Re: How does wheel size affect skateboard performance?

The answer to your question is "Friction, Friction, Friction!" You really asked two questions, one about friction on the wheel bearings, and one about friction with the ground.

For a given speed, a small wheel has to go at a higher rotational speed than a large wheel to keep in contact with the ground. The formula is w = v / pi 2 r. The rotational speed (w) is inversely proportional to the straight-line velocity of the skateboard, and proportional to the radius. Say you were driving a car with noral-sized wheels. At 10 miles an hour, the car is going about 176 inches/second. At that speed, a 15" radius wheel will spin at 1.8 revolutions/second, or about 112 rpm. At 60 miles per hour, the wheel will spin at 6720 rpm. You get the idea. The problem comes in with the wheel bearings. The wheels have a certain friction associated with the design of the bearings, and the friction is directly proportional to the rotational speed of the wheel. So the faster you go, the more resistance the wheel feels from the bearings. So if you want to go fast, you need as little friction as possible. This means that while you want to cover a lot of ground quickly, you also want to have the largest wheels possible. That translates to minimizing the rotational speed of the wheel, and minimizing the friction from the bearings. Large wheels introduce a number of problems of their own, with intertia at the top of the list.

The second question is about the width of the wheels. The wider the wheel, the larger the contact with the road. This means that you will be able to stop faster, but that is because there is a larger area of contact with the road. The larger contact area means that the coefficient of friction is larger. That means for any speed, the friction caused by the wheel rubbing on the road is proportional to the surface area of contact. If you have a narrow wheel, the contact area is minimized, and the fiction is in turn minimized.

So, for efficient, speedy running, you want large, narrow wheels.

Hope this answers your question!
-Fred