Chain skipping

[Giff]

I have a SRAM 12spd Eagle cassette and it has just started skipping on the 7th cog up from the smallest. The others seem ok. The chain is 50% worn. Does this sound like a worn cog / cassette or is there an adjustment that will help. Thanks to anyone who knows.

 

[steve_sordy]

Could be (send an in focus close-up pic), could be your cable tension needs adjusting. Don't be tempted to alter the end stop screws.

 

[Giff]

Hi Steve
Thanks for your reply. Here are a couple of pictures. The chain seems to ride up and not engage the sprocket. It is using an SRAM AXS wireless deraileur.

 

[Giff]

Here is the cassette with the wheel off the bike.

 

[Rod B.]

Hi Steve
Thanks for your reply. Here are a couple of pictures. The chain seems to ride up and not engage the sprocket. It is using an SRAM AXS wireless deraileur.

View attachment 92431

View attachment 92432

View attachment 92433

Giff,

The cassette looks okay to me, I don't see any broken teeth in the pictures.

I have a SRAM AXS derailleur also. Occasionally, I've accidentally I've hit the micro shift adjustment button located on the shift pod or had a rock kick up and hit the derailleur arm. This causes the derailleur shift alignment to get thrown off and will cause the chain to ride up and drop like an out of tune derailleur. It's a quick and simple process to re-align the derailleur using the microshift adjustment function. Attached is a video produced by SRAM for the AXS system. Refer to 5:57 in the video for derailleur adjustment procedures.

While your are at it, re-check the B screw adjustment just to make sure it hasn't fallen out of adjustment. The B tension screw on my AXS has backed out on one occasion. Use the B screw adjustment gauge that came with your AXS derailleur to check for correct B gap. Have somebody push down on the bike or release air from the shock so it's at correct sag, i.e. 20-30% while you check the B adjustment using the tool. Refer to 5:18 in the video for B adjustment/check procedures.

Sometimes, if you miss a shift, the motor's torque can damage a chain. To check for a bent chain link, put the bike on a stand or turn it outside down. Spin the wheel backwards and watch the chain links as they move across the small derailleur pulley. See if any of the links bind and don't bend freely around the smallest pulley. If a link doesn't move freely across the pulley, this will indicate a bent/binding chain link or a link plate that is about to separate from it's rivet.

 

[RustyIron]

When you clean your bike, you're not only removing filth, but you're able to spot potential problems before they get out of hand. Additionally, clean machinery just runs better. In this case, the cassette is jam packed with filth. Clean the cassette, clean the chain, clean the whole bike. Then lubricate and ride. Only then will you be able to diagnose whether the problem is a mechanical failure, maladjustment, or lack of maintenance.

 

[RustyMTB]

That doesn't look right.

 

[Giff]

Hi Rusty MTB Do you mean the angle of the link ?

 

[Giff]

Giff,

The cassette looks okay to me, I don't see any broken teeth in the pictures.

I have a SRAM AXS derailleur also. Occasionally, I've accidentally I've hit the micro shift adjustment button located on the shift pod or had a rock kick up and hit the derailleur arm. This causes the derailleur shift alignment to get thrown off and will cause the chain to ride up and drop like an out of tune derailleur. It's a quick and simple process to re-align the derailleur using the microshift adjustment function. Attached is a video produced by SRAM for the AXS system. Refer to 5:57 in the video for derailleur adjustment procedures.

While your are at it, re-check the B screw adjustment just to make sure it hasn't fallen out of adjustment. The B tension screw on my AXS has backed out on one occasion. Use the B screw adjustment gauge that came with your AXS derailleur to check for correct B gap. Have somebody push down on the bike or release air from the shock so it's at correct sag, i.e. 20-30% while you check the B adjustment using the tool. Refer to 5:18 in the video for B adjustment/check procedures.

Sometimes, if you miss a shift, the motor's torque can damage a chain. To check for a bent chain link, put the bike on a stand or turn it outside down. Spin the wheel backwards and watch the chain links as they move across the small derailleur pulley. See if any of the links bind and don't bend freely around the smallest pulley. If a link doesn't move freely across the pulley, this will indicate a bent/binding chain link or a link plate that is about to separate from it's rivet.

Thanks Rod. That was really helpful and I think I have fixed it with your suggestions. When I turned the crank backwards a link seemed to click/stick on one point on that problem cog. On very close inspection it seemed as though 2 teeth were very slightly bent inwards and touching the outer link of the chain (possible as I had an off the other week and the problem started since then). I tapped the tooth out a bit with a drift and also micro-adjusted the derailleur outbound as well. On the stand it seems to have fixed it but I will tell over the weekend if it is ok.Thanks again for your very detailed help.

 

[RustyMTB]

Hi Rusty MTB Do you mean the angle of the link ?

Yes, it should track the sprocket teeth. You may have a dirty drive train a good clean will sort it or a bit of chain stretch in which case, new parts time.

 

[steve_sordy]

There are no glaring burrs on the teeth, just some very small ones. Big burrs can snag on the teeth and cause problems with shifting. Of more concern to me is that the roots of the teeth are extended, which to me shows that the cassette is worn, (but not necessarily worn out). Is this the first chain on that cassette?

@RustyMTB pointed out in pic 2 that the chain is standing up, which indicates that the chain is longer than the teeth it is spanning (ie it is badly worn). If it is that worn then you should be having shifting problems in most gears. Your pic 1 appears to show that the chain is mid-way between shifting from one gear to another.

I am assuming that you are using a chain gauge (many do, including bike shops because it is quick and easy). I have never placed much faith in those short chain gauges. I very much prefer the method of removing the chain and measuring 100 links with a metal tape measure to an accuracy of 1/32 of an inch (0.06%), which is more than accurate enough. Technique is important. If your chain is brand new, then your 100 links will measure 50" exactly. If it is 0.5% worn then it will measure 50.25" a very obvious difference. I was told by my LBS that my chain was almost worn out (below 0.75%). I removed the chain and it was at just under 0.4% (50-3/16"). So I kept the chain on.

One of the problems I started to have with 12-speed transmission is that it is much more sensitive to the position of the top jockey wheel. Any bang to the mech during riding, even from dragging vegetation, can bend the mech or even the mech hanger. The hanger is designed to bend or break before the mech does, but it is tucked away and usually looks OK, until you measure the mech position with a gauge. My first 12-speed bike went back to the LBS for shifting problems I could not fix until the LBS adjusted the mech hanger alignment, which is a quick and simple thing to do.