Bicycle Mechanics - Jumping chain under load mostly in cogs 4-6

I've just put together my ALAN model cross bike. It's set up as a 9 speed, cassette is 12 - 32. The Sora long cage is new as is the KMC 9 speed chain. The Veloce compact crank and (probably SRAM, lower end) cassette are mildly used. The 9 speed indexed DA bar end shifter is new.

On my trial ride the lowest and highest cogs were smooth but the middle ones 4-6 jumped like crazy. Under load they'd jump/crunch up and down a cog every 1 - 2 seconds. I did run the RD adjuster back and forth with no improvement.

I'll look closely tonight at the cogs but the cassette was perfectly clean and I suspect very lightly used.

I intend to switch to friction mode for a trial run. My Fuji has a wheel with large range cassette (probably 12-30) that I'll also try.

Does anyone have other suggestions on diagnosing this shifting issue?

Thanks

Nine out of 10 times this is a symptom of a worn chain or cassette or both. It's especially likely if the skipping is on the most commonly used sprockets.

It could also be derailleur related or a mis-aligned hanger, but a tip off is how and when it skips. Worn chains or sprockets will be OK and skip under load as the chain slips forward and climbs out. Derailleur or hanger issues will show up at any load and can usually be heard or seen as uneven feeding of the chain onto the sprocket.

I have a similar problem with an old schwinn I'm restoring... I put a new chain on it and it only slips in one gear in the rear... however the teeth don't look very worn. I haven't tried it yet but I suspect it could also be related to chain tension. I'm going to try and take a link out of the chain before replacing the freewheel.

Would a chain that is slight to long also cause the same issue?

Would a chain that is slight to long also cause the same issue? [skipping under load]

No. A derailleur system involves a tight chain pulling the sprocket on the top, but the bottom is basically slack with minor tension provided by the RDs idler cage. If it depended on any tension on the bottom loop it simply couldn't function. Skipping has two general causes.

1- chain pulls forward relative to the sprocket under load and climbs out. This happens only with with wear as the chain pitch increases and the shape of the sprocket no longer holds the rollers down at the bases of the teeth. You can see this by using a small screwdriver and lifting the chain away at the midpoint of the wrap. A new chain won't lift because the pitch is just enough to reach the next position, but worn chains or sprockets allow the chain to pull up on the nearby teeth allowing it to lift away.

2- chain isn't fed properly and instead of engaging the sprocket lifts and rides the tops of the teeth. When this non-engaged section comes around to the top the tension on the upper loop pulls it forward causing a skip. If in the meantime it had settled back at the bottom the skip will be short, usually only one inch or so. If the chain is still riding the tops it can skitter along there for a while before catching and dropping back into engagement. This was a common problem before index when shifting under load.

Nine out of 10 times this is a symptom of a worn chain or cassette or both. It's especially likely if the skipping is on the most commonly used sprockets.

It could also be derailleur related or a mis-aligned hanger, but a tip off is how and when it skips. Worn chains or sprockets will be OK and skip under load as the chain slips forward and climbs out. Derailleur or hanger issues will show up at any load and can usually be heard or seen as uneven feeding of the chain onto the sprocket.

On the stand it indexed smoothly through all 9 cogs. The chain is new; I'm not sure when I got the cassette but it was spotless and looked new. I'll check it tonight. It's a lower end, probably SRAM, so a higher quality replacement would be good.

On the stand it indexed smoothly through all 9 cogs. The chain is new; I'm not sure when I got the cassette but it was spotless and looked new. I'll check it tonight. It's a lower end, probably SRAM, so a higher quality replacement would be good.

Read my post above, and start by deciding whether you're dealing with #1 which I call a load skip, or #2 which I call a feed skip. The causes and cures are totally different for the two so deciding which is the first step.

if the cassette was worn and new chain or viceversa you would not notice it in the stand.

but I agree it may be your cassette.

PS sounds like a great ride.

A cog can look brand new but still be worn significantly enough to skip on a new chain. You can't tell by visual inspection until the cog is grossly worn. The real test is how it shifts and runs under load. If it skips, it's appearance is meaningless.

Read my post above, and start by deciding whether you're dealing with #1 which I call a load skip, or #2 which I call a feed skip. The causes and cures are totally different for the two so deciding which is the first step.

#2 chain not feeding properly - Do you mean not threaded through the RD, not aligned (RD or drop bent), or not adjusted correctly for indexing? I don't think that applies in my case - it's probably #1. The chain only skips in the middle cogs and only under load.


if the cassette was worn and new chain or viceversa you would not notice it in the stand.

but I agree it may be your cassette.

PS sounds like a great ride.

I'm hoping she'll be a great ride. I posted in C&V (no pics yet). I also have a issue with the front fork (Al) shuddering on braking. I'd hate to have to use mini-Vs since the cantilevers are so sharp and work great. A fork mounted guide won't work (no mounting hole on fork). Any comments on the downsides of the shudder apart from being annoying - I suspect it's bad for the headset.

#2 chain not feeding properly - Do you mean not threaded through the RD, not aligned (RD or drop bent), or not adjusted correctly for indexing? ....

I mean not feeding onto the sprocket evenly and tending to ride the tops of the teeth. Only what happens between the chain and sprocket matters. What happens at the RD level before it feeds onto the sprocket may cause the chain to climb up, but isn't the problem itself. You can see it when the feeding section stutters as it engages instead of settling smoothly to the roots of the teeth.

I swapped out the wheel/cassette for another one and the skip went away. I suppose the next step is swap cassettes and see if the trouble follows the cassette or wheel. The smart money appears to be on the cassette. Could it be improperly installed?

I swapped out the wheel/cassette for another one and the skip went away. I suppose the next step is swap cassettes and see if the trouble follows the cassette or wheel. The smart money appears to be on the cassette. Could it be improperly installed?

Not much you can do wrong installing a cassette that would cause skipping. And the wheel is also innocent unless it isn't chain skipping after all and there's slippage in the freewheel ratchet.

Switch the working cassette to the wheel you want to use and it'll be fine. Give yourself some peace of mind before spending dough by putting the suspect cassette on the other wheel and confirming that it is indeed the cassette itself.

That's what I did this morning. The suspect cassette failed dramatically on the alternate wheel. The alternate cassette worked smoothly on the original wheel. End of case.

I ordered a nice SRAM 11-32T today. Now to mount the cantilever brake cable guide lower to alleviate fork shudder.