# The Perfect Chainline



## fishcreek (Apr 10, 2007)

Since most of SS'ers start from DIY's, I added a little instructables on how to achieve the "perfect" chainline. No more guess works and eyeballing so grab that ruler.

The instruction is for SS conversion from geared cassette and having the front chainline as the constant, meaning that the chainring is fixed and the cog will be the one to follow/adjust.

1. Measure your front chainline. This is the distance between the center of the frame/tube and the chainring. It's a bit tricky to find the center of the frame, look for bottle mounts. I gave mine as an example, 50mm.









2. Measure your rear hub spacing. Most mountain bikes have 135mm rear hub spacing, when in doubt, measure from locknut to locknut. Image courtesy of Sheldon Brown.









3. Reminisce grade school.

135mm / 2 = 67.5mm, this is your imaginary center of your rear spacing.

I think you know what next. Subtract your front chainline to the center of rear spacing.

67.5 - 50 = 17.5mm

17.5mm is your "perfect chainline", measure this from your locknut to your hub similar to figure 2. Take note that the center of the cog must fall into this measurement.

Congratulations! I promise you that there will be no more skipping, no more guessing, no more headaches.

4. Congratulate yourself with a bottle of beer or two.:thumbsup:


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## ginmtb (Apr 16, 2004)

Too logical. I prefer the SWAG approach. Nothing beats slamming your chest into the stem or knee into the stem when you chain jumps off the cog... 

Great instructions. Woulda been handy when I first started SSing.


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## Sparticus (Dec 28, 1999)

Way to tell it!

--Sparty


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## tpm7 (May 14, 2007)

This'll be VERY useful for me... well that is if my parts order ever comes in for my conversion...:skep: ... Thanks!


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## atomiclotusbox (Feb 2, 2004)

thanks! can't wait to go home and use my ruler.

chain skipped on my new build on the way to work this morning...


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## obi_twan_kenobe (Dec 5, 2005)

Good instructions. Endless sent me these exact instructions with my spacers.


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## flankwood (Dec 4, 2006)

One prob with this idea: As tension pulls on my chain tensioner, the sprocket on the tensioner is not perfectly perpendicular (90 deg.) to the ground so my line is always off. Whats the best way to make my SS the most efficient?


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## fishcreek (Apr 10, 2007)

flankwood said:


> One prob with this idea: As tension pulls on my chain tensioner, the sprocket on the tensioner is not perfectly perpendicular (90 deg.) to the ground so my line is always off. Whats the best way to make my SS the most efficient?


i guess you have a bent derailleur hanger, its either replace it or straighten it back to normal.


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## moschika (Jan 12, 2004)

i figure if it doesn't fall off, it's perfect :thumbsup:


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## atomiclotusbox (Feb 2, 2004)

followed the instructions last night and my chain is running smooth and straight...

i even got rid of my tensioner!

thanks fishcreek!!


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## gmx (Nov 12, 2006)

Cheers! Took the newly built up bike out today and hammered (until I had to walk) up steep hills and no chain problems.


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## fishcreek (Apr 10, 2007)

*67.5 Rule*

Here's a quick tip to check whether your chainline is correct or not. Let's call it the 67.5 rule. 67.5mm is your number if you have 135mm hub spacing, 60mm if you have 120mm etc... you know the drill.

This instruction will take you less than a minute to do.

1. Measure front chainline. Center of the frame to middle of chainring. My example: 50mm










2. Measure rear chainline. Middle of cog to where the locknut of axle and frame touches. Mine: 17.5mm










3. Add the two measurements and it SHOULD be 67.5mm

4. Adjust cog/chainring accordingly to achieve your 67.5 number.

5. Celebrate and get drunk.


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## Velobike (Jun 23, 2007)

Or the inveterate bodgers method....

Put the cog on the cassette without any spacers. Put on the chain. Turn the cranks a few times with a bit of drag from the brakes to put tension on the chain. 

This will put the cog in the optimum position. Mark the cassette around the cog with a pencil. Add spacers to suit.

That's it. Perfect spacing.


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## fishcreek (Apr 10, 2007)

Velobike said:


> Or the inveterate bodgers method....
> 
> Put the cog on the cassette without any spacers. Put on the chain. Turn the cranks a few times with a bit of drag from the brakes to put tension on the chain.
> 
> ...


never thought of that, i guess mine is the anal method.


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## GrumpyOne (Jan 7, 2004)

There's an old write up by Matt Chester on 63xc.com: http://www.63xc.com/mattc/setup2.htm (Scroll down a bit.)

He uses a slightly different (maybe more accurate) method of measuring your front chainline.

jw


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## Natedogz (Apr 4, 2008)

Great info, thanks Fishcreek!



fishcreek said:


> never thought of that, i guess mine is the anal method.


