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
I thought they were called “speed wobbles”….but maybe that’s just bicycles
Pretty sure it’s a sudden and catastrophic change in the center of gravity which can’t be resolved.
Centrifugal forces keep the bike upright as does the body acting as a counter weight
Do you mean the death wobble?
Well because conservation of momentum. Also, the bike *does* fall over if they get bad enough. The natural way a bike rolls is straight, and the wheels *really* want to go straight, so unless you get some serious wobbles where the entire center of mass gets shot one way or another very quickly, you can usually recover from it by just slowing down.
Partly because of the rake of the forks. The wheel can rotate left to right, but only around the axis of the front forks.
But, uh that’s all I recall.
A motorcycle wants to go straight. It will naturally correct its course by going to opposite the direction that would cause it to fall over.
So if there is sudden traction with the handlebar turning to the left, the bike will lean the opposite direction, which will inevitably turn the handle bar far right to counter. Which, inevitably makes the bike fall to the left, which then will jerk the handlebar far to the left.
This occurs if coming down from a wheelie and the front wheel was turned to one side. This also occurs if the front wheel hit a pothole or a bump while turned.
Usually a tank slapper will resolve if you speed up the bike.
The wrong move to do, as a rider, is to try to counter the tank slapping by trying to hold on to the handlebar harder. This will transmit the oscillation to the rider, which will transmit the oscillation to the body of the bike, further causing instability and ultimately a crash.
A bike, when encountering a tank slapper, will generally resolve itself if the rider relax on the handlebar, and speeding up. If speeding up is not feasible, then, letting go of the handle bar completely can resolve the tank slapper.
It’s important to remember that a tank slapper will send your pistons way back into the calipers.
After the tank slapper has resolved itself and if you are still on board, it’s essential to gently and repeatedly pull the brake lever until the pistons contact the disc again.
Failure to do this before actually needing the brakes could cause you to crash.
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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