Eli5: Why do speed wobbles happen on bicycles travelling around 40mph or higher?
In: 8
They… don’t? Unless something is wrong with the bike. Just look at the Tour de France. These guys go down hill at speeds in excess of 60mph and everything is just peachy.
Essentially your bike, as a system, gets into a mode with a resonant frequency. Due to the bike being slightly flexible the forces can cause it to vibrate like a piano string. For this to happen the forces have to be high enough that the frame will actually flex, so it doesn’t happen at low speeds.
It doesn’t happen while pedalling because a changing system doesn’t have a resonant frequency.
[This is a good explanation.](https://cyclingtips.com/2020/07/bicycle-speed-wobbles-how-they-start-and-how-to-stop-them/)
> Data from this latest study shows something less obvious too: oscillations at the seat stays are out of phase with those on the top tube. In other words, when the front of the bike is accelerating in one direction during shimmy, the back of the bike is accelerating in the opposite direction, effectively twisting the bike beneath you.
> The fork, meanwhile, twists slightly upwards in the direction the wheel is facing during shimmy. The handlebar twists downwards in the same direction, creating torsion in the front steering assembly.
Regulations on car safety and design could probably fill many large books, and for good reason – all the accidents and deaths over the last hundred years prompting said regulations to improve vehicles in an effort to reduce accidents and deaths.
Bicycles are just required to have brakes that meet a specified performance level, non-sharp edges, no loose screws/bolts/nuts, protected cables controlling shifting/braking, and no dangerous objects/”protrusions” near the crotch area. That’s it. No testing to make sure that the bicycle is machined and assembled well enough to perform at high speeds. Some manufacturers will do this themselves, but it’s not required of them.
Also a matter of weight distribution; which usually results from poor geometry or design. There’s a video somewhere on YT that shows differences in stability at speed between a setup with weight centered and a setup with weight too far back. With the weight too far back it creates a pendulum effect from the rear that causes instability at the front.
In the case of a bike or motorcycle the rear end won’t necessarily go swinging out of control like you’ll see in a trailer setup but you’ll still get the instability on the front end.
Short unswer. You holding handlebar too tight.
When you ride bysicle motorcycle front wheel goes sideways whenever obe oass any curveture in irder to maintain stability. If rider holds handlebar to tight they fight initial microturn and that force is added up to other side when microturn changes durrection, thus length if oscillation increased. That happens because microturn oscilation happans faster than human reaction allows to proceed and controll.
Two way to fight it. Propper one relax you hands and stabilise you legs and seat position. In case if ibe absolutly needs to hold tight greep (through u cannot imagin a reason as woblibg not happens in situation where grip required) lean you body as close as possible to hadlebar (you still need to stabilise legs and bottom, never hang on handlebar)
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