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Converting from triple to double

BG2000BG2000 Posts: 517
edited July 2010 in MTB workshop & tech
I've just ordered a 28T inner and 42T middle to fit my existing triple crankset (SRAM XX clone :D)

I'm going to need to order some narrower chainring bolts for the middle ring and just want to check the type I need to ask for.

So far, I've only come across 'normal' chainring bolts and 'Single Ring' chainring bolts.

I need the captive/threaded part (the nut) to fit into the LHS of the chainring, as normal - but I'm concerned that it may be designed to press against the inside of the spider for single outer chainring setups.

I've never paid much attention to single speed MTBs - where do most people mount their chainrings ?

Posts

  • nicklousenicklouse Posts: 50,675 Lives Here
    the nut needs to pass into both the ring and the spider.

    5.6mm ones tend to do. Or there are washers that can be used as well.
    where do most people mount their chainrings ?

    not much choice with the rings you have.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • gs3gs3 Posts: 249
    BG2000

    Velosolo do the sort of thing you are looking for. This link below has both the single ring bolts and the washer set to allow use of existing chainring bolts (look towards the bottom of the page).

    http://www.velosolo.co.uk/shopcrank.html

    Hope this helps!

    .
  • BG2000BG2000 Posts: 517
    Thanks for that link - it's a useful site, and I'll bear that in mind next time I need more bits. Seems like a good supplier of rings for my track bike too.

    Being a bit of a poser, I wanted red anodised bolts, so got some off eBay. If they give me problems, I'll go back to velosolo and get some stainless steel ones, like a sensible person would do.

    I can't wait for my TA Chinook rings to arrive (42T & 28T). I just want everything to be perfect for SITS...
  • stu8975stu8975 Posts: 1,334
    It depends on what chainset you are using for the conversion as to what bits you need, most certainly shorter bolts for the middle or 2mm washers, you may also need to space the granny ring to give the correct gap between granny/middle.
    Eg..My XT 770 only needed shorter bolts for conversion using TA 26/40 rings.
    XTR 970 chainset needs shorter bolts on the middle and spacers on the granny ring to give correct gap between rings when using TA's or the chain hits the middle ring/bolts when in the granny.
  • BG2000BG2000 Posts: 517
    Thanks Stu - I'll think about spacing the granny to suit my derailleur/cable pull ratio.

    I'm fitting my TA rings to a Truvatic Stylo GXP crank, so the new bolts should be OK.

    It seems like we've both gone for a 14T jump setup at the front. I basically copied SRAM's XX chainset, and thought if they can do it, so can I. I'm running a 12-32 at the back, so 28x32 is plenty low enough.

    My other concern now is with the front derailleur. SRAM and Shimano both make MTB double FDs in top-pull: the XX and Saint. The XX is pricey and the Saint is 83mm BB only.

    So I'm hoping my XTR FD will be OK. In theory, I just need to raise it up the seat tube by about 1cm. As it's designed to cope with a jump of 14T, so it shouldn't matter what size the actual rings are, although the arc of the inner/outer FD plates are probably optimised for a smaller granny and middle ring. But none of this should be an issue with a double setup as the inner plate won't ever have to pass over the middle ring. I'd have preferred a road triple FD but the cable pull ratio is much less (13mm compared to 18mm I think).
  • BG2000BG2000 Posts: 517
    This is the result:

    chinooks.jpg

    Great gear ratios and perfect chainline. Front middle x middle sprocket = straight chain. Or granny x 8th largest sprocket = straight chain.
  • AnonymousAnonymous Posts: 79,667
    that doesn't look like an XX clone at all. YOU LIE, sir, you LIE! :lol:
  • The RookieThe Rookie Posts: 27,802
    Sweet!

    YHMcG is right though as it looks much nicer than an XX........

    Simon
    Current steed - Whyte T129, 2013 frame, mongrel Revelations, Giant dropper, Stans S1 wheelset. 12, Magura Trail Sport brakes, 1x11. 12.8Kg
  • BG2000BG2000 Posts: 517
    that doesn't look like an XX clone at all. YOU LIE, sir, you LIE! :lol:

    Fair enough - it was a bit of a lie !

    But I did save about £250 over buying an XX chainset, and I reckon the performance isn't far off.

    As long as I don't get any chainsuck, I'll be happy.

    Glad you like it TB.
  • stu8975stu8975 Posts: 1,334
    Schweeeet...as for your FD and wanting a roadie one, plenty people use Dura Ace FD's on their 2 ring MTB's, cable pull is determined by the shifter not the FD. But most of them are bottom pull so you would need an adapter..eg..Speen.
  • AnonymousAnonymous Posts: 79,667
    What causes chainsuck anyway? The only time I've suffered it is when the rings have been properly gunked up.
  • njee20njee20 Posts: 9,613
    I wouldn't bother with the road FD, I had a Dura Ace one for a while, no real advantage over an MTB one, they have very slightly different cable pull, so you're likely to get some rub in the extremes.

    That chainring set up does work well though, I ran 28/40 like that for a while, I personally prefer having a slightly smaller big ring, or a slightly larger cassette, so you spend less time shifting on the front and just keep it in the big ring.
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