UKH

In need of some technical advice... For the last few weeks whilst out riding, whenever I hit a steeper uphill section and put a little more power on, my chain jumps/slips (feels like it has snapped). Just enough to scare the life out of me that the chain has snapped and I'm about to come off, then all fine again. As far as I can tell the gears etc. are all functioning well and I'm pretty up on my (very) basic maintenance and regular cleaning etc.

Any suggestions? I wondered if the teeth on the derailleur sprockets might be worn. Is this even a thing? 

How old are the chain, the chainring and the cassette?

Worn chainring would be my guess. Certainly mis-match in wear between chain/cassette/chainring.  Feels horrible and scares the shit out of you. 

I'd agree with the others bit would also check that you haven't got a seized or stiff link in the chain

I would check that the gears are indexed (ie the chain aligns nicely to the gears) and then that there aren't any stiff links as above. If not then likely the cassette and or chain are worn. 

I would check:

Chain hasn't stretched,

Cassette isn't worn (caused by not changing chain enough)

Chainrings aren't worn (caused by not changing the chain enough - but rarer than cassette wear)

Remove cassette and freehub body, check the freewheel - clean it, and look for signs of wear and/or sticky pawls cleaning and regreasing (not to heavy a grease) should sort this.

Lastly I would look at dérailleur sprockets... I've had these go and cause issues, but iirc symptoms were general noise and jamming and poor shifting

ETA: If chain is worn to extent it's slipping probably worth changing cassette as well. If cassette is worn then make sure you do the chain as well, if chainrings then most likely you will need to do the lot... worn cassette and/or chainrings will rapidly stretch a new chain.

If its jumping off the chainring, my guess a worn chain ring. Just had similar on one of my bikes. Changed the chain and cassette before realising it was the chainring! Changed the chainring (actually whole chain set as it was out of production XTR) and all was fixed.

Check chain stretch first. You probably need a new chain and cassette. Maybe chainrings too. Can you share a photo?

I'll third what has been said previously -

If it jumps under pressure then it is most likely a worn cassette, chain and chain rings, change all components together otherwise it will wear faster. Personally I find the cassette wears faster but the chain rings jump earlier.

But if the wear isn't too bad consider the gear indexing is dodgy (this will be making noises even if not under load though) and other culprits I've had were a bent derailleur hanger (flexed under load) and a single bent tooth on the middle chain ring which would snag the chain under load and throw the chain.

FWIW I found SRAM chains last way longer than Shimano ones...

Get a chain wear tool or take to your LBS for assessment. If the chain has stretched by less than 0.75% it may not need a new cassette.

There are other tests you can Google to see if you need a new cassette or chain ring.

Look at the wear on the teeth of the chain ring, does it have a characteristic hooked wear profile? 

And if you put pressure through the pedals with the brakes on, can you see the chain riding up the teeth of the cassette?  You can only really tell this one once you've changed the chain; if the cassettes worn you'll still get the slipping and this test will come up positive. 

Definitely sounds like worn out chainring and or cassette. New chainring chain and cassette do the job

Thank you everyone for all the advice, sorry for the late reply! From what you are saying it does sound like a worn chain ring - the jumping under pressure etc. 

How fast would you expect them to wear? I've only had the bike for 5 months (granted its done a few miles since then). Is that about right or would it just be shitty parts? I've upgraded a fair bit on the bike but not touched the chain/gears.

Chains wear first, then cassettes, then chainrings, usually the outer for me.

I get about 3 chains to 1 cassette and 3 cassettes to one chain ring.

if it is worn so much that it is slipping then I would guess chain and cassette as a minimum. These things do wear out and need replacing, bit like brake pads and tyres.

Age is not really relevant. Mileage and in particular usage are more important. a dirty chainset can work like grinding paste and reduce the expected lifespan considerably.

Depends!  How often do you clean the chain and gears?  How often do you ride through mud/dirt/dust?  Have you ridden through the beach of slag near Peterlee?  5 months seems well early for road riding, correct alignment and regular cleaning.

She gets cleaned fairly regularly, but mud and dirt is pretty much an everyday feature. Sorry should have specified we're talking off-road riding. Maybe a new chain is the way to go first. 

A new chain can make things worse if the cassette is worn. I would tend to replace both chain and cassette if the cause was wear. However:

Since you say it is a relatively new bike...The symptoms you originally described sound similar to what was happening to my bike (after a few months ownership) it was caused by cable stretch, and was driving me mad,  I fixed it by realigning the chain at the rear derailleur. (2 minute job that I'd put off doing for a month)

If you look at the teeth on the cassette, do they look symmetrical or are they shark fin shaped? If the latter then you've worn the cassette badly. At 5 months old I'm surprised this would be the case unless you've done a lot of miles or you abuse the bike a don't clean it regularly or ride in really bad conditions. 

The teeth on the cassette look OK, wouldn't say there was much wear there. Like you say for the age of the bike I'd be surprised if I'd worn them that much, but then again who knows what standard the factory parts are nowadays? 

Would that interfere with the gear shifting as a whole? She switches through the gears pretty smoothly at the moment but maybe a misalignment or cable stretch is more likely given the bikes age... Stripping her down this afternoon to see if I can get to the bottom of it

Cables might stretch a little but cable outers settle in the ferrules and everything slackens and needs tweaking soon after you buy the bike.

Chains don't stretch, they wear.

clean chain, chainrings and cassette and lubricate. Then check front derailleur setting. I'd be surprised if you've worn it enough in 5 months to need replacing. It took 5 years of abuse and sandy filth to wear out my chainring.

Agree. The chainrings should be Ok at 5 months. Check chain, replace if  stretched and go from there. If it's not worn chain/casette, then it might be the freehub.

Derailleur misalignment might cause some skipping between gears, but tends not to be the more violent lurch you get from a worn drivetrain when you put the power down.

If the alignment isn't right, correcting it is not going to cause a problem, if its fine you know straight away as it will cause a problem and you put it back. The adjustment I'm talking about is achieved using a small tensioner on the rear derailleur and requires no tools or major effort. If it works, great, if it doesn't, look at other things. I only suggest it as your symptoms (as described) matched mine very closely.

...and referring to it as stretch is standard parlance in cycling when talking of wear.

"Chains don't stretch, they wear."

I think we may have reached peak pedantry now at UKC... (or should that be peaks pedantry)

Five months sounds short - are we talking crappy unbranded components? In which case, replace.

Good advice above

I may have missed someone saying it but also get a chain wear indicator - they're only a fiver

Use it often enough and you get more life out of the cassette & chainrings

Chain and sprocket wear has nothing to do with the time span. 
I cycle off road 2 - 2500 miles a year in all conditions and replace my chain and chainring 2 - 3 times a year. A mate who raced mountain bikes changed his chain every two races. 
You could just try for a new chain but if the chainring is worn you will get chain suck. I once changed everything only to find that it was the pawls inside of the free hub that were sticking open.