Questions about derailleurs

Discussion in 'transport' started by teuchter, Aug 10, 2018.

  1. teuchter

    teuchter je suis teuchter

    The front derailleur and chainset on my bike has served me well since the mid 90s but increasingly erratic shifting has led me to decide that I should probably replace at least the chainrings, as they do seem to have something of a sharks fin profile to the teeth, which youtube tells me is not good.

    My main question:

    If I replace the chainrings, and/or the whole crankset, how much do I need to worry about compatibility with the mech, and the rear derailleur, and/or anything else?

    Is it pretty much anything goes, or are there things I need to look out for?

    If I get chainrings that are slightly different sizes from the existing ones does it matter (I fancy swapping the big one for a bit of a larger one)?

    If it makes any difference the existing is Shimano Altus C20 and the rear derailleur is Tourney TX (I replaced it at some point).
     
  2. weepiper

    weepiper Jock under the bed

    You'll have to replace the chainrings, the chain and the freewheel/cassette on the rear wheel all at the same time. If the derailleurs are Altus and Tourney there's a very strong chance that the chainset will just be riveted and the chainrings won't be individually replaceable. If the derailleurs are the same age as the rest of it they're probably knackered too so if you want to change chainring sizes, just stick a new front derailleur on too.
     
  3. weepiper

    weepiper Jock under the bed

    If you do replace the chainset, you might have to also replace the bottom bracket as the chainset dictates the necessary axle length. Basically what I'm saying is this is going to be a lot more expensive than you've bargained for.
     
    mrs quoad likes this.
  4. teuchter

    teuchter je suis teuchter

    Thanks. I think I've already worked out that the chainset is riveted and therefore I have to replace the whole thing.

    I guess I need to find out whether the bottom bracket will need to be replaced. Is that easy to check?

    I actually just had the chain and the rear cassette replaced a week or so ago and have only ridden about 70 miles since so hopefully I can get away without having to do that.
     
  5. weepiper

    weepiper Jock under the bed

    Ok, probably it'll be fine then. Is it slipping under pressure? Take a photo of your chainset and I'll probably be able to tell you what length the BB is.
     
    mrs quoad likes this.
  6. teuchter

    teuchter je suis teuchter

    It's not slipping I don't think, just slow to change between cogs, sometimes I have to crank round a couple of times before it'll do it. If I adjust the derailleur enough to give the necessary extra nudge then it will be adjusted too far and start rubbing on another cog combination.
     

    Attached Files:

  7. teuchter

    teuchter je suis teuchter

    Of course it might just be my derailleur adjusting skills that are at fault and the chainrings are fine but this is what they look like close up -


    Screen Shot 2018-08-10 at 22.03.38.jpg
     
  8. weepiper

    weepiper Jock under the bed

    Ok. The BB will be a 122.5mm axle. The only ring that looks worn is the smallest one. The front derailleur is sitting a bit too high off the chainrings, it might shift better if you lower it a few mm. Or it might just be fucked. If you grab the cage and wiggle it back and forwards there shouldn't be play, if there is I'd replace it. Edit, it looks like it's bent actually, the rear of the cage is sitting further from the chainrings than the front is. It should be roughly equal distance all along. This will mean the chain is not being caught by the shifting ramps on the inside of the cage. New derailleur time.
     
    Sprocket. likes this.
  9. mrs quoad

    mrs quoad Well-Known Member

    The middle ring looks like it has a fair few chipped teeth. If actually used, would that not be well likely to skip?
     
  10. weepiper

    weepiper Jock under the bed

    They're not chipped, they're made that way from new as it helps the chain to shift up to the bigger chainring.
     
    mrs quoad likes this.
  11. weepiper

    weepiper Jock under the bed

    Worn out chainrings look like this, they go very sharp at the tip and get scooped away on the side that is pulling the chain forward

    3435036007_a4778e0903_z.jpg
     
  12. teuchter

    teuchter je suis teuchter

    Here's the smallest chainring
    Screen Shot 2018-08-10 at 22.39.02.jpg
     
  13. teuchter

    teuchter je suis teuchter

    Here's the derailleur. Do you mean it's sitting further from the chainrings at the back/left than it is on the front/right, looking at it face on like this?

    Screen Shot 2018-08-10 at 22.41.52.jpg
     
  14. DownwardDog

    DownwardDog Riding a Brompton with a power meter.

    Clean the fucking thing before you do anything else. Let's see how it shifts when it's not covered in manure, tar and sawdust.
     
