Corner balancing is key to achieve optimum handling
|Ron Pace, right, with Brian Redman, legendary Porsche
factory racing team driver, during the 2007 Kohler
International Challenge at Road America. Ron is currently
our region secretary and also is the region's chief driving
instructor. He is a past president and a long-time board
When we refer to the handling characteristics of a car we usually think about springs, shocks or sway bars. Get the right combination and the car ought to stick like glue – right? Well, maybe. Just like the best engine parts may not be the hot set up, if not tuned properly, so too does the right suspension stuff become worthless unless the right tuning is in place.
Small changes in the suspension settings can make a world of difference in the handling characteristics of the vehicle. While the hot alignment settings and the right tire pressures can be helpful, they won’t amount to a hill of beans if the vehicle’s weight distribution isn’t balanced. And to make matters worse, improper corner weights can make the handling quite different when turning left versus turning right. The car may over steer in left turns but under steer when turning right. So you play with suspension settings and tire pressure. Now you have your pride and joy going around left turns like it is on rails in a very predictable fashion. But now the car is evil in right turns.
The likely cause of such behavior is improper static corner weights. According to legendary Corvette racer Dick Gullstrand there are three fundamentals to getting the car’s weight distribution properly sorted out. The three aspects that need to be looked at are: rear weight distribution; left weight distribution and the cross weight percentages.
The values are found by the amount of weight resting on each tire patch. The car must be weighed on a flat level service, tires inflated to the same pressure, the driver or their equivalent weight must be in the driver’s seat and the fluids should be topped including a half tank of fuel. First determine the total weight by adding up the total of the four individual wheel weights (an accurate set of electronic scales is essential). Next calculate the left side weight distribution by adding the left front and left rear wheel weights and dividing the sum by the total vehicle weight. For road courses or an autocross you want this percentage to be 50 percent. This will equalize left hand and right hand handling characteristics. Now it is highly unlikely that you will get to precisely 50 percent, but get as close as possible.
Now let’s tackle the rear weight. Add the two rear wheel weights and then divide this sum by the total car weight to find the rear weight percentage. The ideal front to rear weight distribution is not as straightforward as the left to right balance (which should be 50 percent). If the car is front wheel drive or rear wheel drive it changes the desired balance. The ability to get maximum acceleration depends on having the weight biased toward the end of the car with the drive wheels.
So after you have optimized the left /right distribution to as close to 50 percent as you can (you might very well have to actually add weight to one side or another) it is time to get the cross weights correct. Here you add the right front weight and the left rear weight and divide the sum by the total vehicle weight. If the cross weight is over 50 percent, the car has wedge. If the percentage is less than 50, the car has reverse wedge. The more wedge a car has, the more likely the car is likely to under steer or “push” in a turn. More wedge also lowers the ride height at the left rear which helps the car come out of a left hand turn better. However, coming out of right hand turn, the car tends to be unpredictable. As you move away from a 50 percent cross weight balance the handling tends to vary from left versus right turns and further, the farther it gets away from 50 percent, the more unpredictable the handling gets. For predictable handling you want the cross weight to be 50 percent plus or minus 1 to 2 percent. Occasionally a road racer will bias the car to handle better in one type of corner. For example, Blackhawk has more right hand corners than left hand corners. So the hot set up is to bias the handling for right hand corners.
So, now that you have gotten the rear/front weight distribution to match the car drive system (front wheel drive versus rear wheel drive), it is time to adjust the cross weights to 50 percent. Put the car on the scales with the tires adjusted to the same tire pressure and the sway bars disconnected. Now you are going to literally adjust the ride height on the corner to get the cross weight to 50 percent. Raising the ride height on one corner increases the weight on that corner as well as the diagonally opposed corner, while the other corners will go down in weight.
If you have coil springs with adjustable perches, the task before you is actually pretty easy. For those with coil springs that don’t have those adjustments, add spacers to accomplish the adjustment. Don’t cut the springs as you will change the spring rates. Leaf springs are more complex. In this case, you have to actually change the spring eye attachment points to the spring shackles. Those with torsion bars can be adjusted at the chassis attachment point.
Needless to say, these adjustments require some sophisticated scaling equipment as well as some fairly complex adjustments. Read this to mean these moves are not for the shade tree mechanic. It will likely cost you a few hundred dollars to get the car properly corner balanced, but you will find the handling much improved and far more predictable.
|This is the caption for the photo above.|