front dynohub - wheel building

i'm trying to calculate the spoke length for a wheel with a front
sturmey archer dynohub and a sun CR18 26x1-3/8 rim.
in the instructions for wheel building
http://www.rickadee.net/~zephyrus/ad4935/ad4935.html (thanks!) they
write that the rim should be off-centre.
does anyone know the reason for that and how much off should it be?
better still, the exact lengths of the spokes?

dan

Dan Limon writes:

I'm trying to calculate the spoke length for a wheel with a front
Sturmey archer dynohub and a sun CR18 26x1-3/8 rim. In the
instructions for wheel building:

http://www.rickadee.net/~zephyrus/ad4935/ad4935.html

they write that the rim should be off-centre. Does anyone know the
reason for that and how much off should it be? Better still, the
exact lengths of the spokes?

Use the standard spoke length programs available on the web and enter
cross zero for the lacing pattern. The 'dish' of the wheel is
immaterial to spoke length, the cosine of small angles being
essentially one. The offset will develop itself as you center the rim
between the locknuts on the axle as is standard for wheels.

Jobst Brandt

wrote:
Dan Limon writes:

I'm trying to calculate the spoke length for a wheel with a front
Sturmey archer dynohub and a sun CR18 26x1-3/8 rim. In the
instructions for wheel building:
....

Use the standard spoke length programs available on the web and enter
cross zero for the lacing pattern. The 'dish' of the wheel is
immaterial to spoke length, the cosine of small angles being
essentially one. The offset will develop itself as you center the
rim
between the locknuts on the axle as is standard for wheels.


Dan- Damon Rinard's Spocalc has values for many hubs and rims already
in place. I checked mine (an older download) and it has values for the
Sturmey Dynohub and a Sun CR20 rim. If those aren't close enough, you
can measure the rim yourself and manually input values. It's available
at http://www.sheldonbrown.com/rinard/spocalc.htm . As Jobst says, any
assymetry will resolve itself as you true the wheel.

Jobst- why "cross zero"? Surely you aren't recommending he build the
wheel radially. The keyhole spoke holes on the small flange would make
it difficult to keep the wheel tensioned and true, I think.

Jeff

Jeff Wills writes:

Use the standard spoke length programs available on the web and
enter cross zero for the lacing pattern. The 'dish' of the wheel
is immaterial to spoke length, the cosine of small angles being
essentially one. The offset will develop itself as you center the
rim between the locknuts on the axle as is standard for wheels.

Dan- Damon Rinard's Spocalc has values for many hubs and rims
already in place. I checked mine (an older download) and it has
values for the Sturmey Dynohub and a Sun CR20 rim. If those aren't
close enough, you can measure the rim yourself and manually input
values. It's available at:

http://www.sheldonbrown.com/rinard/spocalc.htm

As Jobst says, any asymmetry will resolve itself as you true the
wheel.

Jobst- why "cross zero"? Surely you aren't recommending he build the
wheel radially. The keyhole spoke holes on the small flange would
make it difficult to keep the wheel tensioned and true, I think.

I don't see any reason to cross lace spokes on a steel flange of that
diameter. I have one like that and the small flange should be cross
three so that the double keyhole spoke holes in the hub will give
their best support to the spokes.

Jobst Brandt

wrote:

I don't see any reason to cross lace spokes on a steel flange of that
diameter. I have one like that and the small flange should be cross
three so that the double keyhole spoke holes in the hub will give
their best support to the spokes.

Jobst Brandt


OK- radial on the large flange side, 3X on the small flange. It'll look
cool!

FWIW: there's a drum-braked single-speed cruiser running around town
with a radial-spoked front wheel. I don't think it gets heavy braking
on that wheel...

Jeff

Quoth Jobst Brandt:

[Sturmey-Archer DynoHub with huge flange on one side]

I don't see any reason to cross lace spokes on a steel flange of that
diameter. I have one like that and the small flange should be cross
three so that the double keyhole spoke holes in the hub will give
their best support to the spokes.

There's no techincal reason, but as a practical matter, it may be
difficult to find appropriate length spokes for this at a typical LBS.

Sheldon "253 mm?" Brown
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