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Name: Richard
Status: educator
Age: old
Location: N/A
Country: N/A
Date: 2000-2001

I have been interested in the physics of bicycles and so was dismayed by the answer of Obiwan (sic) on motorcycle turning. Turning the wheels briefly to the right always and at all speeds causes the motorcycle to turn left. Why does Obiwan say it only happens above a certain speed, and what speed???

It has nothing to do with the angular momentum of the wheel. There are by now many good references on the web and elsewhere so I won't give the arguments here. If you would like further explanation, I would be happy to oblige.

Best, Dick Plano, Prof Physics, Rutgers U, emeritus..

Well, I used to ride a motorcycle, and I remember when I took my motorcycle Riders' Education course, we were taught that at low speeds, you turn a motorcycle "like a bicycle," by turning the handlebars in the direction of the turn. At higher speeds, they said, it is necessary to countersteer, that is, cause the bike to lean by pressing on the handlebars opposite the direction of the turn. As an avid bicyclist, I found that explanation rather amusing. Bicycles work just like motorcycles, and countersteering is definitely the way to go at high speeds.

Of course, you are right in pointing out that countersteering will, at all speeds, cause the bicycle and motorcycle to turn in the desired direction. At low speeds, however, it just isn't as noticeable. Since the centripetal acceleration is so low in a low-speed turn (as you know, the centripetal acceleration is proportional to the square of the tangential velocity), all that a rider really notices is that, during the turn, the front wheel is pointing in the direction of the turn. The torques on the handlebar are too small to notice. As the tangential velocity increases, it's impossible to miss the force you have to apply to the handlebars to maintain the turn. I think that's why it's common to see descriptions featuring different effects iat different speeds.

Actually, I'd appreciate if you can point to some web sites dealing with the physics of making turns on a bicycle. I'm not entirely satisfied with my own understanding of why the handlebar torques are the wy they are...

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois

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