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While riding her bike my daughter's pedal came off. It appears to be stripped. This has happened to her with a total of three bikes. What could be causing this to happen?

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A total of three bikes, meaning three different bicycles have lost a pedal? Are these bicycles being put together by the same person? –  WTHarper Jun 7 '13 at 23:39
    
Yes three different bikes and they were assembled at the store. All bought at different stores at different times. Two of the bikes were one year old and the other was new. –  Lynda Jun 7 '13 at 23:41
    
Is there any chance that some mechanically inclined person is giving the bikes a tune-up that involves tightening the pedals? –  WTHarper Jun 7 '13 at 23:44
    
No. She takes it out and rides it then brings it back in the house. We don't leave our bikes outside and we don't attempt maintenance until something goes wrong. –  Lynda Jun 7 '13 at 23:50
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I don't think it likely that these bikes were of such poor quality that the pedals just shear off spontaneously. (Though, I find it less likely that some vandal is targeting just the pedals of your daughter's bicycles.) Even cheapo resin pedals will go four or five years before breaking... and even then it takes a special dedication to strip a pedal out of a crank. Is it always the left pedal, or the right pedal? –  WTHarper Jun 7 '13 at 23:58
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1 Answer

The only time I've seen pedals in a newish bike strip out of the crank was when the mechanic who assembled the bike overtightens the pedals (the mechanic was a younger me, but you live and you learn.) Often, people will forget that left pedals are left-hand threaded and will strip them out when they try to remove them.

The mechanical motion of pedaling causes metal-metal mating surfaces to fret - microscopic motion back and forth causing noticeable wear. If this were a bike from the 70's with aluminum cranks it wouldn't be a surprising failure mechanism. However, since these are new bikes, the most likely explanation is that who ever assembled these bikes tightened the pedals too much. There is no way a child could be riding the bicycle enough to cause a pedal to strip out. To have it happen three times is incredibly unlikely, though.

I'd avoid buying department store bikes and refer to your local bike shop for any repairs or replacements.

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