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I'm riding an 09 Genesis Vapour, original short cage 105 rear mech, Sora compact. I've been using it as a wet weather bike/commuter with a 25-12 cassette, but when I wrecked my MTB recently I put the Cross tyres back on and changed the cassette to a Shimano 28-12 so I could go XC on some steeper tracks.

The existing chain (2 months old, not worn) skipped a bit on a few gears, so I replaced it, but no joy. Now the cassette and chain both have about 20km on them. While the highest and lowest gears are fine, all the gears in the middle of the range skip badly under load. I've replaced the rear gear cable, stripped and cleaned the rear mech jockey wheels and as far as I can tell, the indexing is OK and if I put the old 25-12 cassette back on the problem goes away, even with the new chain.

My thinking is that the 105 short cage mech can't handle 28-12, even though it has no problem in getting to the innermost cog. However the guy in the (Edinburgh Bike Co-op) shop reckoned it would be fine, but the 32-12 wouldn't. This seems to be the common reasoning elsewhere too.

Any suggestions as to what to try next? Chain length? Derailleur hanger angle adjustment? New jockey wheels?

7 comments

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stewieatb [292 posts] 4 years ago
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Hmm. Have you tried re-indexing the mech from scratch? Odd that it's the middle range that's going wrong.

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Gordon Watt [6 posts] 4 years ago
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When I put the new cable on I removed cleaned out the barrel adjuster and reset the indexing from scratch, so I don't think that's it.

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stuke [335 posts] 4 years ago
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What condition are your chainrings in? i had the same issue after replacing chain and cassette a couple of years ago on one of my bikes, it only happened in the big ring though. I replaced the chainring as the teeth were pretty worn and it fixed the problem straight away.
jockey wheels could be worn and leading to this problem also

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arrieredupeleton [574 posts] 4 years ago
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Apologies if this is stating the obvious but have you considered the 'B-screw'? The upper pulley needs to track the cogs of cassette as close as possible and if you have a widely spread cassette there will be a wider gap in the middle compared to the extremes (12 and 28?) at each end. Think of it as a concave section. I've had a similar issue with a 11-28 Ultegra cassette I bastardised.

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Gordon Watt [6 posts] 4 years ago
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Genius - thanks for that. I'll have a go this evening but that makes a lot of sense...

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Gordon Watt [6 posts] 4 years ago
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Yay! B-screw loosened a couple of turns and all is good in 28-12 world. Thanks for the suggestion...

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arrieredupeleton [574 posts] 4 years ago
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Glad it worked out for you Gordon. Looks a nice bike that Genesis.