Starman, you can see from the pictures the leverages that are at work. They are:
1. Percentage of distance between chassis pivot and pushrod mount to distance between chassis pivot and lower ball joint. 2. Angle of pushrod to wishbone. 3. Angle of pushrod to rocker. 4. Relative lengths of rocker legs. 5. Angle of rocker to shock.
Something to watch out for with your angles is that at 90 degrees you have 1:1 leverage, so if you are approaching 90 degrees you have a rising rate, and if the angle is falling away it is decreasing rate.
Guesstimating some values to illustrate using modernbeat's setup, we have:
1. About 0.85 2. Call it 60 degrees, so sin 60 = 0.866 3. 90 degrees; sin 90 = 1.0 4. Looks like about 3:2 or 1.5. With motorcycle shocks the outboard leg will typically be the longer one. 5. Call it 90 degrees for a factor of 1.0
Multiplying these together we have 0.85*0.866*1.0*1.5*1.0=1.104 which would be the motion ratio. In other words, raising the wheel 1" would compress the shock by 1.104".
To relate wheel rate to spring rate, we would divide the desired wheel rate by the square of the motion ratio to get our target spring rate. Let's say we want a wheel rate of 100#/inch; that would give us 100/(1.104^2)=100/1.219=82. So, an 82# spring would provide a 100# wheel rate.
Again, a motorcycle shock setup will have a lot more leverage, where a 1" bump will compress the shock 0.5" or less, to convert a spring rate of 550# to a wheel rate of 100# or so. Also, with a motorcycle shock I think I would want a sturdier, wellbraced double shear mounting for the bottom of the shock due to the greater force being applied to it.
This estimate was very rough and intended only to illustrate the principles. In reality, you'd want precise measurements and to model the action in CAD or with a scale mockup, checking your motion ratio in steps from resting position to full bump, in order to make sure that the wheel rate is constant or perhaps slightly progressive.
_________________ Dreaming the dream since 2003. Haynes chassis, Miata uprights, everything else from 1987 NA RX7 donor.
