John Haas said:Ok, I'm a novice wheel builder and there are several things I don't
understand yet. If any of you verterans out there can help me I would
sure appreciate it.
A couple of weeks ago I rebuilt an old set of wheels with new hubs and
had fun doing it. Of course I was armed with Jobst Brandt's book which
made things fairly routine. On these wheels I didn't have to worry
about determining spoke length.
This first wheel project emboldened me to attempt something I've been
wanting to do for a couple of years. I've had a brand new pair of
Mavic Open Pro 700c 36 hole rims laying about that I've been wanting
to build up. The only major stumbling block was what spoke length to
use. I downloaded Spocalc and proceeded to enter in the rim/hub
(Shimano 105) info and it told me that, on the front wheel, the spoke
length should be 294.8mm. The rear wheel needed 292.4 and 291.3mm
spokes. This is with a 3X pattern throughout. I rounded the numbers
off to the nearest whole millimeter and ordered the spokes. I
attempted to build up the front wheel but quickly determined that the
spokes were too long. I don't know what went wrong with the spoke
calculations but probably the spokes I ordered for the rear wheel are
too long also. I'm guessing these spokes are in the neighborhood of
3mm too long but I don't know for sure.
My questions are...
What did I do wrong with Spocalc to come up with a too high spoke
length?
What is this with a crossing pattern value that allows decimals? How
can you have a crossing pattern of 2.25 for example?
Has anyone, by chance, put together a set of wheels using Mavic Open
Pro 700c 36 hole rims and Shimano 105 hubs who could just flat tell me
the proper spokes lengths to use?
Thanks!
John,
One subtle aspect of spokcalc is that it considers ERD to be the
distance from nipple bed to nipple bed PLUS the thickness of the "nipple
head". The reason it does this is that there are still good threads in
there, and that's what counts in terms of spoke length - not nipple bed
to nipple bed, but 'end of nipple to end of nipple when seated in the
rim'.
From the instructions:
----
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!
----
So how could this explain too long spokes? Well, maybe it can't, but
in my experience, if I use the ERD database in Spocalc, or the method of
measuring where I include the nipple head, my spokes tend to be almost
too long. So I typically directly measure the ERD and use the true ERD
of the rim in spocalc, not counting the nipple heads.
Kyle
