A properly balanced chassis...
A properly balanced chassis will markedly improve any Corvette's cornering prowess. (Photo by Tami Traeger)
A significant portion of the Corvette's appeal lies in its ability to go around corners way faster than you (or the highway engineers) ever thought possible. C5s and C6s are amazingly adept at this right off the showroom floor, even with hockey-puck rubber, tire-life-maximizing alignment, and God-only-knows crossweights. Huh? Crossweights? What are those?
Glad you asked. One of the keys to a properly handling car is balance, and the way to improve your car's balance is by tuning corner (aka cross) weights. The term crossweights refers to a comparison of the car's weight distribution at the corners. It's determined by comparing the weight of the left front plus right rear corners to that of the right front plus left rear corners. The idea is to jack weight to where it's needed to achieve a 50/50 balance between these corners. Such balance leads to a neutral chassis that exhibits the same behavior, whether turning left or right.
When the chassis isn't balanced in this manner, it will most likely understeer in one direction and oversteer in the other. The good ol' boys in NASCAR refer to this disparity in crossweights as "wedge," and it's something they do intentionally on oval tracks to make the car more stable and easier to turn. While this is great at Bristol or Martinsville, where there are only left turns to contend with, it's definitely not the hot setup for a road course, an autocross, canyon carving, or even daily driving.
As you might imagine, crossweight...
As you might imagine, crossweight adjustments require the car to be measured on four scales simultaneously. It's very important that the scales are on a perfectly level surface. High-end scales like these at LG Motorsports even incorporate ramps so the car can be easily rolled on and off.
Many people assume that balancing crossweights changes the car's weight distribution, but it doesn't. Changing weight distribution (i.e., changing front-to-rear or left-to-right weights) can only be accomplished by physically moving mass-in this case, components or ballast-around the chassis. While this is something you can do, it's more often performed by the hard-core guys running dedicated race cars in T1 or similar classes. For a street-driven car, you'll get the most benefit by simply tuning crossweights.
Last month, the LG Motorsports crew installed a set of its coilover shocks and a pair of GM Performance Parts T1 sway bars on D6C. While simply bolting these parts on and calling it a day would have resulted in a much-improved car in terms of cornering grip, lessened body roll, and quickened transient response, I wanted to maximize the components' potential. The key to doing this was to crossweight the car.
Luckily, GM engineers are well aware of the benefits provided by a properly balanced chassis, and they've given us the means to adjust this balance, even on a car with stock springs. While many (including this author) use the adjusters to lower the car, these parts' real purpose is to enable crossweight adjustments.
Before getting started with...
Before getting started with the crossweighting project, you'll need to set the tire pressure at all four corners. Since the tires effectively act as springs, adjusting them to consistent pressures removes one variable from the weighting equation.
Prior to starting the crossweighting project, you'll want to prep the car as you would for its intended use, be that on the road or at the track. If preparing it for track use, you'll want to bolt on the race wheels, remove the floor mats, empty the trunk and glovebox, and remove whatever else you normally take out of the car.
Another weight consideration is the car's fuel load. For general road use, half a tank is a good compromise. Autocrossers may want to go as low as a quarter-tank, minimizing weight. You open-track lap dogs will have to use your best judgment, based on experience. If you're unsure, go with half a tank.
"But wait!" you might say, "Don't our Corvettes have dual fuel tanks?" Indeed they do. However, it's nearly impossible for the average enthusiast to determine how much fuel is in each tank at any one time, and the amounts are constantly changing anyway. So, again, while you can chase your tail worrying about how much of your car's fuel is in the left tank versus the right one, unless you are dealing with a purpose-built race car, this is probably a waste of time. D6C had a third of a tank of fuel when its crossweights were set.
The third consideration is driver weight. If you're having a professional race shop scale your car, the techs will want your butt (and even your helmet, if you really want to be precise) in the driver seat. If you're doing the project yourself, you must add the appropriate amount of ballast to simulate your weight in the driving position. The same goes if you ordinarily have a passenger or an instructor in the co-pilot's seat. Don't underestimate the importance of this, as the additional weight will make a tremendous difference in the car's balance. The weight of the driver or passenger can skew corner weights by up to two percent in some cases.