Hi guys, currently building up a bike SRAM apex 12 spd. The set I bought is slightly used but the cassette (11-44 Apex cassette) hasn't run more than 500-600k. I bought a ZTTO 12 speed chain from AliExpress that is completely new. However the chain is skipping on the second lowest cog. Other than that it shifts absolutely crisp from lowest to highest gear and runs smoothly on every other cog. I've had about 10 ZTTO chains before and never had any problems whatsoever with the chains however faulty chains can of course be a thing but I reckon then the rest of the gears wouldn't run as smooth
Several other things I suspect could be:
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Previous owners has almost exclusively been running the drivetrain on the second lowest gear, but the gear itself doesn't show much shark-toothing, so I'd be surprised if that is was the case.
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The previous owner has missed a spacer when he took off the cassette and forgot to include it ? Couldn't confirm yet if the 12 spd xplr cassette needs a spacer between the lowest and second lowest gear but I'd be surprised if it did, as with all the other cassettes I know (many many cassettes from 7-11 spd) do not use a spacer between the lowest and second lowest cog. However it looks like the chain does not have enough space and thus is skipping and jumping
Anyone have a clue whats going on here ?
SRAM Apex 12 spd mechanical – chain jumping 2nd lowest cog
byu/Onda185 inbikewrench
by Onda185
4 Comments
It’s not the chain if it’s fine in all other cogs.
It’s (almost certainly) not a spacer issue. It would be worse than that / just visually look at the spacing. It would be extremely obvious if you were missing a spacer.
Having a single cog worn could possible cause the skipping. Visually inspect the cog as you pedal extremely slowly (with the chain on a different cog so you don’t hide it). Are any of the teeth even slighly bent or misaligned?
Many times, routing can cause misshifts that show up on the second / third smallest cog. (the first is excluded as the high limit allows an overshift to let the shift succeed). Having too short of a derailleur loop or having a misaligned derailleur hanger or having a bent derailleur cage can cause this.
Check lower limit screw. Could be set too high
It wants to climb so try dropping cable tension. If that causes problems elsewhere on the cassette I suspect an ever so slightly bent hangar.
Start with loose cable, then shift all the gears. Gradually increase cable tension until you can pass all the gears.