BG2

So here a bit diffirent q'n an a common problem.

I have a shimi 600 tri colour crank and it came with a BBUN72 116 mm.

However the frame i bought is oversized (frondriest EL-OS with BBUN51 123 mm) and according to sheldon this requires a longer BB.

When i fit the crank to both bb's and use the frontderr outhershield over the big gear as a guideline i measure a difference from 2 to max 3 mm's.

Is this ok reminding sheldons words that this has a os seat tube of go for the smaller 116 one.

When i put a 8 speed wheel in it it all seems ok and the problem occurs possibly when i ride constantly on the biggest cog and thereby accept more chainwear or is there a snatch i overlook in this.

furballi

Check the chain line. Actual could deviate about 5 mm from ideal. If you can live with this, then there is no issue other than poor shifting and faster wear of the drivetrain with certain gear combo. A new BBUN55 is less than $20.

bradtx

BG2, Have you test fitted the 116 mm BB? What you want is a straight as possible shot from between the two chainrings to the middle of the cassette.

Brad

Road Fan

+1 to this! As long as the threads on the BB fit the frame you should be able to at least test-install the 116 mm BB and the crank, measure the chain line (gotta be sure to torque it correctly), and check for chainring and crank arm clearances to the frame. If those things work AND you can adjust a front derailleur to fit and shift right, you don't need to worry about any wider BBs just 'cuz Sheldon said so.

You and probably most of us don't know ahead of time what the correct BB dimensions are for that crank on that frame. The only sure way to tell is trial and error. You have two BBs in your hands, give them both a try. If you try the narrow one first and the clearances are too tight, measure them before you disassemble. That will give you a guide to what the width of the next BB you try should be.

BG2

I have checked the BB's and the 123 one go's on the (from small to large) 4'th from the smallest and is as straight as a arrow.
The 116 one is on the 5'th (from small to large) so either will work out ok.
The point is that the 116 is 68 mm and the BB is 70 mm (wich is the 123 BB).
Also the cogs on the hub are such that i use the 6'the the most so there is little of no need of the biggest cog and thereby excessive chainwear.
Due to this i can opt for the 123 BB knowing this is not the perfect one, but technically it's no problem like explained in the above.

If it turns out that poor shifting is the case than i will replace it with the correct 116 one, but i need to ride it first so for the moment it's on hold.