Why is it when I am riding a bicycle it feels so stable (look ma no hands), but not when it is still. The torque from the wheels would not be large enough to provide a real force to stablise a person?
Contrary to “intuitive” physics, it is not due to the angular momentum provided by the rotation of the wheels. It is due to the “trail” of the front wheel, as demonstrated by specially made bicycles having “negative trails”. These bikes look perfectly normal but are almost impossible to ride, behaving rather like out-of control shopping trolleys.
Imagine you had a bike with fixed forks - no movable steering (and imagine for a moment you were able to ride it without going arse over tit) - if you leaned to the right, the bike would still swerve to the right because of the curvature of the wheels (try it with the rim of a dinnerplate on a table) - if you leaned to the left, you would swerve left.
But because the centre of mass of the system is quite high up, as it swerves to the right, the centre of mass wants to carry on moving in a straight line, ‘over the top’ of the bike, causing it to tend to stand up again.
Now with a bike that has movable steering, the same is true, but additionally, steering to one side from a completely upright position means the centre of mass of the system tends to tip the bike over, but tipping it over causes the wheels to swerve back under the centre of mass again.
There was a staff report this a while ago. And here is the lively followup. It seems that trail is the answer.
tx for the links. never knew it was so complicated.
You might also read this thread: http://boards.straightdope.com/sdmb/showthread.php?t=248077
Note the comments of someone who’s tried to ride a bike on rollers without being able to steer the front wheel back under him.