A Weighty Subject

Many aspiring autocrossers seek to boost the power of their vehicles with engine and fuel system improvements, and spend large sums upgrading the suspension to improve cornering performance. Much improvement is available without adding anything at all.

It is a fact that increasing the weight on a tire will increase its traction, but the relationship is not linear. The actual weight upon a tire will of course be counteracted by an equal force at the contact patch. But increasing the mass of a car will increase centrifugal force in a turn, according to Newton's second law. As mass increases, the centripetal force generated by the mass of the car will tend to overcome any increase in traction the added weight may have given to the contact patch. What this means is that doubling the weight will not double the traction; there is a diminishing rate of return. The reverse is true inversely: halving the weight will decrease the traction less than half.

The performance of a car is dependent not merely upon available power and traction but upon the relationship between mass, power and traction. By decreasing the weight of a car, there is less inertia to overcome, so braking and acceleration are improved. Because traction has decreased less than the percent decrease in the weight of the vehicle, cornering power will increase.

It is a fact that this type of performance improvement costs less than any other type. It is also less likely to add a lot of points to your vehicle and bump you up in class.

It should be downright refreshing to think that you can get a performance boost from taking stuff off your car more easily than bolting new things on. The main types of weight control are removing and relocating. Some of the heaviest items in your car can be reduced without point penalty; remove your spare tire. Leave the washer bottle empty. Arrive at the autocross site with miniumum fuel. A full tank of fuel may weigh 100 pounds. Take out the floor mats.

Some items have small point assessments: remove the rear seat (1 pt.), remove the passenger front seat (1 pt.) That represents about 80 pounds, and maybe a couple of tenths of a second. Add a set of sticky tires to your stock rims and many BMWs will still be in class A. A lighter car will require less traction, and generate more traction in proportion to its weight. It will also have a higher power to weight ration. It will accelerate and brake quicker, and because the suspension has less mass to manage, it will handle better, respond more quickly and lean less in turns than a heavier version of the same car. (Don't raise the bridge; lower the river).

Some heavy items in the car cannot be removed, but can be relocated to advantage. Most front-engined cars tend to plow in turns at an autocross. Anything that gives the front tires less work to do will help the car negotiate turns with less plowing. Relocating the battery to the trunk is a popular way to balance a car; most storage batteries weigh 30-35 pounds. Set way forward in a corner of the engine compartment is has maximum influence on the inertial resistance of the car to change direction. Relocating it over the rear axle can make a difference that will show up on a stopwatch.

Please send questions or comments to: drautox@comcast.net

Last Modified December 25, 2000