Wheel building/spoke length question

I plan on building a set of XT M760/Sun CR18 36H, 3x. Wheelsmith
calculator gives 295/296mm for the rear, and spokecalc gives
294.9/396.5mm. My wheelbuilder uses the Wheelsmith calculator with DT
spokes and always adds 1mm to their calculation, which would result in
296/297mm. I plan on using DT 15/16 spokes. The spokes I have on
hand are 297/298mm. Opinions please.

Thanks,
Robin Hubert

Robin Hubert Wrote:
I plan on building a set of XT M760/Sun CR18 36H, 3x. Wheelsmith
calculator gives 295/296mm for the rear, and spokecalc gives
294.9/396.5mm. My wheelbuilder uses the Wheelsmith calculator with DT
spokes and always adds 1mm to their calculation, which would result in
296/297mm. I plan on using DT 15/16 spokes. The spokes I have on
hand are 297/298mm. Opinions please.

Thanks,
Robin Hubert

I would caution you that Sun isn't the best in keeping ERDs the same
within the same "model".
When I run spocalc I get 297.6 mm front, 296.3 mm left rear, and 294.8
mm right rear for you planned build.
If you have the Wheelsmith rods or appropriate spokes/nipples, I would
check the actual ERDs of the rims in question.

I think that the spokes you have on hand will end up protruding from
the back of the nipples when fully tensioned. Also remember that you
will get more compression from the CR18 as it isn't quite as radially
stiff rim when compared to Velocity Dyad (around the same weight, but
stiffer due to it's profile)
I would use 296 mm front, 295 left rear, and 293 right rear; but it all
depends on the actual CR18s and the specific nipples you use.
You could manually enter the data into the DT caculator once you have
rim ERD. DT's calculator includes specific nipple and expected spoke
stretch into their calculations.


--
daveornee

daveornee wrote:
Robin Hubert Wrote:

I plan on building a set of XT M760/Sun CR18 36H, 3x. Wheelsmith
calculator gives 295/296mm for the rear, and spokecalc gives
294.9/396.5mm. My wheelbuilder uses the Wheelsmith calculator with DT
spokes and always adds 1mm to their calculation, which would result in
296/297mm. I plan on using DT 15/16 spokes. The spokes I have on
hand are 297/298mm. Opinions please.

Thanks,
Robin Hubert


I would caution you that Sun isn't the best in keeping ERDs the same
within the same "model".
When I run spocalc I get 297.6 mm front, 296.3 mm left rear, and 294.8
mm right rear for you planned build.
If you have the Wheelsmith rods or appropriate spokes/nipples, I would
check the actual ERDs of the rims in question.

I think that the spokes you have on hand will end up protruding from
the back of the nipples when fully tensioned. Also remember that you
will get more compression from the CR18 as it isn't quite as radially
stiff rim when compared to Velocity Dyad (around the same weight, but
stiffer due to it's profile)
I would use 296 mm front, 295 left rear, and 293 right rear; but it all
depends on the actual CR18s and the specific nipples you use.
You could manually enter the data into the DT caculator once you have
rim ERD. DT's calculator includes specific nipple and expected spoke
stretch into their calculations.

The ERD for that rim is "612". I just built a set 3X with the XT M752
Rear hub in 36 hole, and an XT M753 Front hub in 32 hole. I used
Damon's spocalc.xls to figure the lengths and ended up with 295/296 in
the rear and 300 in the front laced 3X. When I run this with 760 hub,
34.2 Width from center to flange, and 38.0 diameter 36 hole, 3X I come
up with 297.7 for the front length, so 298's should work fine there. So
far I'm liking the wheels. Of course, the hard part was figuring out
which spoke hole to start with to get the valve stem, hub label, wheel
label in line...


--
Mark Wolfe Lakeside, ca http://www.wolfenet.org
gpg fingerprint = 42B6 EFEB 5414 AA18 01B7 64AC EF46 F7E6 82F6 8C71
Life's a switch() and then you die().

Robin Hubert Wrote:
I plan on building a set of XT M760/Sun CR18 36H, 3x. Wheelsmith
calculator gives 295/296mm for the rear, and spokecalc gives
294.9/396.5mm. My wheelbuilder uses the Wheelsmith calculator with DT
spokes and always adds 1mm to their calculation, which would result in
296/297mm. I plan on using DT 15/16 spokes. The spokes I have on
hand are 297/298mm. Opinions please.

Thanks,
Robin Hubert

I entered 612 mm as the ERD into DT's calculator with the following
results:
Front Spoke length
precise le. 297.90 mm ri. 297.90 mm
rounded
(incl. Corrections) le. 297 mm ri. 297 mm
Weight front n/a

Rear Spoke length
precise le. 296.20 mm ri. 294.80 mm
rounded
(incl. Corrections) le. 296 mm ri. 294 mm

DT's results are 1 mm longer all around than what I originally
suggested.
I strongly prefer to have the spokes 1 mm short than have the
possibility that they will be 1 mm long. Nipple material is soft
enough when you run out of threads to allow the spoke to "keep going".
However, this doesn't make for an optimum build.

If you deliver the parts with the lengths I specified and have me build
them for you, see what you think of the results... and then we can talk.


--
daveornee

Mark Wolfe wrote:snip

Of course, the hard part was figuring out
which spoke hole to start with to get the valve stem, hub label, wheel
label in line...


that bit's easy. if it's a 3 cross wheel, count 3 holes from the hub
label on the left side. on the 4th hole, insert the first spoke, head
out, and put that on the leading side of the valve hole.

2 cross is 2 empty holes before the spoke.

Robin asks- Wheelsmith
calculator gives 295/296mm for the rear, and spokecalc gives
294.9/296.5mm. BRBR

I write-"296 and 298 is what I would use".

I round and then add 1 using DT spokes.

Peter Chisholm
Vecchio's Bicicletteria
1833 Pearl St.
Boulder, CO, 80302
(303)440-3535
http://www.vecchios.com
"Ruote convenzionali costruite eccezionalmente bene"

Robin Hubert wrote:

I plan on building a set of XT M760/Sun CR18 36H, 3x. Wheelsmith
calculator gives 295/296mm for the rear, and spokecalc gives
294.9/396.5mm. My wheelbuilder uses the Wheelsmith calculator with DT
spokes and always adds 1mm to their calculation,...

That's because DT length is rated as effective (in tension), rather Wheelsmith's
untensioned rating. Your 297/298 probably measure 296/297 on the average
untensioned.

which would result in
296/297mm. I plan on using DT 15/16 spokes. The spokes I have on
hand are 297/298mm. Opinions please.

You might run out of threads. Get the right spokes.