Is 2mm crank arm chainstay clearance too tight?- Mtbr.com
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  1. #1
    os2
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    Is 2mm crank arm chainstay clearance too tight?

    Quick question! I'm just starting a singlespeed 29er mtb build and hoping to use a left over BB/ triple crank from a previous road bike. I put a Salsa 34T SS chainring in the middle position, and adjusted the BB (a 125mm symmetrical Phil Wood) so that the chainline is 47.5mm. The crank spins no problem! However, while I've got about 7mm of crankarm clearance on the chainring side, there's only about 2mm of clearance on the non drive side. Is this too close? Phil Wood also makes 3 larger size BBs up to 135mm, so buying a longer BB is an option. Another buy option is the new RaceFace Deus crankset, since I'm not sure if roadbike crankarms (these are 180s) simply flare out less than a mountain bike. Any recommendations or thoughts on minimum crankarm/chain stay clearance? Thanks for the help.

  2. #2
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    My gut

    says 2mm is too close.
    -Chris

  3. #3
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    Quote Originally Posted by os2
    Quick question! I'm just starting a singlespeed 29er mtb build and hoping to use a left over BB/ triple crank from a previous road bike. I put a Salsa 34T SS chainring in the middle position, and adjusted the BB (a 125mm symmetrical Phil Wood) so that the chainline is 47.5mm. The crank spins no problem! However, while I've got about 7mm of crankarm clearance on the chainring side, there's only about 2mm of clearance on the non drive side. Is this too close? Phil Wood also makes 3 larger size BBs up to 135mm, so buying a longer BB is an option. Another buy option is the new RaceFace Deus crankset, since I'm not sure if roadbike crankarms (these are 180s) simply flare out less than a mountain bike. Any recommendations or thoughts on minimum crankarm/chain stay clearance? Thanks for the help.

    That's too close. You need a longer BB.You should have about 10mm (give or take 2) on each side. Also should be the same space on each side.

  4. #4
    And He was Not
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    Space It

    I have used a 2-3mm spacer between the frame and drive side cup. This might solve your problem and even out you crank arms clearance. I'm not sure which BB you have but this might help. By going to a longer crank spindle you might gain the extra clearnce but your cranks are prolly still going to be off center. The actual chainline on a s/s doesnt really matter that much as long as the chain is straight. I've run cranks with less than 5 mm clearance on the stays and never had a problem. If your BB is adjustable, center the cranks in the frame and space the front ring to line up on your freewheel or in the event you are using a freehub (cassette type ) rear wheel, space it to line up on the chain ring.

  5. #5
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    As long as it does not hit while riding any amount of clearance is fine. There is going to be some flex so 2 mm is marginal. You do not need 10 mm and it does not need to be the same on both sides
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  6. #6
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    Quote Originally Posted by Enoch
    I have used a 2-3mm spacer between the frame and drive side cup...
    That would move it the wrong way. His clearance problem is on the non-drive side.
    mtbtires.com
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  7. #7
    And He was Not
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    Quote Originally Posted by shiggy
    That would move it the wrong way. His clearance problem is on the non-drive side.
    OOpps, I miss-read, Thought he said Drive,
    My Bad

  8. #8
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    2mm is probably not enough. If you're just spinning around the block it might be OK. However, if you start cranking uphill and flex the BB or the crank arm you may whack the frame and it's likely to take some paint. Clearance needed is a factor of how much the crank flexes, how much the BB/frame flexes, chainline, and length options for BB you run (simply can't get a BB in 1mm increments).

  9. #9
    os2
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    I think you're all correct on the crankarm/chainstay clearance -- flexing the cranks makes 2mm seem borderline. So...I tried to calculate what a longer BB length would do measurement-wise and going to a 130mm spindle (from a 125mm) should get me to 7mm of clearance on both sides (with a 47.5mm chainline, which lines up with a WI ENO rear hub.)

    However, now that I'm worrying about too close... Does 4mm seem like enough clearance between the chainring and the chainstay? (That's where it is right now. <sigh>) Going to a 135mm spindle would get me to 9.5mm crankarn/chainstay spacing and 50mm chainline.. Better?

    Thank you for the really useful help everyone!

  10. #10
    Nouveau Retrogrouch SuperModerator
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    Quote Originally Posted by os2
    ...However, now that I'm worrying about too close... Does 4mm seem like enough clearance between the chainring and the chainstay? (That's where it is right now. <sigh>)...

    Thank you for the really useful help everyone!
    4 mm is about what I have.
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  11. #11
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    i shoot for 5 or 6. i'm setting up profiles now that are about 3mm from hitting on the driveside. that freaks me out. so i think i'm gonna go pick up a spacer tomorrow.

