For anyone interested, here is what Sachs, Italy says about the stock shock's spring:
K1= 8 kg/mm from 0+/-54 mm F1= 432 Kg
K2= 10,5 kg/mm from 54+/-90 mm F2= 810 Kg
And their recommendation for a rider of 170 pounds is to increase 2 mm preload over stock and adjust 2 clicks rebound.
If anyone has thoughts on whether the above is or is not a suitable spring rate for a 170# rider on an M620, please weigh in.
I;ve been told the 748 Showa's spring is pretty soft in comparison (and my butt shock dyno agrees although I don't have preload set properly yet). I think I'm going to swap the sachs spring to the showa shock body and see where we go from there. I'm also going to add in one of those Traxxion preload roller bearings to allow easier adjustment of preload. Its such a pita to get at, anything you can do to make it easier to adjust seems like a good idea...
The 748 shock certainly feels a bunch better than the stock sachs did -- I can actually feel both sides of bumps now instead of being launched over them. If the spring swap works well, this is a pretty cost effective modification for newer monsters....
I just bid on and won a showa off ebay. Is there anyway to tell what spring is on it? The auction said 916 but who really knows. What are the preload roller bearings you are talking about?
P.S. thanks for posting your research results. No replies are a good indication no one knows the answer. :-/
Other than with a shock dyno or a homemade scale and a big ass lever and , I don't know of a way. And if you call Showa with the part # of the shock body, its not definite b/c someone could have always swapped the spring.. In my case, I know the sachs spring is stock and I now know the rate for that spring, so at least I'm cutting down a couple variables.
BTW, Ohlins recommends a 10.19/kg per mm for 170# rider on this bike. Looks to me like the stock sachs spring (8-10.5) is a bit on the soft side but with properly dialed in preload should work just fine....