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-   -   Crank spin question (https://www.bikeforums.net/road-cycling/753143-crank-spin-question.html)

pgjackson 07-19-11 10:07 PM
Crank spin question
 
Last night I was doing some bike maintenance on my daughter's BMX bike and my Giant. With the chain on I noticed that the crank on the bmx would spin around several times with just one big backwards throw, while the crank on my bike would only spin about 1 1/2 times around. Is this normal?

tagaproject6 07-19-11 10:26 PM
Friction caused by the RD and its subsequent pulleys. BMX cranks only have the rear sprocket/cog to work with. I can't back that up, I just pulled that out of my ass :P

NathanC 07-19-11 11:42 PM
It's correct, though.

pgjackson 07-20-11 09:25 AM
That makes sense. Thanks.

rollin 07-20-11 09:35 AM
Originally Posted by pgjackson (Post 12955175)
one big backwards throw, ?
?

Originally Posted by tagaproject6 (Post 12955253)
Friction caused by the RD and its subsequent pulleys. BMX cranks only have the rear sprocket/cog to work with. I can't back that up, I just pulled that out of my ass :P
Tis the BF way.

rollin 07-20-11 09:36 AM
Originally Posted by pgjackson (Post 12955175)
one big backwards throw, ?
?

Originally Posted by tagaproject6 (Post 12955253)
Friction caused by the RD and its subsequent pulleys. BMX cranks only have the rear sprocket/cog to work with. I can't back that up, I just pulled that out of my ass :P
Tis the BF way.

pgjackson 07-20-11 10:00 AM
Originally Posted by rollin (Post 12956854)
?


Meaning, spin the crank backwards forcefully.

markwebb 07-20-11 10:30 AM
On my roadbike with Campy group I can give cranks a light spin and they'll spin several times - the bb feels like it has no friction at all. I think it should spin more freely than what you described.

pgjackson 07-20-11 10:35 AM
Originally Posted by markwebb (Post 12957153)
On my roadbike with Campy group I can give cranks a light spin and they'll spin several times - the bb feels like it has no friction at all. I think it should spin more freely than what you described.
Is that with the chain on?

markwebb 07-20-11 10:47 AM
Yup. Spins smooth as can be. I have found that Shimano FSA etc don't spin as friction-free so it may depend on bb and maintenance level of bb.

LowCel 07-20-11 11:11 AM
I have found that the freewheel can have a huge effect on how the crankset spins. With one wheel mine only spins once or twice, with a different wheel it will spin several times. Pretty simple to decide if it is the freewheel limiting the spin or the bb. Just take the chain off. :D

thump55 07-20-11 11:58 AM
Originally Posted by tagaproject6 (Post 12955253)
I can't back that up, I just pulled that out of my ass :P
This goes without saying...you don't actually need to type it.


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