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Stability of a moving bicycle
By David Loeffler (P865) on Wednesday, August 2, 2000 - 09:00 am:

Why is a moving bicycle stable, but a stationary one will always topple over?
This has been puzzling me for a while.

David

By Sean Hartnoll (Sah40) on Thursday, August 3, 2000 - 02:08 am:

Believe it or not, I think this problem was only solved fully a couple of years ago!

It is because of conservation of angular momentum, sort of like a spinning top (which doesn't fall over because it is spinning about its axis). I can't remember the details offhand. But if there were no wheels, it would fall over.

Sean

By David Loeffler (P865) on Thursday, August 3, 2000 - 11:52 am:

Thanks, but I'm still confused. The problem with the conservation of angular momentum argument is that it doesn't seem like the size of the wheels should be important. For example, on those "Go-ped" scooters where the wheels are only about 10cm across you still get the same speed-stabilising effect. Also, don't ice-skaters rely on the same thing?

By Sean Hartnoll (Sah40) on Friday, August 4, 2000 - 03:47 am:

Well, I just had a long conversation with a group of physicists, and the only conclusion was that we didn't know why a bicycle worked...

I think it is related to the rotational motion somehow, ice skates depend on the person maintaining a balance, I think. I am not sure though, whether it depends on the facts that the rotational motion is touching the ground or not...

Sorry for not being helpful,

Sean

By David Loeffler (P865) on Friday, August 4, 2000 - 01:48 pm:

OK, I know I'm now answering my own question, but there is a good site at http://rabi.phys.virginia.edu/HTW/
entitled "How Things Work: The Physics of Everyday Life," which has a section on bicycles. I still think there must be something else involved - maybe the skating was a bad example, but what about the Go-Peds? I must ask my friend who owns one!

Thanks

David