Ji,
Yes, accounting accurately for dynamic loading is all-but beyond me too these days. 30yrs ago I may have relished it, but I've long since (in a moment of madness) thrown out my dynamics text books & forgotten the formulae. And the 'grey matter' is more than a little sluggish these days.
The Load Factor in #2 formula does seem to offer some accounting for dynamic loads tho, even if thro a whole lot of assumptions/approximations. It's just a rule of thumb that seems to get you in the ballpark, but needs to be checked w time on the track(s).
Just reviewing the thread so far, there seems to be a few oversights in several of the calcs, eg:
Spring rate for doing a wheelstand is going to be diff for when the fr wheel is just off the deck to when the bike is at say 45deg, cos the movement of the rear axle is almost 90deg to ground in the former but around 45deg in the latter. (More variables!) The worst-case scenario would be the former, which is probably what you calc'd for. The figure you got for it sounds too high tho.
If I perform the same jump and allow the spring to completely compress or bottom and have no extra travel left what spring rate would I need.
Answer 140lb/in
It is obvious that the perfect spring rate on a bike with 4inches of travel is unattainable.
Thus for my CZ400 I would need a progressive spring with the rates of 82-140lb/in
This should give me my ride height and just bottom out over a good jump.
Unfortunately there's a slip-up there. Its not sound reasoning. Equivalent spring rate of an 82-140lb progressive spring (& hence total compression force at bottoming) is something considerably less than for 140lb straight-wound, so according to yr calcs, if it just bottoms w a 140lb staright-wound spring it would bottom terribly w an 82-140lb progressive.
I may well be wrong, but there also appears to be someting amiss in a 0.5" preload changing the required spring rate from 126lb to 82lb. That seems excessive. Working on a 4" shock stroke, I would have tho't an 82lb spring w 0.5" preload equates to a 92lb spring w'out preload when the shocks are just bottomed. But perhaps I'm still brain dead.
However I like yr Table in Reply #38. The results seem a bit hi (by about 15%) for what was widely used back in the day, but its reasonably in the ball park & perhaps accounts for us all being somewhat heavier these days. Or maybe we ran w more than 30% 'live' sag in the 70's. (We never checked/measured in those days. We just worked on what just bottomed occasionally on big-hits.) More likely tho, the diff also accounts for the amount of friction in the system &/or compression damping in the shocks. Perhaps you could use a factor like that (say 10-15%) & write it into yr formula to account for friction & comp damping in real-life situation. Or that could be another "what-if" variable.
I have reservations about working out spring rate just by sag/ride ht, but as has been said, its another way of arriving at a starting point & will have to be adjusted w time on the track(s). Y're perfectly right in saying dynamic forces are a "moving target". There are so many variables
