Fried Bottom Bracket?

Question

Is the bottom bracket is the sealed bearing that the crank arms attach to?
Anyway, after two yrs. and 3000 mi. my cranks are loose. The bike shop says I need a new bottom bracket. If this is true, how do I keep it from happening again, or will I just need to shell out the money every 2 years?

Answer 1

Depends - most often when this happens, it's because the bolts that hold the cranks to the BB became loose, allowing wobble which ruined the BB axle / mounting area. Even a tiny little bit of looseness/wobble will do it.

Could be caused by lots of things, either it wasn't tightened correctly when installed, or if you're doing lots of jumping type stuff.

To prevent that, find out exactly what torque is required on those bolts (the shop should know) and check it periodically with a torque wrench.

A normal, decent quality sealed BB should last for way more than 3k miles, as long as you're not riding thru creeks or using the car wash to clean it.

Answer 2

It's not a very expensive repair, but 3000 miles seems a bit soon to be replacing it. I've had a Trek 520 for about 4 years and had to replace the bottom bracket last summer after about 15,000 or so miles. When they replace it, have them put in a good quality bottom bracket. Maybe the new one will last a little longer.

Answer 3

If you have an ISIS 'bb' then it probably is your bottom bracket - and by the speed at which your bb has died, I'll assume it is.

ISIS bb's have smaller bearings due to the larger axle diameter, I have had 2/2 ISIS bb's die on me - I now use a steel dmr crank/bb all in one set up, similar to profile cranks (the best in the biz').

They (shimano, race face, truvativ e.t.c) now make external bearing bb's to counteract the initial flaw in their splined bb designs, this allows for larger, stronger bearings, try these if you are really fussed about weight, otherwise invest in a set of Profiles and have a single chainring (32-38Tooth) up front, you should never have any problems ever again.

Alternatively, invest in a high quality square taper setup.

Answer 4

I'd be surprised if the cranks are actually loose. Do you mean that there is play in the bottom bracket. If there is play, there is not much you can do to newer bottom brackets. I'm surprised that it didn't last very long, but they aren't too expensive and can't be repaired.