Determining correct bottom bracket dimensions
As is often the case with my DIY projects an attempt to tighten my bottom bracket led to me breaking it. It was a prefectly good Shimano UN25 68mm/117.5mm according to the info stamped on it. So I figured that if I ordered the nearest replacement BB I would be OK. A Shimano square taper UN53 68/118mm duly arrived.
However, when I fitted it it became obvious that the drive side crank is about 3-5mm further away from the frame than with the original BB. The effect is I am unable to shift to the smallest chainring. Despite the axle lengths on both BBs being the same the crank does not go as close to the frame with this new BB. So my question is how do I know what BB to buy to fit?
It's a Specialized Rockhopper Comp from about 2004 and I've measured the chainline at 48.75mm if that helps.