CM autocross spring rate

[Davey]

Hi I have been autocrossing the last year and a half in a 83 Van Diemen. Ya I know it's one of the ugliest FFords, but it was a deal I couldn't pass up. I know different cars are going to have different spring rates, but I wanted to know generally what do people run. I have just been running the springs that came on the car. I got the car running for $500 running and have just been have a blast autocrossing it the way it is. Now I want to tweak it a little bit. I know the car to car to win CM is a DB1, but like I said I'm having a blast just trying to make it has fast as it can be.......thanks Davey

[PCalhoun]

There are too many variables to properly answer the question, but in general think softer than road racing to maximimize mechanical grip.

[na94]

A good starting place is having your wheel rate be about 55% of static corner weight (with driver and fuel, of course). You will need your motion ratios for this.

[Dick R.]

You may want to bump this question again after the Holidays.

FYI at one time "wheel rate equals corner weight" worked pretty well for my 85 VD but that was 2000/2001 on tires which apparently were less stiff than newer ones. After a MANY year layoff I found that for this RUSTY driver those rates were too stiff on newer tires and went back to approximately "wheel rate equals 50% corner weight". The car was always very driveable with those rates and, hopefully, will be with an old driver and current tires.

Let us "know" if you aren't positive about how to determine motion ratios and how to calculate wheel rates from spring rates, etc.

FYI NA94: Are you "involved" with the second place CM car at Nats this year?

Dick
CM 85
85 VD

[na94]

You may want to bump this question again after the Holidays.

FYI at one time "wheel rate equals corner weight" worked pretty well for my 85 VD but that was 2000/2001 on tires which apparently were less stiff than newer ones. After a MANY year layoff I found that for this RUSTY driver those rates were too stiff on newer tires and went back to approximately "wheel rate equals 50% corner weight". The car was always very driveable with those rates and, hopefully, will be with an old driver and current tires.

Let us "know" if you aren't positive about how to determine motion ratios and how to calculate wheel rates from spring rates, etc.

FYI NA94: Are you "involved" with the second place CM car at Nats this year?

Dick
CM 85
85 VD

LOL you could say so, it is my car. Yarko is the faster driver tho

[Dick R.]

I figured you know "something" about wheel rates for a fast CM car.

I may try stiffer again someday if I can get sharp enough to drive the car well again but in the meantime, the 50 percent range worked for me for a long time. The car sure had fast reaction time with the 100 percent range though on the older, apparently mushier, Hoosiers.

Dick

[Jim Garry]

I know the car to car to win CM is a DB1 ...

Hi Davey,

Just wanted to address your statement, above. Yup, DB1s have won the C Mod National Championship their fair share of the time. But the class is really wide open as far as competitive makes go. Since 1990 (when the class was revamped to its present make-up) we've had the following makes win the CM open class championship:

Citation: 6
Crossle: 1
Lola: 2
Reynard: 5
Swift: 4
Van Dieman: 1
Tiga (S2000): 1

IMHO, the class can be won in pretty much any well set up FF, as long as it is well driven.

Merry Christmas to all!

Jim

[Larry]

Motion ratios make my head hurt. If you want to simply try something different, consider this:
1. Most formula cars use a lighter front spring than rear.
2. Determine what your existing spring rates are. Use a scale to compress them, but throw out the first 1/2", as it isn't accurate due to the flat spot of the spring. Determine how many pounds compress the spring 1/2", and then how many compress it 1 1/2". That measurement is your current spring rate. (alternatively, it may be written somewhere on the spring.)
3. Put the front springs on the rear, and buy a set of fronts that is 100 pounds less than the rears. Give 'em a try!

This will give you an idea of the resulting change without having to purchase two sets of springs. BTW, I have a LOT of used springs of various rates. $40/set.

Larry Oliver
International Racing Products
703-759-0567 most evenings until 9:00 PM--but not tonight!

[Dick R.]

FYI in regard to Larry's comment about most formula cars using a lighter front spring rate:

Larry knows a LOT more about more cars than I do (I only know my specific car)!

However, you need to check your specific car before making assumptions.

I don't know about 83 VD's but the motion ratios on my 85 VD are such that I need stiffer front springs to end up with a softer front wheel rate than rear wheel rate. For example, I need 300 lb front springs to get about 105 wheel rate. Rear requires 200 lb springs to get about 170 wheel rate. Watch out for the front/rear spring free lengths also. These numbers about 50% of corner weights. If you go much softer you may need to use silasto bump rubbers to keep the car from bottoming.

Dick

[Mark Walthew]

You can also determine a spring's rate by doing some calculations based on measurements. I was never any good at that so I just use this handy web site to enter the spring data.

http://www.stockcarproducts.com/pstech10.htm

[Dick R.]

The British made springs that I own have the spring rate etched in the "flat" at one end. Fortunately with a formula ford the springs/shocks are easy enough to get to and in order to measure the motion ratio you probably need to remove them anyway.

Dick

[Roland V. Johnson]

The marking on older British springs usually has no resemblance to reality. If you check them, as suggested you will discover the rates are all over the place.

[Dick R.]

The marking on older British springs usually has no resemblance to reality. If you check them, as suggested you will discover the rates are all over the place.

Good point. Never assume a "marking" reflects reality.

Fortunately with mine they end up the same loaded length side to side when corner weights on each side are equal and the anti-roll bars are disconnected. So even if they are wrong, they are consistent. The free lengths are correct also (i.e. 6, 7, or 8 inches depending on the particular spring). I "think" that I checked actual rates using a scale and "ruler" but the "accuracy" of that method is obviously questionable when I am the measurer

I've done the math to verify that the compressed lengths are consistent with corner weights and motion ratios squared.

I'm known (to me at least) for using lots of low precision measurements to verify "ball park" accuracy of stuff.

Dick

[Davey]

Thanks guys, I have some homework to do........Davey