At Home Mechanic

It is not to say that cars do not come from the factory predetermined to behave in a particular manner in a given situation. Nearly every new car that is sold in the United States comes from the factory designed to under steer (under steer is the phenomenon where you turn the front wheels and the car continues to go straight) under extreme limits of traction on dry streets. While this may seem counterintuitive (I want my car to turn when I move the steering wheel, but I am funny that way) but the justification is that the understeer only happens when the limits of adhesion had been exceeded. The alternative is to have cars that over steer (the tail end tends to swing out as traction limits are exceeded.

Oversteer was a problem for early rear engines cars with a simple rear swing axle design. Ralph Nader made a name for himself by pointing out the potential for disaster of the Chevrolet Corvair’s rear engine/swing axle design in the hands of unaware consumers. Rather than suggest that the consumers were at fault for driving their Corvairs beyond the limits of the design’s intention, Mr.Nader persuaded Congress that tail-heavy, over steering designs were unsafe at any speed. Congress responded by passing a comprehensive series of laws regulating automobile safety. While there is an argument to be made on both sides of the debate the car companies have since chosen to build cars with a bit of built-in under steer in the design.

I like to drive my hobby car at speed on racecourses where the conditions are controlled and there are as many provisions for safety as possible. To get the most speed out of my car on the racetrack, I prefer that my car have neutral handling, neither under nor over steering. Knowing that my Honda CRX Si came from the factory with built-in under steer, I attacked that problem (detailing how I eliminated under steer is the subject of another posting that will come shortly) and my car now is very neutral and gets the most grip from both ends of the car.

Mechanical considerations aside, the driver still has the primary responsibility for the car’s behavior on the road. The driver’s right foot has a tremendous amount of influence upon whether a car under or over steers. “Weight shifting” is the name of the technique of changing the balance of a car as it swings through a turn.

Adding or subtracting power while making a speedy turn (speedy for the conditions, your actual speed make me relatively low) shifts the weight of the car forward and backwards. Imagine that a car is going around a turn and that the weight of the car is loaded equally between the front or rear tires. By adding power while turning, the car’s weight is thrown towards the rear of the car. The extra weight on the rear tires will cause them to overload and begin to lose traction. Conversely, by letting off power while turning shifts the car’s weight forward which overloads the front tires causing under steer.

These effects are magnifies by extreme weight imbalance; front wheel drive cars have the majority of their weight over the front wheels. These cars are particularly prone to understeer and are extra sensitive to adding or subtracting power while turning at the limits of traction (this is another way of saying, “They are going too damn fast for the turn.”)

The best rule of thumb is to brake in a straight line before you need to turn the steering wheel. If you find yourself in a turn and feel you have entered with too much speed... you are screwed.