Bottom bracket wear
 

I was at my local bike shop today and they notified me that my bottom bracket on my single speed mountain bike was old and that's why I feel a really faint wobble in the cranks. I'm trying to decide whether to get a new bottom bracket or just keep this one for now so I need a few questions answered. Any help would be appreciated.

• Will the bottom bracket in it's semi-worn condition be likely to damage any of the other components on the bike?
• How much wobble is too much in the cranks?
• Will the rate at which the bottome bracket wears be any faster than the rate at which a new one would wear?

- No, the wear will be limited to the replaced parts. Unless the wear gets so great that the rings/cranks hit the frame or cause chain link issues.
- When the wobble causes the rings or cranks to hit the frame of the chain gets link issues. When the wobble is too great the chain wants to shift between the moving side to side rings.
- Worn and roughened surfaces will degrade at a faster rate then smooth surfaces. Look at a road with smooth asphalt. It will last a long time but as soon as a pot hole starts the hole quickly becomes much larger as it's edges crumble away.

If the current BB is a classic cup and cone style it could be cleaned and regreased with (excepting possible crank arm remover) no special tooling. This would give you much more info as to the actual condition as well as whether when it came time to replace the BB completely you would want to do it yourself, or have a shop do it. Andy. (Who is assuming that the current BB is not just coming loose within the shell).

It's probably a $20-$30 part. Why not?

As Andrew noted, the only consequence of riding with a loose or worn Bottom bracket is that you'll kill it off. But since it's just about dead and not savable, that's no loss. As for how far you can take this, it depends on your tolerance for chain rub at teh front derailleur and/or sloppy shifting there. (RD is unaffected either way).

This does bring up another possible consequence. If you're tolerant of chain rub and allow it to continue, you'll eventually saw through the cage, so if the wobble is bad enough that you cannot trim the FD for no rub, replace the BB sooner rather than later.

Did the LBS say there was anything actually wrong with the BB besides it being old. If it simply loose and is the old style with loose bearings, all you need to do adjust the pre-load so it doesn't wobble, but I would suggest you also clean and re-grease the bearings.

They tightened it and said I would need a new one if I wanted it to stop wobbling

It's a single speed. ;)

That makes it simple, the only consequence of riding with a worn and wobbly bottom bracket is faster wear and eventual failure. Since the only consequence of failure is the need for replacement, you might as well ride it as is until you can't.

Preventive replacement, when the only thing you're preventing is the need for replacement is just plain dumb.

..assuming that you have a backup plan for how to get back from wherever you are when the bike turns unrideable....

Sure, outside a racing situation, the span for what's a "useable condition" can be quite wide, but it does make sense to replace worn parts somewhat before they fail and leave you stranded. Unless all your rides are within comfortable walking distance of your home of course.

The failure mode of worn crank bearings is one where there is a very unlikely possibility for the crank to "work" one moment and not the next. Thereby stranding the rider with a no longer working crank.

I've seen REALLY worn out BB's. Like when the balls are sitting on the shell's bottom and the unit is then using the spindle and cups as a plain bearing. As when the unit is taken apart the cups' spindle hole is no longer round, but elongated from becoming the bearing surface. The crank/rings are by now moving side to side and "wobbling" all over the place, the chain is having problems staying on the rings (any of them), the ft der cage is by now about sawed through. Any reasonable rider would have considered the bike broken and unrideable long ago. BUT the rider in question was still going. It's amazing sometimes what necessity does. Andy.

The operative word was ONLY. If the only reason to replace was to prevent future replacement.... Avoiding being stranded is another reason, and a valid one. BB failure is rarely sudden so if the OP uses this bicycle within 50 or so miles from home, he'll have plenty of warning as the bearing worsens. OTOH, I wouldn't start a multi-week tour with a BB in this condition.

The extent to which we replace parts with remaining life depends on circumstances. For example, I used to replace the perfectly good water pump in my car whenever I replaced the timing belt. The pump was cheap, not likely to last until the next time I serviced the belt, and the labor of doing wither job was significant. So replacing the pumps prematurely saved dough in the long run.

Same with worn tires. For riding locally, I'd wear them till they gave up. But I wouldn't start a tour, with anything not expected to last until I got home.

My guess is your LBS wrenches don't deal with loose bearing BB very often and probably don't know how to properly pre-load the bearings (or don't want to put the time in); it requires an experienced touch to get it right. The correct way needs a certain amount of trial and error. To take out the slack you need to loosen the lock ring, tighten the NDS cup, and then tighten the lock ring again. As I said this will take some trial and error as tightening the lock ring will affect the pre-load. If after adjusting the BB and tightening the lock ring, it is too tight/loose, you need to re-do the process and loosen/tighten the NDS cup a small amount. Most LBS wrenches these days are simply parts replacers, which explains why they normally suggest you replace and entire component.

Wobbly cranks would drive me nuts...and I would hate to worry about the impending failure (Murphy's law and all that). I'd fix it or replace it now. My .02