Numpty gear compatibility and chain length question

[srw]

I've put together a (nearly all) brand new all-expense-spared drivetrain for the old mountain bike that's been hanging around for far too long. But I can't seem to get a good clean shift, no matter how much playing around with the adjuster barrel I do, and there's quite a lot of chain jumping under a bit of stress.

Ah. I've just looked at the boxes, and I think I might just have spotted the embarrassing schoolboy error.

The cassette is an 8-speed Deore thing - something like 12/35. The front mech and derailleur is the original mountain bike Deore triple. The rear shifters are Tourney 8-speed Revoshifter.

And the rear derailleur is Shimano Deore RD-M592-SGS with a little "9sp" logo on the box. That means 9-speed, doesn't it? And 9-speed isn't compatible with 8-speed, is it? It's smaller shifts, IIRC.

Oddly, I can get a pretty clean set of shifts on the big ring, which is why I wondered whether shrinking the chain would help.

To save my blushes, could someone link to the right 8-speed derailleur for me?

 

[Tim Hall]

But aren't the pulls dictated by the shifter, rather than the mech (at least for 8 and 9 speed)?

 

[Tim Hall]

Pace screenman, have you checked the hanger alignment? And did you use new cables?

 

[Flying Dodo]

Tim's correct. You can use a 9 speed derailleur with an 8 speed cassette. I'd look carefully at the alignment.

 

[srw]

So it's a different kind of numptiness...

It's a brand new derailleur hanger, derailleur, and cable inners. Bent anything seems unlikely after less than 50 miles over mostly unchallenging terrain. I'll try the loosen-everything-and-retighten-it to make sure it's seated correctly technique.

To come back to the implied half of my question - does chain length matter much?

I'm expecting something a bit more agricultural than my nice fancy internal gears and Ultegra on my other bikes, but not this agricultural - it's noisy and I'm getting regular chain skips.

 

[Tim Hall]

Stiff chain link? I don't think too long a chain would give such symptoms.

 

[mjr]

Capacity is 45 teeth and range is 11-36 so it's not that. Chain ring capacity is 22 teeth so what are you using?

I'd expect more skips from a too-short chain than a too-long but how did you size it?

 

[srw]

I haven't counted the chain-ring teeth, but I'd be surprised if it was as low as 22 - and I've mainly been riding on the flat, hence middle and large rings. I simply got the chain out of the box and put it on - and decided it looked about right! I'll check for a stiff link; it's a KMC chain with a quicklink, so that might be a culprit.

 

[srw]

Oh, and one other thing - the gear changes unwillingly. In order to get from 2nd to 3rd I often have to go up to 4th and then back down. That could be a symptom of improper adjustment.

 

[Tim Hall]

Changing up is , rapidrise mechs excepted, done by the sprng in the mech. This can get affected by dirty, worn or bent cables as they add friction to the mix, which the spring has to overcome. You say new inners. What of the outers?

 

[srw]

Changing up is , rapidrise mechs excepted, done by the sprng in the mech. This can get affected by dirty, worn or bent cables as they add friction to the mix, which the spring has to overcome. You say new inners. What of the outers?

Ah. I was lazy. I bought, but didn't bother with, a new set of outers. I have a feeling another fettling session is on the cards.

 

[Ajax Bay]

there's quite a lot of chain jumping under a bit of stress

does chain length matter much?

it's noisy and I'm getting regular chain skips.

I simply got the chain out of the box and put it on - and decided it looked about right!

Not addressing the shifting issues which others have given advice on (except agree with @Tim Hall 's outers suggestion - it may be that dropping GT85 down both ends of each old outer bit may help, but better off just replacing).
When you say 'jumping' and 'chain skips' I'm assuming you mean that 'under stress' the chain 'gives' ie this is a fore/aft issue not a 'shifts onto a different sprocket' issue.
I often refurbish bikes like this. And often the cassette and chain have worn, the latter more than the former. From your quotes I infer that the cassette is the 'original' (if this is wrong then clearly my advice below is nugatory). Overall chain length matters, not because it will cause skating but because it needs to be long enough for large/largest (inadvertent) and short enough for the RD to wrap without cage/chain interference small/smallest (inadvertent). Chains (in a box) are invariably sold long enough to cater for a 52/36 and longish chainstay, so one nearly always needs to take a few links off before using the quicklink. You can undo the quicklink quite easily, take out a few links with your chain tool, and then reuse the quicklink to reconnect. AASHTA.
Consider changing the cassette. If it has significant wear caused by the old chain, then the new chain you've put on will skate, as the symptoms you've described suggest.

 

[mjr]

You can undo the quicklink quite easily, take out a few links with your chain tool, and then reuse the quicklink to reconnect.

Check the quicklink - not all are reusable. I even encountered a single-use single-speed quicklink on a Taya chain recently! They give you two but was anything really wrong with the 3-part quicklink for single-speeds?

 

[Ajax Bay]

KMC connectors link, but other discussion suggests that even the links specified as non-reusable are, in practice, reusable.

 

[srw]

Not addressing the shifting issues which others have given advice on (except agree with @Tim Hall 's outers suggestion - it may be that dropping GT85 down both ends of each old outer bit may help, but better off just replacing).
When you say 'jumping' and 'chain skips' I'm assuming you mean that 'under stress' the chain 'gives' ie this is a fore/aft issue not a 'shifts onto a different sprocket' issue.
I often refurbish bikes like this. And often the cassette and chain have worn, the latter more than the former. From your quotes I infer that the cassette is the 'original' (if this is wrong then clearly my advice below is nugatory). Overall chain length matters, not because it will cause skipping but because it needs to be long enough for large/largest (inadvertent) and short enough for the RD to wrap without cage/chain interference small/smallest (inadvertent). Chains (in a box) are invariably sold long enough to cater for a 52/36 and longish chainstay, so one nearly always needs to take a few links off before using the quicklink. You can undo the quicklink quite easily, take out a few links with your chain tool, and then reuse the quicklink to reconnect. AASHTA.
Consider changing the cassette. If it has wear caused by the old chain, then the new chain you've put on will skip, as the symptoms you've described suggest.

Thanks for the suggestion - but it is a new cassette. Been there, done that....

I'll need to get the bike up on a stand - which won't be until next week at the earliest. I'll replace the outers and do some checking for proper seating and see what that does, and then see whether the chain needs shortening.