Caster in the rear suspension of an LMP 1-2 type car

[Chlsmaier]

I am building my own closed cockpit style "prototype" sports car. It will resemble the latest Lola/Judd that Sebah racing runs. I am stuck on a few geometry subjects in the rear of the car. Running caster in the rear of a double A arm suspension? Some call it trail. Should I? if so, how much and why? Static or so many degrees at say, 1" compression..?
Question #2 If I'm running 2" of static ride height and were to set up a push rod assembly for the dampers, would you run 1:1, digressive or progressive rates from the wheel to the shock/spring & why? I "think" I would like to run lower spring rates and keep the ultimate force up..? Finally, I have been to Le Man a few times and looked over the wide variety of pick up points for the suspension. Is there a good logical reason for the A arms to go forward to the center of the car vs, even spread or towards the back of the car. Standing at the RR looking at the transaxle, I am currently laying out the upper and lower A arms to run 2" behind the axle and about 22+" forward, just in front of the motor; like an old GT 40. Thoughts??
Thanks!!

[Mystery Steve]

Is there a good logical reason for the A arms to go forward to the center of the car vs, even spread or towards the back of the car.

This is mostly driven by packaging and load path efficiency. The most correct answer is "it depends..." If this is to be used for racing purposes, the required weight of the a-arm should be considered. If this is just to be a big toy, then I would probably opt for whatever is easiest to make and that fits within the overall package.

would you run 1:1, digressive or progressive rates from the wheel to the shock/spring & why?

I would NEVER run digressive rates. (A side note on terminology: spring rate is defined as the force required to displace the spring a unit distance and wheel rate is defined as the force required to displace the wheel a unit distance through its travel) With a digressive wheel rate, this means that less and less force is required to move the wheel as it travels in jounce. This means that as the car rolls in a turn and the suspension is compressed it will feel unstable and want to compress the suspension more the further it compresses. However, with a progressive rate it requires more force to compress the wheel as it travels in jounce and this feels much more stable when you're in the driver seat. A common control analogy used for stable/unstable is a ball:



Figure 1 would be the example describing progressive rates, and Figure 2 is the digressive rates. Before you start building/designing I would recommend picking up a copy of Tune to Win by Carroll Smith as a starting point for learning about suspension design (there are other books to look into, but I would start there). It will more than pay for itself in the end.

[Chlsmaier]

#1 Thank you!
Did you have any thoughts on Caster in the rear?
When I said digression, what I really meant was less wheel travel and more shock travel. Like 1" wheel travel and 2" of shock without the spring rate increasing comparitively. Yet, your answer guided me, thanks!
-Yes, this is a serious project race car. My brother is the suspension mind in our team; i'm trying to catch up and learn. He also recommended Carrol's book;-) I have all my back ground in carbon fiber parts.
All the best!
Charles

[Jersey Tom]

First things first...

Caster and mechanical trail are related but NOT the same... they are independent by means of longitudinal spindle offset.

On the rear, I'd probably want fairly low caster angle so I can make rear toe changes without changing static camber.

With regard to total trail, that depends on what you want from your compliance rates. Running a large mechanical trail will generally increase steer compliance since there is more torque about the axis of rotation. Usually that's a bad thing and you want to keep compliance rates low.

Sometimes there's a benefit to it. If you don't know how to take advantage of it though, I'd go with zero mechanical trail. Maybe even slightly "reverse" to offset tire pneumatic trail.

Second... you are talking about two different things. The motion (or installation) ratio you choose is a function of a variety of things. Personally I'd want to use as much shock travel as possible (more damper travel than wheel travel). It lessens the effects of friction in the damper (seals, etc) to increase mechanical grip, and also means you're developing a worthwhile amount of damper velocity so you can actually do some tuning.

There are limitations of course... with spring and damper rate availability and how soft you can go.

Progressive, linear, and digressive rates are something completely different. They relate to how much your motion ratio changes through travel. Linear is the simplest to tune to. If you're clever and really understand the system, there can be benefits to running various amounts of progressive (or even digressive) rates in your front and rear rockers. However, it's also going to mean that your roll stiffness distribution is going to be a function of chassis roll (in addition to everything else) which can really throw your tuning for a loop if you aren't totally aware of what's going on.

You can also use progressive-wound springs, with linear ratios, for the same effect.

Another benefit of a progressive spring or rate is that you have small force variations for a given displacement when the spring is extended (like driving in a straight line), but then can still give you a lot of ultimate stiffness when you're really leaning on it in a corner.