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On the BMX 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?

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2 Answers 2

up vote 36 down vote accepted

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.

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+1 great answer! –  jv42 May 22 '11 at 14:14
    
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. –  Neil Fein 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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@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 8:36
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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.

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Coaster brakes are also popular in cruisers. –  Neil Fein May 23 '11 at 13:04
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...and when I originally installed the spacers incorrectly in my gearset, my freewheel bike functioned just like a fixie. :-) –  Brian Knoblauch May 26 '11 at 19:07
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