Why is it easy to balance on a moving bike, but almost impossible to stay upright when it stops?
(Continued)
I was pleased to see that the importance of the gyroscopic effect on the stability of conventional bicycles has been downgraded in previous answers, but was surprised that the most important design aspect that makes bicycles easy to ride has been overlooked. Every cyclist knows that above a certain speed it is possible to ride hands-free. Most will also be aware that once a bicycle without a rider has been given a push to a little over walking speed it will stay upright for quite a while. This is because a conventional bicycle is designed to be inherently stable.
Advertisement
This stability is mainly due to the trail. Trail is the distance that the point of contact of the front wheel with the road lies behind the point of intersection of the steering axis with the road (see Diagram). This comes from the built-in tilt of the steering axis and the rake of the front fork. The effect of trail is similar to the action of castors. If the bicycle leans to the left, the contact force at the road will turn the front wheel to the left. This allows the hands-free rider to achieve a degree of steering control by leaning slightly one way or the other. Dynamic analysis shows that trail, together with a gyroscopic effect, can produce stability above a critical speed, trail being the most important factor.
H. R. Harrison, Loughton, Essex, UK
The usual notion is that to steer to the left you push the handlebars to the left. However, experiment by holding the handlebars with your fingertips and you find that pushing the handlebars to the left by a couple of centimetres makes the bike turn right, rather than left. This counter-intuitive effect arises because turning the handlebars a little to the left makes the bike lean to the right, which then 鈥渢urns鈥� the bike to the right.
Mark Pettigrew, Sheffield, South Yorkshire, UK
For those who would like to see a bicycle that cancels out gyroscopic effects, Hugh Hunt of the department of engineering at the University of Cambridge has posted some images at . In 1987, New 杏吧原创 reported the work of Tony Doyle, then at the University of Sheffield, UK, who built a bike that not only cancelled out gyroscopic effects but also had no trail, and so no castor effect (30 April 1987, p 36). 鈥淥nce [riders] had overcome their initial impulse to scream, they could ride the destabilised bike easily,鈥� ran the article. 鈥淏ut whereas a normal bicycle stabilises itself almost instantly, when the riders were left to make the corrective movements for themselves, they could do so only after a delay.鈥� Doyle also describes the sequence of events needed to turn a bicycle: to begin turning right when travelling at a fair speed, cyclists do indeed push the handlebars to the left, and continue doing so throughout the turn. 鈥� Ed