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.
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.
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.
Disconnect one endlink on each sway bar. The job of a sway bar is to resist dynamic weight transfer when the chassis is loaded. Disconnecting one endlink from each bar will allow the chassis to move freely as you adjust weights. (If you don't disconnect the sway bars prior to adjusting the crossweights, you'll put the bars in a state of tension, where they're pre-loaded. This will do all sorts of wacky things to the way the car handles.) After the crossweights are set, the endlinks should be adjusted so the bolts can be easily re-inserted. Obviously, adjustable endlinks are needed to achieve this.
It's now time to roll the car up onto the scales and determine its current numbers.
To get the true corner weights, the four scales need to be situated on a flat, level surface. One way to check this is by stretching strings, in the form of an X, between the left front and right rear scales, and between the right front and left rear scales. Adjust scale height until the strings just touch in the middle. An easy way to adjust scale height is with square linoleum tiles. LG's scales have levelers built in and had been pre-leveled.
D6C's corner weights were off by a lot. The LF/RR showed 52.8 percent, which is way outside of acceptable. Of course, you're trying to achieve a perfect 50 percent. If you're adjusting a car that still has leaf springs, anything inside 51 percent is considered pretty good. Coilover cars can generally get to 50 percent, as there are no rubber bumpers to contend with.
Set your desired front ride height and then adjust rear ride height to achieve a 3/8- to 3/4-inch rake. Rake is important in order to avoid running into lift at high speeds. The idea is to minimize the amount of air going under the car. Measure rake at the jack points between the wheels. Choosing ride height is a personal preference. Low is good, but if you go too low, you'll run out of suspension travel (assuming stock springs and shocks are used). Again, coilover-equipped cars generally don't have this issue. Be sure the front ride height is equal from left to right prior to setting rake.
Once you have the front ride height and rake where you want them, you can jack weight into (most likely) or out of (fairly unlikely) the right rear corner while the car is on the scales. If using stock springs, reaching the rear adjusters is fairly easy to do. Unless your car happens to be off by an extraordinary amount, you should only need to adjust the right rear corner. Again, in most cases, you'll be adding weight by raising this corner. The right rear is almost always the corner with the lowest weight. As you adjust, you'll notice the opposite corner's weight will decrease.
One area in which coilover-equipped cars are at a disadvantage (however insignificant) is the inconvenience of removing a wheel to make a ride-height adjustment.
The actual adjustment of a coilover is quite easy. Loosen the lockring and spin the spring seat (down to increase ride height or corner weight). The lack of rubber parts means that adjustments with coilovers are far more precise than with the stock springs and adjusters.
Patience is a required tool in setting corner weights. It'll take several rounds of adjustments to get the weights just right. Be sure to bounce the car to resettle the suspension after each adjustment and repeat until you're satisfied. Picky bunch that they are, the LG crew was satisfied only when D6C was at a perfect 50/50.
Adjusting the ride height has little effect on camber on a Corvette. Here, Louis Gigliotti verifies that camber is still in the ballpark. Look for an alignment story in an upcoming issue.
Ride-height changes do impact toe settings significantly. It's a good plan to head straight to the alignment rack after performing so much suspension work. It's the last key in maximizing the effectiveness of all the shiny new parts hanging under the car.