DG's Autocross Setup Secrets

Autocross to Win

V1.0 Jan 03 2007

This is a work in progress, and it has been a very long time coming. I intend on working on it for the next little while, dumping as much of my brain and hard-earned knowledge into the public record. It is, however, NOT public domain; I reserve the copyright for myself. You may NOT duplicate this elsewhere without my express permission - and this is the age of Google, folks, I WILL find you if you post it elsewhere. Otherwise, read and enjoy, and put this all to good use. Go fast!. DG

Calculating Corner Weights

Adjusting corner weights is one of those things that is part of your baseline setup, but finding any sort of agreement on just how to set corner weights is next to impossible. It seems everybody has their own favourite formula and magic sequence.

I'm no different.

My method takes as a given that your car is, being production based, almost certainly left side and front end heavy, thanks to the packaging of the driver (offset left) and the engine/transmission (offset front). There are, of course, exceptions, but for the majority of us, the physical distribution of weight inside the vehicle is asymmetical - and so then will be the corner weights.

No amount of twiddling spring perches or cranking down on load bolts can compensate for 200+ lbs of driver offset 10 inches left of the vehicle centreline. Unless there is a corresponding mass in very nearly the name location (in plan view) the corner weights will be offset left.

So then, the question becomes how to best minimize the effects of the asymmetric weight distribution. If we have to live with it, at least we can reduce the effects and keep things reasonably balanced.

Unless you only have left turns to deal with, you don't want to talk about "cross weight" or "wedge". Those can be helpful in setting up a car that has purposeful lateral static weight distribution bias because all turns are in the same direction, but we don't play that game. It doesn't help at all that most electronic racing scales sold in North America are intended for the roundy-round guys and so will happily show you cross weight like it was the magic ticket - it isn't.

Instead, I like to set up a car so that it has equal left weight front and rear; that is, the portion of the left side weight on the front wheels is the same as the portion on the rear wheels, by percentage. If the car has 52% front left weight, then it should have 52% rear left weight. Why? So there's no diagonal bias and no static twisting moment. It seems to keep the car more predictable.

The Javascript calculator here is provided to help work through all the various corner weight calculations. This is a Javascript version of the spreadsheet I use to set up my own car.

Here is the sequence:

1. DISCONNECT THE FRONT AND REAR SWAY BARS! Do NOT forget this step, or you will spend a tremendous amount of time chasing your tail as the sway bars transfer weight laterally. (guess how I figured this out?)
2. Set the ride height to where you want it. On my car, this was 1.79" of clearence between the top of the tire and the interference point on the fender. The idea here is to set the car at the low point, and then come up on the light corners. Don't forget that the driver must be in the car, or an equal representative weight. Fuel level should also be representative of the fuel carried during a race.
3. Roll the car on the scales, and enter the numbers into the calculator. If the goal delta is less than 5 lbs (the numbers will turn green) stop.
4. Pick a light corner, and raise it a couple of turns. Remember that raising a corner increases the corner weight on that corner and its diagonal opposite, and decreases the weight on the other two corners. We aren't changing the mass of the car; that must remain constant. We're just redistributing the load.
5. Bounce the car, and roll it back onto the scales. Enter the new numbers into the calculator. Make note of the goal delta numbers and continue tweaking the ride height adjusters until you hit the goal.
6. I would try to avoid fixing my attention on one corner; I'd switch back alternatively between alternate corners until I hit the goal. It normally didn't take much; a couple of turns on the adjuster normally hit the target.
7. Once the weights are set, reattach the sway bars and reweigh. Any major difference (more than a couple of pounds per corner) indicates preload in the sway bar - adjust the tension in the end links to eliminate it.

Corner Weight Calculator