How a bicycle works

Do you know how to ride a bicycle and do not know why you can do it? A widely accepted theory behind bicycle stability is the gyroscopic effect. David E. H. Jones went on a journey to find out the truth. In his paper “The stability of the bicycle” published in 1970, he proved that gyroscopic effect is not the only ingredient to the magic. Here he is on the experimental bike with a counter rotating wheel: 

 

He added an extra counter-rotating front wheel to cancel the gyroscopic effect and according to him the bicycle stayed stable even at lower feels. The only difference was that it felt a bit strange riding it due to a large moment of inertia about the front forks.

So what makes a bicycle going without falling over? “On the stability of a bicycle on rollers” by P.A.Cleary and P.Mohazzabi does an excellent job of summarizing the “Trail” effect that contributes to the stability of a bicycle. I drew free body diagram of the tire patch to demonstrate the key idea:

Imagine you are riding a bicycle with its front wheel configured for positive trail. Now imagine leaning to your right. Can you picture what would happen to the wheel? Remember that the wheel can only rotate about the steering axis. So if you were leaning on the right, the ground will push your wheel to the right about the steering axis i.e. the wheel turns with the right lean of the bicycle. How does this help in stability? It lowers the center of gravity which adds stability. If you also add centrifugal force acting on you when making a right turn/lean, the bike is being pushed to left while the ground forces the wheel into your turn or lean. This is what gives you the confidence when going around a corner on your bicycle, centrifugal force and the trail.

Now imagine you are riding a bicycle with its front wheel configured for negative trail. Imagine leaning to your right. And as we know the wheel can only rotate about its steering axis, the point of contact with the ground will push the wheel to the left when you want to lean to the right; ladies and gentleman, this is a recipe of a face plant. 

I hope now you know what it takes to ride a bike: a positive trail, centrifugal force from god, and a helmet.