Wednesday, January 07, 2009


So after footering around to try and find a way to get the chainline sorted, i looked up old friend 63xc. Matt chester may not be flavour of the month, but he describes very nicely how to measure chainline. You need vernier gauge and a good ruler. ideally mm measurements.

Taker vernier gauge and measure diameter of seat tube. Half it. This is A.
Measure distance from middle of tooth on chainring to edge of seat tube. This is B.
Add A and B.

Then with ruler measure distance from inside of drop out to middle of cog at rear. This is C.
When measuring, measure with all bolts/creaks tight as they would be riding.

If you are using a 135 hub spacing, then the number to note is half this, or 67.5mm. Adjust for hub width as required.

C+A+B should equal 67.5 or as close as dammit.

So as there is limited movement with a screw on rear cog laterally, you are down to adjusting the B number for chainline. If you use a tomicog or london fixie cog then you can alter both.


matt said...

I used to use that method, but now I just guesstimage the correct spacing, put wheel on the bike, grab a long spirit level (or long straight thing, metal rule, piece of wood etc.

hold one end of the rule against the edge of the chainring (just above the chainring bolts, and flat against the side of the ring) and the other end of the rule will be near the cog, and if you have spacing correct the rule will perfectly meet the side of the cog, if not you have a 'by eye' measurement of spacers you need to remove or add.

(quicker to do than to explain)

I'm not really gay said...

"pedant mode'

a ruler is a king or queen, you meant to say a rule, ruler was guarenteed to get you a kick up the arse back in the days of apprenticeships ;-)

"end of pedant mode"

I'm seriously taken by those big fat front wheels, next time we meet make sure you have that bike please !

Shaggy said...

Steve: apparently that is no longer the case. The dictionary has been updated to reflect common usage :-)

Mat Chester's method is all well and good but I bet a lot of frames don't have alignment accurate to a mm...

grant said...

Gold cog....nice.

Anonymous said...

Something wrong with your eyes?

dRjON said...

in the interest of public info. new chain and chainline sorted. last night wet muddy, hoopy steep ride - dropped chains=none.

dave (chops optional) said...

Dropped/tossed riders - countless....