in black shows this more clearly.
In hindsight I should have pulled a new 0.90 kg/mm spring from the shelf to use for this comparison. The springs I usually use are between 260 and 297 mm long, depending on brand. But, unless I used a thick plastic spacer, I'd possibly end up with an even more varied result if I had a longer aluminium preload tube. The plastic tube is hard to find in this sort of size (38mm od ish) and suitably thick wall thickness, and the closest thick wall orange electrical conduit is on the small side OD wise for my liking.
The next graph sums up my frustration with the way the Ducati forks are set up as they leave the factory. The red curve is oem at the 105 mm oil height spec and orange the ST linear spring with 125 mm oil height. The green line represents 30 kg loading, which equates to 40 and 30 mm compression respectively, chosen because it represents an approximate loaded sag setting. The blue line is 145 kg, which is the force at 120 mm compression with the linear ST spring. I chose this as it is probably a good representation of the max load the fork will see (they bottom at 122 mm travel), and it illustrates my point nicely.
If we assume 30 and 145 kg represent our end points in on-road use, then the oem setup operates between 40 and 105 mm of travel, or 65 mm total effective travel. Over the same load range the linear ST spring uses nearly 90mm of travel. This is the point that I don't understand. Why make use of a little over half of the available travel? Using more of the travel with an overall softer rate at the end point will help it absorb bumps when loaded heavily. Specifically, braking hard into a bumpy corner. With the oem setup, the effective spring rate in that case would be over 3 kg/mm. With the linear setup, it is half that or less. This will allow the suspension to work, whereas the oem setup will be more likely to just bounce over the bumps and unsettle the bike.
I believe this is why sport bike forks run linear springs and are going to lower oil levels, increasing the compliance at close to full compression. From a design point of view, it may be the separation of influences that is the big plus. Much like the Showa BPF damping separation, anything that can be done to clarify influences and reduce interference between them is desirable from an engineering stand point.
Bringing the progressive spring point up again, the above example leaves me with no understanding as to why they are so popular. The effective spring rate in the oem Monster fork changes by a factor of 7 or so over the total fork travel, and leads to a reduced range of travel. It seems to me that most progressive springs start out too soft, which just gives you excessive sag. Excessive sag is just using up one end of the travel range, for no reason I can see. Even the linear ST spring gives an effective spring rate change of a factor of 2 or so over the total travel range with 125 mm oil height. So I guess the question is "how much progression is desirable?"
The linear spring gives an effective progressive rate as tested, and raising the oil level would increase that just as effectively as changing the spring would. The big qualifier there being "within reason". The relationship between internal component volume and oil height will also be relevant. All forks will be different in that aspect, and the results in this report apply only to these forks. I'm sure the Showa forks, with their reduced spring and preload spacer volume, will require higher oil levels to show the extreme increase the 105 mm oil height does in these Marzocchi forks. Going back to the 90's Monster 40 mm Marzocchi and 41 mm Showa forks, oil levels in them were 80 to 90 mm. And there is an early M900 service bulletin to add another 30 ml of oil to them to reduce dive under hard braking.
The other side of that is lowering the oil level until it starts bottoming out, then go back up 10 mm. But the spring is the first thing to get right.
One thing some do to try to help a too soft spring is to increase the preload. I have been told that a soft spring overly preloaded will give a harsh feeling towards the end of the travel, so I made another graph with the oem Monster spring preload increased. I think it's representative and reasonably accurate.
I've added 13mm of extra spring force to the measured load to give curves for 105 and 145 mm oil height and compared these to the ST spring with 125mm. As you can see, with more preload and less oil it's not too dissimilar to the ST curve in the working range. I might try to physically measure this setup, and maybe drop the oil level even more.
The extra preload may help reduce the dive under brakes too. Without any real compression damping function in their design, any help you can get there is a bonus. Raising the oil level will possibly help that too, but given these results it's a variable I'd be going the other way on for all the other requirements.
Which is not to say that the ST curve is ideal. It's just more like I would do myself, but now I may think about that more too.
I'm very pleased with the test results. I wasn't expecting such an extreme rate change with the higher oil levels. I have heard of people saying you can restrict overall fork travel with oil level, but I think personally I'd rather do it with spring rate. On a bike that is designed from a marketing viewpoint, such as the Ducati Hypermotard for example, the suspension setup (soft, long travel) is what is expected by the market, not what is desirable. and you could certainly raise the oil level on those forks to reduce the total usable travel. But you're still stuck with the excessive sag and oft associated dive under brakes, and that's the thing that annoys me the most. Replacing the springs and leaving the oil level at a normal height would give a much better result in how the bike performs. In my opinion, anyway.
