"Commander Kinsey" wrote in message
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I knew steering played a big part (I thought about 50%) because I've seen
a trick played where people are given bicycles with fixed steering, and
it's very difficult to balance on them.

Also there have been tests where a perfectly balanced human-size/weight
dummy is attached to a bike, and it falls over even when it is travelling at
a speed that a human could ride easily. That suggests that minute
adjustments to balance and steering are a major factor in staying upright.


I'm sure the wheels play a big part too - if you remove a bicycle wheel
and keep the hub, then spin it holding the wheelnuts, it's difficult to
turn over. Mind you, maybe that's not enough to prevent the weight of the
rider falling over too.

That's the gyroscopic effect. If the wheel is rotating and you twist the
axle side-to-side (in a horizontal plane), there is a force that tries to
twist the axle up-and-down (in a vertical plane). It acts so as to correct a
tendency to fall over: if you start to lean to the left and the front axle
turns to the left, the gyroscopic force acts to try to move the bike back to
vertical, and its magnitude varies with speed.

I can see why people though that this was the only force that mattered,
since it does play a small part in keeping balance - it's just that it's not
enough on it own.

I read of an experiment where a bike was fitted with wheels that had discs
of equal mass to the wheel that were rotated (electrically) in the opposite
direction so as to cancel out any gyroscopic force. And the bike was still
rideable, though it was slightly harder to keep one's balance.