Clockwork BB Question.

[brocklanders023]

Hello all. Need a new BB for my 1993 Clockwork project and I'll be running XT M730 cranks on it. The frame originally had a Shimano UN52 68 with 127mm axle length. Is this the size I need for thr replacement?

 

[drystonepaul]

127mm? That sounds a bit long for M730 cranks. Normally you'd be looking at 122.5mm for them uns.

 

[Splatter Paint]

I'm using a 68 x 118mm on 500LX cranks on my 1992 Prestige.

SP

 

[brocklanders023]

Cheers for the replies gang. What's the best way to work it out?

 

[FluffyChicken]

Best way to work it out is to fit a 68x122 (.5) and go for a ride ... er from there I mean.
Than go ooh that seems to work and job done.



OR fit what you have, measure the chainline and alter it to be 47.5 as best you can.

So if you use the 127.5mm bottom bracket, you may have a chainline of 50mm while that is perfectly fine and in spec it's not ideal for your setup. Assume your still using a 26.8diameter front mech (not shimmed) then 47.5 is the perfect chainline for the mech and the chainset bike gears etc..

So 127.5 - 2x(50-47.5) = 122.5 ideal length.

 

[toadoftoadhall]

I use a UN72 68 x 122.5 on my 93 prestige and my 97 clockwork.
I hope this helps.

 

[brocklanders023]

Looks like 122.5 it is then. On the shimmed front mech issue... How much difference will it make if I use a shimmed mech? Might be a case of needs must as a top pull M750 in the correct size seems hard to come by.

 

[FluffyChicken]

Looks like 122.5 it is then. On the shimmed front mech issue... How much difference will it make if I use a shimmed mech? Might be a case of needs must as a top pull M750 in the correct size seems hard to come by.

In use, not a diddly squat that you'll notice.
Just as you'll not notice if you use your current 127.5mm bottom bracket as it's 'within spec'.

But the wider mech bands puts the front mech further away from the frame, and a shimmed mech is just a wider one made to fit. So you should either go for the chainline best for the chainset to rear shifting (47.5) OR go for what's best for the front shifting (smallest band 47.5, middle band 48.75 , fattest band 50mm) Shimano apparently always went for what's best for front mech shifting and hence there are often 3 sizes of bottom bracket. e.g. 107/110/113 would be a standard set for super low profile cranks, such as XTR M900's

Hence though 122.5 is the ideal bottom bracket for M730 you can use 125 and 127.5's

Personally I would run your current bottom bracket, it's fine for now. Only get a narrower one if you see the need and you're getting a lighter one (good weight saving for little cost can be had in my opinion).

Read the link I posted on measuring chainline, there's far more on that page. Should give you some insight, plenty of other books n stuff available as well.
If you can get hold of < cough >
Sutherlands 6th & Barnetts and use them together. You've got a retro bible there

 

[brocklanders023]

Cheers fluffy, makes sense now. Forgot to mention the original BB is shot so it'll have to be a new one.

 

[David Gibson]

bottom bracket

For future use, you can look at this table to find the best compatibility for different kinds of cranks:

http://www.bikepro.com/products/bottom_ ... able.shtml