Singlespeed & Fixed Gear - Rear Wheel, bottom bracket chainline question

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Chef23
01-13-05, 01:50 PM
I am building my first fixie and am about to order a rear wheel for it (I am thinking an IRO as it seems like a good deal). What issues will I run into regarding the bottom bracket and spindle length. I need a BB as well and am confused about what spindle length I should get to ensure proper chainline. Can I adjust it as needed with different size spacer bolts on the crank set? Should I order the wheel first and then can I do some sort of measurement to determine the right spindle length?

Thanks from a newbie for your help.

willfcc
01-13-05, 02:10 PM
The IRO MTB hub has a 52mm chainline, and the track hub has a 42mm chainline, according to their website. So:

Measure the distance from the seat tube to the center of your current chain ring. (x)
Measure the diameter of your seat tube. (y)
x + (y/2) = current crank chainline.
Subtract hub chainline from your current crank chainline.
If the result is negative, increase BB width by twice the result.
If the result is positive, decrease BB width by twice the result.
Take off cranks, measure BB width.
Buy the closest size BB to the calculated size.

Example:
Seat tube to chain ring = 30mm
Seat tube diameter = 31.8mm
Chain line = 30 + 15.9 = 45.9mm
45.9 - 42 (track hub) = 3.9mm offset
Current BB width = 120mm
120 - 7.8 = 112.2
You need a 112.2mm BB, get the closest size you can find, probably 113. The resulting chainline error will be less than 1/2 mm.

Will

H23
01-13-05, 02:12 PM
Sheldon Brown's website has chainline measurements for common crank-spindle combos and common hubs. I would use that as a starting point. You can then adjust w/spacers to get things just right.

Another option is to get it right with one of those adjustable phil wood BB. I would do that if the bike was very nice and worth it.

BlastRadius
01-13-05, 04:30 PM
Question is what cog is used for the 42mm chainline of the IRO hub? Different cogs have different thicknesses.