Please --Need confirmation of chainline calculation for DH hubs--- Mtbr.com
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  1. #1
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    Please --Need confirmation of chainline calculation for DH hubs--

    I might as well ask here since y'all usually deal with chainline for SS.

    I'd like to know if my calculations are correct about the approximate placement of a rear cog to match a 65mm crankset (flipped DM ring) chainline on a 157mm hub.

    According to Sheldon and others the approximate chainline for 150/157mm hubs is 55mm. Also a 10 speed Shimano cassette is 37.2mm wide, and the spacing between cogs is 3.95mm.

    So, if a recommended chainline centers the chain to the middle of the cassette, then 55mm gets me between the 5th and 6th cog. Half of the distance between cogs is 3.95/2 = 1.975.

    So the 6th cog should be at a 55+1.975mm chainline, or 56.975mm. If I wanted to match a rear cog with a 65mm chainline, I should place it around the location where the 8th cog should be, or (55+1.975(6th) + 3.95 (7th) + 3.95 (8th) = 64.875mm, with room to spare where the 9th and 10th cog should go? It is my understanding that singlespeed cogs are wider at the base to protect the free-hub body?

    Is that correct or did I miss something? Please help

  2. #2
    Always in the wrong gear
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    You're making it hard on yourself with all that guesstimation math.

    Take a wooden stick. I used a bamboo BBQ skewer.
    Your hub is 157mm wide. divide that by 2, to get how many mm it is from the dropout to the dead center of the bike left-to-right.
    mark and cut your stick at that length. (approx 78-79mm for a 157mm hub.)
    Edit: I can't math after beer. corrected.
    Now with your cut stick, place one end on the apex of the seat tube so it's a tangent pointing right through your chainring and mark where the stick crosses the it on the stick with a pen. That's how far the chain sits away from the 'middle' of your bike.

    Now you know how far it is from the center of your bike to the chainring, and the stick is 1/2 the width of your hub.
    Put the stick against the dropout where the hub sits with the wheel and cog (no spacers) on the bike.
    determine where the cog needs to be based on where the mark is on the stick, and add in SS spacers to match.

    After I install the spacers, I get a decent straight-edge and lay it on the chain, as well as eye-ball the line from behind the bike to visually confirm.

    I've only been riding SS for a handful of years, but it's the only bike I ride, I ride a fair bit and I learned this method from a guy who used to race SS at a pro level.

    It's never failed me.

    I usually refer to it (as it was told to me) as the "stick method". I saw a picture walkthrough here on MTBR, but a google search didn't show what I wanted. If you need pics maybe I or another can post up. It really is the easiest way and is independent of cog base width. You might be mathematically off by a mm or two, but it's not enough to affect part wear.
    Donít modify the trail to match your skills, modify your skills to match the trails.

  3. #3
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    The stick method makes sense and it is straightforward, I'll give it a go, thanks for sharing!

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