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I have been wondering how spring rate (at wheel) is chosen for a race car...

To simplify the discussion, I am refering to the spring rate at wheel (or I should say at the tire contact point), as the spring rate of the actual spring is dependant on the suspension geometry, it will be converted to the wheel spring rate eventually.

How to correctly choose spring rate?

If we consider the static load, based on vehicle weight and the weight distribution, each corner has its own load which can be calculated or measured. We set a min static ground clearance requirement, then we can change this rate to make sure the spring is not all the way compressed. That can set a lower limit for the spring rate.

How to set the upper limit? You obviously don't want the spring rate to be too stiff.

And what are the other considerations? say dynamic behaviours, tire load on accerlation and braking, load while conering, etc

Is there any methodologies on this? Anyone can share with me a link or something?

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Miata UBJ: ES-2074R('70s maz pickup)
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LxWxHt
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You're talking about "wheel rate"… the amount of force it takes to move the wheel at the contact patch.

The way I've done it over the years is a simple formula to get me in the ballpark and then make follow up adjustments depending on car behavior.

I start with a multiplier based on the tires I'm using (tire choice goes hand in hand with what you're doing with the car, yes?)

The multipliers I use are:
1 for slicks
.7 for R compound tires
.5 for performance summer tires

(The multiplier needs to evolve as tires evolve, but these are close enough for today's tires to get a baseline)

You also need to know:
Corner weight (how much weight is on each tire)
Motion ratio

Motion ratio is how much the spring moves relative to wheel movement. Typically (but not always) the spring moves less than the wheel. For example, if when the wheel moves 1", the spring compresses 3/4" then the motion ratio would be 75%.

Here's the formula:

(Multiplier x Corner Weight)/(Motion Ratio x Motion Ratio)

Example :
(.7 x 300)/(.75 x .75) = 373.3333

You would need a 373lb/in spring

It's a place to start. No matter what formula and variables you use there should always be an expectation for final tuning.

My method is certainly old school. MV8's frequency method is more modern thinking and has advantages.

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Peace, Ron

An old racer told me that no matter how well you think you've designed the suspension, expect to end up with three sets of springs: the first (designed) set, the second set that when too far, and the third set that's on the car. Darned if he wasn't right. The reason is that what suspension stiffness "feels right" is very subjective.

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Midlana book: Build this mid-engine Locost!, http://midlana.com/stuff/book/
Kimini book: Designing mid-engine cars using FWD drivetrains
Both available from https://www.lulu.com/

And then there is the driver issue. The same driver, in the same car needs to have personalized the suspension to get "his" best out of the car. There is not one right answer.

_________________
Chuck.

“Any suspension will work if you don’t let it.” - Colin Chapman

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