Showa 43mm adjustable fork design and impact of oil level on spring rate, Part 1: M900ie

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Or, keeping the same title so as to be consistent with the previous playing with forks.

So I got to play with a set of 43 mm Showa adjustable forks from a 2000 M900Sie today.  Well, I'm halfway through it.  I have serviced the forks in this bike once previously, and filled them with 5 weight oil to the spec height of 108 mm.

I thought to start with I'd do the same travel vs load graph (mm across the bottom, kg up the side) as for the Marzocchi forks, and then thought I'd add the original curve for the 43 mm Marzocchi with oil set to the spec height (105 mm?).  Turned out to be a bit spooky, frankly.

Red is the Showa with preload at maximum, yellow Showa with preload at minimum and blue Marzocchi.  Not really what I was expecting, and a wee bit eerie.  Obviously someone at Ducati thinks this is an ideal fork setting.  The difference between minimum and maximum preload is 15 mm, with this translating to 19 and 34 mm of total preload in the assembled forks.

More to come.  It's going to get some 0.85 kg/mm springs (rider 80 kg) and I'll drop the oil to 140 or so mm.

Ok, so on to the more.

I was very surprised by the fact that the air spring effect in the Showa was pretty much identical to the Marzocchi.  Looking at the parts, the Showa has a longer spring (349 mm) that sits at the bottom of the fork around the cartridge, with a long, but thin steel preload tube (277 mm) above it.  The Marzocchi has a shorter spring (249 mm) that sits on top of a stepped washer that itself sits on top of the cartridge, with a thick wall plastic preload tube (38 mm OD, 30 mm ID, about 100 mm long) sitting above (or maybe below, now that I think about it) the spring.  So perhaps it's just the visual weight of the shorter, but thicker Marzocchi preload tube that gives the impression.  Anyway, it was certainly not what I expected.

I replaced the original springs with a shorter, mostly linear rate spring (with a couple of tight coils at one end) which are meant to be 0.85 kg/mm.  I tested them on my amateur rig and came up with the curve below.  At 50 mm compression they average 0.85 kg/mm, but by 100 mm compression they're up to 0.90 kg/mm.  The OEM and ST series springs from the previous Marzocchi report are also included.  The two OEM springs are pretty close overall.  The Showa spring curve starts a bit higher, as it was a touch too long for how I had my rig set up, and also after midnight.  Dimension wise, the Showa spring has less coils (harder), but thinner wire (softer) and they're both 38.6 mm OD.

I fitted the 0.85 springs with 10 mm of preload (adjuster at minimum).   These springs are 282 mm long, requiring longer 335 mm preload tubes.  This is usual when replacing fork springs.  The only springs I know of that come at the same length as the originals are Ohlins, where they generally have a uniform length for a given application.  For the preload tubes I use 32mm aluminium tube with a 1.6 mm wall.  Unfortunately, the original steel tubes have a slightly larger ID, which means, when using this aluminium tube, I have to machine down the plastic end pieces Showa use to allow them to slide inside the aluminium.  The chuck ID on my little lathe is just a touch too small to allow the tubes to slide inside and be held at the machining end, so it's much less destructive to do the plastic pieces.

I set the oil level to 140 mm, thinking this would be a good starting point.  I did check the difference in oil quantity between 120 and 140 mm, so I could make changes based on quantity once the forks were all together.  This saves having to strip them all apart again.  At 120 mm oil height, the fork had 475 ml of oil in them.  Dropping the level to 140 took out 27 ml.  So if I wanted to go back to 120 mm I would just add 27 ml.  Or, conversely, if I wanted to go down to 160 mm, I could just suck out another 27 ml.

So, to the curve for 0.85 kg/mm springs with 140 mm oil height.  Not exactly what I was hoping for comparatively.  Red is OEM with max preload, yellow is OEM with minimum preload, blue 0.85 with max preload, green 0.85 with minimum preload.

