Why would a BB unscrew?

[procel]

Just had to re-tighten the bottom bracket on my bimbling bike. Its a standard shimano cartridge type in a standard British thread BB shell. But riding yesterday, the drive side screw came undone (non-drive-side was fine). The thread direction is correct (pedalling should tighten it, if anything). The crank with sprocket remained perfectly tight on the square taper. Can't logically work out why it would have come unscrewed - almost completely out - any ideas anyone?

 

[DCLane]

+1 to that - mine did on the commuter bike last week. Still unsure why it did it.

 

[KneesUp]

With the cartridge bearings the bit you tighten doesn't have bearings running against it. With the old cup bearings, the balls ran on the inside of the bit you tighten up so there was always a small amount of friction trying to tighten it up. I guess that meant they never got loose.

With the newer ones, nothing rotates against the bit you tighten up, it's just there to keep the cartridge in place, so it doesn't have the constant small amount of friction tightening it. Once it comes loose, the vibrations will shake it - some will shake it clockwise, and some anticlockwise. As it's easier to undo it rather than tighten it, on average the shocks will tend to loosen it further, until it's totally loose.

That's my theory, anyway.

 

[cyberknight]

I had a similar experience , turned out i had over tightened the non drive side.Since i redid it with the correct spacings both side mine has been fine.

 

[ColinJ]

I had it happen on my Cannondale. I was getting clunking from the bottom bracket and thought it must be worn out. I was replacing it anyway with a longer one to suit a new triple chainset and when I went undo the wotsit, I discovered that it was so loose that I did not need a tool to remove that side! (The BB itself seems fine now I have it out of the frame.)

 

[Brommyboy]

The drive side has a LH thread. What stops it from becoming loose is that the non-drive side screw (which has a normal RH thread) locks it tight.

 

[tyred]

The threads may be a bit iffy in the first place.

 

[wisdom]

With the cartridge bearings the bit you tighten doesn't have bearings running against it. With the old cup bearings, the balls ran on the inside of the bit you tighten up so there was always a small amount of friction trying to tighten it up. I guess that meant they never got loose.

With the newer ones, nothing rotates against the bit you tighten up, it's just there to keep the cartridge in place, so it doesn't have the constant small amount of friction tightening it. Once it comes loose, the vibrations will shake it - some will shake it clockwise, and some anticlockwise. As it's easier to undo it rather than tighten it, on average the shocks will tend to loosen it further, until it's totally loose.

That's my theory, anyway.

And a good theory It is too.I would go along with it.

 

[accountantpete]

My drive side Campagnolo UT BB cup works loose - I think that physics plays silly buggers and works in a logical way some of the time.

 

[themosquitoking]

My drive side Campagnolo UT BB cup works loose - I think that physics plays silly buggers and works in a logical way some of the time.

Physics i understand up to a point, after that it's all witchcraft.

 

[MontyVeda]

With the cartridge bearings the bit you tighten doesn't have bearings running against it. With the old cup bearings, the balls ran on the inside of the bit you tighten up so there was always a small amount of friction trying to tighten it up. I guess that meant they never got loose.

With the newer ones, nothing rotates against the bit you tighten up, it's just there to keep the cartridge in place, so it doesn't have the constant small amount of friction tightening it. Once it comes loose, the vibrations will shake it - some will shake it clockwise, and some anticlockwise. As it's easier to undo it rather than tighten it, on average the shocks will tend to loosen it further, until it's totally loose.

That's my theory, anyway.

That's pretty much what i reckoned had happened to mine a month ago... I'd simply forgotten to nip it up and it must have shaken loose.

 

[mrvandango]

Had this happen on two bikes over the years, both on the left side. In fact, it was three thinking about it, the first was an old Raleigh Banana that I sent back because of this happening, and then the replacement kept doing the same. Didn't matter how tightly nipped up it was, it would work loose over a 10+ mile ride. Guess I could have tried thread lock.