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Down gearing a single speed MTB

Discussion in 'BMX & Dirt Jumping' started by Daniel_Burton, Jun 5, 2012.

  1. Daniel_Burton

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    Hey, I own a Haro Thread 1 2009 but I have a little problem. I bought it single speed as standard but the gearing is far too high for me. It is currently running a 16t cassette and a 34t chain ring and I was wondering how I could down gear it. I want to down gear it by quite a lot almost trials bike gearing I'm talking <25t chain ring. I'm happy enough to leave the cassette as standard I would just like to swap out the chain ring with something smaller.

    Another way of doing it would be to get some BMX cranks and sprocket and put that on it which would work fine and I could do but I don't know whether BMX cranks would fit on a MTB. Any help with that would be great too.

    Any advice, links, suggestions, ext would be highly appreciated.

    Cheers
    -Dan

    [EDIT]

    Just found this picture online. This is EXACTLY what I wanna do and this guys bike came standard like this
     
    #1 -   Jun 5, 2012

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  2. WestCoastHucker

    WestCoastHucker Turbo Monkey

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    wow, 34/16 a little high for you? that's already a little girl gearing...

    it seems like you have already found your answer. this is essentially what we all have done...
     
    #2 -   Jun 5, 2012
  3. ATXguy

    ATXguy Chimp

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    You can run BMX cranks just make sure they are compatable with your bottom bracket. If you just drop from 34 teeth to 25 teeth keeping the 16 cassette you will have to paddle like mad and have a lower top speed. You should also drop to at least 12 Teeth in the rear but that is my personal preference and that may not be yours.
     
    #3 -   Jun 5, 2012
  4. Daniel_Burton

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    Will a BMX sprocket fit on an MTB bottom bracket though because I notice on BMX bottom brackets there is a clear circle section for it but there isn't on an MTB.
     
    #4 -   Jun 5, 2012
  5. cmc

    cmc Turbo Monkey

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    when you say "down gearing" do you really just mean you want a small sprocket up front (for looks)? or are you saying you want to pedal faster (i.e. cover more ground with a stouter gear ratio) or do you mean you want it spinnier (cover less ground... i.e. like a trials bike). :confused:



    34/16 is a good ratio for 26" wheels. it's roughly equivalent to what comes stock on most BMX bikes. i'm guessing you just want a smaller sprocket "look". in that case, you should be going for 28/12 or 25/11 with 26" wheels.

    (the wheel size matters when determining the "gear inches." 25/9 for example on 20" wheels will be waaaaay stouter on 24" or 26" wheels.)
     
    #5 -   Jun 5, 2012
    Last edited: Jun 5, 2012
  6. cmc

    cmc Turbo Monkey

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    post a picture of your cranks. .
     
    #6 -   Jun 5, 2012
  7. WestCoastHucker

    WestCoastHucker Turbo Monkey

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    the bike comes stock with Truvativ Ruktion cranks...
     
    #7 -   Jun 5, 2012
  8. cmc

    cmc Turbo Monkey

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    a friend of mine got this bike. it's not a bad bike. a little heavy but very solid and good geometry. just ride it as is :thumb: then when you're ready, sell it complete and build a custom with bmx cranks, bmx sprocket, new wheelset etc. etc.

     
    #8 -   Jun 5, 2012
    Last edited: Jun 5, 2012
  9. eaterofdog

    eaterofdog ass grabber

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    Sounds like he's setting up a street trials bike.

    Most bmx cranks will not work if you cannot adjust the chainline in the back. They are intended for 110mm rear spacing, so chainline will be way off. You CAN use bmx cranks, but you need a 6 inch spindle and outboard bb. (IE profile stuff)
     
    #9 -   Jun 5, 2012
  10. Daniel_Burton

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    Just to clarify I want to be doing street trials so I need more pedal strokes to less ground covered. As close to a 1-1 ratio as possible
     
  11. WestCoastHucker

    WestCoastHucker Turbo Monkey

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    unless i'm really missing something, i was under the impression that one used the spacers provided to adjust the chainline for this very purpose. albeit it's not adjusting the rear cog, but the same effect is being achieved...

    in over 20 years, i've never had an issue putting bmx cranks on mtb frames...

    while the 6" spindle part is correct(well, mostly correct, but not always needed), the outboard bottom bracket part is incorrect...
     
    #11 -   Jun 6, 2012
    Last edited: Jun 6, 2012
  12. jonKranked

    jonKranked Press Button, Receive Stupid

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    for trials you want a touch over 1:1. for street trials even a little higher.
     
  13. eaterofdog

    eaterofdog ass grabber

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    With a 5" spindle, the cranks sit really far out on the spindle once you space it out far enough. I won't ride it if there's only 10mm of overlap. 5.5 spindles work sometimes.

    You are right, outboard is not needed, but it's a lot better setup if you are buying a BB anyway.
     
  14. WestCoastHucker

    WestCoastHucker Turbo Monkey

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    i'm pretty sure the spindles come in various lengths depending on the manufacturer. either way, i agree 100% on the interface overlap issue...