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chainline

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JimK:
A month or two ago I was chasing a clicking at frequency of my cranks. Turned out to be pedals. But I was worried it might be the bottom bracket. That's probably the part of a bike I understand the least & so I have been studying up a little.

This morning I made a little measurement on my Nomad. I measured from the center of my down tube over to the chainring. I came up with 60mm. Now, what I measure was my chainline, right? The ideal chainline for the Rohloff is 54mm, yes? Surely 6mm is no big deal, but still.

I am trying to understand how all these measurements fit together. E.g. suppose I wanted to fit this crankset:

http://www.rivbike.com/product-p/cr7.htm

I imagine the chainline for a double chainring is measured to the halfway point between the rings? and the standard chainline is 47mm, i.e. the Rohloff is 7mm outside the standard?

Probably the spindle is centered. So if I want the drive side to stick out an extra 7 mm then I need a spindle that is 14 mm longer... well, if I would be using the 100 bcd ring then I wouldn't need the full 7mm... maybe just 4mm.. so maybe I want a 118 mm spindle or thereabouts?

I have no plan to swap anything at this point. But I am thinking about buying something to have it in house in case of failure. Anyway if anybody sees a flaw in my reasoning.. I would appreciate any tips!

JimK:
I should collect the previous posts on this question!

http://www.thorncycles.co.uk/forums/index.php?topic=3898.0

Andre Jute:
Lot of speculation in that thread, Jim, as we all stumbled (very agreeably though) our way to enlightenment. The post you want is this one: http://www.thorncycles.co.uk/forums/index.php?topic=3898.msg19603#msg19603

If you measured right and you're honestly 6mm out, you don't want components on standby, you want to act and soon, before you have to change the entire drivetrain.

A Rohloff chain is supposed to run straight to within 1mm. Mine is less than half a mil out. You make fine adjustments with thin shims behind the bottom bracket lockring.

One more point. That XD2 crank is good, and commonly available from about a sixth of the price those boys want. Spa Cycles in the UK will sell you a set of arms for about twenty pounds to which you can then fit a Surly stainless chainring and have money left over to buy several more of the same, Thorn has something with their own brand on it which looks suspiciously like it (which would make sense as Thorn's central philosophy is about excellence at realistic prices), Stronglight packages the same thing as their Impact line (try XXCycles in France for good prices and delivery). Rivbike appears to be pretty pricey.

JimK:
Thanks, Andre. I can see that the measuring process is a bit tricky so I will have to repeat that a few times & maybe the time has come to shop for some calipers. I expect that coarse adjustment can also come from moving the chainring. Mine is now on the outside. If I just moved it to the middle ring position, seems like that'd go a long way toward straight.

JimK:
eek! Moving the chainring from outer to inner position seems to move it 10 mm. So I go from about 60 mm to about 50 mm.

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