As I said, I'm no suspension expert. But I do find it interesting up to the points that I can understand.
.
Disclaimer: I'm not a suspension expert, just curious. If any experts would like to comment on my rambling, I'd be more than happy. Plus I keep updating the text, now into its third day of writing and editing based on some more info from Rick. Also with Rick's replies I have realised that this leg is (possibly, maybe it never had it) missing the inner spring guide, which is a plastic slotted sleeve that goes over the damper rod above the cartridge to help support the spring from the inside. See the circled part in the parts catalogue diagram below. Also note that this diagram is shown for all the 43mm non adjustable Monster forks from 03 - 07 that I looked at, and is possibly a Showa from memory with the spring located slotted collet.
There was a post on the Monster Forum about the Marzocchi 43mm non adjustable forks fitted to a M800ie. I have an old M400ie fork leg at work, one with nasty external damage that I expect came from rubbing against something in the container on the trip over from Japan. So I figured I'd strip it and have a look at the valving system.
Typical of the springs Ducati fit to many of the non SBK models, it is a two stage style. I wouldn't call it progressive, as that implies something some see as desirable and useful. Don't see either myself.
In terms of spring rate, you get an overly soft spring until all the tight coils have bound, then you get a fairly sudden change to a much harder rate. Spring rate calculations are based on the wire diameter (bigger = stiffer), coil diameter (bigger = softer) and number of coils (more = softer). So when the tight coils bind, you go from 22 to 14 and the rate changes. The measured graph is as below, mm of compression along the bottom, kg up the LH side:
Adding two constant gradient lines to the graph shows the rate change more clearly, starting at 0.63 kg/mm and changing at 90 mm of compression to 0.91 kg/mm. In a Monster I'd usually fit 0.85 to 0.95 kg/mm springs, depending on rider weight. I don't understand why they choose these springs. But, if you cut off the tight section, you get a 0.9 ish kg/mm rate spring that is quite usable (possibly a bit short though, the Showa ones are better to cut down in my experience).
On to the internals. The cartridge has heavily swaged ends, so is very much non rebuildable without getting medieval. And destructive. But it's always nice to see what's inside, and the pipe cutter was happy to accommodate. I didn't take a photo of the assembled cartridge, I forgot. The following photos and text are a fairly basic description of the cartridge system.
The four small holes you can see are the rebound ports. The four shims to the right are the rebound shims. They act like a circular leaf spring, and the oil being forced through the ports as the piston moves bends the shims up to allow itself to pass through. On the left is the compression plate. The photo below shows the piston ports. The large ones are the compression transfer ports. The size of the rebound ports can be a restriction in themselves, and is why lighter oils are often used when revalving work is carried out. One of the advantages of the Race Tech Gold Valves, for instance, was the increased flow area. More flow area means you need heavier shims (usually just the same as shown previously, but stacked with more layers) to handle the increased flow, but also increases the ability to widen the range over which you can have effective control of the damping rates.
So, to summarise. The fork cartridge has a basic rebound damping shim stack and no compression damping function. I have heard of the damping being different left to right in these forks, but the parts catalogue shows only one cartridge part # for both legs. Having one compression dedicated leg and one rebound leg is not a new idea, but it is becoming increasingly popular again. For example, the Showa BPF (big piston fork).
I have read posts from Rick at Cogent Dynamics saying that, in his opinion, the valving in these forks is quite good for what they are, and certainly equal to the Showa non adjustable forks also used by Ducati during the era.
I thought I should ask Rick, instead of just misquoting him. His reply was:
"Who knows what I said??? ;-) It is true that while the Marzocchi fork is not serviceable or revalvable, the damping design is better than the crappy Showa, even the adjustable ones from the era. The Showa can be fixed but the list of crap wrong with those cartridges is a long one. The adjustable ones on many of the late 90s to even now for all I know are super crappy in that the rebound adjuster bleeds cartridge pressure into the cap where it effects compression damping as much as rebound. Also, from what I see on the dyno, the damper rod pumps its self up with air and causes lag in VERY hard use.
We do have our own replacement cartridges that fit right in to the
Marzocchi forks making them very good and also adjustable if wanted.
We have a couple sets running around down there on the bottom of the world with
you.
The spring/oil height combo really can improve the fork feel, those
progressive wound springs kinda work some for everyone but not too good for
anyone. Not too a bad choice if you're Ducati, I would say."
