UKH

As above, I took my bike into Halfords and they think the problem might be a bent derailleur but they're not sure. They've ordered another derailleur. They don't seem to be clear at all though, just say they'll try a new rear derailleur and see if that's the problem. Does this rear derailleur seem bent to you? Puzzled how one mechanic there said it was bent, but the other didn't think it was.

 

It looks bent inwards to me. Comparing the angle of the cogs and the angle of the rear derailleur hanger.

It's a bit tricky to tell, but there's a decent chance that your derailleur hanger's bent rather than the derailleur (IF anything's bent).
Have you had any crashes where the bike's wound up on that side/ have you dropped it onto that side?

Other things to consider:
Worn rings on those gears (you probably do most of your cycling on one of the smaller rings at the back I would guess?)

Chain has stretched

Chain is too long and the derailleur's not able to take up enough slack from the chain (unlikely)

Halfords should have been able to spot chain stretch or worn rings, however the competence of their bike mechanics is a bit uneven (some know what they're doing and love it, some don't understand that bolts are generally meant to be tightened up).

Did they check the derailleur hanger was straight before they ordered you a new derailleur?

One guy said the rear derailleur was bent, the other guy wasn't sure it was. Decided to see if the new rear derailleur fixes the issue as can't get an independent place to look at it for a week or so and I need it for work.

Thanks, yeah I read about worn rings. Right, I'd cycle on the smaller rings. Food for thought, will see if they manage to fix it. If not, I'll take it to an independent place.

The picture is really poor, so it’s difficult to tell if the hanger or rear mech are bent.

There are loads of things that could cause a chain to skip, as other have mentioned above but a few of extra possibilities are;

Is the chain too long?   Are there any stiff or damaged links in your chain?  Are you riding small chainring and small cogs on the cassette?  Are the chain and cassette compatible?

Looks like the chain it too long as the bottom jockey wheel tucked in low, by adjusting the "b" screw forward might help tension on the chain.

And take s couple of links out of the chain, but test first on big ring and cogs.

Thanks, yes I did wonder if that was the case. 

Is it skipping under load? The small cogs skipping under load is the first tell tail of a worn drive-train (replace cassette and chain).

Edit to add: https://www.sheldonbrown.com/chain-wear.html. If the situation feels a bit baggy (i.e. like removing links might help) but wasn't before then the chain stretching is a reasonable hypothesis.

Hanger definitely looks bent.

Chain looks filthy, how worn was it last time you checked?

Other contender is to make sure that the derailleur moves far enough over to engage the smallest sprocket fully.

Worn chain also probable cause.

Bent derailleur unlikely - that would cause problems elsewhere too. If anything's bent, to will be the hanger not the mech.  Surprised (not really) that Halfords didn't check the hanger.  The tool's not cheap, mind

Most likely is that the screw needs to be adjusted to let the mech move over a bit more.  Hang it  up so that you can spin the gears freely.  Adjust the screw to move the rear mech out until the chain starts to clatter.  Then move it back a touch so that it runs quietly.  If it won't move over, your cable tension is too tight, so ease that off a bit.  The check all the gears.  Check the screw at the other end  - pull the cable by hand and check that the chain runs true on the largest sprocket.  Then check that the gears shift properly and adjust cable tension if not

Get yourself a chain wear checker, they're only about £3 on ebay and will save you £s on components

Put a thin straight edge along the back of the large sprocket and look to see if the hanger is parallel. This being the case, most of my sprocket jumping on a couple of cogs has been caused by the indexing being out slightly. Adjust at the lever. 
If you check the chain and find it to be worn then the worms will start leaving the can.

A bent mech can be pulled straight if you are careful not to snap the hanger, which of course it is meant to do.

that's right yeah, under load. Sorry for late reply

Oh it's a new chain

I common mistake is to fit a new chain on a worn cassette.  If that’s the case, they’ll not so with each other and will skip under load, more than the old cassette did.  If this is the case, then it’s likely that the cassette also needs replacing.  If you can, try a mate’s wheel (same type, manufucaturer, number of gears etc).

Another typical issue is fitting a new chain at full length, without considering how long it needed to be.  This would lead to less tension on the rear mech, which can also lead to skipping (and gears jumping around),  I imagine there will be a ton of videos on YouTube on how to get the chain length correct.  

