I've just swapped the cranks on my son's 20" bike to a 130mm / 32t single ring one as the stock 150mm ones were too long. The originals were running on a 120mm spindle JIS square taper one and the chainline was good.
The chainring on the new cranks is further outboard and mounted on the current BB, the chainline isn't great on the two biggest rear cogs (Shimano 6-speed freewheel). The new crank manufacturer's website suggests a 116mm BB which I am presuming would bring the ring 2mm closer to the frame?
That still seems less than ideal if the goal is to get the chainline aligned mid-cassette. Am I right to think that I can use a 113mm spindle BB? Clearance looks ok.
The bike has a 68mm BSA BB shell and 130mm rear dropout spacing.
Have you got a 113mm? If so just put it in and see what happens.
I'm in the Tweed Valley and have a spare one if you're local and want to try it for size?
If you've still got the old cranks and chainring, would be easiest just to measure the distance from the centre of the seat tube to the ring, then swap and do the same with the old set. That'll show you the size of the new one you need.
You may want to double check this, but I think that with square taper BB's the difference is all on the drive side, the non drive side is fixed, so the difference on the drive side between a 120mm and 116mm is 4mm not 2mm, so the manufacturer may be right.
Thanks all. I've ordered a couple of cheap ones and will try both sizes.
You may want to double check this, but I think that with square taper BB's the difference is all on the drive side, the non drive side is fixed, so the difference on the drive side between a 120mm and 116mm is 4mm not 2mm, so the manufacturer may be right.
This is not correct. Square taper BB's are symmetrical, so going 4mm shorter will be 2mm each side.
I stand corrected, it's been a while since I had to replace one on my last bike still using one.
are you sure the old and new BB axle and crank are all JIS? If any are ISO square taper the crank will sit differently by 4mm
are you sure the old and new BB axle and crank are all JIS? If any are ISO square taper the crank will sit differently by 4mm
@wzzzz yes, both sets of cranks are Prowheel (Taiwanese brand) which run on JIS.