There are five hub dimensions:
dL, dR,
S, WL, WR
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d, flange diameter is measured between centers
of opposite holes in the hub flange. It is usually between 38
and 67 millimeters. Note that it is NOT the outside diameter of
the hub's flange. Left and right flange diameters are often, but
not always, the same.
S, spoke hole diameter, is the through diameter
of any spoke hole. It is usually 2.6mm.
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W, width from center to flange may differ
between left and right sides of the hub. In this illustration,
WL and WR are the dimensions you enter into
the spreadsheet for the left and right sides of the hub, respectively.
You can use a hub listed in Spocalc's database, or measure your
own hub by following these steps:
- Measure OLD (Over Locknut Distance).
- Measure Dimension A.
- Measure Dimension B.
- WL = (OLD/2) - A.
- WR = (OLD/2) - B.
Not a lot of precision is needed in the
width dimensions. A few millimeters error in width will only
lead to a fraction of a millimeter error in spoke length.
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I usually measure A and B dimensions by holding
the hub lock nut against a flat surface (like the edge of my work
bench), then measuring from there back to the hub flange with
a ruler or caliper.
Note that you must hold the lock nut (not
the axle end) against the edge.
Of course, I use two hands for better accuracy.
In this photo one hand is working the camera!
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There is one rim dimension: ERD
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Effective Rim Diameter (ERD) is the diameter
on which you want the ends of the spokes to lie. Most people prefer
it near the end of the spoke nipple. If you want to measure your
own rim (recommended, just to be sure), then follow these instructions:
- Insert two old spokes into holes exactly
opposite each other on the rim. Count holes to be sure.
- Screw some nipples onto the spokes.
- Pull them tight and measure dimension
A in the figure (the diameter to the edge
of the nipples, where the spokes dissapears into them). Do
this at several spots around the rim and average the measurements.
- Measure the length of a nipple (dimension
B in the figure) and add it twice (once for
each nipple). The result is Effective Rim Diameter (ERD).
Thus,
ERD = A + 2B.
Effective Rim Diameter (ERD) is the dimension
you type into the spreadsheet for "ERD, effective rim diameter".
Of all the dimensions you actually might measure, ERD is the
most critical dimension affecting spoke length, so it makes
sense to measure it a few times at different places around the
rim. Always count to make sure you use spoke holes that are
actually opposite each other!
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Off Center or Asymmetric
rims require a slight adjustment for spoke lengths. I
don't know enough about Excel to make Spocalc smart enough to
compensate for rim offset, but I have two work-arounds:
the easy way and the exact way.
The Easy Way: Calculate
spoke length as if the rim were symmetrical. Then add 1mm to
the right side and subtract 1mm from the left side on rear wheels,
or add 1mm to the disk side and subtract 1mm from the non-disk
side of front wheels.
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The Exact Way: The idea is to adjust
the hub widths to mimic the rim offset. (The change
in spoke length is the same whether the rim moves relative to the hub,
or the hub moves relative to the rim.) To do this, just change the left
and right flange offsets by the rim offset.
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