I had figured that the 0.85 kg/mm springs would need about 5 mm of preload to give the right sag, which would place them about 1/3 of the way between the green and blue lines.  But the overall result is still a rather high rate of increase in effective spring rate near full compression.  Adding two further curves from the Marzocchi fork report, being the OEM spring with preload increased by 15 mm in orange and oil set to 145 mm and the ST series spring with oil set to 145 mm in purple, shows what I ended up with there.

So we come back to the question of how much load will the forks see?  Peter at Promecha told me that the maximum load (non impact if you like) the forks will see is if the bike is standing on its nose under hard braking with the rear wheel in the air.  And also that you don't spring them that hard, as it doesn't really work that way.  Which makes sense now that I've seen the impact of the air spring.

If we assume this M900Sie weighs 200 kg ready to go, and with the rider weighing 80 kg, plus 5 kg for gear, and taking off 5 kg to allow for the unsprung weight of the front wheel and fork lowers, that's 280 kg or 140 kg per leg.  These forks have around 115 mm of travel.  If we sprung it assuming the springs themselves supported all the load, 140 kg at 115 mm of travel gives a rate of 1.22 kg/mm.  But once we introduce the air spring effect, the required spring rate drops quite a lot.  With the 0.85 kg/mm springs, 145 mm oil level and my planned 5 mm of preload, that load will only give around 100 mm of compression.

How the bike was set up when it arrived, with the preload set to maximum on the OEM springs (the red curve), the same load would have given 95 mm of compression.

So it looks like I need to lose some more oil.  I'm quite disappointed with how this is turning out.  Not at all as I had expected.

Back again after losing some oil.  I sucked out 40 ml of oil, which should relate to 30 mm.  The change is somewhat as expected, as below: purple 140 mm, red 170 mm.  Both curves with minimum preload.

With my intention of running more preload, I made another calculated curve based on adding 5 mm of preload (1/3 of the maximum) and raising the oil level to 155 mm, 1/2 way between the two tested levels above.  The result wouldn't be exactly 1/2, as the change in air spring rate is slightly exponential.  I gave it 60% as a (random) guess, and came up with the curve below.  It's the yellow one.

I haven't ridden this one as yet, but given the chances of me exploring the end limits on a customer's bike during a suburban road test are non existent, I'll have to wait and see how the owner finds it.  The graph below shows how it arrived in red and how it is now in yellow.  There's not a lot of difference to show for my several hours of messing around, but the change in the last 1/3 of travel is what this has all been about anyway.  I would hope it's much more compliant, and less likely to make the big bumps feel big.  If that's not a poor way to describe it.

I got to thinking today that I had seen a fork oil level spec somewhere that always seemed pretty odd.  Turns out it was the 888.  The 888 has 41 mm Showa adjustable forks, much the same internally as the 43 mm forks fitted to this Monster.  The carburetor model 900SS used the same forks as the 888, albeit with the soft crappy dual rate springs in place of what I recall are much firmer, linear rate springs in the 888.  The oil level specified for the 888 is 162 mm and for the SS it's 108 mm, the same as this Monster.  I wonder why there was so much difference?

The 916 oil level is 135 mm, and the manual says "Oil quantity affects fork behaviour at full compression.  Compression load increases or decreases with oil level."  It also gives a minimum level of 150 mm and a maximum of 106 mm.

Which brings us to the end for now pretty much.  I'm curious to try some of the other forks I have kicking around, being the 40 mm Marzocchi and the 41 mm Showa non adjustable forks fitted to 90's Monsters.  Both of these had quite high oil levels specified, 90 mm for the Marzocchi and 80 mm for the Showa.

Plus there was a service bulletin for the M900 with the 41 mm Showa that recommended adding 30 ml of oil on top of the 80 mm spec.  This was aimed at helping the fork performance at full compression.  I'm curious to run them on my test rig to see how they behave, as it just seems like way too much oil.

More rambling to come.  Lucky you lot eh.
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