interesting SRAM problem

[vqdriver]

i recently completed my switch to sram red with the cranks being the last piece of the puzzle, and ran into a strange problem on the test ride. when crossed over on the big ring and any of the last three largest cogs, the chain starts skimming the top of the chainring teeth and eventually jumps off down to the small ring. i fiddled with the FD position on the ride and messed with the cable tension, all the while believing it was a FD problem. but i discovered that the cage never touched the chain, no rub at all if i hit the trim. it seems that the chainline on the larger half of the cassette is "pulling" the chain across the big ring and is essentially pulling the chain off towards the frame. i can't think of a single thing that i can adjust to compensate for this.

crankset bolt is TIGHT. no side to side play in the crank at all.
chainring looks good, no visible damage to teeth or overall shape

does the SRAM red crankset actually sit farther out from the center of the BB? or is this my imagination?
please let me know if i'm overlooking something, ideas on a fix are welcome.
fwiw, no issues previously with the same FD and shimano cranks. same kmc x10 chain.


btw, let's keep this thread focused on the problem. no sram bashing please. :)

[Vinci]

What is the range of the cassette you are using?

Is it possible that the chain isn't sized correctly (too small) and is getting too tight towards the larger cogs and is getting pulled off?

[oliver1850]

I assume that the rings are running true, no lateral wobble. I would first check chainline for whatever crankset/BB you have against the specs here:

http://www.sram.com/sites/default/files/techdocs/road_cranksets_fram_fit_specs_gen_0000000004165_re v_b.pdf

[vqdriver]

hmmm. that's a good thought.

but the crank gearing didn't change. went from 50/34 shimano to 50/34 sram so i don't know why the chain would be too short.

[Vinci]

Is there anything else different about the crank? Were you using a BB30 crank before and now a GXP crank with adapters, maybe?

[vqdriver]

nope. english hollowtechII to english gxp.
same gearing, same chain, same wheels, same cassette, different color

oliver1850 - yes, no side to side wobble.

[Smiley]

chain length measurements are a bit different as the instructions for SRAM are different for chain length measurements.

[jemoryl]

hmmm. that's a good thought.

but the crank gearing didn't change. went from 50/34 shimano to 50/34 sram so i don't know why the chain would be too short.

Maybe the distance the chain travels through the SRAM pullies is different compared with your past setup (due to cage length, chain wrap) so you might need a few more links?

[thegunner]

methinks perhaps a cassette spacer might solve everything :p

[vqdriver]

keep in mind that i'm getting this problem across multiple cogs, the biggest 3 cogs, not just the totally crosschained combo. so a spacer or short chain doesn't fully explain what's going on. but who knows.....

i'm going to revisit this tonight when i get home with the suggestions here in mind. hopefully it's something simple.

[oldpotatoe]

i recently completed my switch to sram red with the cranks being the last piece of the puzzle, and ran into a strange problem on the test ride. when crossed over on the big ring and any of the last three largest cogs, the chain starts skimming the top of the chainring teeth and eventually jumps off down to the small ring. i fiddled with the FD position on the ride and messed with the cable tension, all the while believing it was a FD problem. but i discovered that the cage never touched the chain, no rub at all if i hit the trim. it seems that the chainline on the larger half of the cassette is "pulling" the chain across the big ring and is essentially pulling the chain off towards the frame. i can't think of a single thing that i can adjust to compensate for this.

crankset bolt is TIGHT. no side to side play in the crank at all.
chainring looks good, no visible damage to teeth or overall shape

does the SRAM red crankset actually sit farther out from the center of the BB? or is this my imagination?
please let me know if i'm overlooking something, ideas on a fix are welcome.
fwiw, no issues previously with the same FD and shimano cranks. same kmc x10 chain.


btw, let's keep this thread focused on the problem. no sram bashing please. :)

Where is the small ring in relation to the chainstay? About a 'normal' distance?

Frame rear triangle straight?

General chainline? Small ring and about 6th cog up the cogset a line about parallel to the top tube?

How does the chain sit on the big ring? Engage all the teeth normally, no bent teeth?

[Hartlin]

methinks perhaps a cassette spacer might solve everything :p

I would try that as well.

[SpeedyChix]

Does the rear der. hanger thickness fall w/in the SRAM specs? I think I've read where some hangers can be too thin and a shim needs to be put in behind the RD. Is the rear der. cable length cut to spec? B-screw adj good?