Wednesday, May 21, 2014

Showa 43mm adjustable fork design and impact of oil level on spring rate, Part 2: 748 / 916 / 996 / etc

Today while I was doing a major service on a 748 I took the opportunity to throw a fork in the rig and see how it compared to the others.  I really wasn't sure what to expect, but was surprised anyway.  It's always nice to be surprised.

While the 748 forks (same as 916 and 996) are also 43 mm Showa adjustable, they differ internally to the M900ie forks by having the long rebound adjuster rod as discussed long ago in the 1000SS fork post.  All 916 on SBK have this style fork (except 749 Dark).  Assembly wise, there is a 100 mm long thin wall steel tube spacer and spring supporting washer below the spring and the usual Showa style preload tube above the spring.  I set the oil level with the lower spacer and washer fitted, as the manual doesn't specifically say to not fit them.  I'm not sure if that's right or not, but it's not mentioned and will result in a lower final oil level and I'm liking that these days.

The 748 forks have an oil spec of 132 mm.  I suspect this one was more like 142 mm before I pull them apart.  I couldn't get an accurate measure as the bottom plastic bush on the preload tube came off in the forks, making it pretty much impossible to get the spring out without tipping them upside down and shaking.  But the level with spring and bottom plastic bush in was about 10 mm lower before compared to after when set to 132 mm.

The spring in the 748 is a linear spring 285 mm long and the rate calculated at 1.00 kg/mm.  I didn't have time to measure it, but the calculation is generally very accurate.  So while it's a bit heavier than the others, it's fitted with only 8 mm of preload with the adjusters at the minimum setting.  Like the M900ie fork, the adjuster range is 15 mm.

To the curves: red is 748 with minimum preload, blue 748 with maximum preload, yellow M900ie as was originally with maximum preload and green is how I set the M900ie fork up at the end.  I don't feel so bad about my M900ie setting now.  Again, the coincidence of unrelated results is quite amazing.

Also again, it amazes me that you can hit (so near to) the same target with such variation in spring rate.  Although the rate variation in this case is a little less than the variation in the M400ie leg.  And the unknown is the variance in internal fork volume once assembled, as the different springs and spacers all contribute sometimes vastly different volumes.

This graph shows the collection of springs as encountered in these reports so far.  For this report, purple and green are the contributors.

One point that just occurred to me is that of fork swaps.  Often you'll read forum threads of where someone has fitted SBK forks to their Monster or other and the poster will be told of how the spring rates will be wrong.  I might even have said it myself.  But the above comparison of the 748 "as delivered" curve and what I decided was a good curve for a M900ie shows once again that, unless you actually check something, you really don't know whether you're talking through your arse or not.

Of course, all of this is separate to damping rates and their influence.  I'm not getting into damping rates at all.  That's completely out of my realm of experience, and it's an area where specialist experience is what matters.

No comments: