![]() ![]() “If the front of the car is not rising fast enough for proper load transfer, you can do the opposite and lower the polar moment of inertia,” Bickel said. “The car must pitch rotate around the rear axles, so mount ballast and heavy components as near the axles as is possible.” To do this, you can relocate any added ballast and heavy components (engine, battery, fuel cell, etc.) toward the extreme ends of the car. One way to slow the rise of the car’s front-end (pitch rotation) is to increase the polar moment of inertia. Both wheels weigh the same (200 pounds) and are perfectly balanced, but the one with the higher polar moment of inertia will require more effort to accelerate to spin.” “Assume that the two example wheels are mounted on precision bearings (see illustration below). “The further that weight is located from the CG, the greater the inertial forces when motion is involved,” Bickel said. Polar moment of inertia is a measurement of how the mass is distributed throughout the car. Everything here is dynamic, and no amount of number crunching in the shop can guarantee performance.īickel tells us other factors, such as the car’s polar moment of inertia, become important in a dynamic (moving) situation. But no matter how much work you do you do in the shop trying to figure out the CG, it all goes out the window when the car goes down the racetrack. When you analyze static center of gravity (CG) in your car, it can help to predict what the chassis will do. If the car lacks the power to create significant chassis roll, rear-end torque rotation tends to increase traction to the left rear wheel.” Polar Moment of Inertia This adds traction to the right rear wheel. This is due to the differing degrees of chassis roll between these cars. High horsepower cars roll the chassis significantly, lowering the right side pickup point (instant center). You can see this pre-load when the car is electronically scaled.īickel goes on to say: “Nearly all drag race cars must have at least some initial chassis pre-load in order to make the car run straight. This is because of the unbalancing traction effects of chassis roll and rear-end torque rotation. Lower horsepower cars tend to require right rear pre-load and high horsepower cars require left pre-load. When one rear wheel is pre-loaded, the opposite front corner of the car will also have a weight bias.” “Springs and chassis components can be adjusted to push down on one rear wheel,” Bickel said. Corner-Weight Distributionīickel points out that corner-weight distribution refers to the amount of weight carried by diagonally opposed pairs of wheels. That’s why many well-built drag cars actually have a place to mount ballast on the right door brace or “X” on the roll cage. To offset the driver weight, you can add or move ballast. However, when you place a driver - particularly a big-boned one - into the left front seat, then the left-side weight bias of the car will change significantly. From a bare-bones perspective, it usually means there isn’t much difference in the side-to-side weight distribution when the car is placed on a set of scales. ![]() Of course, when building a new drag car, the same applies as you tend to center the heavy bits. With any rear-wheel drive car, the heaviest pieces are situated on or near the centerline of the chassis. If the wheelie bars unload the slicks, then the car will spin. If there’s too much weight on the back-end (for example, the car hits the wheelie bars hard), the car will be prone to wheelies. It won’t transfer weight if it’s turning the tires. If there is too much weight on the nose, then traction suffers. The catch here is there must be enough hook for the car to get moving and transfer the weight. When a car leaves the starting line, acceleration forces create load transfer from the front to the rear. This puts more load on the back tires and simultaneously increases traction. Even purpose-built cars, like a contemporary Pro Stocker, have more weight on the front-end than the back. Because it’s located at the front, the vast majority of door cars have a weight bias that favors the nose. The engine is typically the heaviest piece in any door car. It influences traction, handling, and performance. Chassis builder Jerry Bickel says weight distribution is at least as important as total vehicle weight. ![]() Weight distribution is extremely important, and mostly misunderstood. But even if you can’t take pounds out of the car, it’s a really good idea to think carefully about how weight is distributed. All things being equal, a lightweight race car will always be quicker and faster than a heavy one. ![]()
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