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# Why is it easy to balance on a moving bike, but almost impossible to stay upright when it stops?

Question #117485. Asked by star_gazer. (Sep 12 10 7:59 PM)

ceetee

It has to do with wheels moving beyond the base of support Check out this link

http://www.howeverythingworks.org/bicycles.html

 Sep 12 10, 9:00 PM
Baloo55th

Inertia has something to do with it. Inertia is the tendency of objects to keep going as they are until something acts to upset this (gravity, friction, wind gusts, bricks, etc). At speed, the inertia of the bike and rider will tend to keep it upright. This is why on approaching a corner, you have to exert force to tilt the bike - and similarly on coming out of the corner. Without the inertia, it is easy to wobble over. To stay upright without the benefit of inertia, you need to keep the 'centre of gravity' (as it used to be called) within the area defined by the contact of the tyres with the road. This is normally very small in breadth - and it's the breadth that counts here (unless you are trying to cycle down a wall or tree, which is not recommended). You overhang this tiny safe area by an amount depending on your size. (I once saw a 27 stone woman riding a Honda 50 - not a pretty sight; I knew her and was present when she stepped on a bathroom type scale and broke it...) You are also fairly high up - those reclining bikes are theoretically more stable because the CoG is much lower. Conversely, you get less leaning power because you're flat not upright.

Basically, the broader the tyres, the easier to stay up. And the faster you go, the more the inertia will help you. (But the faster you are going, the harder it is to avoid the bricks...) For more, see http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics (

Baloo going to brag now and say that he read that link after he'd written the above from his cycling experience. Baloo has a day-glo orange and lime-green mountain bike which is clearly visible well into dusk - and which is regarded as well cool by local kids with new but less brilliantly coloured machines...)

 Sep 13 10, 8:34 AM

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