simonkinahan

I cannot figure this one out. I must be missing something, so hopefully some forum members can point me in the right direction.

I have an 24-speed Specialized Rock Hopper from around 2006. It was skipping between the the smallest and second smallest rear gears regardless of cable adjustment. The gears were very worn, so I switched out the rear cassette and the chain. Initially it seemed this fixed the problem, but after a few rides the problem has returned in a new form.

Now, I can get the shifting perfect on the repair stand by adjusting the barrel adjuster, test ride the bike and it seems fine, but as soon as I ride more than a mile or so it will either start to skip from the smallest to second smallest gears, or vice versa. What could be causing this?

Iride01 

Smallest gears being the smallest physical size or the lowest ratio gears?

simonkinahan

Sorry for the ambiguity - smallest physical size

Andrew R Stewart 

The first thing I think of is that the replacement cassette probably fits on the hub slightly further out from the spokes. So that the high gear limit screw is no longer set correctly. I suspect that screw is not letting the chain run onto the small cog as smoothly and as centered as it should. With soft pedaling the "trying to sort of shift but not quite" limit screw setting generally won't be an issue. But add pedal pressure and this slight misalignment can be the cause of the chain trying to shift but not making it.

While the industry does try to make every cassettes of the same cog count fit on the freehub body exactly the same, this just isn't the real life case. So with a different cassette (and/or wheel) one should double check the der adjustments and make (minor usually) corrections as needed. Bicycle gear systems are not a plug and play system. Close but not exactly. It's up to the person doing the work to confirm and correct as needed. Andy

simonkinahan

Thanks for your help Andrew. You may be right. I reset the H limit screw and re-indexed the rear derailleur this afternoon, then rode for amount a mile and no switching back and forth between 7 and 8 so far. It has seemed fixed before and then the symptom has come back, so I will do a longer ride tomorrow and see how it stands up to more hard peddling/

schwim

I think you'll find this to be the root of your problem. Drivetrain components wear in sympathy to their siblings. As the chain slowly stretches, the teeth on both the cassette and chainrings wear at a rate that keeps everything in sympatico with the other parts. If you replace only some of these parts, you'll find that those that you didn't replace do not play well with the newer arrivals. You changed the cassette and chain because they were "very worn" but it seems you didn't replace the chainrings up front. I have had the same issue occur as you in the past due to the very same offering of new parts and the issue was resolved once I replaced the front rings. You're likely only noticing it at the smallest gears on the rear because those have the least teeth to try to make the chain behave.

If I were you, I'd replace the chainrings as well. I posit that will resolve your issue.