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On the Mountain bike I have, pedalling backwards moves the chain but doesn't drive the wheels of the bike backwards like you would expect it to. Why is this?

Ambo100
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2 Answers2

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Do you hear a "tick" sound every time you spin the wheel anti-clockwise? that's the freewheel mechanism composed by two small parts called "pawls" and when you spin it on that direction, those pawls turn loosely until they find the "dent" in the inner mechanism (the ratchet body), that's when the "tick" sound happens.

pawls

When you ride your bike, those two pawls do exactly the opposite, they catch the "dents" and force the inner mechanism to rotate! it's wonderfully ingenious and simple at the same time.

"dent"

I've disassembled and assembled several and it amazes me every time.

jackJoe
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    Do you mind also explaining why bikes have this feature? – Ambo100 May 22 '11 at 19:50
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    @Ambo100 - Because if there were no freewheel, you wouldn't be able to coast. – Goodbye Stack Exchange May 22 '11 at 20:10
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    @Ambo100 exactly as @Neil Fein said. You have another thing called a "fixed gear", which allows you to pedal backwards! Now, on this topic, think of the strength and stress the pawls have to endure to make the wheel rotate, it's amazing, two little metal parts that hold hundreds (I think it's about hundreds) of Kgs! – jackJoe May 22 '11 at 20:21
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    I think I understand now, The pawls prevent the pedals from spinning around like crazy and mangling your feet? – Ambo100 May 22 '11 at 20:49
  • @Ambo100 no... The pawls are the thing that when spinning clockwise will lock the inner mechanism of the free-wheel and so force it to spin the wheel, this can only be done via pulling the cogs and that's done via the chain, which is done via the pedal movement on that direction. When spinning it on the other direction it is "free" thus not spinning the wheel and also not spinning the pedals. If you have a system without some sort of pawls (like the fixed gear) the pedals do spin like crazy and those can "mangle" your feet. – jackJoe May 23 '11 at 08:29
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    @Ambo100 edit: depending on your perspective, having pawls does prevent the pedals from spinning like crazy as you said, but of course only on the anti-clockwise direction, at least the way these pawls work means the pedals won't move when spinning in that direction. On the other hand, even if you spin the wheel clockwise and stop pedalling, the pedals won't move either,in this case the pawls have no action (when not pedaling and moving forward). The pawls only work when stressed, pulled. – jackJoe May 23 '11 at 08:36
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    Dunno what y'all are talking about regarding mangled feet. I ride a fixed gear at the velodrome, and have yet to mangle anything. ;) – Stephen Touset May 23 '11 at 14:52
  • @Stephen Touset Please notice that I did write the "mangle" part inside quotes :) I also have ridden a fixie and never mangled anything. That phrase was meant to explain to the OP the extreme of not having pawls and was in the context of the OP comment. – jackJoe May 23 '11 at 15:34
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The freewheel allows forward pedalling to lock and drive the bike, but spins freely when reversed.

Some BMX style bikes (and kids bikes) have coaster brakes, where if you pedal backwards (well usually they do not actually pedal backwards, just the motion is begun) and it brakes the bike.

I remember well locking the back wheel and trying to generate skid marks on the sidewalk.

geoffc
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