Depending on the accuracy you require, you will find that the motion ratio is not a number but a function of the suspension location.
Only when the shock is tangent to the lower control arm can you directly use the pivot locations, everywhere else requires some trig (and calculus if you want to create a complete mathematical model) to determine the actual change in shock length VS change in tire position.

 

Having geeked out on all this stuff for a while, I can say you're most likely wasting your time. All that this theory is good for is getting a starting point, and it's much more efficient to just ask around for sound advice.

A more productive use of geek-out-time would be fitting all your shocks with transducers and logging. Unfortunately, that gets very expensive quickly. But at least here there's (sort of) a target to shoot for. With the calcs, the shortcoming is: "what's the goal?"

 

I think for the most part, this is pretty true. I guess it depends on ones level of knowledge and experience on the subject, the time one is willing to commit and the cars and type of racing in question. You are right though, its only a starting point. But at least the math can get you in the ball park so you’re not chasing your tail. Ride frequencies are pretty useful to get an idea of what kind of ride you can expect. With cars that are not strut based, this is particularly hard to do without some math and the right formulas. High and low speed shock valving the same.

At least with the last two cars I had shocks custom valved for(the last ones where Penske) there were three shock 'experts' that had completely divergent valving theories and standards... and they would not share their dyno info...” just pay up, trust me Im the best.”

Im different than most guys in that Ive read quite a bit on suspension and shock valving theory over the last 10+ years, designed and fabricated several suspension related parts, and created a XLS that takes suspension measurements, weights and calculates ride frequency, critical damping, and shock dyno curves based on that info and another XLS that helps size brakes and bias based on forces from the pedal to the contact patch. (If anyone wants a copy, shoot me a PM of email and I will dig them up) All of this work, failure, and success was while competing and winning both regionally and nationally in road racing and Solo 2.

At least for me, its been to my benefit not to trust the 'experts' and learn it so I can be pissed at myself if the results suck . These were not Porsche projects so no flame please.

It may be that Porsche folks that compete have good combinations and are completely transparent with their data. That hasn’t been my experience in the past.(again, not related to porsche)

At the very least, the more you know, the better you can converse with folks who have more experience than you which is always good.

David

 

Not to mention having a better idea in your head of what is going on.

I was once involved in restoration of a vintage sports racing car with a SLA suspension where we had no idea where to start with spring rates. So we used the method Chris Walrod mentioned, but without dial indicators, just trusty old tape measures. This was easy in this case because the car had the body off. The derived wheel rates gave us a good place to start and the rates have not changed much from that ever since.