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Name: Shannon T.
Status: N/A
Age: 16
Location: N/A
Country: N/A
Date: 2001-2002


Question:
I am studying physics and was wondering what a skid is, how it happens and how you can recover - from a physics perspective. I am also required to research 'hand-brake turns' and have come up with no information. Can you please help?


Replies:
Shannon,

A skid occurs when the road and wheel cannot exert enough friction to hold onto each other. A skid is very much like your foot sliding on ice. It is the same as a box sliding across the floor. If a car slams on the brakes so that the wheels stop rolling, the car may stop immediately. If the road cannot apply enough friction to make the car stop, the car's will skid to a stop over more time (it will not be immediate). In a sharp turn, sideways force is needed to make the car change direction. If enough friction from the road cannot be provided, the car will "spin out" rather than making a turn.

Dr. Ken Mellendorf
Illinois Central college


I believe that this has to do with the difference between static and dynamic friction. When a wheel is rolling, it clings to the road via static friction (at any moment, there is a patch of tire that is touching the road, but is not sliding against it). When the brakes are applied too hard, the wheels lock up. This would cause the patch of tire touching the road to slide. Since it is now sliding across the ground, the friction involved is now dynamic friction. For most materials, dynamic friction is only half that of static friction. This means that the car will not stop as quickly. More importantly, when the tires lock up, the car will slide (skid) in the direction it was going. Since your wheels are no longer rotating, you have no control of where the car is going.

Anti-Locking brakes solve this problem by allowing you to brake in pulses. During a pulse, the tires lock up, and you will slow down. When the pulse ends, you get a little bit of control as the wheels are momentarily allowed to rotate.

I believe that a hand-brake turn is when you use the parking brake to help in turning. Make a sharp turn in the snow. When your car begins to turn, apply the parking brake. This will lock up your rear tires, causing the car's back end to swing forward. When it is facing the direction you want to go, release the parking brake.

I have never tried this move before though, and do not recommend anyone trying it either. I have heard that it works best with rear wheel drive cars with manual transmissions.

If anyone have a better answer, please let me know.

-Wil



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