Bottom Bracket length for a triple?

[Tommy2]

Had my bike in for a service at LBS, he said the front derailiuer is a little bit awkward to shift properly, he said it was because the bottom bracket doesn't seem to be the correct length for a triple?

can anyone shed any light on this?
i.e does it sound plausable and what should i be looking to measure/change to rectify it?

i only ask as the lad at the bike shop doesn't seem very confident when he is talking to me about it, whether that is because he's not confident in talking to customers or not confident in his work i'm not sure at the minute.

 

[Tommy2]

It changes to all the chainrings if that's what you mean.

 

[MacB]

depends on what sort of chainset you have

 

[Tommy2]

he said it was too long i think, does that mean he seems to thing it will have problems shifting to the big ring because a shorter BB would take into account the increased overall width of an extra chain ring?
it's all campagnolo mirage 9 speed exadrive.

 

[MacB]

Ah, I know nothing about Campag but if the chainset is three pieces, as in two crank arms and a separate bottom bracket with axle then it is feasible that the wrong BB axle length could be used.

 

[Tommy2]

would i be ok using this http://www.ribblecycles.co.uk/sp/ro...olo-triple-centaur-bottom-bracket/campbtbr675
or do i need to find a mirage specific BB?

cheers

 

[woohoo]

IIRC, the Campag Veloce and Mirage square taper BBs have the same fit but Centaur and better types differ from that. The length of the BB is dependent on the seat tube diameter. If it is oversized, you need the longer BB (as that Ribble link says). This is covered in the Campag technical documents on their website. If you are managing to shift to all three rings, I suspect that it's probably a case of fine tuning the existing setup.

PS I had to do this BB change on one of my bikes but the chainline was horrible and I went back to a compact but with a larger bottom cog on the cassette.

 

[boydj]

I'm running an older Centaur triple chainset on a 115.5mm Veloce BB. The Campag square taper is a slightly different shape from other standards and I would expect any Campag chainset to fit fine on the Centaur BB.

It is correct that you have to have the correct axle length to give you the right chainline, depending on whether you have a double or triple chainset.

 

[woohoo]

IIRC, it depends on how old your Centaur chainset is. If you look at the latest square taper Campaq BBs (before the moved to UT/PT) the Centaurs BBs differ from the Veloce/Mirage BBs (see Campag website). The correct axle length can give a rubbish chainline if you have to move to the longer BB (115.5mm) due to having an oversized (35mm) seat tube. (The oversize seat tube causes the front changer body to move a bit too far away from the frame centre line which means that the chainset has to be moved out a couple on mm to allow the shifter to reach the inner ring.)

In any case if the changer can reach all three rings then the current BB should be OK.

 

[Hacienda71]

Sheldon always has the answer

 

[woohoo]

Sheldon always has the answer

.. usually, but not in this case

 

[BimblingBee]

Did you buy the bike new or is the BB an original to the bike? If so, then its very, very likely to be be correct as is is.

What problems are you having to cause you/them to think the BB axel length is at fault.

TBH I'm not convinced that a BB axel length would cause the problems they are describing.

 

[Tommy2]

Bike is second hand so don't know if it is the original BB.
tbh I didn't notice it was causing a problem, the guy at the bike shop said something about it not shifting on the front mech very well but was kind of unsure as to whether it needed replacing or if I could live with it without much bother.
I will wait until I get it back and run it for a while and see how it is.

 

[BimblingBee]

Have a look at this for an idea for calculating. I'm not convinced there is a problem though.

http://www.sheldonbrown.com/bbsize.html#chainline

 

[woohoo]

Given that the bike is secondhand, it may be possible that the original Campag (ISO standard taper) has been replaced by a Shimano etc (JIS standard taper) type. If this is the case then the Campag chainset would not fit as far onto the BB as it should which would effectively lengthen BB.

… but if you don’t notice a problem before the LBS mentioned then I would leave it as is.