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?
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2 Answers
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.
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.
I've disassembled and assembled several and it amazes me every time.
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7@Ambo100 - Because if there were no freewheel, you wouldn't be able to coast. Commented May 22, 2011 at 20:10
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4@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!– jackJoeCommented May 22, 2011 at 20:21
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2I think I understand now, The pawls prevent the pedals from spinning around like crazy and mangling your feet?– Ambo100Commented May 22, 2011 at 20:49
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1@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.– jackJoeCommented May 23, 2011 at 8:36
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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4Coaster brakes are also popular in cruisers. Commented May 23, 2011 at 13:04
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3...and when I originally installed the spacers incorrectly in my gearset, my freewheel bike functioned just like a fixie. :-) Commented May 26, 2011 at 19:07