And now you tell us!

If you’d said that at the beginning everyone would have told you you need a new cassette.

Either change chains regularly or buy a new cassette when you replace a chain that’s been used for awhile.

Oh didn't realise that. Halfords seem to think it's a bent derailleur. Well one guy did and they've ordered a new one. The other guy wasn't sure. Was tempted to just say don't bother and then take it to a good independent bike shop I can trust. Don't mean to be nasty but I haven't had great service.

I’d say 99% chance it’s the classic new chain old cassette problem and 1% chance it’s something else. I reckon you’d be wasting money on a new derailleur.

Did this 'skipping' problem also happen before you changed the chain?

Have you still got the old chain hanging around?  If so, you could (temporarily) re-fit it, then see if that 'cures' the skipping problem.

I'd agree with other folk, New chain on old cogs is almost 100% the issue. The old chain meshed with the old cogs but the new chain is slightly shorter and does not mesh with the old cogs which will be worn and as such the 'gap' between the teeth on the cogs is bigger. If you turn your bike upside down so its on its seat and handlebars, and then push pedal with hands and resist the wheel rotation (with another pair of hands - though can be done by yourself), you will see after half a rotation or so the chain will begin to ride up ontop of the cog teeth (because chain does not fit well) and then clunk back into the teeth again, thus skipping. You'll note that the skipping does not occur when at low loads (like pedaling on the flat) but happens LOADS when pushing uphill when using the smaller cogs. [to get the skip to occur with bike upside down as described above does require a fair bit of resistance to the rotation] Also small cogs will be worn more (a/because we use them more, and b/there is less material to wear out!). Best solution is a new set of cogs to go with a new chain, and then replacing the chain fairly regularly. OR putting back on the old chain and accepting that the system is now worn out and you'll need the full set up (which you do anyway).

I've done that on a few bikes, and ended up using pretty knackered chain and cogs for a surprisingly long period of time!

Depending on mileage done, you may have also worn out your front ring and you can get skip under load from that after new chain and cassette. Guess how I know.......mind that was circa 20,000km

I wouldn't accept paying for a new derailleur on a 'it could be bent' suggestion. I'd want the hanger alignment checked properly first. Any decent shop would do that as it doesn't take much to knock them out either vertically or longitudinal. I'd expect them to have had the check/adjust tool on first. 

I also had a look at a friends bike that had been in a bike shop that skipped like hell on small cogs, because the hanger was fitted wrong, pointed backwards instead of down 🤔

Mark

Right, thanks, I asked how they knew it was bent, whether they used a tool/machine or just eyeballed it. They said they just eyeballed it. Really haven't had good service at all. Will see what they say tomorrow but most likely will take it to an independent place. All the posts above have confirmed that's the best thing to do. 

Out of interest, did Halfords also fit the new chain onto the old cassette? 

They sound like total numptys with this guessing that the derailleur is bent business, but that would really take the biscuit.

https://www.parktool.com/en-int/product/derailleur-hanger-alignment-gauge-d...

This is the check - adjust tool. It screws in the hanger where the derailleur fits then there's a pin hat touches the wheel rim, you check top to bottom and then 90 degrees at horizontal, giving it a twist if required. 

Cheaper versions are available.....

Mark

Yes, they fit the new chain onto the old cassette.

I contacted an independent bike mechanic and he agreed from talking to him that it's probably a worn cassette. He has ordered a new one and I'm seeing him tomorrow. 

Sorry if you’ve already explained but when you say skipping what do you mean?

Jumping back and forth between cogs?

When you shift gear it skips a cog?

It is slipping on the same cog when you apply power?

Plus did you answer when did it actually start? A bent hangar or derailleur is only going happen if it’s had a hard impact such as in a crash. A bike falling over is unlikely (but possible) to damage either to any great extent.

It's slipping on the same cog. I'm taking it to an independent bike mechanic today.

It started about 2 or 3 weeks ago I think. I haven't had a bad crash. The bike has fallen over from being stood up once or twice.

That’s a classic new chain on worn cogs symptom.  Almost certainly a new cassette will resolve it.