Bottom bracket/chain-line advise requested ...

Hi all - here's the Q:

I replaced my b/b with an equal length axle UN-71 unit (112mm), as that was what came out of the bike (although, that said, the old one didn't seem to be quite centered in the shell, being a tad skewed toward the drive side).

So, new one is installed perfectly centered, bike built and off I go ... except, if I am in my granny gear and try to shift through the cogs on the back (7 spd), I can only get into the 4th cog before the chain starts slightly rubbing against the side of the second chain-ring (starts rubbing when in the 5th cog), and in 6th and 7th it really rubs.

I am guessing that who-ever had re-built my back back in '99 (I had a shop do it for me back then) had installed the b/b but found it the wrong length, and turned it a few threads out toward the drive-side of the frame, in order to get the chain-line functioning without rubbing. That said, would replacing the 112mm unit that I have in it now with a 115 mm long one solve my issues, as I am surmising that it should be about 3 or 4 mm that three or so threads in a 68 mm shell would comprise?

Any knowledgable information or insight would be greatly appreciated, as I am bout to strip the bike down almost completely in order to switch a number of components to a 'closer to' period correct groupo (the frame is an '89 Brodie Romax) ...

and thank you in advance for your help :!:

KR

There are ways and means of working out what length bottom braket you need.

It's simple measuring, bit of maths and probably some string involved. The basic is 48 to 50mm from middle ring to centre of seat tube for a triple crank. The bigger the tube the bigger you will need as it pushes the front mech over .

Measure what you have...if you struggling to find the tube centre wrap a bit of string round it and the head tube in a loop. It makes the centre easy to find. I bet your a bit short.

However, there are other factors, such as hub width and packing the cassette that can effect it.

If you really want to know see sheldon browns chainline article.

Hope that helps.

That said, really 567 on a granny is probably going to rub. As will 123 on the big ring.

Bimly":3sq2ba4o said:

That said, really 567 on a granny is probably going to rub. As will 123 on the big ring.

Click to expand...

Thank you for the prior advice Bimly; yet nope ... it didn't rub at all in any gear combination before.

I have found that as chains have got better and thinner the rubbing has got less. That's I plus point for modernity!