Rear cassette/mech skipping

[grellboy]

Got a second hand (but virtually unridden) bike for my turbo. It's great, but using it on RGT cycling - and going up the 20% Paterberg - had me reaching for the granny gear. It's only 8 speed (Claris) so gear changes quite clunky anyway but on the the two largest cogs (probably 32 and 28 maybe?) there is quite a bit of slippage. It will go onto the largest cog but then slip back (and back again!) onto the next one down, making it quite hard to maintain momentum. When this happens I can the derailleur kind of jerk forward and spring back as the chain moves between these two cogs. As I said, all the parts are virtually brand new so it can't be put down to wear, so could anybody give me any ideas on what to adjust to smooth these changes? I have made a short video (no special fx, I'm afraid) Not sure if it will upload, but will do so if possible.

 

[All uphill]

The three things I would start with are:

1 Is the gear cable in good condition and free of friction?
2 Are the gears correctly adjusted
3 Is the derailleur hanger precisely vertical?

If you are handy and have access to tolls there are loads of videos to help you; if not I'd suggest a visit to your local bike shop.

 

[Ming the Merciless]

Adjust the b screw to ensure enough clearance between derailleur and big cogs.

 

[fossyant]

New cables and gear adjustment. They won't be the best cables anyway, and depending upon storage, and lack of use may be slightly sticky. Always worth replacing. if it's changing in other gears OK, then I'd go for this. Check limit screw and that the mech can shift to the biggest sprocket.

 

[Rusty Nails]

The fact that it actually goes into the largest cogs tends to show the limit screws are OK. If it slips down to smaller cogs it suggests the tension is too slack in the gear cable, but I would have expected some slippage across the whole of cassette.

It is simple to tighten the tension to find out, either using the tensioner on the derailleur or the retaining nut, but rear mechs can be a bit frustrating to sort out precisely without experience. I agree with all the comments above, although I usually associate sticky cables with not dropping back down to smaller cogs, and would look at some video tutorials before starting. A new cable is not expensive and a good starting point.

 

[grellboy]

The fact that it actually goes into the largest cogs tends to show the limit screws are OK. If it slips down to smaller cogs it suggests the tension is too slack in the gear cable, but I would have expected some slippage across the whole of cassette.

It is simple to tighten the tension to find out, either using the tensioner on the derailleur or the retaining nut, but rear mechs can be a bit frustrating to sort out precisely without experience. I agree with all the comments above, although I usually associate sticky cables with not dropping back down to smaller cogs, and would look at some video tutorials before starting. A new cable is not expensive and a good starting point.

Also of relevance (I'm guessing)

 

[grellboy]

Also of relevance (I'm guessing) is that it will stay on the largest cog if I keep the shifter pressed in. By which I mean, when pedalling it will slip back to the next cog, but keeping the gear shifter pressed in this keeps the derailleur in the correct place and stops the chain slipping down, if that helps to clarify things.

 

[Rusty Nails]

I always believe in starting off with the most common cause of chain dropping down the cassette, which is usually cable tension. It is not the only cause so then tick the other possible causes off in turn.

 

[si_c]

The fact it stays in the top gear when you shift the lever and hold it indicates the stop screws are set appropriately, but that there is insufficient tension in the cable to move the derailleur over properly. You could also not be using the same brand of cassette on the rear wheel and trainer, causing alignment discrepancies. There is also the chance that the derailleur hanger is not aligned properly, it could be a slight twist stopping it shifting into place on the lowest sprocket. Check that first.

Otherwise, solution: Reindex the gears on the trainer.

 

[Paulus]

I would still slacken the limiter screw by a quarter turn to make sure the derailuer is going over sufficiently, as the chain stays put under pressure from the shifter when held.
Then I would check the cable tension and then make sure the indexing is correctly adjusted.

 

[T4tomo]

upper limit screw and then possibly cable tension - be guided by how its doing the other changes for that