SRAM 12 speed GX skipping in the 2 highest gears (smallest sprockets)

[smorgasbord]

As part of getting ready for the riding season, I've been going through the bikes making sure everything's setup right. You may have seen my threads on brakes (success!) and Bafang (Ultra) speed sensor (waiting for part to arrive).

Now one of the bikes on a test ride is skipping with high power in the top two gears. This is the SRAM 12-speed GX setup (10-50 cassette) What should my debugging steps be?

0) Drivetrain (chain, cassette, derailleur, shifter) are the same age - about 1500 miles. Seems too soon to be wear related.
1) I suppose checking the derailleur setting, especially the "B" screw is the first step. Any advice there beyond using the SRAM plastic gauge thingy?
2) I'll check for chain wear, but again, I don't suspect anything there.
3) There's no clicking while in any gear - that is, the derailleur appears to be centered well on each sprocket and it's not trying to shift up or down on its own.
4) I should probably check the hanger for being bent, but I don't know how that might have happened while the bike was stored.
5) I suppose it's possible the derailleur cage is bent, too, but don't know how to check that.

Anyway, if you've got suggestions for what to check (and in what order!), I'd be most appreciative.

 

[mschwett]

check chain wear first for sure. when you hear hoofbeats, think horses. for a powerful mid drive 1500 miles could easily wear the chain and small cogs enough to cause this…

 

[Stefan Mikes]

check chain wear first for sure. when you hear hoofbeats, think horses. for a powerful mid drive 1500 miles could easily wear the chain and small cogs enough to cause this…

I concur. Chain skipping on the two smallest cassette cogs is the typical symptom of having them worn by a seriously stretched chain.
The last time I forgot to check and replace the chain on my big Vado in time cost me replacing as many as four smallest cogs. I had ridden for 2,000 miles on a single chain to regret it later. Meanwhile, I was checking the chain stretch on my less powerful Vado SL regularly, and was replacing the chain as soon as the stretch exceeded 0.5% (typical for that e-bike is 1,000 miles). The cassette in Vado SL is still sound after 4,300 miles ridden. (However, I rarely ride it in the smallest cogs as the motor power is low there).

 

[Tom@WashDC]

Yeah, 1500 miles and my chains are stretched.

 

[AHicks]

I guess mine must be babied. 1300 miles on an Ultra equipped bike, and original OEM chain is still within specs.

Noteworthy likely, is this bike is only run on the 11t top gear on very rare occasions. Front gear sized to run speeds I want for MY riding preferences WITHOUT that 11t in play.....

 

[mschwett]

I guess mine must be babied. 1300 miles on an Ultra equipped bike, and original OEM chain is still within specs.

Noteworthy likely, is this bike is only run on the 11t top gear on very rare occasions. Front gear sized to run speeds I want for MY riding preferences WITHOUT that 11t in play.....

depends hugely on choice of lube, chain cleaning, road conditions, riding “style” etc. wouldn’t be surprised if the range is 500-5000 miles.

i found that my cleaning routine was cleaning the outside of the plates very nicely, but not doing jack for the interior surfaces and rollers which are of course what matters. conditions here seem mild (no road salt, not rainy) but it turns out coastal fog and the occasional windblown sand from the beach are not so hot.

 

[mclewis1]

No solution follows ... just some commiseration.

I also run a SRAM 12spd setup, only in my case it's with a hub motor so less wear and tear on the cog set and chain (which currently has over 3000kms and still within spec and shifting fine ... but I'm watching it closely). I did notice a year or so ago that there was a kit being offered online for the SRAM 11-50 cassettes that replaced only the lower three cogs/sprockets (11, 13, 15). It looked like a knock off of something SRAM once offered too but alas I can't find any current source for what would be an ideal solution to replace only the stuff that really wears (new chain and the smallest 3 cogs).

I keep hoping the market will catch up with the demand ebikes place on specific components ... at least by the time I need to do my upgrades ... and in cost effective forms ... sigh.

 

[JRA]

SRAM does have this available to replace the 11-13-15 cogs only. They wear out due to the high torque and low amount of engagement compared to the larger rings.

 

[Bobber]

Same thing happened to me. I discovered my chains wear out in 1000 miles or less. It's real steep around here. But chains are cheap. I got mine for $17 from Bezos.

 

[mclewis1]

SRAM does have this available to replace the 11-13-15 cogs only. They wear out due to the high torque and low amount of engagement compared to the larger rings.
...

Yes, that's it ... but can you find the package in stock anywhere? I even asked a dealer about a year ago about the SRAM branded one you've listed and at the time they said they couldn't even order it (wasn't actually available from SRAM).

 

[smorgasbord]

New piece of information. Turns out that since my bike is having some speed sensor/electrical issues, the motor is always assisting at the highest PAS level. So, that's putting more stress on the smallest cogs than ever before.

I'll check for chain stretch next, but I'm wondering if I don't just have too many links in the chain. I'll post some pictures soon.

 

[Stefan Mikes]

I'll check for chain stretch next

First
I do not think the chain length has anything to do with the chain skipping on the smallest cogs. Went through the issue for too many times (always due to my negligence).

The first replacement of the two smallest cogs on my Vado ever. Interestingly, watching the old cogs didn't show any apparent signs of wear. However, the chain was skipping on the old cogs, and a new chain did not on the new cogs.

 

[mclewis1]

The wrong chain length can in some circumstances cause skipping (not enough chain wrapped on the smaller cogs from a derailleur at the wrong angle) but it's highly unlikely in this situation. Overall I agree that chain stretch (from wear) is the most likely culprit.

 

[fooferdoggie]

Now this wackiness happened to me I had taken the wheel off my trek and when I out it back on the brake was a bit off. I put the wheel up it felt solid and on right. I adjusted the caliper. but had skipping on one middle cog and I thought it was worn out. well when I changed to tubeless again the brake was off but it was back to normal and when I rode no skipping. now shifting was perfect.

 

[smorgasbord]

I measured my chain lengths (both bikes) using the digital calipers method as described in this post:
At 5.008", I have 0.16% of stretch, and 0.5% is the earliest anyone says to replace the chain. I'm using the SRAM X01 chain, which did the best in the some wear tests a few years ago.

I have two almost identical bikes with the identical drive train. The one that does not skip had a slightly short chain (as measured by looking at chain position in the smallest cog), so I removed the minimum 2 links from the skipping bike's chain. It's dark now so I won't get to test until later.

But, at any rate, it's not chain stretch.

 

[smorgasbord]

Unfortunately, the bike is dead due to a bad wiring harness connection, so it may be a while before I get to find out if removing the 2 links helped or not. It works fine on the bike stand, but then it worked fine before, too. It was only under power that it skipped.

 

[smorgasbord]

Well, we've done a few rides (probably about 90 miles) on the bike since I got the motor back (repaired controller board) and with the two links (got to keep an even number of links, obviously) removed. Not a single skip. So, maybe it's just that she doesn't use full power in the tallest gear or maybe what I did fixed things, but you know what they say: "If it works, don't fix it!"

BTW, the chain has over 2000 miles and with the wear numbers I posted up-thread, I'm very happy with how well the SRAM X01 12-speed chain holds up, especially considering these are high-powered Bafang M620 motored bikes (we do have shift-sensors installed).