The Exasperating World of Motion Ratios

[Christopher Crowe]

There seems to be a consensues that a good motion ratio to shoot for is around 1:1. However, I'm finding it very hard to get that ratio totally constant, from the first .1" of movement to the last .1". That is, no matter how I seem to configure, there's a measureable amount of rate change (rising or falling) through the rocker's total travel from full bump to full droop.

I know that alot of rising or falling of spring rate creates all kinds of complexities... so just how much rate-change is acceptable through the suspension's full travel? If there is a progression from say, 1:1 to .9:1 over the course of that travel... is that acceptable? What about .8 :1 from ride-height to full bump (etc., etc)? In other words, just how criitical is this measure.

Again, I'm just finding it very difficult to get a totally flat spring-rate through the full range of motion -- full droop to full bump -- and need some idea of what folks feel the limits are.

Thanks for any help in this tough area.

[David Ferguson]

What is your design suspension travel?

[S Lathrop]

You should be able to keep the ratio within .05:1 of your target. Ratios between 0.8:1 and 1.1:1 are acceptable but many good cars have ratios below 0.8:1. The demands on shock valving increase with higher ratios. In short you will move the shock further for any chassis displacement, thus any error of shock valving is magnified.

The actual ratio you want is more dependent on the tires you are going to use and the wheel rates that best fit the characteristics of those tires.

Now if you want to really muck you brain, look at the motion ratio at various ride heights and roll angles. This is how things actually work.

The trick with a winged car is that your ride height varies with speed. I plan around 0.5 in. difference in the cornering ride height at various speeds. On fast straights, the car will have no more than 0.25 in. ground clearance.

[Don Conner]

Having just done all this when working on our JDR car, I'll throw my two cents in here. You should work backward from your target wheel rates and damping rates and choose your motion ratio to get what you want. What you'll find is that what you will want depends highly on the car. The trouble you can run into is if you go to high (damper/wheel) you may not be able to get soft enough spring rates (within reasonable lengths) or soft enough damping. Also if you are going to have a non-linearity you should error on the rising rate side. Think about this...the Van Dieman one of the most successful small cars for 14 years and running has MR's that are much less than 1/1 (around .7 to .8) and the front is noticeably rising rate going as high as .9 to 1 under normal conditions. Swift FA MR's are on the high side around 1/1 slightly more I believe but then again that's a car which due to the high downforce would demand stiffer wheel rates than a lower downforce or lighter car. Also just to note the JDR is "linear" front and rear.

Don Conner

[Christopher Crowe]

Thanks to all.

And Don, are you folks gonna put together a JDR website? I'd love to see more photos of your creation.

Again, thank to everyone for the input.

Chris

[Don Conner]

I'm not much of a web designer but I put one together.

www.jdrmotorsport.com

[JohnPaul]

really interesting sidpods. Looks like you take full advantage of the body width rules. Post some pictures of them if you can. Particularly of the space between the side panel and air scoop on the side pod. That is cool stuff

[Christopher Crowe]

Thanks for the link. Very cool car.

Does the chain run through a tensioner sproket?

Chris