What length bottom bracket?

[mike]

I couldnt see it in the FAQ.. sorry.

what length bottom bracket do I need?  the dimensions I have are:

the distance between the dropouts is 130mm
the distance from the inside of the LH dropout to the centre of the cog is 111mm

so do just I need a 111mm bottom bracket?  Or am I missing something fundamental?

(it's a square taper stronglight chainset, 68mm bottom bracket shell, english thread - any suggestions?)

[Paul D]

You need to think about centrelines

Measure to the cog from the centre of the hub. This is the distance you need to recreate on the centre of downtube to chainring measurement. The negth of the BB will obviously be a factor in this, but first you need to find out where the chainset sits on said BB ie 111mm axle is the length to the end of the axle - the chainset will sit a certain distance up this axle, and different chainsets have different shapes...the ring could also be on the inside or outside position (for a converted road double) for instance...

[mike]

nuts. 

so I need to do what the bloke in the LBS said - guess, buy one & see if it fits, then buy another one longer or shorter if it doesnt.

thanks Paul!

[Martin109]

The job can be made easier by obtaining an adjustable BB, such as this one.  There are more expensive offerings, but this one has served me well.

[iakobski]

You don't mention the hub, do you have one yet? If not, buy the same size BB as the bike had previously, measure the chainline, then choose the hub to match. Apparently, the Phil BBs have +/- 2mm adjustment available, which could give you extra leeway. If the wheel will be single sided, build the wheel with spacers/slight dishing to match.

Alternatively, get hold of an old BB of the right taper, measure the chainline and work out the axle length for the chainline you're after.

[mike]

it's a new hub - an ENO excentric one. I'm fixing a previously geared bike.

I've ordered a middling-length 10 quid stronglight hub.  if it's a bit out, I'll order another one...

[iakobski]

That has 47.5 mm chainline and is supposed to match to the middle ring of a triple on 111mm BB.

[alexb]

Check the long list on the London Fixed Gear and single Speed site:

Transmission database - London Fixed-gear and Single-speed