Measuring chainline

Posted on
  • what is the standard accurate way to measure your chainline? assuming that it's pointless to measure it when the bike is taken all apart i couldn't figure out how to measure it accurately with the spokes and the cranks and everything in the way. so how do you fellas do it?

  • I'm having some trouble with chainline too. I was wondering how much of it is to do with how far the crank is pushed on to the BB taper. I tightened the bolt fairly hard but I've no idea how you're supposed to know when it's 'right'. Is there any
    chance of stripping the thread or should I just go as hard as I can with an adjustable spanner that's about 170mm long?

  • edmundane what is the standard accurate way to measure your chainline? assuming that it's pointless to measure it when the bike is taken all apart i couldn't figure out how to measure it accurately with the spokes and the cranks and everything in the way. so how do you fellas do it?

    Same as this fella.

    http://www.sheldonbrown.com/chainline.ht­ml

  • re cranks - erm, if you don't titghten them properly, they'll work loose and the spindles will get rounded out, meaning dead cranks. tighten up as tight as you can the first time you put them on, using a proper wrench, then put some allen key bolts in and don't tighten them up again even if the bolts feel a little loose (the cranks will squirm a little up the taper as you use them, which is what makes the bolt loose, but they're jammed tight onto the spindle so they're fine).

    chainline - if its for the kuwahara, best thing is to assume the same chainline as you'd get on a track bike, because you'll not find a 126 rear hub - is a lot easier/cheaper to just squeeze in the rear stays to fit a 120 hub.

    btw - chainline was okay but noisy when i was running some cheap sugino cranks. when they broke i bought some miche - and the bb they come with can be dialed in/out, which made it a lot easier to get a good line. now it's very quiet... of course, no reason you couldn't get the cheap cranks and the nice miche bb, it was hardly the most expensive bb.

  • kuwahara bmx or 20 incher? or what? sounds interesting

  • I personaly would meassure the distance between your RHS drop out to the center of the cog, lets call thaT X and your over locknut messurment lets call that Y then you chainline will = (Y/2)-X

  • TheBrick(Tommy) I personaly would meassure the distance between your RHS drop out to the center of the cog, lets call thaT X and your over locknut messurment lets call that Y then you chainline will = (Y/2)-X

    I love it when you talk dirty.

  • I think my frame might be slightly bent.

    I measured my chainline front and back as best I could: it's between 42 mm and 42.5 mm at the front which I measured 2 ways. Firstly, using a metal ruler, I held one end against what looked like the centre of the seat tube and read off against the centre of the chainring. Secondly, which involves less guess work I think, I measured from the chainring side of the seattube by butting the ruler up against it (28mm), and then from the far side of the seattube using a flat surface held against both the tube and the end of the ruler (57mm). Chainline = (28+57)/2 = 42.5mm. It would be better to measure the diameter of the seat tube but I don't have a vernier caliper. It should be 57-28=29mm.

    At the back, the spacing is 120mm while the cog centre is 19mm in, so the chainline is 120/2 - 19 = 41mm.

    So in theory, it's out by 1 or 2mm. But when I check it by eye, sighting along the chainring, it's much more, 4 or 5mm. So does this mean my frame is bent? Or maybe just the track ends? Does anyone know a good way to check?

  • Tommy - normally that's right, but with the Kuwahara frame he's got (it's an old 9 speed I think, I have one that looks near identical) the rear spacing is 126mm, and it's cheaper/easier to put a 120mm rear hub in there and squeeze the stays in. That gives you a roughly 42mm chainline, but it's best to get a BB that you can dial in/out so that you can fine tune it.

  • natureboy I think my frame might be slightly bent.

    I measured my chainline front and back as best I could: it's between 42 mm and 42.5 mm at the front which I measured 2 ways. Firstly, using a metal ruler, I held one end against what looked like the centre of the seat tube and read off against the centre of the chainring. Secondly, which involves less guess work I think, I measured from the chainring side of the seattube by butting the ruler up against it (28mm), and then from the far side of the seattube using a flat surface held against both the tube and the end of the ruler (57mm). Chainline = (28+57)/2 = 42.5mm. It would be better to measure the diameter of the seat tube but I don't have a vernier caliper. It should be 57-28=29mm.

    At the back, the spacing is 120mm while the cog centre is 19mm in, so the chainline is 120/2 - 19 = 41mm.

    So in theory, it's out by 1 or 2mm. But when I check it by eye, sighting along the chainring, it's much more, 4 or 5mm. So does this mean my frame is bent? Or maybe just the track ends? Does anyone know a good way to check?

    is it really that much off though? even 1 or 2mm will look quite off to the eye if you squint down the chain because of foreshortening. 5mm would show a really obvious angle looked at from above, not just when you squint down the chain.

  • is it really that much off though? even 1 or 2mm will look quite off to the eye if you squint down the chain because of foreshortening. 5mm would show a really obvious angle looked at from above, not just when you squint down the chain.

    True, it's difficult to be sure, but when I sight along the chainring, the line clearly intersects with lockring thread, not the sprocket thread. Anyway, I'll wait till I get some wheels sorted out before I do anything (hub is currently rimless) - at least then I can cycle it to frame builder to have it checked properly!

  • as well as Sheldon http://www.sheldonbrown.com/chainline.ht­ml
    also look to see if your teeth mesh with the chain dead center
    finally it should be silky smooth 'swoosh' nice and quiet

  • check hubjub for chainring spacers lock ring spacers or axle spacers with a combination of some or all of the above it should get into line !!

  • yeah why didn't i check sheldon... but hey that's detailed information, h2o!

    the question concerns both bikes i've got, the track frame and the kuwahara (not a BMX, julian, i know you got all excited by the exhibition!).

    my plan is to get new cranks + b.b. for the track frame and move the old ones onto the kuwahara, if possible. the chain's quite noisy on the track frame so i'm thinking it's not quite right. it's also 3/32 chainring + a 1/8 cog. i should have known better before i paid the guy for the whole bike...

    bet i might change over to 3/32 chain on when the kuwahara finally gets built up, and have the 1/8 chain set dead on on the track frame :)

  • have u got a new frame already natureboy?

    thanks dicki i'm a purist i want a bike with no spacers, i bet you do too :)

  • yes, and most of a transmission too, but it's going nowhere without a back wheel.
    i'll have to bring the geared bike if we do the prologue ride tomorrow :-)

  • well of course i do !! no spacers on mine ... just like my sentences

  • haha thats exactly what i mean

  • Post a reply
    • Bold
    • Italics
    • Link
    • Image
    • List
    • Quote
    • code
    • Preview
About

Measuring chainline

Posted by Avatar for edmundane @edmundane

Actions