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Cranks spin freely backwards...

1K views 6 replies 5 participants last post by  PCC 
#1 ·
I just picked up a used mountain bike for my 6 year old son. It's a Trek MT220 Mountain Bike.

The bike works fine, there is just one thing that has me puzzled, and I'm not sure how to clearly explain this....

When you take your feet off the pedals, the cranks freely spin backwards so that the cranks are up and down. The weight of the back pedal causes the cranks to spin backwards. I would expect the cranks to just stay where put, not freely spin backwards.

Where should I look first?
 
#2 ·
DrAxeman said:
I just picked up a used mountain bike for my 6 year old son. It's a Trek MT220 Mountain Bike.

The bike works fine, there is just one thing that has me puzzled, and I'm not sure how to clearly explain this....

When you take your feet off the pedals, the cranks freely spin backwards so that the cranks are up and down. The weight of the back pedal causes the cranks to spin backwards. I would expect the cranks to just stay where put, not freely spin backwards.

Where should I look first?
This is normal if you've got a freehub on the rear hub. As opposed to a coaster brake (cruiser/bmx type).
Yeah, I just looked up your model, as it should be. There's essentially a ratcheting system in the freehub so that the wheel spins freely when not pedaling (You probably know this, no?).
On some bikes, there's enough friction in the drivetrain that this doesn't happen.
 
#3 ·
Let me get this straight. If you position the cranks so that they are horizontal the back pedal will pull down until the cranks are vertical? Is it only one pedal or the other or both? If it's one pedal then the obvious question is, are they the same pedal, left and right? Is there something on one pedal that will make it heavier than the other?

If it's either pedal that this happens with then I'm going to suggest that you call the Ghostbusters.
 
#5 ·
Thanks everyone for the help. I should give this about 30 seconds more thought before posting. The bike has a broken crank arm and that is the obvious reason for this behavior.

Feeling kinda dumb....

Well, since I'm feeling dumb anyway, does anyone know where I can buy crank arms? I've looked at a few online shops but all they have are the full cranksets. I only need new arms.
 
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