Adjusting front chainline (external BB)

[citoyen]

I've done some proper careful measuring and it seems there's a 2.4mm discrepancy in my chainline (rear is 2.4mm further out than front).

I figure the easiest/cheapest way to get a perfect (or at least more nearly perfect) chainline would be with some kind of bottom bracket spacer. Does that sound sensible?

If so, would one of these be appropriate? http://www.chainreactioncycles.com/Models.aspx?ModelID=36416
[edit: it's an FSA MegaExo BB]

How and where do I fit the spacer - between BB and frame? or between BB and crank?

And should I fit one on each side so that the cranks remain equally spaced?

Will I be able to do this with the BB already fitted (will it be able to accommodate the extra width), or will I need a wider BB?

Or is there a better solution? Or do I need to bother at all?

d.

[gordon taylor]

I don't know... but I do have spare BB spacers. The external ones always come with a selection, which I ignore. 

I'll shove a couple in an envelope and post if you want to experiment.

[fuaran]

Or you could add a few spacers on the chainring bolts.
eg http://www.charliethebikemonger.com/chainring-spacers-1297-p.asp

[Gog yn y De]

I figure the easiest/cheapest way to get a perfect (or at least more nearly perfect) chainline would be with some kind of bottom bracket spacer. Does that sound sensible?

I'm about to do the same. My chainline is spot on but I'm about to fit a freewheel, just out if interest. However, the freewheel is a touch wider than my fixed gear sprocket so will knock the chainline out. What I'm about to do tomorrow is to fit the appropriate sized spacer on the drive side BB, between the frame and the BB. I'll see how it goes - I may fit one to the other side to balance things out. When/if I return to fixed wheel mode, I'll leave the BB spacers as they are but fit the same sized spacer between the rear sprocket and the wheel to keep things in line.

If so, would one of these be appropriate? http://www.chainreactioncycles.com/Models.aspx?ModelID=36416
[edit: it's an FSA MegaExo BB]

Should do - I'm using these

Will I be able to do this with the BB already fitted (will it be able to accommodate the extra width), or will I need a wider BB?

They'll fit, no worries 

I've just done some fettling and fitted a 2.5mm spacer on both sides of the BB. However, this meant that when I tightened the crank arm, it pressed tightly into the BB bearings instead of the seat of the spindle (that's due to my SRAM crank - wouldn't happen on a Hollowtech)

So, I decided to fit the 2.5mm spacer to the drive side only - seems ok. I couldn't complete the job though as I couldn't get the sprocket off the wheel (bent a chain whip in the process) - a job for the LBS tomorrow so it'll be their fault if they bust it!

[citoyen]

Completely forgot about this thread after starting it… Thanks for the replies. Sounds like BB spacers are the right solution.

d.

[tiermat]

FWIW Raceface provide spacers, and a handy little chart, with their version of the Hollowtech BB.

I needed them when building up the InBred.  The BB shell was too narrow to fit the crankset on it's own, sans spacers.  I didn't use the Raceface BB, but used a 105 instead (apparently the RF ones are bobbins and break after only a few hundred miles, so I was playing safe rather than sorry).  The problem is that the BB external cups on a 105 BB are wider than those on the RF one (which could go someway to explaining why the RF ones are pants).  So the standard spacers wouldn't work, as they only provided 2.7mm ones(IIRC), and I needed 0mm on the NDS and 4mm on the DS (so that the middle chainring didn't foul the chainstay).  I visited my LBS and they gave me 2*1.5mm and a 0.7mm which did the job nicely.

And yes, they go between the external cups and the BB face.