Finding out what bottom bracket length is needed

[montage]

I've measured both my chainlines, front =54 and the rear =40ish, but I can go up or down on this by 2mm.
I'm looking at a bottom bracket 4F suggested:
http://www.chainreactioncycles.com/Models.aspx?ModelID=17473

Cheers

 

[RecordAceFromNew]

Since you are looking at shortening the bb spindle, if I were you I would first check to see what clearance do I have currently i) between the chainset and the bb at their closest point, ii) between the chain ring and the chain stay and iii) between the crank arms and the frame on BOTH sides. What we want to be sure is getting a suitably shorter bb (14mm on the drive side, ?mm on the left if indeed such a thing is available) will not cause clashes, or we may have to look at other options also to improve the chainline.

Then we will also need to know what the dimension of the bb shell is, and the specification (thread e.g.) and dimensions of the existing bb (see Sheldon's bb database). This will help giving us a starting point to work with.

 

[montage]

Thanks Record, looks like I will need to get back with some of that stuff tomorrow, complete with photos. Fingers crossed there will be no redishing of wheel needed!

 

[montage]

currently using a 120mm spindel.
i) there are only a few mm (5 maybe?) between BB and chainset - but looking at the UN54 shimano BB, it looks like there is room to accomodate the chainset "inside" the frame slightly if that makes sense (i.e. it looks concave)

ii) bags of room

iii) looks fine

Thread on BB is english 68mm

Doing the maths, the 107mm BB doesnt seem to work - but it's the only real option I can see without wheel redishing

 

[montage]

Hmmm on a side note, that lockring doesnt look too happy!

 

[MacB]

Monty can you not mount the chainring inside the spider rather than outside?

 

[montage]

that still won't do it, but if definately an option. I'll start with the 107mm BB, then try the chainring on the inside - though this will look fugly

 

[MacB]

that still won't do it, but if definately an option. I'll start with the 107mm BB, then try the chainring on the inside - though this will look fugly

you could get yourself a trendy, hipster, bash guard for the outer position, alternatively you could stop being a tart and go with what works!!!

 

[RecordAceFromNew]

currently using a 120mm spindel.
i) there are only a few mm (5 maybe?) between BB and chainset - but looking at the UN54 shimano BB, it looks like there is room to accomodate the chainset "inside" the frame slightly if that makes sense (i.e. it looks concave)

ii) bags of room

iii) looks fine

Thread on BB is english 68mm

Doing the maths, the 107mm BB doesnt seem to work - but it's the only real option I can see without wheel redishing

From your figures, I would have thought even utilising the concave in the UN54 would still not make the 14mm you need?

Regarding your comment re redishing, am I right that you have flip-flop at the back? if so shifting the hub/redishing is most unlikely to be the answer, but the sprockets might be capable of being moved a little bit further out.

A different chainset/bb is likely to work. The reason I am saying this is because standard track hub chainline is 42mm.

 

[montage]

I've ordered the new bottom bracket, 68-107. If that doesn't work it is chainring on the inside, if that doesn't work then more extreme measures shall be needed!

Cheers for the help, I will let you know how it gets on in a couple of days after BB is installed. If it doesnt work then my chainset is an oddball