The Mysteries of Corner Balancing Revealed
One of the services performed at AP&C for our track enthusiast customers is corner balancing. For the uninitiated this term is not the least bit intuitive. So let’s go to a restaurant.
How many of us have been sitting at a table with our friends enjoying dinner or even just a cup of coffee and become annoyed by a table that rocks. As one of you leans on the table it moves and spills your coffee. You would reach for a book of matches to shove under one of the table legs, but no one smokes any more. So you move the table around or find a substitute to jam under the short leg so that you can get back to your coffee and conversation without being interrupted by a tilting table.
The same sort of thing can, and does occur on track with a race car. Absurd, you say. How can that be when each wheel is supported on springs? Aren’t all the tires touching the road all the time?
Yes, at rest, they are all touching, but they are not always carrying their fair share of the load. Like the rocking table, one corner may be carrying less of the load than another. To find out we place a set of specially designed scales under each tire to see just what is going on with static weight distribution. We find out that most cars are heavier at one end than the other. This is to be expected since there are large heavy objects, such as engines and transmissions that are not all crowded in the middle of a chassis. In most cases more weight is on the front of the car’s chassis than on the rear.
So how do we figure out which corner of the car is not carrying its fair share if the car is not perfectly balanced in the first place? Instead of trying to get the weight balanced on the chassis as a whole we can figure out how well the car is balanced by measuring the cross weights. That is the sum of the right front weight with the left rear vs. the sum of the left front and right rear and then look at the percentage of total weight the cross weight is. Ideally we would like to see 50%. That would tell us that corners are each supporting their proper share of the car’s total weight.
“So what?” you might ask, “The car is already suspended on springs and they will adapt as the car corners and weight transfers.” That is true to a degree, but on a race track consistency is paramount to being competitive as a driver and an unbalanced car will behave differently on a right hand corner than it will on a left hand corner.
Back to the garage. We have a track car with springs and shocks that allow for ride height and preload to be adjusted. These are often called coil-overs, where a coil spring is mounted on a threaded shock absorber with adjustable spring perches. These allow for changes in weight distribution on each corner.
Let’s say that we have a race car that weighs 2000 pounds with a full load of fuel and a driver. If it were the ideal race car we would probably like to see that the scales under each tire show that they are each carrying 500 pounds of weight. Realistically, in the real world, we might see weight distribution more like this:
650 600
390 360
Telling us that 1250 pounds of the car is at the front and 750 pounds is at the rear.
If we sum the diagonal weights we get 1010 pounds for one diagonal and 990 for the other. In percentages that is 50.5% and 49.5%. Close, but not ideal. The right rear is not carrying its fair share and this race car will not handle consistently on right and left hand turns.
While we can’t just shove a match book under the short leg we can change the ride height of each corner and get the car back into balance. So we can first start with the light corner and lengthen it by changing the ride height. The threaded shock absorbers allow us to do just that. With them we can add a little less than another quarter inch to the ride height on the right rear. After the car is settled and we roll it onto the scales again we find that the weights have changed and are now:
630 575
420 375
We are getting closer with the percentages now 50.25% and 49.75%. We could either add just a bit more to the right rear again or we could take away a bit from the left front corner. So we shorten the left front just a tiny bit and get these results:
628 583
417 372
Now we are at the perfect 50% cross weight and we will have a happier driver who now has a more consistent handling track car.
Corner weighting is just one aspect of getting a track car to perform at its best for a driver. The suspension also needs to be adjusted and aligned so that maximum grip can be obtained at the tire patches. Tire pressures, spring rates, anti-roll bars, weight distribution, and many other factors will limit or enhance handling performance.
AP&C has the tools and the experience to help you achieve your goals on a road racing track whether it is to prepare your street car for a weekend of HPDE (high performance driving education) or to help you set up your track car for a Chump Car event. Drop by or give us a call and we will work together to help you get the most out of your ride.