Engineers over at Cornell University have designed and built a riderless bike which seems to have no other purpose than to debunk the theory that the gyroscopic precession of a spinning wheel, and caster action of the front wheel has nothing to do with the self-steering properties of a bicycle.
I am sorry but this is not a bicycle yet, it is a push toy; when you put pedals and a rider on it, then it will be a bicycle.
I believe that giroscopics and caster action contribute to balance and steering; however, that is not the whole story. Simple momentum has a lot to do with it; take a surf board for example that has no wheels.
A surfer has to initially move his body constantly to remain upright, but when he gets up to speed it becomes easier to maintain balance and his body is practically stationary. A bicycle too, the faster you go the easier it becomes to balance and hold a straight line.
Another natural and intuitive thing to do is to lean into a corner; in the direction we are turning. We even do it while running, and for that matter so do all animals.
While running, if you lean to the left you automatically step to the left to maintain balance, and so you turn; it is an intuitive movement without thought. So if a bicycle is designed to steer in the direction it is leaned, it too becomes intuitive.
It is the intuitive nature of the way a bicycle handles that makes it relatively easy to ride. Riding at slow speed it is almost impossible to ride in a straight line; as the rider falls to the left he steers to the left bringing the bicycle back under his body to maintain balance.
Much in the same way you balance a broom on your hand; you constantly move the hand to keep it under the center of mass to maintain balance.
At very low speeds the cyclist will physically steer the bike left or right by turning the handlebars; as speed increases the rider will steer by leaning to the left or right. This is where the self steering properties of the bicycle kick in.
As the bike falls to the left the head tube of the frame moves to the left; so too does the steering axis.
The steering axis reaches the ground at a point ahead of where the front wheel contacts the ground.
This is known as “Trail,” because the wheel trails along behind the steering axis.
Leaning to the left will cause the front wheel to steer to the left; leaning to the right will steer to the right.
On the Cornell model the exact opposite is true. They deliberately placed the steering axis behind the front wheel’s point of contact. (Negative trail.) They did this in order to prove a point, but in doing so created a reason their model works. As it falls to the left, it steers to the right thus correcting its direction of travel. (Watch the video on their site.)
In order for the Cornell model to work they have added weights. The momentum of a weight on a pole ahead of the contraption pulls it along. Another weight placed low on the front steering falls at a quicker rate causing the front wheel to turn in the opposite direction. (Go to article link above and click on the picture to see an enlargement.)
As I see it all that has been proven here is that by adding carefully placed weights you can make something self-steer even though its steering axis is behind the front wheel’s point of contact.
Gyroscopic precession and caster action have been removed, but this had to be replaced with a system of weights to achieve the same end. It is simply a different way to skin a cat; it proves nothing
As a model this apparatus is a cleaver idea, but will not work as a bicycle; here is why. As I have already mentioned to balance on a bicycle as you fall to the left you instinctively steer to the left to bring the bicycle back under the center of mass, which is the rider.
On the Cornell model as you fall to the left it steers to the right, the rider will be unceremoniously dumped in the road. Even worse as you intuitively lean into a corner the bike will steer in the opposite direction.
Not only have Cornell taken away the intuitive handling nature of the bicycle, they have designed an unridable bicycle. Don’t believe me? Build it and see.
There is some more reading on Head Angles and Steering here, and on Trail, fork rake and a little bit of history here.