Bicycle Mechanics - Getting a straight chainline... road double on FG

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musicsucks
06-05-07, 12:52 AM
okay, i feel like a dink for asking this here since i know that it has probably come up a million times- but i can't figure this out and it's making me go insane.

I want to buy the cheep ultegra crankset that i keep seeing all over ebay and such (http://cgi.ebay.com/SHIMANO-ULTEGRA-FC-6500-DOUBLE-CRANKSET-CRANKS-NEW_W0QQitemZ160123177455QQihZ006QQcategoryZ56195QQrdZ1QQcmdZViewItem)
which means that i have to get an octalink BB...

So! I have an IRO rear hub (36.0, according to sheldon) and a Surly Cog (good for another 5.07?) so by my math i need a 41.07mm chainline?

If i get the octalink 109.5 V1 blah blah, i get a chainline of 43.5.

Does this math add up to good, or bad? Is being 2mm out terrible? When you measure a road double, you're suppose to measure the middle ring, but i'm pretty sure i want to use the outer ring, so does this all not mean anything anyways?

Soil_Sampler
06-05-07, 06:56 AM
If i get the octalink 109.5 V1 blah blah, i get a chainline of 43.5.

Does this math add up to good, or bad? Is being 2mm out terrible? When you measure a road double, you're suppose to measure the middle ring, but i'm pretty sure i want to use the outer ring, so does this all not mean anything anyways?
Chainline is measured between the chainrings on a double. It will be 41mm +/- on inside chainring.
Get a 105 crank(cheaper), sell the chainrings and get a non-ramped/pinned ring and put it on the inside(small ring side) of the crank. Get single ring chainring bolts.

Coupon code CR231 10% 0ff
.

HillRider
06-05-07, 07:41 AM
...... get a non-ramped/pinned ring and put it on the inside(small ring side) of the crank. Get single ring chainring bolts.
I have no way of checking this myself as all of my bikes have triple cranks and the middle chainrings have the ramps/pins to aid shifting to and from the granny ring.

However, isn't the inner ring of a double crank "flat", i.e. it doesn't have the shifting enhancements which are only needed on the big ring? If that's the case than any double crank can be used by just removing the outer ring and using single ring bolts as noted.

The only problem may be if the inner ring is smaller than you want to use.

vpiuva
06-05-07, 07:53 AM
okay, i feel like a dink for asking this here since i know that it has probably come up a million times- but i can't figure this out and it's making me go insane.

I want to buy the cheep ultegra crankset that i keep seeing all over ebay and such (http://cgi.ebay.com/SHIMANO-ULTEGRA-FC-6500-DOUBLE-CRANKSET-CRANKS-NEW_W0QQitemZ160123177455QQihZ006QQcategoryZ56195QQrdZ1QQcmdZViewItem)
which means that i have to get an octalink BB...

So! I have an IRO rear hub (36.0, according to sheldon) and a Surly Cog (good for another 5.07?) so by my math i need a 41.07mm chainline?

If i get the octalink 109.5 V1 blah blah, i get a chainline of 43.5.
Does this math add up to good, or bad? Is being 2mm out terrible? When you measure a road double, you're suppose to measure the middle ring, but i'm pretty sure i want to use the outer ring, so does this all not mean anything anyways?
As S_S points out, your chainline to the inner ring position will be 41mm with this setup. If you're talking about using the outer ring postion, not just the outer ring, you will be at 46mm and off 5mm, too much. Even if you could buy a shorter spindle, using the outer ring position is difficult in many situations, so go with the inner.

Soil_Sampler
06-05-07, 07:55 AM
The only problem may be if the inner ring is smaller than you want to use.
Limited sizes. Usually 39 or 42T non-ramped/pinned.

HillRider
06-05-07, 08:04 AM
Limited sizes. Usually 39 or 42T non-ramped/pinned.

Right, and the 42T rings are usually part of a triple crank which the OP wants to avoid.

Soil_Sampler
06-05-07, 08:19 AM
Right, and the 42T rings are usually part of a triple crank which the OP wants to avoid.
Not always. 42T is an option on Shimano doubles and triples!