‘Driving Tips’ Series

Volume 1: Oversteer

In the mid to late 1970’s, the Porsche 935 dominated sports GT racing in the United States and Europe. After its first successful year, it seemed that every team switched to the 935 for its superior combination of power, braking, economy and handling. Before long, watching sports car racing was little more than watching a train of 935’s snaking its way through corner after corner.

This dominance came in spite of a devilish handling trait unique to the 935 – excessive oversteer could be prompted with the slightest wrong move of the driver’s right foot, resulting in many 935’s collecting the catch fencing, backwards at over 100 mph. In experienced hands the 935 was potent and hard to beat. In inexperienced hands, the 935 was frequently converted into very expensive junk.

Slot cars exhibit similar traits to the 935, but for slightly different reasons. The 935 was susceptible to this condition because it had over 60% of its weight hung over the rear axle. When a driver entered a corner too hot, the driver’s natural instinct would be to lift off the throttle. Unfortunately, this natural response unbalanced the tail-heavy car, breaking loose its grip in the rear. The tail of the car inevitably raced the nose as the car hurtled sideways, then backwards on its way to an early (and sometimes painful) retirement.

Slot cars display this behavior, also when entering a corner with too much speed. While a slot car doesn’t have the hideous weight distribution of the Porsche, it acts similarly when the throttle is lifted. The tail races the nose in the same manner, not due to weight, but because the front has a guide inserted in the slot. The car carries momentum into the corner, and while the momentum of the car’s nose is kept in check by the guide, the tail is set free like a preacher’s daughter on prom night. In addition, the guide causes friction, and when the driver lifts off the throttle, this friction slows the front of the car faster than the rear of the car. The end result is that the rear swings around, frequently causing a spin or even a flip.

So what can a driver do to avoid this fate? Just like a real driver, the solution takes courage – courage to stay on the throttle. Take a look at the following picture:

Red Arrow: Negative acceleration (deceleration) due to friction of the guide in the slot

Blue arrow: Forward momentum

Yellow arrows: Components of acceleration from the driving wheels

As Raikkonen enters the corner, his car carries a great deal of momentum (blue), and he is still on the throttle (yellow). He releases the throttle (hence no more yellow arrows), but he still has momentum. As he gets deeper into the corner, you can see that the momentum is no longer directed along the slot. Since the front is being redirected by the guide, the front of the car is changing its direction of travel. The rear of the car is relying only on tire grip to change its direction of travel, and the car begins to slide. If Raikkonen relies on the grip of his rear tires to keep that momentum in check, the car will continue to slide towards the outside of the curve, until the car de-slots.

Instead, Raikkonen applies just a little throttle to accomplish two things: first, to exert a forward-pointing force on the guide, countering the rearward friction, and second, to exert a force pushing the rear of the car towards the slot again. Should you find yourself sliding after a late-braking maneuver (I’ll call it a maneuver, assuming that there’s no way that you might have accidentally stayed on the throttle too long), apply partial throttle. Touching the throttle may not totally reverse the slide, but it will buy you a little more time to feather the throttle well into the corner. The resulting powerslide balances delicately between sliding the tail too far, and carrying too much speed (which would cause a de-slot). In the real world, this powerslide was very expensive in terms of tire wear for the 935’s, but saved a lot of embarrassment, twisted metal, and possible injuries. With slot cars there is very little tire wear, and even if you mess up the slide, there is little to no chance of a fatality. So what are you waiting for – get sliding!