nascar cross weight

If more of the car's weight was added to the right-front wheel, it might not have lifted off the ground as we see here. But that adjustment might also add too much weight to the left-rear wheel.

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Cross-weight in a NASCAR Race car

We've seen how important it can be for suspension springs to help tires maintain a sturdy grip with the road. It's especially important on a racetrack where every second counts and accidents can end in disaster. To accommodate such delicate conditions, pit crews perform wedge adjustments to alter the tension on the springs. But if you notice, these crews adjust the wedge only on the rear wheels. Actually, it turns out that adjusting the pressure on just one rear wheel is often all a race car needs. That's because, as we mentioned earlier, adjusting the tension of one spring can affect the weight distribution on all tires.

­To understand why this happens, think of a stable, four-legged table where the weight is evenly distributed among the four legs. If you tuck a small piece of folded paper under one of the legs, the table will wobble. Now the wedged leg and its diagonal leg carry more of the weight than the other two legs [source: Guerrero].

­The same thing happens with a race car.­ Compressing the spring of a left-rear wheel or adding wedge puts more of the car's weight on that corner. This adds pressure to that end of the car just like putting the paper wedge underneath the table leg. As with the table, the corresponding diagonal corner of the vehicle gets more of the car's weight. So if you increase the tension in the left-rear wheel, the left-rear and right-front wheels will hold a larger share of the car's total weight than the right-rear and left-front wheels.

The reverse happens if you reduce the tension on the left-rear wheel's spring or subtract wedge. In our analogy, that would be equivalent to cutting short a table leg. It would increase the weight on the right-rear and left-front wheels. This is why a crew may need to adjust only one wheel when a race car needs to add or subtract wedge.

The diagonally related weight between the left-rear and right-front wheels is referred to as cross-weight or simply wedge. It is often measured as a percentage of the vehicle's total weight. When more than 50 percent of the car's weight is on the left-rear and right-front wheels, the car is said to have more wedge.

­Now that we've learned how an adjustment on one corner of a vehicle can affect the weights on the other three corners, let's take a look at how this uneven weight distribution affects a race car's handling.