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JLeather wrote:

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Here's my calculation page (sorry, I'm at work so I had to scan it to a PDF and take a screen-shot to post it here).

http://i86.photobucket.com/albums/k113/JLeatherman_CF/ShockForceCalculations.jpg

This is assuming a resting force of 250# at the back wheel. The equation is there, however, if someone feels their bike distributes 300# or 350# to the back wheel. If anyone knows the spring-constant (k-value) of the stock shocks I could tell you how much it will compress between resting and you getting on it. The stock shocks have lienar springs I think, not progressive springs, which makes it easier. Point is, at 45* it's not really that significant of an increase (100# of force total, or 50# more per shock). At 30* it doubles each shock. 30* would be significant, but still do-able with stiffer shocks. I'm sure you could find shocks with double the spring-rate of stock that could be made to fit.

Would it work with stock shocks? No, atleast not very well. Technically even lowering blocks or a 2" stretch swingarm changes the force of the stock shocks but lots of people do that without issue on chops. I can't tell what shocks are on the bike in that auction, but it CAN be made to work.

Once again.....

Define "work". What counts as it "working"?

If it sits on the bump stops and rides like a rigid, then it doesn't work.

You post some working out that shows you need a 40% increase in static force for a 45 degree shock angle, and yet you still haven't addressed the issue of what's going on in the relationship between vertical wheel movement and linear shock travel.

Just to make it easy to follow, assume that the relationship betwwen vertical wheel movement and linear shock travel becomes 2:1, in other words, for every 2" the wheel moves vertically, the shock gets 1" shorter.

Say the stock set up used 100 lb/in springs.

In the stock set up, a force of 100 lbs applied to the wheel will move it vertically 1"

But....

Wth the 2:1 wheel to shock ratio moving the wheel vertically by 1" only compresses the shock by 1/2"

Hold on a moment. If 1" of movement at one end results 1/2" of movement at the other end, that's a lever isn't it?

As any idiot knows if you've got a lever with a 2:1 ratio, then a force of 100lbs on the "2" end, will need a force of 200 lbs on the "1" end to resist it, or balance it as we usually say.

BUT.....

Because the shock only moves 1/2" to apply that force, we need a new spring rate of 200lbs / 1/2in, or, 400 lbs/in. So that's the square of the difference.

Harking back to "theta" for a moment, as the swing arm rises, "theta" decreases. Which is a bit unfortunate, because the rubber sleeved top mount and metalastic bottom mount on the shock aren't actually designed to cope with much in the way of angular movement which is why they'll wear quickly. That niggly little point aside, as "theta" falls, the ratio between vertical wheel movement and linear shock travel falls too.

Which means that the perceived spring rate at the wheel is falling, as the square of the change in ratio.

That's some pretty fucking stiff shocks you're going to need there Cochise.

Point is, math is like a box of wrenches.

Pick the wrong one, you'll screw the job up.

For instance, using vector analysis to analyse suspension sytems......

So what you're saying there, is that despite having spent all this time wittering on about doing the math, you haven't actually done the math...