Can someone explain wheel rates

[Crimson Racing]

I understand that
motion ratio = wheel travel/spring travel and
wheel rate = spring rate/(motion ratio)^2

However, I don't understand what to design for. I've found several posts that say a wheel rate of 100lb/in is a good place to start, but why? Also, what would the differences be between front and rear rates?

We're designing around a 650lb car (w/ driver)

[Crimson Racing]

I understand that
motion ratio = wheel travel/spring travel and
wheel rate = spring rate/(motion ratio)^2

However, I don't understand what to design for. I've found several posts that say a wheel rate of 100lb/in is a good place to start, but why? Also, what would the differences be between front and rear rates?

We're designing around a 650lb car (w/ driver)

[CappyUMD]

The usual definition is motion ratio = spring travel/wheel travel (they were flip flopped)

which makes wheel rate = spring rate * (motion ratio)^2

I suggest reading the appropriate chapters in Milliken's Race Car Vehicle Dynamics. Anyway, here's one method:

Decide how much vertical movement you will tolerate on each axle during steady state braking and acceleration and design your wheel rates (or more accurately ride rates) accordingly. Then add anti-roll bars to minimize the roll angle and allow easy LLTD adjustment. A stiffer car will better control camber change and roll center movement. A softer car will have less normal load variation over bumps. If you have tire data and suspension geometry you can determine appropriate rates.

[Kurt Bilinski]

There is no magic number; every team has their own idea of what works, and pretty much, they all do.

Another vague rule of thumb is that wheel rate be somewhere between 50%-100% of unsrung weight, for a non-aero car. Than again, if you find you have blurred vision from the hard ride, or the car leans too much, then the springs have to change.

[mtg]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by kb58:
There is no magic number </div></BLOCKQUOTE>

Taken out of context, that's pretty funny.

As far as finding baseline wheel rates, you want to first pick ride frequencies. That is the normalized suspension stiffness in bump. Then you don't have to rely on rules of thumb which you have no idea where they come from.

Look at the system from a frequency response standpoint of natural frequencies and damping ratios and it's pretty straightforward to pick what you want. Then back calculate to a spring stiffness.

[Jersey Tom]

If you dont have a baseline from previous cars, you may want to start around 2-2.5 Hz sprung natural frequency and buy springs that are stiffer and softer. When you get out driving, play around, see how things react, may want to go stiffer. We wound up going appreciably stiffer on our past two cars.. but that's just what worked for us. A lot depends on what ride height you'll be running, and how much pitch and dive you want.

If you go to much stiffer springs than designed for, your ARBs will likely not have a big effect on adjusting balance of the car (overall roll resistance will be dominated by spring rate).

[Kurt Bilinski]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by mtg:
As far as finding baseline wheel rates, you want to first pick ride frequencies. That is the normalized suspension stiffness in bump. Then you don't have to rely on rules of thumb which you have no idea where they come from.

Look at the system from a frequency response standpoint of natural frequencies and damping ratios and it's pretty straightforward to pick what you want. Then back calculate to a spring stiffness. </div></BLOCKQUOTE>

Eh, so you choose a wheel rate frequency. Does this mean the calculated spring rate is a non-negotiable magic value? Since the math says it has to be that number, you aren't allowed to change it?

If your driver says the car hops around too much, what are you going to say, "Sorry, the math says it has to be this number, tough." No, it's a starting point, which is what a rule of thumb is.

[Chris Allbee]

kb58,

In no way did I interpret mtg's comments as saying that you can't alter the spring rate to affect handling change...

What he is saying is that the aspect of the vehicle that is being altered when you change the springrate, and therefore the wheelrate, is the natural frequency of that end of the car. That frequency coupled with your damping coefficients give you a frequency response, which is what the driver is trying to describe when he uses terms such as "hopping around", "falling over", etc...I believe mtg was merely saying that you should approach the problem with the idea that the primary issue (in this case frequency response) should be altered. If you have data acquisition you can use that in conjunction with driver comments to determine where the problem lies (could be damping issues, wheelrate issues, roll rate issues, etc), then decide what behavior you would like to see/feel from the car, quantify it, and then determine what points on the car need to be changed.

Furthermore, all that was given from mtg was a method to find a starting point, as you so noted. He mentions nothing about not being able to change it because the math says it should work.

Your confusion may come from the reference of "the magic number", which is a term used by Claude Rouelle and others to mean the percentage that the front roll stiffness contributes to the over-all roll stiffness of the vehicle. thats why mtg commented "Taken out of context, that's pretty funny."

In short, run calculations to get a baseline, then go test and tune the hell out of it.

[mtg]

Yep, Chris is right.