i have concluded that i have got no idea how to do this. i have ordered my new pompetamine frame, with 135mm spacing to replace my 120mm current one.

how am i supposed to work out what size i need in order to get the chain line right? this is a complete mystery to me… 😆

Depends on the cranks you intend to use really!
If you're using HT2 cranks then just measure the BB shell width (68mm or 73mm)
Octalink and Sq taper, erm…..

well they're truvativ elita isis cranks, so do i just get a truvativ isis BB and space out the ring to get the chain line right?

You can do, but what hub are you running? assuming you're running it SS? As you can always space out the sprocket to suit on the hub instead.

i've got a WTB Laser Disc hub, yet to be built up. With the lockring on there is still about 5mm of thread on the outside, so i assume there would be space for a spacer inside the cog.

so the conclusion, then, is a 68mm truvativ isis BB and potentially spacing out the cog at the rear if required?

Is it a SS specific rear hub, or a normal 9speed version? If it's not an SS one, you can obvisouly move the sprocket around to adjust. If it is an SS hub, you'll need to adjust on either BB axle and/or change where the chainring is bolted onto the crank, either inside or outside the spider.

SS specific hub. doesn't look like the ring can be bolted to the outside so i guess it's a case of spacing out on the axle?

cheers for the help, by the way. 🙂

It should bolt on the outside ok as the bolt pattern is the same for both outer & middle ring, but obviously you'll need single ring chainring bolts if you haven't got them already.
You can't space out on the axle itself. Try the BB you have already & if that's too long/short you'll need another shorter/longer to try in it's place.
Hope that makes sense, if not, feel free to ask.

nice one, will do. 🙂

The way that bike shops do it is to find out from the frame manufacturer what the chainline is (e.g. 52mm, 53mm, etc) and use calipers to measure that distance from the centre of the seat tube to where that should put the middle chainring (or single ring if SS).

You should therefore ask On One what the chainline is for this frame.

Since they should already have a lot of info about whatever cranksets they fit at the factory as standard to this frame they should also be able to give you a pretty good answer to your question about what BB axle length you require. I'd be astounded if they couldn't.

Frames don't have a chainline. Cranks and hubs do….

I might be completely wrong here but as travellingman said you should surely be able to use the same bb/crank and hub combination on any frame and the chainline would be the same.

The only part of the frame that would affect chainline would be the width of the bb shell and the rear spacing (which should be 135 on a mountain bike). I would have thought they would all be broadly similar (within a couple of mm) otherwise certain hubs/bb wouldn't fit properly.

Unless you're saying that the bottom bracket shell width may vary and this will dictate bottom bracket axle length accordingly and thus alter chain line? Aren't the actual bottom brackets a standard width though and it's just the axle which is longer/shorter in order to accommodate different spider designs which alter the chainline?

The way that bike shops do it is to find out from the frame manufacturer what the chainline is (e.g. 52mm, 53mm, etc) and use calipers to measure that distance from the centre of the seat tube to where that should put the middle chainring (or single ring if SS).

interesting, never heard of that before

Chainline (or rather axle length required to give recommended chaineline) is determined by cranks, speak to the crank manufacturer, and get a BB with appropriate axle length. BB Shell width will be set by frame builder but wont affect over all axle length 68mm or 73mm for most MTBs).

Sheldon – Gone but not forgotten

ok, so the conclusion is that it should be a 68mm BB, and i should get in touch with truvativ to confirm what axle length is needed?

if it's a truvativ isis crankset, does that mean i can only use trvativ isis BB with it or are others sufficiently similar? 😕

guys? 😀

come on chaps, final hurdle 🙂

Isis is a standard so it wouldn't have to be a truvativ bb

thanks – i have realised in this whole enterprise that i have never bought anything other than shimano BBs and chainsets 😳