SRAM Omnium crankset

I bought new bb and it works great now! Thanks for advices, love this crankset btw.

HELP! (Or, great condition Omniums for sale...)

Used (but great condition) Omniums, new GPX BB, new chainring, new chain, new sprocket, old and abused Fuji Track. Fitted in October, and ridden every day since but never without an issue.

• the cranks need re-tightening every 6-10k on the road, to the point that I even carry a full sized allen key. Next step is too get the frame re-faced, but any other suggestions on why this might be happening?
  
• Hit a pothole yesterday, one heavy clunk later, and I've got some serious clicking issues which I hoping might be BB related (see linked video) but has anyone else come across similar?
  
• Cranks definitely do not turn freely (see end of linked video)
  
  

Anyone have any thoughts? Please let me know if I've missed any important information or worse, missed a relevant comment at the top of the thread - I'm sort of hitting my limit and considering a return to trusted square taper!

https://vimeo.com/126398056

Maybe it's the video, but your NDS bearing cup doesn't look square to the BB shell. So much so it could even be cross threaded.

Yeah, something ain't right there. When I had Omniums, the GXP BB had a lot of drag, but that is silly.

Do you have the tools to disassemble?

Getting the BB shell faced would be my first call of action.

Do the bearings feel ok when you take the cranks out? When you installed the cranks, did the crankset axle want to go through the NDS bearings without fuss?

Also what is your BB shell width? Have you used any spacers?

I run only omnium on my 2 bikes and bearings are just rubbish, only 20£ and i need to change them every 5k miles. If the cups are correctly installed (hard to see on the video), i would say change the BB.

Spacer or not shouldn't affect except chainline, but if you over tighten then you can push the arm too much in contact with the cup

Great replies - you're all giving me hope.

upstart - cups went in really cleanly, so doubt (re: praying) it's not cross threaded
Pifko - if GXP drags, any thoughts on alternative BBs?
Santino - interesting thought, I have been known to run my chain a bit tight (although have been riding super slack since christmas in an attempt to stop the cranks from loosening off)
Hovis - can't recall, i suspect its 68 but I'll check and spacers are there

I'll clean up and disassemble tonight, come back with any pictures / or extra detail.

Thanks everyone

I don't get the whole need to face the BB shell for external BB. Isn't the alignment of the BB dependant on the shell threading, not the face of the BB?

Am I just complety confused?

Facing is particularly important for external BBs. The cups need to sit flush against the edge of the shell (which they won't do if it isn't properly square)

Interesting point. I can only assume that once you tighten up to 35-40Nm, the threads deform enough to allow the cup face to sit flush against the bb shell, thus putting the bearing at a slight angle.

Am I just complety confused?

Yes. If the face isn't square to the threads, there's more than enough clearance in a normal thread fit for the face to pull the thread askew, especially in the case of a BB cup where the face contact diameter is about twice the thread depth. Not only does that set your bearings out of parallel, but your thread flank pressure will vary wildly around the circumference, leading to either sheared threads or loosening in service.

I was close enough...

The cups need to sit flush against the edge of the shell.

Why?

Answered! Thanks all

Hovis - can't recall, i suspect its 68 but I'll check and spacers are there

Do you need spacers on a 68mm shell? I think you don't. If you don't, and you have them in anyway, you'll be crushing the bearings when you tighten down the crank bolt. This will explain the crazy drag. And the horrible crunching noise. Although quite how it worked from the off I have no idea.

This could also explain why the crank bolt is losening off over time.

Get a new BB, remove the spacers, put some locite on the axle / crank arm interface (as it might be a bit sloppy after running loose), pop some blue loctite on the crank bolt, tighten to 35Nm, check the extractor bolt is at 12Nm, enjoy.

You don't need spacers for 68mm

I think we have a winner

You can use spacers to adjust the chainline with those crankset no problem. But indeed you have to tighten it only to the point the crankset stop any horizontal play (as specified in the SRAM manual you get with it new). More and you crush it as howard explain.

There is a tolerance build in the crankset for the bearings to seat in a wider area than necessary so few mm spacers can't do any damage. Remember that chainline is being adjusted with spacers on the non drive side to pull the crankset toward the frame.

GXP cranks - in my experience - can only be adjusted with spacers on the NDS to pull the DS inboard. Spacers on the DS, beyond taking up any space between the shield and the crank spider*, cause the NDS arm to bind the NDS bearing.

* space, not slop, because GXP cranks when installed properly have no slop regardless of shell tolerances. The whole point of GXP is that the crank bolt is only used to attach the NDS arm to the axle, and plays no part in bearing pre-load or removal of horizontal slop.

You can use spacers to adjust the chainline with those crankset no problem

This is wrong. There might be a small amount of wiggle room, but it's so small that it's not worth trying to use it, because chainline offsets that small aren't worth worrying about.

GXP cranks when installed properly have no slop regardless of shell tolerances

This is right. Crankshaft end float on GXP should not happen, because the NDS crank traps the NDS bearing against the step on the spindle. That's the whole reason why they have that annoying difference in bearing bore between DS and NDS.

This

10k miles in total on my both bike with spacer (1 & 2 mm) and it works perfectly. A good chainline allow you to reduce noise and smoothness but the main reason is to move the drive side spider away from the frame (in my case).

As howard illustrate very well here, you can move the drive side by ading spacer on the non drive side. That way i moved the drive side away from the frame otherwise it was rubbing very slightly the frame. A 2mm spacer and you get the necessary clearance.

Spider on omnium are rather bulky and many people bump into that problem, adding spacer allow you to run it and the tolerance of the crankset allow it perfectly. As howard say above, the bolt which lock dive and non drive side is only there to be tighten until the crankset lose any play.

the main reason is to move the drive side spider away from the frame (in my case).

As howard illustrate very well here, you can move the drive side by ading spacer on the non drive side. That way i moved the drive side away from the frame

When you add a spacer under the non-drive side cup, you actually move the spider closer to the chainstay.

If you really want to move the drive side crank further outboard and still have a GXP crankset properly assembled, you have to face material off the NDS of the BB shell. The whole left/right alignment is determined from the NDS face of the shell, as the NDS bearing is located with reference to that, and the axle is located with reference to the NDS bearing.

To move the crankset away from the frame, you just add a spacer on the drive side to push the cup away. As you tighten the crankset only to remove play, it works great.