  12. #12
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    Quote Originally Posted by the Inbred
    i shoot for 5 or 6. i'm setting up profiles now that are about 3mm from hitting on the driveside. that freaks me out. so i think i'm gonna go pick up a spacer tomorrow.
    bear in mind that profiles are one of the most rigid cranks around, so clearance is far less of a problem than with lightweight thin armed aluminium cranks.

  13. #13
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    true, but what about flex in the frame?

  14. #14
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    Quote Originally Posted by the Inbred
    true, but what about flex in the frame?
    I've been trying to work out if the stiffer cranks would make the frame flex more. I guess they would, but it's pretty nominal.

    The times I find crank flex/clearance being a problem is on the upstroke, on my back foot. Pull up hard and it's possible to "clunk" the crank against the chainstay with flexy cranks.

    Chainring clearance is another issue I guess though

  15. #15
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    Quote Originally Posted by os2
    Quick question! I'm just starting a singlespeed 29er mtb build and hoping to use a left over BB/ triple crank from a previous road bike. I put a Salsa 34T SS chainring in the middle position, and adjusted the BB (a 125mm symmetrical Phil Wood) so that the chainline is 47.5mm. The crank spins no problem! However, while I've got about 7mm of crankarm clearance on the chainring side, there's only about 2mm of clearance on the non drive side. Is this too close? Phil Wood also makes 3 larger size BBs up to 135mm, so buying a longer BB is an option. Another buy option is the new RaceFace Deus crankset, since I'm not sure if roadbike crankarms (these are 180s) simply flare out less than a mountain bike. Any recommendations or thoughts on minimum crankarm/chain stay clearance? Thanks for the help.
    Why not flip the ring and put it in the outside position? If it is an unramped/pinned ring with a symmetrical tooth profile, it should work just fine and give you the clearance you need.

    (ff)

  16. #16
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    Quote Originally Posted by shiggy
    As long as it does not hit while riding any amount of clearance is fine. There is going to be some flex so 2 mm is marginal. You do not need 10 mm and it does not need to be the same on both sides
    Yeah, i don't know what i was thinking. Yesterday was just a bad day. 4 mm should be fine.

  17. #17
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    Clearance, Clarence.

    I crashed my road bike and the crank was lined up perfectly with
    the chainstay upon impact.... flexed a DuraAce crank right into
    the chainstay and put a nice crease into the driveside stay. Ug.
    I'd shoot for 10mm if you can get it.

    ZB
    No hairSShirts.

  18. #18
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    adjust BB cups?

    Can you adjust both bottom bracket cups to shift the bottom bracket little to the drive side...more room on the tight drive side and give up a little on the non-drive side? That would alter your chainline a few mm, but that should not be a big problem. i ran into the same problem trying to put the biggest chainring possible on a mtb SS bike for the road. cranks were ok, but the chainring came close to the chainstay: 44 tooth chainring within 2mm of the chainstay. road like that for a while, without dinging the chainstay, but it had me worried.

  19. #19
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    Quote Originally Posted by os2
    Quick question! I'm just starting a singlespeed 29er mtb build and hoping to use a left over BB/ triple crank from a previous road bike. I put a Salsa 34T SS chainring in the middle position, and adjusted the BB (a 125mm symmetrical Phil Wood) so that the chainline is 47.5mm. The crank spins no problem! However, while I've got about 7mm of crankarm clearance on the chainring side, there's only about 2mm of clearance on the non drive side. Is this too close? Phil Wood also makes 3 larger size BBs up to 135mm, so buying a longer BB is an option. Another buy option is the new RaceFace Deus crankset, since I'm not sure if roadbike crankarms (these are 180s) simply flare out less than a mountain bike. Any recommendations or thoughts on minimum crankarm/chain stay clearance? Thanks for the help.
    The only thing I can logically come up with here, after reading all the other posts, is just questioning why your spindle is off so far to the drive side?? In theory, couldn't you adjust that by positioning the chainring in the outer spot...about 5mm difference or so. Is the BB you are using a 68mm...being used in a frame with a 73mm BB shell? 5mm off seems 'right' only if one is trying to get chainline correct...still seems off to me. Center the spindle and space the rear...git r dun!

  20. #20
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    steel and ti frames will flex laterally at the chain and seat stays. 2mm wouldn't be enough clearance on many such frames... it's just not that much room considering how much the frame can flex.

    the stiffer your crank/bb setup, the less clearance you need. but you still have to account for frame flex.

  21. #21
    os2
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    thanks all. i'm definitely convinced that 2mm is not enough. the main issue is that my first build attempt started with a BB I had on hand that's simply too short... So, based on everyone's helpful input, I just ordered a longer BB from Speedgoat, and I should be good to go with centered cranks this time and around 7 or 8mm of crankarm/chainstay clearance. Will post some pics of the finished bike once it's all together. thanks again.

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