  15. weepiper

    weepiper Jock under the bed

    Yep, it's worn. May or may not slip under heavy load. Find a steep hill, put it in the smallest chainring and the second smallest sprocket at the back and pull off from a standing start. If the chain suddenly slips round you need to replace the chainset. Tbh even if it doesn't actually slip yet, it will wear your new chain out prematurely.
     
  16. weepiper

    weepiper Jock under the bed

    Yes. It's got bent outwards by a heavy/clumsy upshift or two. It's never going to shift properly again.
     
  17. teuchter

    teuchter je suis teuchter

    Ok. Thanks.

    So, I definitely need to replace the derailleur, and i should probably replace the chainrings, but the derailleur is likely to be the main source of my shifting problems.

    Is a front derailleur just a front derailleur, or do I need to be careful not to get the wrong thing?
     
  18. Fez909

    Fez909 toilet expert

    I'm pretty messy when it comes to [not] cleaning my bike, but that was literally my first thought. Pure filth that teuc :D
     
  19. weepiper

    weepiper Jock under the bed

    You need to be careful not to get the wrong thing. You need an 8 speed, triple, MTB/hybrid, non-compact 28.6mm band on derailleur.
    This one will do. Get the conventional swing version. It comes with spacers to make the clamp fit your seat tube.

    Edit, actually I can't tell from your pic if it's top or bottom pull. Does the gear cable go into the clamp from above or below?

    Shimano Altus M310 8-Speed MTB Front Derailleur
     
  20. DownwardDog

    DownwardDog Riding a Brompton with a power meter.

    It's bottom pull. You can see the cable guide on the bottom of the BB shell.
     
  21. teuchter

    teuchter je suis teuchter

    Yup the cable comes from below.
     
  22. weepiper

    weepiper Jock under the bed

    Ok. What I said before then. Actually it doesn't matter too much as that mech I linked to will let the cable enter either way. Get the conventional rather than the low clamp version though because that's what you've already got and they last longer.
     
  23. teuchter

    teuchter je suis teuchter

    Ok, thanks for taking the time to answer my questions; you may have saved me from replacing the whole chainset only to find that things still weren't working properly thanks to the derailleur. I'll replace it first and then might do the chainset as well. I'm fairly sure I don't ever have the chain slipping at the front (it was starting to do it at the back before I got the cassette replaced) but point noted that it might wear out the chain more quickly.
     
    weepiper likes this.
  24. mrs quoad

    mrs quoad Well-Known Member

    Oh dear.

    I’ve replaced what I thought was a single speed chainring in the past, on the basis that exactly what looked like that had happened, and the chain had started slipping. Now wondering if I’d ordered a non ss chainring instead :hmm: I recall it had worn out well fast!
     
  25. teuchter

    teuchter je suis teuchter

    I did a 'how dirty is your keyboard' thread a while back. Could be time for a bicycle drivetrain version.
     
    Fez909 likes this.
  26. gentlegreen

    gentlegreen sproutarian

    :eek::eek::eek::eek::eek::eek::eek::eek::eek::eek::eek::eek::eek:
     
    mrs quoad likes this.
  27. DownwardDog

    DownwardDog Riding a Brompton with a power meter.

    The answer to this bit is that you may need a longer chain if you put a larger ring on. The Shimano spec is big-big + 2 links but this doesn't give optimal results on frames with very short chainstays so you might need +3.
     
  28. teuchter

    teuchter je suis teuchter

    With a bit of research I've realised that if I wanted to increase the range at the front I'd need to find a different rear derailleur, because the spec for the one I've got says a maximum range of 20 on the front (the current one is 28-38-48). In fact the front derailluer that weepiper suggested also has a max range of 20. So I'd have to do a bit more thinking on this.
     
  29. maomao

    maomao 四月她爹

    My question about derailleur is how is it pronounced? I've always said 'de-rail-er' or 'de-rail-ee-er' but I keep wanting to pronounce it the French way. Is that too pretentious?
     
  30. weepiper

    weepiper Jock under the bed

    I wouldn't go putting a road chainset on (50t or 52t outer ring) because the chainstay angle on your bike isn't designed for it and the larger chainring will run too close to the stay, possibly even hitting/taking chunks out of the frame. You can stop it doing that by fitting a longer BB obviously, but then whatever derailleur you put on won't be able to shift the chain up to the big ring as it'll be beyond its pull range.
     
    teuchter likes this.

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice