Bottom bracket sizing

Question

I have a square taper BB which has started squeaking.

When I took it out to see what size I have, when I was putting it back in I noticed that when I tighten it to a level which to me felt reasonable, the shaft became noticeably more difficult to turn. I slackened off a bit (and still tight enough to ride just fine) and then it was OK. The BB I have has not suffered any issues with slack cups, at least to my knowledge. Though it has given up the ghost after maybe 600 miles. Whether that's due to incorrect fit or poor quality I don't know - but it's some no-name cheapo.

The BB i have is 68x118mm. I'm considering whether getting a 68x115mm might be a good idea given the above, or whether I should get the same size and go not to so tight. What do you think?

Also, it looks like the Shimano UN55 is the only game in town - are those any good? Is there a realistic alternative? I haven't been able to find any brands in that price bracket that seem worth a try.

EDIT: Looks like I left the most important info out - I didn't realise this was of relevance at the time of initial post 1. The BB is a sealed cartridge type. 2. The chainset is actually a Gates belt drive. The belt tightening mechanism is through a split BB casing - i.e. the BB sits inside an eccentric alluminium case inside a split BB casing in the frame. This allows me to move it left and right and so changing the BB size will not affect the chainline. 3. I can't make out the existing BB make and so have no idea what the correct torque for it would be.

Thanks in advance.

Answer 1

You seem to be laboring under a misunderstanding about bottom bracket sizes.

Threaded bottom bracket shells have standardized widths: 68mm for road bikes, 68mm or 73mm for MTBs. You have a road BB cartridge for a 68mm shell, that's what the 68 in 68x118mm means.

The other number is the axle length. Square taper cartridge bottom brackets come in different axle lengths that work with different crank offsets to get the correct chainline. Cartridge bottom brackets are not offset to one side to achieve a desired chainline as that makes the crank positions asymmetrical.

If the bearings bind when you tighten the lockring, something else is wrong. Cartridge bottom brackets completely contain all the bearings, The lockring just tightens the bracket body into the frame. Check your installation and tighten the lockring to an appropriate torque, you can look up general torque settings.

Answer 2

Old-fashioned cup and cone type bottom brackets have one adjustable cup, with a lockring. The adjustable cup lets you set the bearing tightness, too loose and there is play, too tight and the spindle won’t turn.

With a cartridge B.B., the bearings are self-contained and the cups are installed to a given torque which is normally pretty high as bike parts go. When the two sides are tightly installed, the spindle will turn freely. There is no finessing or adjustment of either cup required.

The problem of the spindle getting hard to turn when you tighten the cup, therefore, is not normal and indicates a problem. Hopefully it means the B.B. is worn out and replacing with a new one will solve it. I would try this first and see what happens.

As for B.B. sizing, 68mm is the width of the shell, you can only use a 68mm B.B.

The length of 118 mm is the spindle length. It determines the position of the cranks, outboard from the centre, and therefore, chainline. Make sure you understand this term. If 118mm gives the bike the correct chainline, then 118mm is the correct size and you should replace with the same size. Spindle length (118mm) is independent of (any) cup adjustment.

Trying to avoid commenting on a given product, you might ask yourself if the only game in town arrived in that position by being crap... probably not.

Update: As you have a belt drive, achieving the correct position of the belt is critical, so you must use the correct size spindle, ie 118mm. The eccentric bottom bracket just allows you to adjust the belt tension, the B.B. dimensions are fixed.

Answer 3

Do not try to change the spindle length to make your BB turn more easily or last longer, since it has nothing to do with it. Spindle length regulates chain line - and unless your bike has been fitted with wrong spindle for some reason it should be kept the same. (and if it has been fitted with wrong one it would result in chain issues, not the ones you describe)

As for BB - try to find better quality cartridge BB, there are many, many to choose from.