How bicycle balances

There are very few engineering machines, where knowing how it works is difficult than actually working on it. Bicycles are one of those things!

It is true that the exact reasons validated by both theoretical proof and experimental evidence has not been found for which all factors contribute to the balancing of bicycles and by how much. Currently we talk only in terms of major and minor factors for this sort-of magical stability of bicycles.

Even then, there are confusion regarding which is the most important factor and how much it accounts for it. In this article, I will be mentioning the major reasons that have been proved experimentally.

There are 4 important factors that contributes to balancing of bicycles.

1. The ability of the front wheel to turn into the fall.
2. The trail of steering axis
3. Gyroscopic Precession (Secondary reason - Earlier this was considered to be the primary reason
4. Operator skill that many times acts effortlessly.

The ability of front wheel to turn into the fall

To understand this reason, try to balance a pen (with flat base) vertically on the center of your palm. If you see carefully, you will notice that you always try to move your palm in the direction in which, the pen starts falling.

The same thing happens with the front wheel. The front wheel turns (rapidly) on the side the bicycle starts falling. You would have also experienced this, while just walking and taking a bicycle from one point to another, by holding the seat (saddle) and the handle bar. If you tilt the cycle and don't hold the handle it falls (we call it turning, but scientifically it is just falling).

Understanding Gyroscopic Precession is easier and helpful in getting the feel of first reason - the trail. So let's start with that.

Gyroscopic Precession

Check this video to understand this seemingly complex topic clearly.

You may like to see this video too - Gyroscope

So a spinning wheel resists change in its angular momentum and prevents itself from falling. Okay, this could be the main reason for the stability of bicycles. Many people believed that this was the case (even now, some people believe the same).

This has been proved wrong, by carrying simple experiments in which another wheel was attached to the bicycle wheels and made to rotate in opposite direction. This cancels the total angular momentum, which in turn removes the gyroscopic effect. Had this been the major factor, the bicycle should have become unstable. But it was not the case. The impact was much less.
It was concluded that while GP effect is there, its effect is a minor factor rather than being a major one.

The trail of steering axis

The point of contact of front wheel "trails" the point where the steering axis meets the road. Because of this, when a bicycle leans, a torque is developed that turns the front wheel. The result is similar to that of Gyroscopic effect but the reason is different.

In fact, the stability of bicycle increases on increasing the amount of trail. The problem that prevents in having excess trail is that it becomes difficult to steer.
In the figure above the trail is referred as positive. Experimental bicycles with negative trail were found to be extremely unstable.

The above three reasons coupled with the skill of operator is responsible for the balancing of the bicycle.