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Discussion Starter · #1 ·
My recent experiences with my brand new drivetrain have not been great. The original spec on my mono pivot dual suspension was full XT with a SRAM 970 11-32 cassette. Bike is a unique to South Africa Raliegh - a bit like a SC Superlight/Haro in design.



As of two weeks or so ago, I have the same new chain (HG93) an SRAM 980 cassette (11-34) and LX steel instead of XT ali for the middle and small rings. Due to some initial shifting issues at the back, the LBS then changed the cable and outers.



Despite this I am still not happy with the shifting. The bike has been back to the LBS and I have tweaked it as well resulting in a relatively quiet rear drivetrain that will still shift form 9th to 8th and shift in one gear increments from 1st to 2nd to 3rd and stay there without rubbing. Its still not as good as I remember it when the old drivetrain was still newish though. I can only think that the rear mech is a bit worn but that’s mighty quick wear in my book. It also ran for a while when the old chain was two links short. Could this have damaged it?



The most irritating problem is not this but rather something I never ever experienced on the bike before – the chain slipping off the middle ring when standing up and pedalling – eg to negotiate a gentle uphill switchback with a few roots/rocks to compress the suspension. This has happened several times, almost leading to a painful face plant . I can’t tell if its coupled with the chain trying to shift at the back on its own, but I can’t see how that would cause it to pop off the brand new middle ring.

It has been suggested to me that my rear deraileur pulleys may be worn. The top one turns more stiffly than the lower one but thats because of the seals isnt it?


Any drivetrain gurus or wannabe’s please feel free to comment at length
 

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So to recap, you've got a brand new drivetrain (chain, cassette, and 2 chainrings), and your chain is jumping on the cassette and skipping off the middle ring under load?

First suspect for me is the chain. Have you measured it to ensure it is indeed new? Not that I think this would be a factor, but are you certain it is sized correctly?

I really can't imagine the derailleur being responsible for what you are describing, unless it is truly tweaked or your frame has a bent derailleur hanger.

 

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Discussion Starter · #3 ·
Thanks, I have checked the chain - its indeed shiney and new and the same lenghth as before. That was of course with a 32t rear cassette. It does howver seem to be quite long enough to shift into just about any combination without overtaxing the rear mech spring so I don't think thats the problem...
 

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headshot said:
Thanks, I have checked the chain - its indeed shiney and new and the same lenghth as before. ...
There's a likely cause. You went from a 32t to a 34t cog, and didn't change the length of your chain. It's too short.

Also look for a tight link. Either flip the bike over, or place it in a workstand (which ever you have) and slowly pedal. Watch the RD, and see if the cage jumps when a link hits it. I had this happen to me once, and it drove me crazy out on the trail. Even my LBS didn't catch it, and those guys are good. I was cleaning my drivetrain and saw it. Worked on it with lube, a chain tool, and a screwdriver. Got it sorta working then said screw it and bought a new chain. Running smooth ever since.
 

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alphajaguars said:
There's a likely cause. You went from a 32t to a 34t cog, and didn't change the length of your chain. It's too short.
Naw, that wouldn't be it... the drivetrain never "knows" a chain is too short until it is operating in that big ring / big cog range, just at the moment something falls to pieces.

A drivetrain will perform badly if the chain is too long, but only in certain small-small or mid-small combos where there is simply too much excess, and a serious lack of tension from the derailleur.
 

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Discussion Starter · #6 ·
I had that kinked link problem before - thats what led to the shorter chain for a while. The RD is only a year and 4 m old so i doubt its knackered, unless the new 9spd Shimano stuff is weaker than the old 7/8 spd. Still the fact that the chain leaps off the middle ring makes me think sonething isnt right with it...
 

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For the rear on a FS bike, the indexing is best done with a rider on the bike. Get it shifting OK on a stand, but then take it down the road to dial it in. I gave up trying to figure the correct 'compensation' for the sag on my FSR's.
 

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SteveUK said:
For the rear on a FS bike, the indexing is best done with a rider on the bike. Get it shifting OK on a stand, but then take it down the road to dial it in. I gave up trying to figure the correct 'compensation' for the sag on my FSR's.
Steve, your post reminded me of a shifting adjustment issue a few riders reported on the NRS in '02 or so. The solution was to run full length housing along the shock stay to the derailleur, which many reported corrected the problem.

This dosen't necessarily account for the chain mysteriously jumping off the OP's ring during a climb, but hell, it's a start.
 

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And yet I've heard some folk berating single length outers on FS bikes specifically because it was affecting their shifting!! I run it on my FSR (no choice, but it's what I'd do anyway) and don't have any trouble at all, even on big hits. The maintenance benefits of single outers are obvious, it just appears that fitting them on a FS bike might not always be straightforward....

I freed myself of front gears many years ago, so can offer nothing by way of ideas for the OP's problems in that area.
 

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Discussion Starter · #10 ·
As far as the rear shifting is concerned, I always tweak this while riding. I have managed to get the shiting perfect in the 1-4 range (the area where it was playing up) but when its perfect there it wont shift from the 11 tooth to the next cog - basically you have to shift two up and 1 down to get there.
At a race this weekend one of the experienced rider/tech experts, told me that after a while a rear dr often goes this way - ie hard to adjust across the range. He suggested that the cage return spring could be a bit slack and suggested I reset the spring for greater tension. I am going to try this and will report back shortly...
 
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