Maximizing Suspension Performance - CHP How It Works
Maximizing Suspension Performance Requires Scaling and Setting Crossweights. Industry Experts Tell You How
From the June, 2011 issue of Chevy High Performance
By Stephen Kim
Photography by Stephen Kim
Few people get thinner as the years rack up, yet older folks are generally in a better financial position to build project cars in the first place than young skinny kids. That means when the resources at your disposal to build a well-balanced street machine are at an all-time high, the organic lump behind the wheel is bigger than ever, making the process of evenly distributing weight within the chassis more challenging than ever before. It’s a rather diabolical automotive truth, but fear not, it can be corrected.
The key virtues of equally distributing the weight of a car from front to back are neutral and predictable handling. However, an often overlooked but equally important metric is how closely balanced a car’s weight is from side to side. Even a car with outstanding front/rear weight distribution can sometimes turn better in one direction than the other. Making things more complicated is that vehicle weight tends to shift diagonally, not just from front to back. To prevent this ugly scenario from playing out, you must first understand how to set the corner weights and crossweights of a car. This isn’t all that difficult, but it does require some specialized techniques, tools, and prep work for the best results. To find out how, we consulted with the most highly regarded suspension experts in the business: Matt Jones of Art Morrison Enterprises, Doug Norrdin of Global West, and John Hotchkis of Hotchkis Sport Suspension. Here’s the scoop.
: Many people install aftermarket control arms, springs, shocks, and sway bars but never think twice about scaling their cars. Essentially, scaling is just another tool to assist in dialing in a chassis, and not something to set and forget. Its purpose is to optimize the weight on each tire and balance a car’s handling when turning left and right, and possibly increase traction on a specific corner when grip appears less than ideal. To get around a road course or an autocross in the quickest duration of time, you don’t want a car that understeers or oversteers more when turning in one direction compared to the other. Obviously, the impact of correctly scaling a car can be large if the car was imbalanced to start with, as it can transform a vehicle from being uncontrollable to predictable. However, if the car is fairly well balanced to start with there may not be much gain, if any.
Doug Norrdin: Having equal weight distribution from front to rear and side to side in a chassis can make a big improvement in handling. It all depends on how serious you are and how much you’re willing to spend to get there. Although it’s often difficult to achieve, 50/50 weight distribution is always a great target to shoot for. Furthermore, concentrating the weight within the wheelbase keeps the polar moment of inertia down, which always assists in handling. The first step in scaling a car is measuring a car’s weight. A very crude way to do this is weighing your car on a truck scale one end at a time. If you weighed your car with the front tires on the scale and the rear tires on the ground, and then with the rear tires on the scale and the front tires on the ground, it would at least give you an idea of the front-to-rear weight distribution. A much more precise way of scaling a car is with a set of dedicated electronic wheel scales that measure weight at each tire. Once you have some baseline measurements, then you can start moving components like the battery and fuel cell around to balance the weight distribution. Beyond that, due to the packaging constraints of most cars, things get much more difficult. Oftentimes, things like setting the engine farther back in the chassis just isn’t practical for most enthusiasts. So while improving weight balance does make improvement at higher levels of racing, it’s probably not a major issue in a typical street car.
: The four patches of rubber contacting the pavement are all you have to rely on in a corner. Handling is all about taking care of those patches. If the corner weights are off and you’re driving at 10/10ths, you can get brake lockup, handling instability, and uneven tire wear and heating. The goal is to get all four tires to do the maximum amount of work you can, and the best way to do that is by getting the corner weights as equal as possible. Consequently, when building a car it’s important to put it on the scale to figure out where to put components like the battery, fuel cell, and oil tank. If you had a mid-engine Camaro, you might be able attain a 50/50 front-to-rear weight distribution, but in reality it’s difficult to achieve. Even so, many car builders are positioning engines farther back and lower in the chassis to get most of the weight behind the centerline of the front axle. Placing the weight toward the center of the car also yields a lower polar moment of inertia.
: There are several different methods for scaling a car, but the tools typically needed are somewhat universal. They include scales, loading ramps, coilover adjustment tools, ballast, and a tire pressure gauge. Optional items include turntables, a laser leveling tool, and 1/8-inch vinyl floor tires. Before any adjustments are made, ballast equal to the driver’s weight should be in the driver seat area, and all the fluids must be topped off, with the exception of the fuel. Most people don’t race their vehicles with a full tank of fuel, so we figure that an average fuel level of half a tank should be used. Be sure to inflate each tire to the optimal pressure, which can be obtained by the manufacturer’s recommendations, or other people who run the same tire. Lastly, disconnect at least one side of both front and rear sway bars.
