Chainline measurements????

Discussion in 'Downhill & Freeride' started by P.T.W, Dec 22, 2007.

  1. P.T.W

    P.T.W Monkey

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    HI ya im finalising the design of my dh frame im going to build next year but im having trouble finding chainline measurments for the popular cranksets out there....gravity,raceface,shimano etc
    Im looking for measurments for both 73mm an 83mm bb shells as im building a few frames.
    As far as i can tell the average measurments are 52mm---73bb an 57.5---83bb
     

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  2. davep

    davep Turbo Monkey

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    BB width has nothing to do with it... chainline is determined by the rear hub (and its offset or lack there of).

    A 135 rear hub = 47.5mm chainline (with zero offset)
    150 rear hub = 55mm chainline (again zero offset)
     
  3. DirtyMike

    DirtyMike Turbo Fluffer

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    What he said, just curious, do you know what cranks youll be running? I know the Truvative stuff Is measuredand sold by chainline alone, I just had to go through and figure it all out for my own bike. I can also add that If you run truvative with an 83 mm shell, you only get One choice for the BB, where as teh 68 and 73 offer a BB for a 51 and 56 mm chainline
     
  4. davep

    davep Turbo Monkey

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    ^^ true, it would be difficult (impossible) to mfg an 83mm bb that would be narrow enough to allow a 47.5 chainline...but the other way around (68 or 73 bb with 55mm chainline) is obviously possible.


    Keep in mind as well, while back in the square taper days, chainlines were exactly 47.5mm (what the technically should be)....while now these cranks (for 135 rear) vary between 47.5 and 50mm....and many wider set-ups are 55 -57mm......some (all) of this has to do fitting external bearings in there.
     
  5. DirtyMike

    DirtyMike Turbo Fluffer

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    Again, Exactly what he said.
     
  6. P.T.W

    P.T.W Monkey

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    ok dave what im trying to find out is where the middle ring on various cranks is going to sit relative to the centreline of both a 73 an a 83 mm bb shell.
    Just trying to avoid any clearance issues with chainguide,swingarm etc
     
  7. davep

    davep Turbo Monkey

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    I understand what you are after..it is just that bb width has nothing to do with chainline...except that you cannot fit the chainline for a 135mm rear (47.5mm) on a 83mm bb.


    68/73mm bbs can have either a 47.5 mm chainline for a 135 rear or a 55 mm chainline for a 150 rear (with varying bbs). An 83 mm bb is limited to the 55 mm chainline due to spacial constraints.
     
  8. P.T.W

    P.T.W Monkey

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    Cheers davep!Not really the info im after but it does clarify a few things for me.I should be able to work it out now
     
  9. davep

    davep Turbo Monkey

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    Sorry I am not answering your question...there is just not one simple numerical answer to what you asked. If you knew the rear hub spacing, that would give a simple number for the chainline.
     
  10. LMC

    LMC Monkey

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    so you want the distance from frame center to middle ring, regardless of the rear hub?

    on a enduro SX the BB shell is 68mm, using truvativ outboard bearing cranks the middle ring is 55mm away from frame canter.

    the chain is however in line with the 3rd gear, in my opinion less than ideal as you would idealy want it in line at gear 5. guess thats the tradeoff with running outboard bearing cranks though. ive just measured ISIS cranks with a 68mm shell, the middle ring is at 50mm
     
  11. Sam B

    Sam B Monkey

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    Truvativ generally run 51 or 52mm chainline, something like that. Most FSA are 50mm like current Shimano. That is all for 135mm frames...

    Classic chainline was 48mm, but Shimano, etc ran into problems getting the front derailleur to drop the chain to the granny ring without the front mech botttoming out on the larger 34.9mm seat tubes on new bikes. So boom... they had to move it out.

    It's easy to calculate based on the width of the dropouts and where the gearing spread or single cog is located from that side.

    The other thing to consider, if you run one ring in the "middle" position that will be spot on with the chainline for that crankset. However, if you run two rings you would ideally want a wider chainline on the crankset to get that phantom middle point in line with the desired part of the cassette. Otherwise you are always cross gearing in the smaller ring.
     
  12. DirtyMike

    DirtyMike Turbo Fluffer

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    Soooo, Get the right chainline, Order the Truvative BB with the 51mm chainline, you rnot stuck with the 68/73 mm BB's Team and xr are both available in a 51 or a 56, I just installed the 51 on my dirtbag, and with E13's assistance, My chain is perfectly straight on the 5th ring on the cassette, BTW the dirtbag is also a 68mm shell, and mine has a 135 hub. In fact, when your looking in the parts book, it tells you to use a 51 for a 135 and a 56 for a 150. It also says that if you do not know which you need, the 51 replaces a 113 Isis, and 56 replaces a 118 isis. Repeat on the last part
     
  13. DirtyMike

    DirtyMike Turbo Fluffer

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    I like to set my chainlines on doubles like I do on a roadie, where you main/middle gear is able to shift throughout the entire cassette without overstressing the chain, and just use the small ring as a granny gear. At least thats how I set my own up, and it works pretty damn well. With that said I think its important to mention that I would just run a single up front except I cannot afford to have a full DH and a xc bike, So I set up My dirtbag with a Double and an E13drs. Paired with an ETA 66 RC2, its perfect for My needs, my setup may not be perfect for someone else, something I feel is important to mention