Cassette and chainring question?

jasonmarkjenkins

Hi i'm new to mountain biking but recently purchased a bike which is great but there has been issues with the rear cassette and the chain jumping under strain on certain cogs?
I took it to a local bike shop who inspected it and said they look fine they just need a clean so stopped at another bike shop who told me the cassette and the front chainring are both knackered.
I have included a few photos and would be very grateful for any advice and opinions






.
Thanks.

karljeff72

I would give the cassette and chain ring a good clean also the chain,if you can remove them and let them soak in some degreaser for a few hours,use a brush to get the oil and grime off then reinstall,if the problem is still there then new parts needed.

reaperactual

Hard to tell with the pictures but cassette teeth profile looks shallow and undefined in the big cog and other most used gears.

The chainring doesn't look as bad but the teeth profile looks to be a little 'shark fin' shaped so not great.

Would recommend replacing cassette, chain and chainring too. Leaving the current chainring on could wear down a new chain and in turn a new cassette prematurely so not worth it for the sake of £20ish for a new one.

I assume the bike is second hand so you don't know how many previous chains the chainring has been used with it, if any.

Generally if you buy a second hand bike you should expect to have to replace these drivetrain components anyway unless you know for sure the parts are new or have been replaced recently by the previous owner before being sold.

steve_sordy

The chainring is definitely worn, you can see the sharkfin appearance in your final photo.
The cassette is harder to tell (photos not too clear), but the gaps between the teeth are elongated - some gears rather a lot.
I doubt a new chain would run on that combo. You need a new chain, new cassette and a new chainring.

if you are not going to do the work yourself, plus setting up the shifting, do not take it to the LBS that said they looked fine.

lesfirth

Sorry Jason but the second LBS are right. New cassette ,chain and chainring are needed.

reaperactual

steve_sordy said:

if you are not going to do the work yourself, plus setting up the shifting, do not take it to the LBS that said they looked fine.

Good point, what was that first LBS looking at when they said it looked fine?

jasonmarkjenkins

Thanks for the replies. The cassette is a cs-m771 model does this mean i need to get the same one or are most shimano cassettes compatable?
As for the chainring it currently has a raceface on it so wondering what other makes are out there?
The set up on it is 30t chainring and 11-42 cassette which i find ok but when on flat ground in the lowest gear my legs seem to be going to fast with little progress and resistance am i right in thinking i need a slightly different set up?
Sorry if i'm asking to many questions..
Thanks again

reaperactual

Short answer on cassette is any Shimano same speed mtb cassette would be a suitable replacement and work with your current derailleur and shifter set up.

There is a lot of choice for replacement chainrings and for your specific low gearing problem. A bigger 32, 34 or possibly even a 36 tooth chainring will help, providing it has the same bolt circle diameter (BCD) most will fit.

The only restriction to a bigger chainring will be frame chainstay clearance and leaving at least the safe recommended 6mm gap to account for any flex when cranking hard.

You also need to be aware that the bigger the chainring the more resistance and better top speed which will be at the expense of your low climbing ratio and making steep climbs more difficult.

jasonmarkjenkins

reaperactual said:

Short answer on cassette is any Shimano same speed mtb cassette would be a suitable replacement and work with your current derailleur and shifter set up.

There is a lot of choice for replacement chainrings and for your specific low gearing problem. A bigger 32, 34 or possibly even a 36 tooth chainring will help, providing it has the same bolt circle diameter (BCD) most will fit.

The only restriction to a bigger chainring will be frame chainstay clearance and leaving at least the safe recommended 6mm gap to account for any flex when cranking hard.

You also need to be aware that the bigger the chainring the more resistance and better top speed which will be at the expense of your low climbing ratio and making steep climbs more difficult.

Where i ride there are lots of climbing and decects but it's mosty moderate flat forest roads so i'm hoping to find an all round set up.
If i bought a 32 and 34 to try would i have to change the chain length for each?
Thanks

mully79

Is your bike a full suspension or hard tail ?
Do not even think about just changing the chainring even for a trial without buying a new chain.

Therefore, the answer to your question is to set the new chain length for the 34t then reduce length should you try a 32t.

If a full sus you need to check that the chain is long enough with the shock fully compressed that it doesn’t over extend the rear mech.

Added complication on a full sus is changes to chain ring size slightly affect suspension anti-squat performance. Most seem to be designed around a 32t so you are unlikely to notice but definitely a consideration going to a 36t.

reaperactual

I'd say you might be better off buying and fitting a 34t chainring with a new chain first and see how it feels for where you ride.

The 34t may be perfect and you will be happy with it so wouldn't consider buying a 32t unless you struggle on climbs then shorten the chain to the optimum length.

jasonmarkjenkins

Thanks for the comments and advice. Would i need a bottom bracket tool to remove the chainring? Thanks

reaperactual

No, bottom bracket tool is only required for direct mount chainrings so just undo the four chainring bolts with an Allen key and you should be able to slide chainring off over the crankarm and pedal.

jasonmarkjenkins

reaperactual said:

No, bottom bracket tool is only required for direct mount chainrings so just undo the four chainring bolts with an Allen key and you should be able to slide chainring off over the crankarm and pedal.

I have tried but it seems impossible unless i'm doing something wrong?
Edit i was doing something wrong it's off .. Thanks for the advice

reaperactual

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