Got a 120mm spaced pompino, surly cog, halo aerorage wheel and very old campag record cranks. Got my local bike shop to install a bb with the idea of getting perfect chainline.... (they told me it was 'almost' perfect, as good as it was gonna get)
Anyway according to sheldon brown i've now got 40mm rear chainline and 44mm front chainline.
What is the best way to measure to get this spot on?
Cheers for any help offered
For a 120mm rear hub it's:
60mm - (centreline of sprocket to inside of drop out)
and for the cranks its just the centre of the chainring to the centre of the seat tube
You can get chainring spacers. Is the chainring on the outside or in?
Measure the distance from the centre of the seatpost tube to your chainring, that gives you your real life front chainline figure. To determine the rear chainline, measure the distance from the inside of the rear fork end (dropout) to the cog, and take that figure from 60 (which is half of your overlocknut dimension). That will give you the distance from the centreline of your bike to your rear cog, ie your rear chainline.
Anything else feel free to ask, but I work nights and I.m going to bed in a bit.
Anthony
I just lie a long straight thing along the chainring and if it points at the cog, good. I use a spirit level or a stick, whatever's closest to hand.
Picture below demonstrating that my Pomp 120mm chainline's a bit off. Ah well.
http://www.flickr.com/photos/allankelly/4334232175/
al.
For a 120mm rear hub it's:
60mm - (centreline of sprocket to inside of drop out)
That's not quite right. 60 mm is the centreline of the bike to the inside of the dropout. To get the chainline you take the rear sprocket-inside of dropout measurement away from the 60mm
Unless you are blind, it's easy to check the alignment by eye.
Just to add, the chainline on my fixie is off by about 1.5-2m/m, and had been for the last 20 years. Never seems to cause any problems.
ok, just to update. midline of seatpost to centre of chainring is 44mm exact and the centre of the cog to the inside of the dropout is 20mm exactly i.e. 40mm chainline . The chainring is on the inside of the crank.
From what i can make out i just need to put 4mm of spacers or better still a smaller bb in to make it perfect?
Thanks for all the responses
Why are you measuring to the inside of the dropout? The chain's a good 10mm inside of that.
Just use a stick.
al.
Does it matter so long as the cog / chainring are pretty much straight in line?
If you have proper single speed cogs and chain, the bike is very tolerant of a small amount of misalignment.
Only really a problem if you are trying to give Sir Chris Hoy a problem.
Still it's nice to get it right. Quieter, smoother.
Bear in mind that using measurements does not allow for some bikes having manufacturing tolerances, so the centre of the OLD may not always co-incide with the centre of the downtube. Which is why eyeball is generally good enough (unless SIr Chris etc). Also listen, ugly noises mean it needs aligning better.
A minor adjustment that you may/may not have tried is to mount your front chain ring on the other side of your spider. Sorted my chainline out!
Same with your rear sprocket if it has a bias. My surly cog is inward bias so could add a few mm if flipped. Should still thread on, like a nut.
fixies are far more tolerant of dodgy chainlines because you run them with a tighter chain than SS's which obviously everyone does, right?
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