Forum home Road cycling forum Workshop

Gears slipping under pressure on mountain bike

Hi Everyone,

My mountain bike had a new chain put on it a few months ago. I took it out the other day for the first time since then and found that the gears were slipping.

I can't describe this any more clearly really... I put pressure on the pedals to turn them and some of the gears slip. I don't think the chain moves up or down, it sort of just shifts along on the same cog.

A person from my Triathlon club replaced the chain for me as I am terrible at things like this.

I walked into the local bike shop to ask and they suggested it could be the chain stretching and to drop the bike in. I've looked online and that is also what I've found as a possible issue. The bike shop also mentioned huge issues with getting parts in due to backlogs and even said some are delayed until next year! :#

What is the usual fix for this... is this something I could try to repair myself? Maybe save a big of money?

The bike is a hard tail, quite old but not used that heavily really. No other major issues, all working fairly well I think. I don't want to spend tons on the bike as it's probably not worth that much - although I am quite attached to it having owned it for a long time.

Any thoughts on this or things I could try?

Thanks all


  • mully79mully79 Posts: 862
    If you ran the bike with a worn chain then it probably wore out the front chainring/s and the cassette.
    If the teeth on either the chainrings or cassette have points like a sharks fin they are toast and need replacing.
  • mully79 said:

    If you ran the bike with a worn chain then it probably wore out the front chainring/s and the cassette.
    If the teeth on either the chainrings or cassette have points like a sharks fin they are toast and need replacing.

    Thanks for the reply. The old chain was replaced as it rusted due to lack of use.

    I don't really know what you mean by using a worn chain. Surely all chains are worn, so when does become an issue?

    From recollection, the bike was fine before I replaced the chain.

    I will check the teeth tomorrow. Maybe post a pic as well
  • DeVlaeminckDeVlaeminck Posts: 8,449
    If you put a new chain on the bike if the cassette or sometimes the chainrings are worn then the new chain will slip - the only fix is a new cassette (or chain rings). It's normally easier to tell if chainrings are worn - less easy with a cassette unless you have a new one to compare it to.
    [Castle Donington Ladies FC - going up in '22]
  • Darn that sounds quite an expensive job. The bike was bought in 2009 I think, for £850

    Will have a proper look tomorrow

    Thanks for explaining
  • ed1973ed1973 Posts: 278
    Chains do stretch but this is over a (normally) a lot of miles. But if you put a new chain on then this wouldn’t be the case. I suspect the issue lies with your cassette (big group of cogs attached to your back wheel). Parts are not too bad to get hold of depending on what you need. If the bike is from 2009 then I suspect you will need an 8 or 9 speed cassette (just count the number of cogs). You can pick these up on various sites if your LBS can’t get hold of one and then. If the old cassette is worn it will very quickly make mince meat of your new chain so look to get it sorted ASAP. If this still doesn’t fix it then like said before it will be your chain rings (the big rings attached to your pedals). Best thing to do is speak to your LBS and ask what parts you need and see if you can get hold of them yourself and then take them to the bike shop for them to fix it. If it is a new cassette then a quick look I saw them for around £25. It’s a 10 min job if that for the bike shop so won’t be a lot at all.
  • NcovidiusNcovidius Posts: 229
    If you’ve been riding with a chain which has stretched past 1% for any length of time / exertion, then you have probably knackered the sprockets in the cassette and / or the chain rings, to the point they won’t engage with a fresh chain properly, leading to the issue you’re describing. If you get yourself an inexpensive chain checker tool, it will potentially save you a lot of money in the long run. Change the cassette and chain together for a start, then Keep the chain and cassette clean and lubed, and periodically check the wear ( stretch ) of the chain with the tool. The inexpensive checkers are fine for most people’s needs. They usually have 2 set markers, one for 0.75% percent stretch, the other one for 1% stretch. If the chain reaches the 0.75% mark, it’s time to think about buying a replacement chain, if / when the chain reaches the 1% marker, replace the chain. If you do that, you should get more chains per cassette. When you fit the new chain, make sure it is the same length as the old chain was when it was new.
  • imposter2.0imposter2.0 Posts: 11,986
    Hi Nick..
  • danxdanx Posts: 27
    As other said this is typical of replacing a chain too late and will probably be a worn cassette and/or chainrings. However before you go out spending there are a few inexpensive possibilities to check first. Basically don't just assume your clubmate got it all perfect.

    Are there any stiff links in the chain? Loosen them with a chain tool if so - cheap to buy.

    Is it the right speed chain for your gearing (9, 10, 11 spd etc..)? The chain will probably have the model engraved on the side of the links.

    Check the chain length and give the gears a set up.

    Look on Park Tools website or similar for how-to guides.
  • Hi,

    Here is a picture of the gears, both front and back. The bike is a Trek 6500 2009 model, according to the Trek website this is a 9 speed. (Link:



    They do look quite sharp and spikey to me... :(

    I've looked up replacements and they come in at £43 and £32 which isn't too bad. I just haven't got a clue how to replace myself, nor do I have the tools. I do have a bike stand though, so I could give it a go... but I am terrible with repairs like this

  • Suspect it's the cassette and not the chainrings.

    It's not always easy to tell how worn a cassette is since some of the teeth on each of the sprockets are shaped differently from the rest in order to aid the shifting.

    The teeth on the chainrings photo don't look like they are the cause.
  • NcovidiusNcovidius Posts: 229
    YouTube is your friend on this one. Wiggle are good for the tools you need.
  • homers_doublehomers_double Posts: 7,618

    Hi Nick..

    You're obsessed...
    Advocate of disc brakes.
  • imposter2.0imposter2.0 Posts: 11,986

    Hi Nick..

    You're obsessed...
    So are you, it seems.
  • homers_doublehomers_double Posts: 7,618
    With what?
    Advocate of disc brakes.
  • imposter2.0imposter2.0 Posts: 11,986

    With what?

  • flycopflycop Posts: 20
    I had the same problem after replacing my chain, as others have said, check the cassette and chainrings.
    In my case I replaced the middle chain ring and all is well. B)
  • To get yourself into a "known good" state after replacing the chain with this problem, I would replace the cassette anyway. It is a mickey mouse job to do and they only run you £30-50 or so depending on the model.
Sign In or Register to comment.