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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.

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    $\begingroup$ Note that so-called "fixies" (fixed-gear bikes) which are preferred by some cycling enthusiasts as the "purest form" of cycling, but are also used by bike messengers (less stuff to break, clean, and maintain), and track racers (because of simplicity and weight) savings, do not do this. They don't have gears, they don't freewheel, they typically don't even have brakes (you brake by "pulling" against the pedals with your front foot or pushing back with your back foot). In this video, you can see what looks like a bike messenger doing what is called a "track stand": youtu.be/AAvEnZKfxQE $\endgroup$
    – Jörg W Mittag
    Commented Apr 27, 2020 at 15:24
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    $\begingroup$ Track stands are not impossible to do with "normal" bikes, but are made easier by fixies since they also allow you to pedal backwards, thus increasing the degrees of freedom you have for keeping balance. Track stands are a staple of track sprinting because the rider in front is at a disadvantage: a) they cannot easily see the opponent and more importantly b) they are in the wind, while their opponent is drafting behind them. In the sprint disciplines in track racing, only the last lap counts, so in the first few laps, you will often see both riders track standing, and whoever loses balance … $\endgroup$
    – Jörg W Mittag
    Commented Apr 27, 2020 at 15:24
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    $\begingroup$ … first is at a disadvantage. $\endgroup$
    – Jörg W Mittag
    Commented Apr 27, 2020 at 15:27
  • $\begingroup$ @JörgWMittag and kids tricycles or unicykles :) $\endgroup$
    – Viktor Mellgren
    Commented Apr 29, 2020 at 11:37
  • $\begingroup$ The older cars with an overdrive option also freewheeled when in overdrive. $\endgroup$
    – Jon Liebermann
    Commented Oct 8, 2021 at 2:55

3 Answers 3

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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)

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

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  • $\begingroup$ Basically the same mechanism as for a ratchet. $\endgroup$
    – got trolled too much this week
    Commented Apr 28, 2020 at 5:46
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    $\begingroup$ @Fizz Not just "the same mechanism". That is a ratchet, and that's how a ratchet works. $\endgroup$
    – Graham
    Commented Apr 28, 2020 at 16:58
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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

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    $\begingroup$ Thanks a lot this is very helpful! $\endgroup$
    – Ankit
    Commented Apr 27, 2020 at 6:13
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    $\begingroup$ The photo in this answer shows a sprag clutch rather than a freewheel, but the concept is similar. $\endgroup$
    – fred_dot_u
    Commented Apr 27, 2020 at 9:57
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    $\begingroup$ I cannot speak for cars with automatic transmissions, but cars with manual transmissions do not, as a rule, have any kind of freewheel mechanism. $\endgroup$
    – Wayne Conrad
    Commented Apr 27, 2020 at 16:56
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    $\begingroup$ Yes, but your answer gives the impression it's common in cars. It isn't. $\endgroup$
    – T.J. Crowder
    Commented Apr 27, 2020 at 18:03
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    $\begingroup$ I would eliminate the citation for cars. Wikipedia mentions only a couple of very odd Saabs from 35+ years ago. Saabs of those era were very different indeed and don't represent nearly any other typical car. $\endgroup$
    – Eric S
    Commented Apr 27, 2020 at 20:00

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