Replace/repair chainset on Specialized Rockhopper

rob190
rob190 Posts: 16
in MTB workshop & tech
The middle ring is skipping quite badly but I don't know whether it's possible to replace just the ring or get a new chainset. I'm rather confused by what's compatible with what.

I believe the chainset is a 'Strongarm'. It's 22/32/42 with a 175mm crank. The crank is marked 'F-7' but I've no idea what that means.

This is an old bike from around 1997 but it works for me so I'd like to keep it going.

The options seem to be

1. 2nd hand chainset off ebay (around £40 or so)
2. New ring. But what type? I've not been able to find exact replacements.
3. New chainset. But are they compatible? E.g. there look to be different chain widths.

What would you recommend?

Comments

  • veronese68
    veronese68 Posts: 27,326
    You need to find out the BCD of your chainset. That's the bolt circle diameter. If the rings are attached with 4 bolts it's the distance from the centre of one to the centre of the one opposite. 5 bolts is a bit harder, you need to measure the diameter of a circle passing through the centre of the bolts.
    Then buy a ring with the same number of teeth, bolt holes and BCD.
    Kinesis Pro6
    Marin Nail Trail
    Cotic Solaris
  • rob190
    rob190 Posts: 16
    Thanks for the info. Yes, it's 5 hole but I was able to find the BCD by measuring the distance between adjacent bolts and using a table to convert it to the BCD (which is 74mm).
    There are spares available but many are marked as '9 speed' or '10 speed'. This doesn't make much sense to me but I'm guessing it has something to do with the cassette. Would I be right in thinking a '9 speed' chain ring is thinner so it can accomodate a narrower chain that would be required for the thinner gears on the cassette? The chain I have is 7.3mm (measured). Would that work on a '9 speed' chainring?
  • webboo
    webboo Posts: 6,087
    10 speed in narrower than 9 speed. The number of sprockets on your cassette will tell you what speed yours is, although given the age it could be even 7 or 8 speed.
  • rob190
    rob190 Posts: 16
    Yes, it's seven. But surely the only issue is the width of the chain isn't it? Otherwise how does the chainset even know about what's at the back?
  • mully79
    mully79 Posts: 904
    You will also need a new chain and probably a cassette.
    The chain is worn (stretched) which has worn away your chain ring.
  • veronese68
    veronese68 Posts: 27,326
    rob190 said:

    Yes, it's seven. But surely the only issue is the width of the chain isn't it? Otherwise how does the chainset even know about what's at the back?

    Yes, not really an issue for you
    Kinesis Pro6
    Marin Nail Trail
    Cotic Solaris
  • rob190
    rob190 Posts: 16
    Right so decided to replace the whole lot: SHIMANO Acera Fc-m361, Shimano CS-HG200 cassette, SHIMANO HG40 chain. Not impressed with the chain (several very stiff links) but the rest seems fine.

    The only problem is that the gears on the chainset are too close to the frame. There is enough room for them to rotate freely but it is impossible to select the lowest gear. The front derailleur is hard on the inner stop, there's no more adjustment and the chain is still on the middle gear.

    What can I do here? Do I need a different bottom bracket and if so what sort exactly? There seem to be so many different types now. Or a different front derailleur? Or a big washer on the bottom bracket to put the chainring back where it should be?

Categories