Red Ed

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.

HillRider

I'm puzzled. The new bb puts the crank further away from the frame but now you can't shift into the smallest chainring? If the crank is further away I can understand you might have problems reaching the biggest chainring but not the smallest. In fact, you should be dropping the chain inside the small ring.

Red Ed

oops sorry yes you're quite right, it's all the other way around. The crank is now 3-5mm CLOSER to the frame.

HillRider

I guess one or the other of the bottom brackets was unsymetrical so, despite the same overall spindle length, one has a longer drive side extension than the other. The next size up AFAIK is 122 mm and that may put your crank in the right place.

Is the crank on the new bb so close to the frame that adjusting the fd's inner stop screw still won't let it shift to the small ring because the fd hits the seattube first?

moxfyre

Can you adjust the front derailer and shifter cable to make it reach all three chainrings properly?

I must say I'm quite puzzled by the difference in the spindles. I thought that all Shimano square-taper cartridge BBs of a given length had the length equally distributed between left and right sides. There is unfortunately a lot of variation in length distribution in cup-and-cone BB spindles (https://sheldonbrown.com/harris/botto...ts.html#axles), but I didn't think the same confusion existed with cartridge BBs. Can you put the new and old BBs right next to each and verify that the TOTAL spindle length really is nearly equal?

Also, how did you break your old BB by tightening it?? With a cartridge BB, the tightness of the mounting rings doesn't actually affect the preload of the bearings, so I can't figure out how you would've broken it.

Red Ed

Well to answer a few questions:

How did it break? Using a micrometer style torque wrench I overtightened before realising that such a torque wrench will not click on a left handed thread. Luckily for me the BB tool slipped inside the BB a few times wrecking the BB by stripping the protusions in its socket before I was able to ruin the threads inside the frame.

Can I not adjust the front mech enough? Unfortunately not. Even with the stop screw removed and no tension on the cable the front mech does not travel far enough to the left to shift chainrings.

Putting the BBs side by side I see that the spindle lengths are the same. The difference is that on the UN25 the drive side spindle has what I can only describe as a spacer on it (but not of the removable type) about 5mm thick which stops the crank coming up as close to the frame as it does with the UN53.

moxfyre

Very interesting. Hmm... I guess your options are (a) get a UN-53 with a spindle a few mm longer, or (b) use say 3 mm of removable spacers to move the current UN-53 BB spindle over towards the right. The first choice is of course better and more structurally sound, but the second may work for you too! Any decent shop should have BB spacers in 1 or 2mm thickness.