[Kornphlake]

Forgive me if this has been posted a bajillion times already but I've never actually figured out how you measure chainline or whatever. People always say that a manufacturer suggests x length bottom bracket but using a saint crank setup they had to use y BB to get the right chain line. I just don't get it, what differance will 3mm make anyway? When you shift from the middle ring to the small ring that's way more than a couple mm so as long as you are close and you adjust your derailurs correctly shouldn't everything be okay?

 

[Rik]

Maybe.
Chainline makes a big difference to the shifting of the bike. The angle that the chain makes coming off the rings/sprockets will affect shifting and wear. Too greater angle, and things go to hell pretty quickly.
If you're running a dual ring setup, I'd say you want the big ring to have good chainline for the lower-middle to top end of the cassette, and the small ring only needs to have good chainline for the top end of the cassette. To me, the very smallest sprocket isn't as big an issue as the biggest sprocket, so I set up my bikes to shift well in my most used gears.
And that's how all bikes should be set up, taking in to account usage and preferred gear combinations. You're going to compromise no matter what, so if it's a bike that you'll climb alot, set the chainline so it's good in the climbing end of the gears. If you're doing speed work, then you want your fast gears to have the best chainline.

 

[Jm_]

I dunno, but my IH with a 100mm BB and 165mm rear hub shifts extremely well, way better than any other long travel bike I've had...

 

[zedro]

I dunno, but my IH with a 100mm BB and 165mm rear hub shifts extremely well, way better than any other long travel bike I've had...

theres only a 1mm difference in chainline between that and a 68/135 setup

 

[erikkellison]

theres only a 1mm difference in chainline between that and a 68/135 setup

How do you figure? You're both about axle length in the rear, and shell length in the front, which is irrelevant - spindle length is what really matters. I see your math, I just think it's missing the point. Really you should be concerned with where the crank ends up on the spindle, which is the determining factor on where the rings end up in respect to the edge of the shell, and ultimately the chainline. You can't change shell width on bikes, so you should be quoting spindle lengths. How about a 145mm BB spindle and a 165mm hub, or a 113mm BB spindle and a 135mm hub.
You also have to take into account where the ring is mounting when you're discussing these matters. Most people with the huge DH BB's use the outer ring mounting plates, or have 1-ring specific cranks (like the new Holzfellers or Saints, I think?), whereas people with the 113mm BB's use the middle ring mostly, but can use the outer or inner as well.
The bottom line is what was said previously by Rik. Make a straight line between the chainring you usually use with the sprockets you usually use, and don't shift to the extremes.
Now that I have thoroughly confused anyone for whom this post was intended to help, I'll shut up.

 

[zedro]

How do you figure? You're both about axle length in the rear, and shell length in the front, which is irrelevant - spindle length is what really matters. .

yeah your right....too quick on the draw...

i had all the exact combos written up when i was designing my bike, and for the most part some of them were analogous. Was just trying to point out that theres no particular magic with the 100/165 combo, from what i remember with the actaul figure :

 

[Curb Hucker]

if i can pedal backwards in low gear, and not have it derail from that cog, its good. At least thats how things have been for me

 

[Brian HCM#1]

The spider also can throw off a chainline.

 

[zedro]

The spider also can throw off a chainline.

speaking of spiders, anyone else get wobbles in their pressed on Shimano spiders?

 

[Jm_]

yeah your right....too quick on the draw...

i had all the exact combos written up when i was designing my bike, and for the most part some of them were analogous. Was just trying to point out that theres no particular magic with the 100/165 combo, from what i remember with the actaul figure :

Yeah, I haven't even tried to figure it out or measure it, but just the way that it shifts as far as how little tension it actually takes, is just hands down better than any long travel bike i've owned or used. It simply shifts "very easy", backpedals easy, and just doesn't feel like it's ever "fighting" itself on the drivetrain...

 

[Wumpus]

To measure rear chainline, the easiest way is to measure the distance from the inside of the rear fork end (or the outside of the axle locknut) to the middle of the sprocket. Double this, subtract it from the over-lock-nut dimension of the hub (or the frame spacing , which should be the same), then divide the result in half, and you have the rear chainline.

http://www.sheldonbrown.com/gloss_ca-m.html#chainline

After you get the rear, then use that measurement on the front measuring from the center of the seattube to the center of the ring(for one ring) or between rings(for two).

Measuring my XC bike -- Rear cluster center is 23mm. 135mm spacing.

135 - (23x2) = 89 / 2 = 44.5mm(This is the chainline.)

 

[Kornphlake]

So is there a tool to measure chainline, it seems like using a tape measure or a ruler would be less than ideal, especially where you're talking about a differance of a couple of millimeters. I understand that with chromoly cranks you have spacers that will allow you to fine tune your chainline, however if you're using an isis drive bb you're pretty much limited to the length of the spindle that is offered. Truvative, for example, only offers 113, 118, 128 and 148mm bottom brackets. It seems to me that if you're chainline is between 113 and 118 you might not be able to visually determine which is correct since the differance is only 5mm, that's less than a quarter inch, if you're trying to measure chainline by guessing the distance between two rings it becomes even more ambiguous. I guess I just think chainline is a bunch of hooey, if the manufacturer reccomends one BB length then that should be the BB that's used, tampering with different lengths and getting better results either indicates poor frame design or a placebo effect. If one assumes that 5mm will really make a big differance in shifting performance then that means that you are optomizing for only one cog because the spacing between the cogs will be enough to give you poor chainline on any other cog right? I can understand using a shorter spindle for better large cog shifting or a longer spindle for better small cog performance but I can't imagine that you'd really be that much worse off by just using a spindle length that gives you a "good" chainline for the middle cog and the rest will fall in place... What's the point of having a nine speed cassette if you can only shift to 5 of the cogs? FWIW I don't have any problems with my chainline, I never have and I've always been able to shift to all 9 cogs in any chainring.

 

[Bulldog]

Perfect chainline for me doesn't have a number. It's when the chainring I use 95% of the time (middle) lines up dead-nuts-center with the 5th of my 9 cogs out back. Part good luck, part excellent bike design, but damn it's so nice.