The next step is to adjust the vehicle’s ride height on a level surface. If your floor isn’t level, use a self-leveling laser to find a common plane for the tires to sit on, and then use 1/8-inch-thick vinyl floor tiles as shims. Be sure to shim an area of the floor where the front lower control arm bolts will be above, as well as the rear lower control arm to frame bolts or front leaf eye bolts. These bolt heads will be used to measure the vehicle’s height. Now that the vehicle is sitting on a flat plane, the ride height can be adjusted. Start off by measuring either one of the front lower control arm bolts to the ground, and record that measurement. Repeat on the rear lower control arm or leaf eye bolts. With all the heights recorded, examine which corners need to be adjusted so that the left and right front lower control arm bolts are level, as well as the rear. If the vehicle’s front-to-rear ride height is supposed to be raked, adjust this now. Once the ride height has been finalized, double-check that the alignment is where it should be and adjust if needed. The car is now ready to have its crossweights adjusted.
: Trying to scale or set the corner weights of a car while it’s still being built often results in inaccurate data. The idea is to replicate how the vehicle will be set up in race trim. The driver must be sitting in the car, or there needs to be ballast to replicate driver weight. Make sure that all the fluids are topped off and that the gas tank is half full. Since you’re never running on empty or racing with a full tank, scale the car with a half a tank of gas. Sway bars can actually preload the suspension, so disconnect the endlinks for your baseline measurements, then hook them back up to see how much the weight readings change. To further reduce potential discrepancies, set tire pressure to where you usually run it on the street or at the track. Alignment doesn’t affect readings too much because you’re not turning the wheel when scaling a car.
John Hotchkis: To ensure accurate readings when scaling a car, always do so with the driver in place. Next, top off all the fluids and fill the gas tank up halfway. These steps will get the car very close to its race weight. Next, even up the tire pressure at each corner since pressure variations can affect ride height. One of the most important steps is to zero out the sway bars, which requires adjustable endlinks. You can’t get true corner weight readings with the sway bars connected because they can preload the suspension from one side to the other. To zero out the sway bars, first unhook the endlinks then set the corner weights. Afterward, adjust the length of the endlinks so that the bolts slide right through with no binding or preload and then reattach them to the car. This procedure helps equalize the weight of the car from side to side, and makes track days much more fun. These are techniques that have been used for decades in road racing, but are now coming into the Pro Touring scene.
: Corner weight, also known as crossweight, is the sum of the diagonal weights of the vehicle. To calculate percentage, add the weight of the left rear and right front corners together and divide that total by the overall vehicle weight. For instance, let’s say the left rear and right front corners on a 3,000-pound car have a crossweight of 1,650 pounds. Dividing the crossweight of 1,650 by the total weight of 3,000 yields a 55 percent crossweight percentage. Increasing the ride height at one corner will add weight on that corner and its corresponding diagonal, while the opposite corners will lose weight. To add weight to a specific corner, increase the ride height at that corner, and lower the ride height at the diagonal. If a car’s handling characteristics are unknown, always aim for a 50 percent crossweight percentage. From there, the crossweight can be adjusted to cure imbalance issues, which is particularly important for road race vehicles. Drag cars could use optimization of corner weights to equalize tire loading or to offset driveshaft torque, but I suspect the benefits would be minimal for most racers.
Doug Norrdin: Corner weight is simply the weight each of the four car tires supports. Crossweight, on the other hand, refers to the total weight of diagonally opposite corners of a car. To help conceptualize crossweight, think of a car’s chassis as a chair. Weight moves in a vector. If you raise the left rear corner of the chair, it puts more weight on the right front leg. Likewise, this also puts more weight on the back legs, and takes weight off of the other two. Crossweight on a car works much the same way. If you have more weight on one tire than another, you’re already overloading it. The goal is trying to get the corner weights from side to side along with the crossweights as equal as possible. In the real world, a 50 percent crossweight isn’t always possible to achieve, but it’s a good target to try to reach nonetheless.