It is interesting if you also plot the top out spring effect at the
beginning of travel (before the sag). When we fit spring to those forks
we reduce the diameter of the spring guide (depending on the wire diameter of the
new straight rate spring). The cartridge you took apart has a base valve
that looks much more simplistic than the one I had apart. The one I had
used a standard type shim stack/ valve and check plate on the base valve.
I have some intact 800 SS cartridges here but I have to dig through my Damper
Dynamometer files to see if and what I have of dyno reports on those…
Moving on, the next stage, and something that I took the chance to spend a couple of hours doing today, was to check the effect of oil height on effective spring rate. In this case, effective spring rate refers to the impact of increased air pressure inside the fork leg as the fork compresses. The air gap inside the fork leg decreases as the leg compresses, leading to a pressure rise. Well, unless the seals are leaking. This pressure acts equally on and perpendicular to all surfaces, so it tries to expand the tubes as well. But what we are concerned with is the pressure acting on the top of the oil (assuming in this instance the top of the outer tube is the fixed portion of the spring system), which pressurises the oil (which is not compressible, unlike air) and thereby transmits that force to the bottom of the fork leg, just as the spring does.
I have an Ohlins chart for this from their manual for the R/T 43 mm forks, as below. In this application, the oil level is specified with the springs fitted. Compared to the Monster settings below, where the oil level is set without the spring fitted, the spring fitted will change (raise) the oil level, probably in the range of 40 to 60 mm from my experience. This system works in the Ohlins R/T 43 because the spring sits at the bottom of the forks and is covered by the oil. In many other forks, the spring sits at the top and protrudes from the oil, both making measuring difficult and introducing another variable (being spring design/configuration).
I did this by reassembling the fork (tack welding the cartridge) as if it were being fitted to a bike and then compressing the leg in the same way as I test springs. This way I measured the overall load on the leg as it was compressed, again in 10 mm steps. I ran 3 different oil heights with the std springs, 105, 125 and 145 mm, as measured without springs, preload tubes or seats. 105 mm is the specified oil height for these forks. Then I replaced the springs with some linear rate ones (oem ST4 from memory) and ran the test again with an oil height of 125 mm. This was as much as time allowed.
There is a small amount of obviously incorrect data in the results (bumps in the graphs), but overall the results were rather interesting.
The first graph shows the overall results, then I'll break it down. The "spring only" curves are for the springs tested out of the leg, and this is the base rate so to speak.
As you can see, even 145 mm oil height increases the spring rate at 120 mm compression from 0.91 kg/mm to around 3 kg/mm. The 105 mm oil height curve ends at 110 mm compression simply because before I got to 120 mm compression the total load went over 200 kg and my scales turned off. At that point the effective rate is over 4 kg/mm.
Next I replaced the original spring with a linear ST series spring measured at 0.83 or so kg/mm. It's shown in comparison to the Monster spring below. As the photo shows, the linear spring at the bottom is longer. In terms of material volume, a basic calculation shows the volume of the linear spring is 41.5 cc and the Monster spring 42 cc, an almost negligible difference. But the black plastic oem spacer at the RH end of the Monster spring is 110 mm long, 38 mm OD and 4 mm thick. Its volume is 47 cc, which is significant. This sort of thick plastic spacer is common in the Marzocchi forks, whereas the Showa have a thin walled steel tube with nylon end supports. The piece of aluminium tube I cut to preload the linear spring is 32mm od and 1.6mm wall thickness. It's what I use for new preload tubes in place of the steel originals when replacing springs in Showa forks. The piece shown has a volume of 4 cc, so the total volume of the Monster spring and spacer combo is around 89 cc, as opposed to 45.5 cc for the linear replacement. That 43.5 cc difference is significant, as the next graph shows. Assuming the ID of the fork tube is 39 mm, it calculates to an oil height difference of around 40 mm.
Both these next curves are for 125 mm oil height, green is the ST series linear spring, purple the oem Monster spring. The 27 cc greater spring/spacer volume of the oem parts relates to maybe 30 mm in oil height at a guess, which is another variable when replacing springs. Dropping the oil level with the oem setup to 160 mm or so might give an equivalent air gap change. Although, as the oem spring volume is more concentrated at the top (you always fit a non linear spring with the tight section at the top to reduce unsprung weight), this may also lead to the air gap volume decreasing at a higher or accelerating rate with increasing compression compared to the linear spring, again compounding the difference. As ever, it's a case of the unknown bringing you undone. Or just confusing you.
Adding the Monster spring with 145 mm oil height curve in black shows this more clearly.
