BB axle length?

[Bob H]

If I change my cartridge BB from one with an axle length of 122mm to 115mm does my drive side crank and therefore the chain rings move in by 7mm, or do both cranks and therefore the chain rings move in by 3.5mm? Or is the answer more obscure?

Replies much appreciated ………….. Bob H.

[Risto]

I came up against this recently. It ain't straightforward! Some BBs have spindles that are offset to the right, some to the left and some are not offset at all. For the old square tapered shimano BB, I've got a feeling (but please check this) that a clue is in the number. Eg UN71 indicates an offset to the right, UN72 indicates no offset (generally used for ATB frames where the width between chainstays is greater), and UN73 indicates an offset to the left. This makes the simple calculation that you would like to make much more complicated. The other thing with square tapered spindles is that some are fatter than others meaning that unless you buy the same make of BB you can never be sure how the crank will seat on the spindle. The simplest solution is to take your cranks to the shop and put them on a range of BBs. I think that you can allow approximately 1mm between a crank that is pushed by hand onto the spindle and one that is bolted on.

[Bob H]

Risto,
Thanks for your detailed response. In light of no other replies I guess I am going to have to accept your theories but it's disappointing to think that engineering for cycle BBs is still at the "bodge it and hope" stage.
Cheers, Bob H.

[Scott]

Bob I think the change would probably be close to symmetrical but it sounds like a drastic change you're making. Please be aware that even 1 or 2 millimetres can change the chain line by too much and cause problems such as the mech hitting the chain stays or overchanging that can't be limited etc. I only know this cos I tried to get away with a BB that was about 2mm different from original (too impatient to wait for the shop to order in)and it was something that eventually had to be swapped for an exact length replacement as it just wouldn't work no matter what I tried.

[Bob H]

Scott ………… perhaps I should have given chapter and verse first off. I needed to replace my worn out BB (old bits and pieces type), axle length 122.5mm, with a sealed unit. I ordered a Shimano UN53 122.5 and found that the slight difference in taper size, fatter as Risto suggests is likely, (angles OK) pushed both cranks, and therefore the chain line out by ca 4mm. I’m trying to get back to my original mid sprocket line, hence my Q. Unfortunately, living out in the sticks in Norfolk means getting to a decent bike shop to try for a fit is a no-no.
Cheers ……….. Bob H.

[Risto]

Bob, you might try the St Johns Street Cycles website. They still stock some "bits and pieces" BBs and I suppose it's just possible they might have the exact replacement. If you were willing to suffer a bit of trial and error, I expect they (and other decent internet retailiers) would also be prepared for you to return any sealed units that don't fit your requirements. I've got a feeling that as long as you stick to the same product code (UN53 I think you said), any reduction in spindle length should be symmetrical. The SJS people should be able to tell you for sure.

[richardirving]

Where in Norfolk? You should try Madgetts in Diss but they may be on t'other side of the county. They will do mail order and they are the most helpful people I know. If you want the best attention ask for Mick Madgett personally and ask him if he isn't too busy first. Far better than the SJS crew IMO.