Wednesday, August 31, 2011

Physics of Driving in the Dirt

On the street, you can usually count on the steering wheel to point your car where you want it to go. On snow, a high speed race track, or in the dirt there's a lot more physics involved.



When I'm not sitting behind a desk blogging about physics you can find me and my son JP out racing through the dirt, mud and snow in JP's 2002 Subaru WRX. He has to drive it to school too, so we do our level best to keep from knocking pieces off during our weekends running with the Sports Car Club of America Rally Cross crowd.

To the casual observer, we may look like a bunch of lunatics thrashing our cars through fields and forests, but there's a surprising amount of physics that goes into the sport. At the beginner's level (where I am right now) you truly have to pay attention to things like velocity, acceleration, momentum - both linear and angular - and friction. I assume that once you get good at it, the physics of rally driving becomes second nature. I have a long way to go before that happens.


Of all the skills there are to learn, the one I'm focused on most at the moment is the slalom. There are several things that go into carving around a slalom cone, but actually turning the steering wheel is far from being the most important or difficult part. Rally cross drivers generally break the process down into five or six steps, executed in rapid succession. The clip above shows JP taking a slalom turn with the appropriate technique. Here's what he's doing . . .

1. As he is about to enter the turn, JP lifts off the gas. Drag and friction kick in instantly, this makes the car lean forward. That puts more force on his front tires, which improves traction up front.

2. Now that he has lots of grip on the front tires, he turns the steering wheel in the direction he wants the car to go, but he doesn't turn it far enough to actually make the corner the way you would while casually tooling around in a parking lot.

3. Once the car begins to turn, JP hits the brakes, which makes the back end begin to slide around while giving the front tires still more grip. Rally drivers say that you use the brakes to rotate the car.

4. As the car goes around the corner, he releases the brake while pressing the gas (that's right, he has one foot on each pedal the whole time).

5. Even before the turn is over, he begins to straighten the steering wheel.

6. When the car is pointed in the direction he wants it to go, JP hits the gas hard to accelerate to the next turn.

All six steps take place in a matter of a few seconds. In rally driver shorthand the sequence is Lift, Turn, Brake, Release (the brake), Straighten (the wheel), Accelerate. Or LTBRSA (pronounced "Litbursa" )* for short.


As you can see, the steering wheel plays an important but relatively small part in the whole act of turning on a rally cross course. Besides being simply the fastest way through a slalom, keeping the front wheel as straight as possible has other important benefits. For one thing, most cars can handle rough surfaces better with the wheels pointed forward. Cranking the steering wheel very far in one direction or the other exposes delicate parts to stresses that may snap them as you go over a rut or bump.

Tires too can take a lot of abuse when they're pointed forward. If you try to go through a slalom by relying on steering rather than the brakes and momentum to carry you around, you will tend to turn them too far and make them act as rudders plowing sideways through dirt and rocks, rather than as wheels riding over them. We and many other drivers have discovered that using too much steering will often pop the tires right off the rims, putting a quick end to your race.

The technique I've described here works for turns on snow, in the rain and even on plain old asphalt when you drive really hard. Basically, whenever the surface is slick enough, or your speed is high enough that turning involves the potential for lots of sliding, you have to think about a lot more than steering to stay on course or in your lane. In fact, rally cross drivers are occasionally asked to conduct accident avoidance classes to teach people to use rally skills to handle driving in winter and wet conditions.

Other turns and surfaces require additional racing skills that I'm just beginning to learn. For now, I'll be happy to make it through a slalom looking as good (or going as fast) as JP.

*Just kidding, nobody says Litbursa.

4 comments:

  1. I think getting your car dirty is actually cool. It only shows how adventurous you really are.

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  2. wrong sequence. first you brake, then you steer. if you steer before braking, all weight goes to the external front tire and front internal has less weight => car is unbalanced. if you brake before you steer the weight is transferred to both wheels and grip increases.

    always brake with straight wheels.

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  3. You're right, provided you have traction. People who race on the dirt will tell you that you turn the car more with the brakes than the steering wheel. You lift off the throttle to throw the weight onto the front wheels, steer to get the car rotating, then hit the brakes to get the car to come around. Many people will use their left foot to brake so they can continue to apply throttle with their right.

    If you have lots of traction, as is often the case on tarmac, then this technique is not as fast. But I find it sometimes useful even when racing autocross, particularly if I'm pushing a set of street tires beyond their limit. It works best when autocrossing on a wet course.

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  4. "Once the car begins to turn, JP hits the brakes, which makes the back end begin to slide around while giving the front tires still more grip. Rally drivers say that you use the brakes to rotate the car."
    - This passage may look simple but it's really very complicated, my brother had failed so many times in this step. It seems I must send this article to him attached a physics course.
    P / S: I like your car, it was great!

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