In a nutshell you can do this because you have two large gyroscopes working for you, ie
your front and rear tires. A spinning gyroscope will stay in it's plane of rotation
unless acted on, hence your spinning tires want to stay vertical. This additional
force helps you maintain your balance and allow you to ride with no hands. This is
also why you have to lean in order to turn your bike, as you know if you ever try to
turn by just using the handle bars. There are other factors that can come into effect,
such as the camber of your front forks and the size of the tires, but is probally more
than you wanted to know.
Answered by: Bruce Lau
Contrary to popular belief, balancing on a bicycle is nothing at all to do with
gyroscopes. It is to do with the shape of the forks and handlebars.
As you ride along, dangerously dangling your hands by your sides, you shift your body
weight from side to side. By tilting the bike to the left, the handlebar and forks
fall naturally to that side.
Similarly, tilting the bike to the other side steers you. (you can test this by
holding your bike stationary and tilting it from side to side).
This is how you steer.
Balancing is a side effect of steering - when you steer, the bike moves under you
allowing you to put it under your centre of balance.
Answered by: Damian Smith, Physics Undergrad, Trinity College, Cambridge, UK
Considering I ride BMX bikes for a living pretty much, I thought it would be suitable
for me to answer this question. I believe the answer lies in something called a Gyro.
Planes and horizonal devices use Gyro's to orientate everything straight. Next time
you take your bike wheel off, spin in really fast in your hands and try moving it. It
is REALLY hard to move because the tire is spinning. The wheel works the same way as a
Gryo! So when you take your hands off on your bike, your tires' rotational inertia ia
keeping your bike up.
Answered by: Josh Ellis, Biological Engineering Major, Oregon State Univ.
The correct answer includes both points of view expressed above. Namely, you can ride your bike without your hands due to the fact that when moving, your bike has a vertical position as it's most stable equilibrium. This is both due to the fact that fast spinning wheels have rotational inertia - meaning that they would much ruther stay spinning in the same plane than tilt, and due to the fact that bikes usually have a front wheel 'fork' designed in such a way as to reinforce this vertical equilibrium.
Answered by: Anton Skorucak, PhysLink.com