ChrisEyles
Guru
- Location
- Devon
Getting pretty confused about BB spindle lengths today!
I bought a replacement crankset for my commuter to bring it back into active service and tried to fit it this weekend, only to find it requires a different spindle length on the (square taper) bottom bracket.
Old set up:
Shimano altus 48/38/28 triple crankset ca 2014
122.5mm shimano square taper BB
Good chain line
New set up:
NOS Shimano STX (? not sure but it looks a lot like it to me) 42/34/24 triple crankset ca 1997 (? again guess work)
Bad chain line with the 122.5mm BB
The drive side crank is sitting well outboard of where it needs to be. The offset is tricky to accurately measure, but my best guess is around 8mm. Basically when the FD is in the top ring position it's currently roughly aligned over the middle ring of the new crankset.
In a sensible world, I'd say to move the crankset in by the required ca 8mm I'd need a 16mm shorter spindle, so a UN55 107mm BB should do the job. There is enough clearance between the chain stays and the crank arms for the cranks on both sides to move inwards by the required 8mm.
But a bit of googling has left me confused - sometimes more of the difference in length comes off the drive side, sometimes the non drive side.
Does anyone know:
1) How to determine what BB spindle length you need when you can't look it up (as I'm not sure of the crank model)
2) Where I can find a good diagram showing the length of the drive side and non-drive side extension of the spindle past the BB shell. I'd be happy with UN55 or UN26 BBs, or anything else recommended here.
3) I guess there's a chance I'm wrong about it being NOS shimano and the new crankset could have an ISO taper interface rather than JIS (which would also place it outboard on the BB) - anyone know of a way to check this?
Thanks for the help, as always!
I bought a replacement crankset for my commuter to bring it back into active service and tried to fit it this weekend, only to find it requires a different spindle length on the (square taper) bottom bracket.
Old set up:
Shimano altus 48/38/28 triple crankset ca 2014
122.5mm shimano square taper BB
Good chain line
New set up:
NOS Shimano STX (? not sure but it looks a lot like it to me) 42/34/24 triple crankset ca 1997 (? again guess work)
Bad chain line with the 122.5mm BB
The drive side crank is sitting well outboard of where it needs to be. The offset is tricky to accurately measure, but my best guess is around 8mm. Basically when the FD is in the top ring position it's currently roughly aligned over the middle ring of the new crankset.
In a sensible world, I'd say to move the crankset in by the required ca 8mm I'd need a 16mm shorter spindle, so a UN55 107mm BB should do the job. There is enough clearance between the chain stays and the crank arms for the cranks on both sides to move inwards by the required 8mm.
But a bit of googling has left me confused - sometimes more of the difference in length comes off the drive side, sometimes the non drive side.
Does anyone know:
1) How to determine what BB spindle length you need when you can't look it up (as I'm not sure of the crank model)
2) Where I can find a good diagram showing the length of the drive side and non-drive side extension of the spindle past the BB shell. I'd be happy with UN55 or UN26 BBs, or anything else recommended here.
3) I guess there's a chance I'm wrong about it being NOS shimano and the new crankset could have an ISO taper interface rather than JIS (which would also place it outboard on the BB) - anyone know of a way to check this?
Thanks for the help, as always!