Stability of Vehicles

You think you do a big job when you are balancing a 2 wheeler while driving it? You hardly do anything! A major role is played by rotating wheel. To see why, just imagine yourself balancing the 2 wheeler that is stationary, for as long as you generally drive! Balancing a stationary 2 wheeler is tough. Do it with cycle, you will have to do some good amount of acrobatics!

Why would a rotating wheel be more stable? What is fall of a bike? Its essentially tilt that doesn't reverse, and continues till it touches the ground. A rotating wheel, when tilted, actually turns - that's how you take turns - rather than falling by side.

A complex mechanics equation and formulae and vectors show why this is necessary. To get a simple idea, think of it this way:- a sledge moving on ice has a tendency to move in the same direction. If you apply sideways force, the direction of motion will be very slightly changed to that side, while it continues to move in more or less same direction.

In case of rotating wheel, it has a tendency to continue to rotate around the same axis - say the axle of wheeel - unless you tilt it. When it is falling, it just shifts the direction of axle - like the small direction change in sledge motion - and continues to rotate around this new axis.

When a wheel tends to fall, instead of falling sideways and continuing to move in the same direction, it will change the direction of travel, and spiral towards fall. This is easily seen by anyone playing with a cycle wheel and pushing it by hitting at back. The wheel will roll on, and depending on which side it bends, it will spiral in that side and fall. What it doesn't do, is to go in straight direction and fall sideways!

Spinning top also shows the similar stability. It can balance itself on a needle end. It will keep balancing and rotating and once friction eats up the energy, it falls down spiralling.
It also shows another behaviour, when, after throwing it may not have landed vertically, and slight tilt causes its rotation axis to go in circle around the vertical axis. Notice that even though its tilted, because its rotating, it goes in circles, rather than falling down. A stationary top will simply fall down. This is called as Precession.

The larger the speed, more the stability. Of course, more speed invites other dangers.Also, this means that falling sideways is much easier on smaller speeds and sharper curves. So be careful!