Hope to get my dedicated track car on the track by march. It has been a 3 year project.

The car now has a full cage, everything that does not add strength or safey has been removed.

The best parts available, within my budget, have been added.

I now need to purchase a set of springs. I will be running Afcoils with weight jacks as this is what I am familiar with.


But before I do, I will set ride height and scale car to determine corner weights, sprung and unsprung weight and center of gravity.

After I do these measurements, is there a formula for determining proper spring rate for a starting point?

 

What sort of track/pavement condition, what speeds, and how's your tolerance for a stiff-legged ride? Do you have a maximum roll angle and estimate of cornering g's?

There are some formulas that can help you select spring rates based on suspension frequency, you can base your choices on limiting your roll to a certain amount per lateral g, or you can use some combination of the two.

Norm

 

Thanks,

Where do I find the formulas?

 

I know this is probably ghetto but I chose my initial rates based off the stock IROC-Z spring rates front/rear, what is it 550# front / 130# rear. I only did it like this because my local track really sucks, might as well be a city street the pavement is so had it. So I knew I was going to be running softer rates. Then once I tried one setup, I changed it to see how the difference felt, and I'm gonna keep doing that until I find the combo I like the most for my application. Right now I'm running 700# front / 150# rear on my street car and 775# front / 175# rear on my track car (with a blown motor hahaha). Spring height is a different issue altogether and to be honest I really wish I had been more technical about choosing my heights. I went just based off the allowable ride heights of the car before having contact, not so much based on the performance of different heights. Again, probably the ghetto method ...

 

I have 775 front and 175 rear (36/24) on my all around car. Great on the road course, horrid for autocross. do not forget to consider swaybar size when sizing springs. By the way I have 8" front and I think they are too short.

 

Thanks,

I am still looking for some type of formula. I have 900lbs front and 300 lbs rear on my current track car. The car is all stock except for suspension and brakes and has full aminities 3464lbs w/o driver ( Church Car) The car handles great and it is a joy to drive on the track. Not worth a damn on the street.

My new car is totally different. It will be significantly lighter and have a better front rear weight % and a much lower CG.

This is why I want some type of formula to get close to a starting point. I could set the car up like my other one, but it would not be right. I would like to get in the right ballpark starting out and fine tune from there.

 

Your best bet is to pick up a copy of Fred Puhn's book ("How To Make Your Car Handle"). Most of the formulas you're looking for can be found there, at least in simplified form. The frequency formulas he presents are for undamped vibration; those for damped vibration will give slightly different results and do provide a better basis for making decisions. A good college text on mechanical vibrations or perhaps a general reference such as Marks' "Standard Handbook for Mechanical Engineers" should have enough information to help write the damped equations.

See the attached pic; the top graph is with performance-oriented shocks in good condition while the lower is for the case where the shocks are not offering nearly enough damping. It's from a spreadsheet approach that's still somewhat simplified (but runs the better part of 1MB filesize). Unfortunately I no longer have either the derivations or the reference list handy (any more I just plug in the numbers and use it). Lower pitch response is one measure of better ride quality. Ideally you'd run a simulation such as this for a variety of speeds and pick a "best overall average" for ride quality.


Also, poke around over at www.optimumg.com for more on this.

Frequency isn't the only thing either. Don't forget that your spring rates (along with sta-bar stiffnesses) are also part of your lateral load distribution transfer calculations, which go to understeer/oversteer handling balance. But it does give you a starting point, and a way of deciding how much you need from the sta-bars. IIRC, Puhn's book gets into this a bit, in a different section.

The Millikens' hardcover "Race Car Vehicle Dynamics" book goes a whole lot deeper into this sort of thing (as it should, given its ~$100 price and availability through the SAE).

Norm

 

Here's the same vehicle, with the rear frequency made higher, but with the same shock damping values.

Expect to make some trade-offs, and some back-and-forth iterating between LLTD and ride frequency calcs.

BTW, Puhn does present some basis for frequency selection. From a bit of experience, the range of 1.0 - 1.25 Hz is generally where OE suspensions fall. 1.3 - 1.5 is entirely liveable (if slightly firm) on a daily basis for a dual purpose car, 1.7 - 2+ is definitely into the hard-core range, and not much goes above 3.0 except for big time race cars with lots of aero downforce. Mid - upper 2's can be found on some of the more serious auto-x cars (not necessarily F-bodies, however).

Norm

 

Is there a formula? No, not really. Frequency is just something that you calculate to figure out if you ended up with something sprung at a rate that the passengers could tolerate and to help shock selection.

If you wanted to come up with one you’d have to first figure out the worst bump that the car would be expected to handle and at what speed, find the wheel rate that would keep that end of the suspension from bottoming when hitting it and then multiplying that by the length of the effective control arm divided by the location of the spring pocket from the control arm pivot. The fact is that there’s enough variables there that most people couldn’t really calculate it because they don’t have the data to do it.

Real world, assuming that you had all the time in the world and a controlled course to tune on, you’d guess at spring rates and then try them without sway bars, and keep trying them till you’ve got the softest springs that prevent the suspension from bottoming under the harshest bumps and stops. Then you would add sway bars and tune the alignment till you get a F/R balance that works well for the driver and the course.

Is that a 3rd gen??? Unless you have a TON of extra weight in the back that’s A LOT of spring rate for a rear axle… I’ve used about the same spring rate in the front on one of my cars and it liked about 190lbs/in in the back on the street and autox and about 210-215 on the road course….

 

Thanks for the help.