I'd appreciate any help or ideas you all might have about this. I'm pretty puzzled and frustrated and not sure what the solution would be.

I got a full group of the new Sram red this past February. I had it put on my 2010 Supersix frame and road it for 4 months. Then I bought an Evo frame in July and had it built up with the same new Sram red group. About a month ago I discovered that, when loaded hard, my rear derailleur was throwing the chain off the bottom pulley, towards my back wheel. The bike would still pedal just fine - it would chatter but since I usually ride with head phones I hadn't noticed. After I realized it was throwing my chain, I looked closer and saw that my chain, when running next to the pulley, had worn down the teeth on the bottom pulley and had ground a chunk out of the derailleur cage.

I took it in to my local bike store and the mechanic said that my rear derailleur hanger was loose, and he thought this must be why it was throwing my chain. But then I looked at my 2011 Supersix, and the rear derailleur hanger was loose on it also.

So my guess is that there must be some problem with the rear derailleur that makes it throw the chain, and that the chatter, over some months, caused the derailleur hangers on both frames to work loose (I'd ridden my old Supersix for 18 months with an old Sram red rear derailleur and the hanger never came loose, and the Evo frame was brand new).

My local bike store guy is saying I should just get a new pulley, but the pulleys for the new Sram red only come in sets and they cost $140 for the set. Also, this doesn't seem like it would solve the problem.

Anybody had the same problem, or have suggestions? Would this be warrantied by Sram?

Have your shop call Sram. I have seen it happen half a dozen times on the Sram XX bottom pulley.

9 times out of 10, it's from a slightly twisted derailleur hanger. It can be exacerbated by using the small ring/smaller cogs. If the derailleur hanger is tail-in, then the chain wants to derail off the bottom cog on approach. It also happens to be that the sram derailleur provides a spot for the chain to sit next to the pulley.
Sram may take care of the pulley If the shop installed the group, they ought to go to bat for you. It would be good for them to accurately check the hanger alignment as well.

i had the same thing happen after I installed a full new red group, but it hasn't happened in a few rides. I checked the der hanger and it is straight, so wasn't sure what to do about it...hopefully it keeps acting right.

One of the main reason of XX failure. Look for chain rub on your cage. It would be warranty.

There's definitely rub on the cage - it's actually dug a groove in the cage, next to the pulley on the wheel side.

The mechanic from my local bike store didn't mention that the derailleur hanger alignment was off (and I think he knows his stuff because he used to travel with pro teams).

And the fact that it shook the der hangers loose on both my 2010 Supersix and now the Evo makes me think it's gotta be a problem with the derailleur.

Does this make sense?

I can't imagine how a derailler could make a hanger come loose. Those things should be screwed and loctited securely.
More likely you really did have two hangers come loose from faulty initial installation.

But I had ridden my 2010 Supersix for 18 months and the hanger didn't come loose. Then when it was built up with the new Sram red it was by a mechanic who said he always checked to make sure the hanger was not loose - and the hanger then came loose within 4 months.

And the Evo was brand new - it had just been stripped. And it was built up by a good mechanic who's been working on bikes for 30 years and I think he would have checked to make sure the hanger was not loose. And the hanger on it came loose within 4 months.

Wouldn't that bee too weird a coincidence?

+1

Based on the failed attempts to fix what is a straight forward fix (i.e. properly align hanger, apply loctite to hanger fixing bolt, check der cage alignment) I suspect your "mechanic" was actually just touring with the team not actually wrenching.

EM3

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My 2011 Cervelo R5

Think of all the vibration, shock, and stresses from various random sources when riding on a bumpy road. How can anything about a deraileur compare to that ?

But the more I thought about this, the harder time I have invisioning exactly how a chain gets thrown off a bottom pulley at all. It seems virtually impossible. But maybe I am just not seeing it. Can you post a picture of the situation ?

Consider the fact that in most gears, the chain goes from front to back at an angle. It then approaches the pulley at an angle, and with enough of a bounce, some misalignment, solar flares, etc, the chain can hop to one side in an attempt to make a more direct line.

Not only can it hop off the pulley, but there's enough of a space between the pulley and the cage on the new Sram red that my chain would ride there quite smoothly, and long enough to where it wore a groove in the cage. I'll post photos of my chewed up cage and the worn down bottom pulley tomorrow.

I actually had this happen with a Force derailleur. The shop thought the derailleur hanger was bent but it seemed like the derailleur cage was actually extra mobile in the side to side direction. Anyways it was warrantied and that fixed the problem.

I've also seen the problem with a SRAM rear derailleur, I believe it was due to a slightly bent pulley cage in that instance.

The design of the SRAM RDs seems to make it far easier for the chain to derail from the lower pulley than with other brand's RDs. Even without a bent cage or hanger, when the shifter gets operated without turning the pedals the chain can easily come off of the lower pulley on a SRAM setup, whereas this seems to rarely happen on other brands (or, at least, on other brands, the chain finds its way back on by itself as soon as you turn the cranks, whereas with SRAM the chain has to be helped back into position manually).

How can a derailleur hanger clamped by a quick release flop around?

If your mechanic is checking hanger alignment without a wheel clamped in place, he/she is not doing it properly.