X2


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## fishcreek (Apr 10, 2007)

thanks, i know this is pretty basic to some but overlooked by others. keep SSing! :thumbsup:


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## tbulluck76 (Jul 8, 2008)

on my current project......I have not gotten to this point yet....what if I have the screw on/bolt on rear wheel....do I add a spacer to the rear and then screw on the free wheel or will it be perfect?


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## Miker J (Nov 4, 2003)

I get chain drop often if I have a perfectly straight line. Power to the pedal pushes the BB "inward" away from the pedaling leg. Running my rear cog slightly inboard significantly alleviates the frequency of chaindrop on most of the SS frames I've owned.


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## Doctorsti (May 25, 2008)

Just got my new Truvativ Noir 1.1 and wanted to make sure my chainline was right using the measurement method described above and turns out my cog was outboard the width of a 5mm spacer. My hubs are SS specific and only came with three spacers all 5mm so I knew I didn't have many options when I first set it up on the stylo 1.1 crankset. I ran that for a few hundred miles with my imperfect chainline. What I noticed on the aluminum bash ring from the stylos were tiny scrapes a various intervals around the edge, I assume from the chain nicking them when under heavy load and frame/bb deformation. I fixed it up so it's dead on by only swapping my Chris King stainless cog inboard a 5mm spacer and now everything measures up. 50mm from center of seat tube to chainring and 17.5mm from cog teeth to outside edge of the hub. I always thought it looked pretty straight before but now I know it's straight and it doesn't seem to make the chain track slightly to the outside of the front chainring like before. Glad I checked as the chain would have eaten up the carbon bash ring faster then the aluminum one I had.


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## -Muz R- (Oct 18, 2006)

This is the smartest method Ive seen to date, cheers.

Is it fair to assume that not all frames are dead straight?, meaning the rear axle may be slightly off centre to the seat tube. Would this effect chainline?


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## 6thElement (Jul 17, 2006)

Yes, but also if your wheel is dished correctly it would be closer to one set of stays also.


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## anno (Jun 27, 2007)

Good info. I have been on a single speed for almost a week now, and I am starting to mess with changing the rear cog to find the perfect ratio for my commute. I got it used off craigslist, and figured the previous owner must have had it right. This morning my chain popped off. I will measure when I get home, but I bet the chainline is not straight. This is a good straightforward approach. Thanks.


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## Fogerson (Oct 16, 2007)

Velobike said:


> Or the inveterate bodgers method....
> 
> Put the cog on the cassette without any spacers. Put on the chain. Turn the cranks a few times with a bit of drag from the brakes to put tension on the chain.
> 
> ...


Be sure that your bike stand, or whatever is holding your bike while doing this, is setup to where the rear axle is level.

My stand leans a little bit and the cog would walk almost all the way to the end of the cassette. But when I shimmed the stand to ensure the axle was level, it walked out to the exact place where I marked it with the measuring method.


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## proline69 (Apr 14, 2009)

Velobike said:


> Or the inveterate bodgers method....
> 
> Put the cog on the cassette without any spacers. Put on the chain. Turn the cranks a few times with a bit of drag from the brakes to put tension on the chain.
> 
> ...


Keeping it very simple , thanks for the info .Working on a loaner bike now, so i'll be trying this out .


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## ozzybmx (Jun 30, 2008)

I actually have this thread on my fav's, its an awesome topic by fishcreek. When i first read it and checked my "perfect" chainline was out by 4mm (8mm overall).


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## Qtep (May 3, 2012)

Why not just use Lazer pens?


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## Cygnus (Jan 7, 2004)

fishcreek said:


> It's a bit tricky to find the center of the frame, look for bottle mounts.


this is essentially what has been posted on the sticky on chainline math.

but here, why eyeball the seat tube mid point? you have calipers.

i still prefer to use calipers to measure the seat tube diam (and not try to eyeball the middle), then use 1/2 the seat tube diameter added to distance from the edge of the seat tube to the middle of the chainring.

but really, all of these methods are better than just standing behind your bike, looking, and hoping for the best.


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## fishcreek (Apr 10, 2007)

surprised that this 7 year old thread is still going


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## TampaDave (Mar 20, 2015)

Qtep said:


> Why not just use Lazer pens?


I was standing there in harbor freight thinking, so how do I measure the offset? Guess I could lay the laser pointer on the floor and shine it on a ruler. Then snug the ruler against the cog. But, my kick-ass cog doesn't have a flat face on it. And how do I know the beam is exactly centered?

And then I saw a set of calipers for ten bucks...


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## kawsakimx6 (Apr 20, 2006)

fishcreek said:


> 5. Celebrate and get drunk.


I started here. working backwards. This should work fine. Long ass ride tomorrow!


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