Spacing GXP bottom bracket for better chainline?
 

Pertinent bike info:
Omnium crankset (GXP bb) w/ original 1/8" chainring
Sram PC-1 1/8" chain
Surly 1/8" cog
Velocity deep V wheelset
Cinelli Mash frame

My problem:
Driveside crank arm isn't mated up against the bb cup like the non-driveside arm is. I believe this is causing poor chainline and thus the odd chain noise that alerted me to this problem.

I have determined that the crank arm can't go in any further because the thick part of the thru-axle is already up against the non drive-side bb cup.

So...
I realize that if I put some spacers in the non drive-side bb cup I would be able to insert the drive-side crank further, and get a better chain line.

Have other people encountered this obstacle with the omnium/gxp crankset? Is this a suitable way of solving the problem?

Most importantly: is the drive-side crank even supposed to be right up against the bb cup?

My Omniums are flush with the BB cup. Are you sure you've got the NDS crank bolt torqued enough?

If you have the spacers that come with the BB, try installing them and see if that fixes your dilemma.

The first thing I did was try pulling the crank arm in deeper by tightening the NDS crank bold -- no luck.

I then removed both of the crank arms, reapplied grease to the DS arm and tried to insert it. It stopped in the same position again. I tried tapping it lightly with a rubber mallet, which also failed to get the DS crank arm in any further.

Weird.

There's nothing to space with that crankset. Sorry.

Do you have one of the new BBs? Mine doesn't sit flush either. I have perfect chainline though.

I pulled the NDS bb cup and the tapped the DS crankset all the way in with a rubber mallet. This confirms my suspicion that the NDS cup was blocking the thru-axle, but I also realized that with the DS crank all the way in the chainring bolts interfere with the chainstay. I'm pretty sure this isn't how the crank was designed to work.

I should have mentioned earlier -- there is only about 2-3mm of space. It's not eye catching or anything, I just wanted to look in to it because the NDS crank sits flush

Mine is the newer type. I'm going to continue to look for advice, but I also realize that I might have to live with the slight noise (the chainline is only slightly off).

I've done about 30 miles on this setup with no chain derailments or anything.

I was going to put a bottom bracket spacer between the frame and the NDS bb cup. Since the NDS bb cup limits how far the DS crank can be inserted, spacing the cup out would allow the DS crank and chainring to move inwards.

I could never get mine perfectly straight with this bb either, though no noise because of it

I think I'm going to leave the cranks as is and attack the chain line issue the other way around by altering the spacing in the rear. Sheldon brown has a spread sheet on cogs including what he calls "chain line from shoulder" which is measured as the inner side of the cog to the center of the teeth. I'm going to try using a cog whose teeth are further outwards.

Have you measured your bottom bracket shell? Is it 68mm wide?

I did measure the BB shell -- it is 68mm

Sure there is. GXP bottom brackets can be used on either 68mm or 73mm BB shells and come with two 2.5mm spacers.

They're not included with Omniums but any BB spacer will work.

Also - it's most likely not your chainline being off by a few mm that's causing the noise. Surly cogs are often very loud.

The point of the spacers is if you are using a mtb crankset and the frame has a 68mm shell. Is this a mtb frame? no. Is this a mtb crankset? no. No need for the spacers.

See how there's a road bike at the top of the instructions here? These are the instructions for road cranksets.

http://www.sram.com/sites/default/fi...ranksets_3.pdf

See how there's a mountain bike at the top of the instructions here? These are the instructions for mtb cranksets.


http://www.sram.com/sites/default/fi..._cranksets.pdf

There is no mention of spacers in the first document, and you shouldn't need any to make Omniums work properly.

No need to be a dick either. I was just saying using a spacer can be done to correct a slight chainline or clearance issue.

Unless they've changed something on the Omniums to allow it to work with a 73mm bb shell, you can't put spacers under either side of the GXP cups. The limiting factor here is the crank spindle. It's designed for a 68mm bb shell and by spacing either cup out, there won't be enough splines exposed for full purchase of the NDS crank.

If you're having clearance issues on your chainstay, you are probably SOL. That crankset is a track crank designed to work with heavily manipulated, narrow chainstays. The spider and/or crank may not clear many fixed gear road frames.

In other words, gospastic, although grouchy, is correct.