How does a “tank slapper” occur? Why wouldn’t a motorcycle simply fall over instead of having the wheel wobble from side to side?
In: Engineering
The front end of a motorcycle utilises what is known as a caster angle. The front wheel does not contact the ground at the same point it rotates around but behind it. Without getting into the specifics this creates a self righting effect. Bike leans one way, the handlebars turn the other way and it rights itself up. You can see this with many videos of runaway bikes or even bicycles. If you’ve ever sent a bicycle rolling at speed you’ll know that it remains upright, and even if it wobbles, it doesn’t fall until it has no speed or if it develops a very violent wobble, enough to lose traction completely.
As the handlebars can turn from side to side, this provides the grounds for a system susceptible to oscillations. Furthermore since the caster angle can produce a countering force, you get a system that has the potential for resonance, since it has two major inputs in its movement that counter each other, the turning of the handlebars and wheel, either by the rider or by their own weight and momentum, and the correcting force produced by the caster angle. If resonance is achieved, the handlebar oscillation and the correcting force produced by the turned wheel can synchronize in a way that feed into each other, making the oscilliation’s amplitude bigger and bigger, meaning in practice that the handlebars will turn from side to side with increasing force and speed. Trying to control it with sheer force is not necessarily a good idea or feasible in some cases, since the rider is very liable to inadvertently feed into the oscillation. The solution is to have steering dampening, which limits the speed at which the handlebars can turn which means the speed wobble cannot increase in amplitude. They provide destructive interference, meaning that they remove energy from the oscillation instead of adding to it.
For the motorcycle to fall it has to completely lose traction so that the tires slide under it and it tips over, but as long as the tires maintain sufficient traction, as much as the bike is bucking and thrasing it will not go down.
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