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Skipping under load
Hi there,

I've got a 2008 Scott Sub 30 which has been ok apart from a common problem i'm seeing which tends to appear between services.

I usual just get what feels like the chain skipping when on the hardest gear with the bike under load. I don't ride the bike hard if at all while the gears are crossed over. It never actually skips down or up gears.

After a service (just lube and gear adjustments i think) the problem went away for a month or so (even without a great deal of riding). At the time they said the cassette and chain looked ok and that it might be the non-serviceable parts in the freehub. It now appears to be happening even in some of the easier gears (say the 2nd front sprocket) and most of the gears apart from the hardest ones are making a great deal of grinding and noise when peddling.

Sounds almost like its trying to change up or down and can't quite make it which kind of makes me think its just a gear adjustment issue.

I normally get the thing serviced every now and again but i reckon i this problem is requiring me to get more services than i should actually require and it is just dérailleur tuning they i can probably just do this myself without the cost and hassle of leaving my bike in the shop for days while they do a full service.

Any ideas what might be causing this ?
Well my brother had the same problem on a Trek fuel ex 7. A new chain solved his problem. I'd suspect your cassette was wearing down, but the shop would have mentioned that to you first. I've never actually had a freewheel fail on me so that would be something new for me.

It's a little hard to tell if a cassette is worn just by looking. The most common source of this problem is worn chain, cassette, or both. It could also be the ratchet inside the freehub, but those typically last longer than a year so it would be a little odd.

If you can get it to slip at slow speed (going up hill in a high gear, etc.) and have someone who can jog next to you, they might be able to see what's happening. If they can see the chain slipping up and jumping over the teeth, probably time to replace chain and cassette. If the chain doesn't jump but the whole gear cluster spins forward for a second and then re-catches, it might be the free hub. If your friend is directly behind you they might also be able to see if the chain is jumping between gears which is a different issue entirely. Good luck

Check the indexing on the rear derailleur… I get similar problems after fitting a new cable as it stretches. You say it happens after service, do they then also fit new cables? The bike’s too young to have a worn drive train (chain and cassette), but I don’t know how hard you ride it…

I’ve (just) replaced rear cassette, chain and middle front chainring with SRAM kit and the same can occasionally happen to me. I will be looking at readjusting the indexing this bank holiday to address the situation. At the moment it can be controlled by slight adjustments on the shifters: i.e. they’re no indexing correctly…
I got the chance yesterday to re-index the rear derailleur and that indeed has solved my problems (BTW my bike is a good quality ten-year-older). Sometimes a little tweak here-and-there improves the performance tremendously. Now I’m cycling to work everyday (Summer time, yippee), I find I make adjustments at least twice a week… Try it for yourself and see if you can improve the slipping…
Good luck (and let us know)…

I have a Trek 2005 Fuel Ex9 (bought used-many miles) - it shifts fine on flat land but once I go up hills, especially hard it feels like it's skipping gears or something, with loud clanking noise.
It's really ruining my MTB experience. My old 1998 Trek MTB is a lot more solid and cheaper.

I've had it it inspected, tuned, and parts replaced at 3 reputable shops (including a local Trek store)
and it still does the same thing.

Parts replaced: rear shift cable, chain, Shimano XT derailluer, Shimano XT cassette

I have even gotten good at adjusting the H & L, b-screw, and fine cable adjustments - but it does NOT help (going up hill)!

Another cycle store suspected the XT dual shifter - recommended clean/lube it or replacement

After looking at this forum, I suspect the freehub/wheel as well. Can this be serviced, ie cleaned and lubed? How would this cause the symptoms?

The chainring seems okay by visual inspection
Freehub can sometimes be serviced, that depends on the brand, though. See other thread for my assumption.
Yeah, i would definitely suspect the freehub. I had a freehub body that had worn away from lots of use.
My opinion: chain rings.
@ Jordan: which part of the freehub body was worn away?
The notches on the freehub body were worn and the cassette was slipping over it. It was an aluminum freehub body.
Yeah, I (so far only) read about this, I try to avoid too lightweight stuff. He'd have realized that when changing the cassette, though. Well, hopefully... it'll also make removing the cassette quite a pain. I believe fuzzrock was thinking about the ratchet mechanism, in which case trying to free it up with some penetrating oil might work when it cannot be disassembled (didn't specify the type).

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