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Bottom bracket shot?(7 posts)

Bottom bracket shot?5stringbass
Nov 4, 2003 8:36 PM
I currently ride an 8 yr old Mongoose IBOC Comp SX. It's a mountain bike that only use on the road since back surgery. I recently had it serviced. However, my bottom bracket still creaks on every powerstroke. It is consistently getting louder and louder. It does quiet down if I pedal in a circular motion vice left-right pushes. Should I anticipate my bottom bracket to need a major overhaul (replacement) soon?
re: Bottom bracket shot?LC
Nov 4, 2003 8:54 PM
It is probally just loose. Take it back to where it was serviced and have them tighten it.
re: Bottom bracket shot?russw19
Nov 4, 2003 9:43 PM
Usually squeaking is your crank arm bolts are too loose. Tighten them. It should either be a 14mm bolt or an 8mm allen wrench nut. Tighten down pretty dang tight and when it gets really tight, use the crankarm and the wrench together and squeeze them against each other to get it tight enough. If you ride too long with loose cranks, you can wallow out the crankarm bolt. The spindle is steel and the crankarm is aluminium, so the softer material will wear.

Usually the non-drive side will loosen faster than the drive side due to the stress put on it from the pedaling force. But check them both and check them once a week or so if they are loose. I check mine every week when I pump up my tires.

re: Bottom bracket shot?oddsos
Nov 5, 2003 3:20 AM
My understanding of cranks is that repeatedly tightening them is a bad idea. The torque required to initially seat the crank is pretty high to ensure that the crank sits snuggly on the bottom bracket axle. There has to be sufficient force to ensure that there is constant contact between the BB and crank during the pedal stroke to avoid damage and wear to the softer material of the crank.

In the use the crank shuffles about on the taper of the bottom bracket axle which may make the crank bolt appear to have loosened. If the crank bolt is retightened regularly the crank will wedge itself further onto the taper and can eventually crack, normally with unpleasant consequences. The dust cap of the crank arm prevents the crank bolt from dropping out if the crank moves up the taper.

If a crank is ridden whilst loose it is likely that it will damaged beyond repair. The crank is able to rotate relative to the BB axle (a miniscule amount) and all the forces due to pedalling are applied over a small area. The stresses generated are suffiecient to plastically deform the crank and develop play. Once this has started there is no way to rectify the situation. It is impossible to tighten the crank onto the BB with enough force to reverse the plastic deformation without breaking the crank bolt.

I hope this helps

re: Bottom bracket shot?russw19
Nov 5, 2003 8:31 AM
Chris, you are correct. My point was to check them for tightness every week, not to grab the wrench and crank down on them every week. If you need to do that, you have other problems. Locktight on the crankbolt threads could help, but yes, if you keep overtightening crankbolts, eventually you will keep forcing the crank further up the spindle taper than you want to and that's bad. However, in the short term, riding a loose crank is going to do far more damage than overtightening a crankarm onto the spindle.

re: Bottom bracket shot?5stringbass
Nov 5, 2003 5:47 AM
I appreciate all the responses. I'll take it back to the shop and see what they can do to tighten it. This might be the perfect excuse for a Christmas present.
re: Bottom bracket shot?5stringbass
Nov 9, 2003 6:33 PM
I grabbed my tools and checked the tightness of my bottom bracket. I noticed that whiled tightening the non-drive side, it make the same creaking noise that it makes during both downstrokes while I'm pedaling. Any guesses what that indicates?