In hindsight I should have pulled a new 0.90 kg/mm spring from the shelf to use for this comparison. The springs I usually use are between 260 and 297 mm long, depending on brand. But, unless I used a thick plastic spacer, I'd possibly end up with an even more varied result if I had a longer aluminium preload tube. The plastic tube is hard to find in this sort of size (38mm od ish) and suitably thick wall thickness, and the closest thick wall orange electrical conduit is on the small side OD wise for my liking.
The next graph sums up my frustration with the way the Ducati forks are set up as they leave the factory. The red curve is oem at the 105 mm oil height spec and orange the ST linear spring with 125 mm oil height. The green line represents 30 kg loading, which equates to 40 and 30 mm compression respectively, chosen because it represents an approximate loaded sag setting. The blue line is 145 kg, which is the force at 120 mm compression with the linear ST spring. I chose this as it is probably a good representation of the max load the fork will see (they bottom at 122 mm travel), and it illustrates my point nicely.
If we assume 30 and 145 kg represent our end points in on-road use, then the oem setup operates between 40 and 105 mm of travel, or 65 mm total effective travel. Over the same load range the linear ST spring uses nearly 90mm of travel. This is the point that I don't understand. Why make use of a little over half of the available travel? Using more of the travel with an overall softer rate at the end point will help it absorb bumps when loaded heavily. Specifically, braking hard into a bumpy corner. With the oem setup, the effective spring rate in that case would be over 3 kg/mm. With the linear setup, it is half that or less. This will allow the suspension to work, whereas the oem setup will be more likely to just bounce over the bumps and unsettle the bike.
I believe this is why sport bike forks run linear springs and are going to lower oil levels, increasing the compliance at close to full compression. From a design point of view, it may be the separation of influences that is the big plus. Much like the Showa BPF damping separation, anything that can be done to clarify influences and reduce interference between them is desirable from an engineering stand point.
Bringing the progressive spring point up again, the above example leaves me with no understanding as to why they are so popular. The effective spring rate in the oem Monster fork changes by a factor of 7 or so over the total fork travel, and leads to a reduced range of travel. It seems to me that most progressive springs start out too soft, which just gives you excessive sag. Excessive sag is just using up one end of the travel range, for no reason I can see. Even the linear ST spring gives an effective spring rate change of a factor of 2 or so over the total travel range with 125 mm oil height. So I guess the question is "how much progression is desirable?"
The linear spring gives an effective progressive rate as tested, and raising the oil level would increase that just as effectively as changing the spring would. The big qualifier there being "within reason". The relationship between internal component volume and oil height will also be relevant. All forks will be different in that aspect, and the results in this report apply only to these forks. I'm sure the Showa forks, with their reduced spring and preload spacer volume, will require higher oil levels to show the extreme increase the 105 mm oil height does in these Marzocchi forks. Going back to the 90's Monster 40 mm Marzocchi and 41 mm Showa forks, oil levels in them were 80 to 90 mm. And there is an early M900 service bulletin to add another 30 ml of oil to them to reduce dive under hard braking.
The other side of that is lowering the oil level until it starts bottoming out, then go back up 10 mm. But the spring is the first thing to get right.
One thing some do to try to help a too soft spring is to increase the preload. I have been told that a soft spring overly preloaded will give a harsh feeling towards the end of the travel, so I made another graph with the oem Monster spring preload increased. I think it's representative and reasonably accurate.
I've added 13mm of extra spring force to the measured load to give curves for 105 and 145 mm oil height and compared these to the ST spring with 125mm. As you can see, with more preload and less oil it's not too dissimilar to the ST curve in the working range. I might try to physically measure this setup, and maybe drop the oil level even more.
The extra preload may help reduce the dive under brakes too. Without any real compression damping function in their design, any help you can get there is a bonus. Raising the oil level will possibly help that too, but given these results it's a variable I'd be going the other way on for all the other requirements.
Which is not to say that the ST curve is ideal. It's just more like I would do myself, but now I may think about that more too.
I'm very pleased with the test results. I wasn't expecting such an extreme rate change with the higher oil levels. I have heard of people saying you can restrict overall fork travel with oil level, but I think personally I'd rather do it with spring rate. On a bike that is designed from a marketing viewpoint, such as the Ducati Hypermotard for example, the suspension setup (soft, long travel) is what is expected by the market, not what is desirable. and you could certainly raise the oil level on those forks to reduce the total usable travel. But you're still stuck with the excessive sag and oft associated dive under brakes, and that's the thing that annoys me the most. Replacing the springs and leaving the oil level at a normal height would give a much better result in how the bike performs. In my opinion, anyway.
As I said, I'm no suspension expert. But I do find it interesting up to the points that I can understand.
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