Calculating motion ratios

[elcaprerunner]

How exactly are suspension motion ratios calculated? I assume it has to do with wheel travel, control arm length, and shock position, but what is the formula to figure it out? Do you want a large motion ratio or a small one? (I assume small) Is this ratio very important to handling characteristics of the car or just semi-important?

[ChuckH]

The most travel shocks have is about 16-18 inches so you have to have a small ratio travel shock, in it also allows you to use a smaller lighter shock like a MX rear suspension or if you don't have much room.

Is this ratio very important to handling characteristics of the car or just semi-important?

Both can be made to work, it depends on what your working with, like with a MX bike, the front is a 1-1 ratio, the rear is about a 1-4 ratio and they both work well, it all depends the design constraints.

Here is the math for a simple set up, a MX bike has linkage that makes it more complex

http://e30m3performance.com/tech_articles/susp-tech/eff_rate/eff_rate.htm

[ChuckH]

Edit, it posted twice for some reason, so i erased the second one

[ntsqd]

There is also the SAW (http://www.swayaway.com/Suspension%20Frameset.htm) page on the topic; which gives less info on the whys, but offers a plug-n-chug.

[grantdcol]

Typically, the motion ratio is not static, it changes throughout the range of travel.

When dealing with velocity sensitive shocks, the idea is to speed up the shock shaft as the suspension approaches full bump, to increase shock damping. This allows for a suspension that is soft and smooth, with good terrain following, but stiff when you need it.

On an A-arm application, the motion ratio might be 2:1 at full droop (the arm moves two inches for every inch the shock moves), and increase to 1:1 at full bump (the shock perpendicular to the arm). These ratios only exist for a single point in the suspenion travel.

-Colin

[elcaprerunner]

Thanks alot guys. This term gets thrown around alot and starts to sound complicated at times, good to know it is simple and something I already knew how to do/use, just never put the name with it. Thanks for the links too, those make it easier to determine. Pretty much just the idea of finding the mechanical advantage of your shock placement.