Science Questions

Why does a bike stay up more easily when itís moving?

Sun, 11th Jan 2009

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Jim asked:

Why does a bike stay up so much more easily when itís moving? Itís very hard to balance when the bike isnít moving.


Dave - It is very hard to balance when a bikeís stationary. Thereís two effects. One of them is if you tip a bike to the left youíll see the front wheel tends to turn into the left as well because the front forks are leaning forwards. Itís more stable if the handlebars tip to the left. Once the handlebars tip to the left then you tend to steer into the corner and the wheel works its way back underneath you.

Chris - So itís like if I give you the old trick of a broom handle you can balance the broom handle in the palm of your hand. Just by moving your hand around you can hold the broom handle vertically. Itís because when the broom starts to fall in one direction you can move your hand to go effectively into the direction of fall and thatís what keeps it stable. The bikeís doing the same thing.

Dave - The bikeís doing the same thing automatically. Thereís also a gyroscopic effect that can help.

Chris - Because the wheels are spinning and so because the wheels themselves are turning they have a gyroscopic moment. Itís obviously difficult to make it deviate. Given that itís most unstable when itís moving slower you can still balance. You probably get more stable when you speed up?

Dave - You do and itís easier to ride when youíre going faster. If you imagine a spinning top it starts to fall over it starts to rotate round and round in circles and the same thing starts to happen to your bike wheel. As you start to turn over it starts rotating and turns into the corner as well.


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