John Hotchkis: A very undesirable characteristic in any car that’s driven on the street or around a road course is a chassis that understeers or oversteers more turning in one direction than the other. The best way to get a car to turn left and right equally is by making sure weight on the tires is as equal as possible. Electronic scales are great tuning tools because they enable adjusting crossweights. If you were to take the corner weight at one side of a car, add it to the weight at its diagonally opposite corner of the car, then divide that total by the total weight of the car, you’d get the crossweight percentage. Ideally, the percentage should be as close to 50 percent as you can get it. This ensures that each corner of the car sees equal loads in corners and eliminates weight jacking so you don’t have more weight on the tires when turning in one direction versus the other.
Matt Jones: One of the great perks of an aftermarket coilover system is the ease in which it can be adjusted. When it comes to setting corner weights, this can easily be accomplished by lowering or raising the spring perch on the shock body. Ride height and corner weight adjustment on cars with OE-style coil springs is a bit more difficult. In these types of applications, you can actually rotate the spring in the lower spring pocket and install stop bolts to keep it in place. Some circle track parts suppliers even offer threaded adjusters to make wedge adjustments easy. Typically, these methods will not be as simple as a coilover for ease of access, but the adjustment methods are the same.
Doug Norrdin: When setting the corner weights and crossweights of a car, aftermarket coilover systems greatly simplify the process. However, adjusting corner weights aren’t that difficult with factory-style coil springs, either. Any coil spring can be clocked, or rotated within the spring pocket to change the weight at that corner. Likewise, spacers can be wedged between the spring and the control arm or frame. Leaf springs are tough to tune, since the only way to adjust the corner weight with them is to change the shackle length. Unfortunately, that’s not always a great idea. Generally, if you raise the right rear corner of a car, it will increase weight on the right rear and the left front corners of the car. Conversely, raising the left rear corner of a car will increase weight on the left rear and right front. Lowering one corner of a car has the opposite effect and reduces weight over that corner and its diagonally opposite corner. In most scenarios, it’s best to balance out a car by moving around the mass within the car first before corner weighting. As a last resort, you can use the endlinks of a sway bar to preload and weight jack the chassis.
John Hotchkis: Increasing the ride height at one corner of the car has the effect of putting more weight on that corner. Conversely, lowering one corner of a car reduces weight at that corner. Adjusting corner weights without the luxury of aftermarket coilovers can be time consuming, but it can be done. You can shim springs or even trim the coils slightly to get the corner weight you want. Some aftermarket control arms have provisions for shims of various thicknesses in the spring buckets. Oftentimes, we’ll just work on optimizing corner weights on the front. If we can get the front where we want them, it’s usually good enough and the car will handle great on the track.
Matt Jones: Front-to-rear and left-to-right weight distribution is heavily determined by how a vehicle is designed from the factory. Moving components around within the chassis for the ideal static weight distribution is best, but adding ballast isn’t preferable if a minimum weight rule applies. Before ballast is added to correct left-to-right imbalances, the builder must first determine if the extra weight added to correct the imbalance will outweigh the shortcomings of additional overall vehicle weight. Just about every vehicle can benefit from a 50/50 left-right distribution, as this will help equalize grip in left and righthand turns. However, the same can’t be said for front-to-rear weight distribution. In my opinion, slightly adjusting front/rear weight distribution should typically be done for tuning rather than adding weight to hit the magic 50/50 figure. Drag cars, of course, can benefit from more rear bias since little is needed from the front tires.
John Hotchkis: Wedge or reverse-wedge simply refers to intentionally biasing the crossweight of a car to help it turn in one direction. If the right-front-to-left-rear crossweight is greater than 50 percent, a car has wedge. If it’s less than 50 percent, then it has reverse-wedge. Since oval track cars only turn left, they run wedge to assist with corner exit speed. This is also done in conjunction with running lots of positive camber on the inside tire. The result is a car that’s so weight jacked in one direction that they don’t even drive straight down the straightaways. You actually have to turn the wheel to the right to get them to turn straight. Although that’s an extreme example, for cars that turn left and right you usually want to get the crossweight as close to 50 percent as you can, even if the road course or autocross that you’re on has more right turns than left turns, or vice versa. Typically, whatever you’ll gain turning in one direction by biasing the crossweight will be overshadowed by the decrease in handling when turning in the opposite direction. If a car turns right more than it turns left, then setting the camber differently on the right front tire is a much better alternative than biasing the crossweight. The next step is tuning a car to the racing surface with tire pressure and shock valving adjustments. CHP