17

I am very confused with something regarding bikes...

How is it that a bike wheel continues to spin (either from momentum or from going downhill) when the rider is not pedalling, without also spinning the pedal?

Similarly, what mechanism allows the addition of this spin from momentum with to the spin from the pedalling that makes the wheels spin faster than they would with just one or the other?

I've literally searched all over the internet for this and have found nothing, so I'd really appreciate the help.

Wasabi
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Ankit
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3 Answers3

36

The soft clicking sound you hear when coasting is the pawl going over the ratchet as in the picture below. enter image description here

Jordon
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26

it is called a freewheel.

Inside the sprocket gear, there is a mechanism that lets the chain engage the rear wheel only when it is moving faster than the wheel is turning; else the wheel will turn freely.

They are either several spring-loaded ballbearings that self deploy when the outer ring turns faster and recoil when it turns slower than the inner ring. or there is a ratchet mechanism.

there are similar gears in some cars as well. so as to disengage the engine when the car is rolling forward by momentum or downhill slope. Here is the Wikipedia page LINK.

free wheel

kamran
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9

Basically several ratchets, below animation shows when you are not pedalling. In the other direction the ratchet will engage and provide force.

If you lift a bike and get the wheel spinning or just leading the bike, sometimes the pedals will turn very slowly due to the friction of ratchets.

Ratchets]

Image borrowed from https://www.notubes.com/technology/neo-durasync-speedsync (no affiliation)

Viktor Mellgren
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