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I’ve been trying to diagnose a serious SRAM Eagle shifting problem with my XX1 Eagle drive-train that no shop or SRAM is able of diagnosing or fixing, and I’m close to concluding that SRAM Eagle has a serious design flaw.

I bought a new 2017 Trek Fuel Ex 9.9 in May and the lower two gears have been crunchy since I rode out the shop. The bike has never been crashed nor has it fallen over. All the basics have been checked, derailleur hanger alignment, chain length, cable routing, kinks in the cable, and proper derailleur alignment was done several times using the red plastic SRAM b-alignment tool (with the shock compressed for sag). No matter how much fiddling, no amount of adjustment is able to prevent the occasional misshift when downshifting to the 42t and 50t sprockets.

Misshift is a mild way of putting it. On a bike stand, shifting from 3rd, to 2nd and 1st gears, the chain will often end up riding on TOP of the sprocket teeth and stay there. Out on the trail, the same thing happens momentarily before the torque from the pedal stroke violently pulls and drags the chain OVER the entire sprocket moving it forward by 1 tooth before it settles into place. It’s actually hard to see this while it’s happening but this is what’s going on when you hear those nasty klunks on those two larger gears.

Reviews online are overwhelmingly positive and read more like press releases, but a few reviews have hinted at the problem.

MTBR mentions :

“an occasional pop in the rear cassette and chain”

but never goes into detail about it .

Singletracks.com also hints at it:

“At one point, I did get the drivetrain to squawk at me: when accelerating out of the saddle, up out of a mud hole, on a stupid-steep hill, and attempting to downshift three gears at once. Ultimately it was a bad call on my part, and while the drivetrain crackled and groaned under pressure and from grit in the gears”

Pinkbike also hints at the problem but completely minimizes it :

“The eight-tooth jump from the 42- to the 50-tooth cogs did sometimes sound a little louder than the other shifts, but the chain always ended up exactly where it was supposed to be, even when shifting on a steep uphill. That jump between those final two gears didn’t bother me out on the trail, although it may take time for riders that are more particular about their cadence to get used to it. As I mentioned, I typically used that cog as a bailout, a last ditch effort to keep my momentum on tricky climbs”

Pinkbike even has a picture of their bike with the chain riding on top of the spocket…

After a lot of analysis, these misshifts appear to be more indicative of a design problem. The two largest sprockets on SRAM Eagle cassettes have a narrow-wide tooth profile. While narrow wide works extremely well on a chain ring, its use on a cassette is questionable. Narrow wide on a single chain ring setup works because the chain never leaves the ring.

Remember how narrow wide works : the chain ring has a series of alternating wide and narrowly cut teeth that integrate with corresponding narrow wide gaps in the male and female links of the chain (I’ll update this post soon with pictures). The chain can only fit around alternating links; and you can try this yourself,  shift the chain by 1 tooth on the chain ring and the chain won’t mesh at all instead sitting on top of the chain-ring. This obviously makes sense as the large tooth is too wide to fit in the narrow link. But as I said, this isn’t a problem on a single chain ring setup as the chain always remains in the same [synchronized] position.

This isn’t true with a cassette, the chain changes sprockets every single time a shift is made. And shifting isn’t a binary process; as you shift, the chain gets pushed to the side of the adjacent cog and  gets lifted by friction. Complicated ramps and tooth profiles help transition the chain as smoothly as possible onto the next gear but timing is still a fuzzy “let’s hope for the best” process, and expecting the chain to be synchronized on every shift is the only way SRAM Eagle can possibly work.

As I’ve experienced, this isn’t the case. I’ve seen several people experiencing similar problems, one of the mechanics says he’s even noticed it on his Santa Cruz but just lives with it. At a Yeti dealer we put one of the Eagle equipped bikes on the stand and were able to easily reproduce the problem, and I have recently come across two reports of broken teeth on the 50t cassette sprockets with no explanation for the damage. I suspect the violent shift of the chain dragging across the sprocket can put so much stress on the teeth that they can shear off under enough pressure :

 

Some people claim their Eagle is perfect, but I suspect these owners are accepting of the occasional clunk in their drive-train. And to be honest, a majority of the time SRAM Eagle does shift very well. Maybe I was spoiled with my previous setup : 2008 Shimano XT 3X, but after thousands of miles it still shifts flawlessly without a hiccup. I naturally expected my 10 year newer 1X “simpler” drive-train to perform at least as well but it doesn’t.

What’s more frustrating is how the 6 bike shops that I’ve gone to (all Trek and SRAM dealers) are incapable of diagnosing the problem and finding a solution. SRAM sent one dealer a new cassette and seemed aware of the problem. And after several more phone calls to SRAM they said that the chain on occasion can ride on TOP of the 42/50t sprockets – but they don’t know how to reproduce it. At least that’s what the bike mechanic who spoke with SRAM told me.

I’m trying not to jump to conclusions, but this appears to be a compromised design and a poor solution to prevent chain drop while backpedaling. If you have any insight into what is going on, please contact me directly or leave a message below.