I was reading an interesting note in Panorama, the magazine of the Porsche Club of America, yesterday, which I feel compelled to share.

David Murray, a well-know road-racer, had written an article on driving techniques. In it, he'd made the comment that taking your foot too quickly off the brake could cause oversteer--the rear end of the car to start to come around.

In a letter to the Editor, one reader suggested he'd said it backward because when you stop braking, some of the weight transfers back from the front tires to the rear tires giving them more, not less, traction. The reader surmised, as I would have, Murray had really meant to say something more like "applying the brakes too quickly can cause oversteer."

Be he was wrong. Murray meant what he'd said. That perked up my ears. Here's how he explained it:

Imagine you're trail-braking at the limit into a turn. (Trail-braking is a technique where you continue braking as you start into a turn, easing off the brakes as more and more traction is needed for turning.) A great deal of weight has transferred to the front tires, giving them some additional traction, but they are usually sliding a bit because they're over-burdened.

They're over-burdened because they're not only trying as hard as possible to stop the car, they're trying to turn it, as well. The rear tires, at this moment, can lose traction fairly easily because they have little weight and, because of that, comparatively little traction to offer. It's easy to spin a car as you turn in under hard braking.

So, both tires are at--or even slightly over--the traction limit. Both ends are probably sliding a bit. At that point, if you eeaaaassseee off the brakes you'll gradually transfer weight to the rear of the car, giving the rear tires more grip and allowing you to start using some of that traction for accelerating.

At the same time, the front tires are losing some grip but they're also losing the extra demands placed on them by braking, so they can hold more or less even in the balance of available- and needed grip. So, a gradual transfer of weight in the transition from braking to acceleration in a turn keeps everybody happy.

Simple enough, but here's where the plot thickens.

I should have listened to the Shoe. Mr. Shoemaker, that is, my high school Physics teacher. He often said, "Use your best intuition to figure out exactly how you think it will work. The answer will probably be just the opposite." That certainly applies here.

When I think about doing all of the above but quickly taking my foot off the brake, my expectation is that the quick removal of weight (read: traction) from the front tires, along with the quick application of weight (traction) to the rear tires, would cause the car to understeer. That is, for the front end to lose traction and try to plow straight ahead instead of turning.

But, apparently, at least under the right conditions, that's not what happens. Even if you stop braking all at once, the weight doesn't transfer all at once. It doesn't take a lot of time but it takes at least a little time. At the moment you release the brake pedal, all the weight is still transferred forward. That means the front tires still have lots of traction and the rears have comparatively little.

But, by not needing to brake anymore, the fronts suddenly can put all their effort into turning.

If they were sliding before, in harmony with the rears, they'll suddenly hook up and turn more sharply. With the rears running at their limit of traction, suddenly turning in more will push them over the edge and start them sliding (or sliding more). With the front hooked up and the rear end sliding, you've got oversteer. Possibly even Terminal Oversteer, which is a nice way of saying you'll be headed backward into the weeds.

Interesting stuff, but totally counter-intuitive. One more reason to be smooth when you're driving.

1. Tom Roberts01/21/2005 02:09:33 PM
Homepage: http://www.wwwebfeet.com

Interesting enough, but why don't you tie down all that stuff that is shifting around when you're breaking and accelerating? It seems like it'd be dangerous sitting in a car with all this "weight" flying around when you're trying to drive.