bottom bracket

[JonB]

I've just built a fixed gear bike which I've been riding for about 6 weeks. It's based around a 90's Raleigh M Trax frame. I used the original Shimano RSX crankset with the outer ring removed and fitted shorter chainring bolts to use the inner 42 tooth ring. I fitted a new UN55 bottom bracket as the old one had a bit of play in it.  I've noticed a creak on each pedal revolution when out of the saddle and applying pressure and when I looked more closely the bottom bracket has play in it.  That's a pretty rapid failure for a unit that has a reputation for durability. 

Any ideas why this has failed so quickly?  is it likely to be something I've done in the set up, I've been a bit worried about chain tension being too loose and am concerned about overtensioning but I've always been able to spin the cranks in the workstand without them appearing to bind.

I'm deliberating whether to buy another UN55 or invest in something different (don't want to spend much as this was always supposed to be a low budget operation to find out whether I like fixed ... I do, but would still like to keep it cheap for now)

[Feanor]

Hmm, are you 100% sure it is actually the 6-week old BB that has failed?
It's not impossible, of course, but highly unlikely.

It's very hard to identify the source of these creaks that only happen under load.
It's almost impossible to observe it, because you are horsing about on the bike.

You say:

I looked more closely the bottom bracket has play in it.

Under what conditions? How exactly?
Are you actually able to feel play in the cranks with the bike on a workstand?

Remember, the frame will be flexing under power too, so small movements of the chainring WRT the frame will happen.

Other sources of creaks under load include chainwheel bolts / seat / seatpost / stem / handlebars / knees...

[Wobbly John]

I'd agree that a creak under pressure would tend to indicate a small ammount of movement rather than something you can detect as play.

The first thing I'd do is to disassemble, smear some grease on the bottom bracket threads and tapers and retighten.

[JonB]

Thanks for the responses. 

Re Feanor's point about the play, with the bike in the workstand or standing on the ground and holding on to one of the crank arms near the pedal I can feel a clunk and a bit of play when I move it side to side, so at 90 degrees to it's direction of rotation. If I hold the other crank whilst doing this I can feel that same play and clunk which says to me it must be the axle moving inside the body and the bearings.  I'm pretty sure that's what I can feel but may well go with Wobbly John's advice for a strip down, re-grease and reassembly.

[Fructify]

When you installed the bottom bracket did you tighten it up to the recommended torque?

Park Tools has that cartridge bottom brackets need to be tightened up to 50-70 Nm.
http://www.parktool.com/blog/repair-help/#article-section-4

In the past I've had a creaking bottom bracket that was solved by taking it out greasing it and then tightening it back up again.

[mrcharly-YHT]

Are you sure the BB shell isn't distorted?

[Oaky]

I recently experienced something on my fixed - a clunk when putting down the power to accelerate from standing, which felt like the chain was skipping.

On checking the chain,  it was reasonably tight, and didn't look as if it could ride up anywhere near enough to skip.

I spent a lot of time with the bike upside down turning the cranks trying to reproduce it, but to no avail.  I then tried manually derailling the chain by pushing it sideways with a pair of pliers (I know not to put my fingers too near a moving fixed drive chain!) but found nothing at first, eventually, though, upon applying enough sideways force, it did make exactly the same king of clunk that I'd experienced on the road, and the chain went slack.  I thought at first that the wheel had slipped in the dropouts, and slackened it off and retensioned.

Turned out the BB was shot - it seems that occasionally, the drive side bearing goes asymmetric (the only way I can describe it), making the BB on that side somewhat eccentric.  My theory is that perhaps one of the balls or ball races has disintegrated somewhat, and this allows the spindle to shift.  This explains why sometimes, after experiencing a slack chain on a ride, I've gone later to re-tension and found it perfectly tight.

I guess one possibility is that this has been caused by excessive chain tension (although I'm careful to check that the chain is nowhere near binding, and has a tiny bit of up and down play when I tension it) but I think the model supplied as stock with this bike has a reputation for dying early (this one has done 5,000 all-weather miles), so I'm going to replace it with a shiny new UN55, just as soon as I've found the time to remove the old one to find out the appropriate size.

[JonB]

Thanks for all the advice. Got round to stripping it down last night, cranks off, pedals off, bottom bracket out. When removing the bottom bracket it wasn't as tight as I was expecting.  I cleaned and greased all the threads, reinstalled and gave it a lot more heave ho when tightening.  No play in the bottom bracket and crucially no clicking under pedal load However, in the dismantling process I managed to mess the paint up on the top tube with my cheapo Aldi bike stand  Still, really pleased to be riding click free and it does illustrate the value of stripping and reassembling ... thanks all!