LóFarkas

So I'm trying to figure out the spoke lengths for a wheel w/ Mavic Open Pro rim.
Sounds easy, but spocalc says, after listing an effective rim diameter of 605mm that "ERD is Mavic's Nipple Seat Dia + 3mm for nipples" for this and a bunch of other Mavics.

So what's that +3mm? The DT caclulator just lists 605. I don't have the rim in my hands yet.

Retro Grouch 

So do the two programs give you different spoke lengths?

"A man with one watch knows the time. A man with two watches is never sure."

mx_599

sounds like its the thickness of the head of nipple. probably so spoke will come just about through nipple

LóFarkas

No, they give the exact same figures. I just want to be extra sure. I'm building a wheel for another guy and DT Revolutions ain't cheap.

mx_599

i have used the dt calc a couple times and always got good results while lacing. so you can probably just use what it says.

LóFarkas

It'd be nice to get a confirmation of the 605 figure. I'm sure a million people have built up Open Pro's here.

Scooper

I built up a pair of Open Pro rims with Campy high flange hubs, and DT Swiss Competition double-butted spokes with DT Swiss nipples. I ordered the spokes based on the spocalc lengths and it worked out well except for bad advice from the LBS. Spocalc said 288.1mm for the front wheel, 288.0mm for the rear left, and 287.1mm for the rear right (drive side). The LBS told me to order 288.0mm for everything, and they were wrong; the drive side spokes were too long (nipples bottomed out on the threads before tension could be achieved), so I ordered 286.0 mm for the right rear and that worked perfectly.

These were 36h wheels laced 3-cross.

LóFarkas

^ They're pretty low profile so 605 sounds credible to me.

Hmmm, maybe I should round one of the lengths down instead of up then... I hate this spoke calculation and esp. that Revos come 2mm apart.

Scooper

Definitely round down instead of up. If you round down by even 2mm, there will still be plenty of thread on the spoke for the nipple to grip to.

Al1943

+1
Yep, I learned to always order driveside rear spokes 1 mm short.

Al

Scooper

Yes. The Open Pros have an ERD of 605 (I just measured a brand new Open Pro rim I have).

Al1943

My mistake, I thought the OP was talking about a 650 rim. Didn't read carefully, I hate it when I do that.

Scooper

Al,

Here's how I measure (from Roger Musson's Wheelbuilding:

Al1943

Kool, thanks.

LóFarkas

Hey, this thread was worth starting for that little diagram alone. Cutting the elbow off is clever.

Thx everyone

cachehiker

I laced up my Open Pros on Ultegras with DT Comp 14/15's using the values given to me by Spocalc and they were perfect. It's been a while so I don't remember the exact lengths but I don't round up unless it's only 0.1-0.2mm. This is especially true given the additional elongation of the DT Revos and Comp 15/16's under tension.

LóFarkas

One of the figures the progs spit out is 291.7 Can't see rounding that down to 290... so 292 it will be.

Al1943

Round down if it's the driveside rear or if the spoke is a Revolution.

Scooper

I agree. I'd round down to 290.

Scooper

My earlier measurement of the Open Pro rim confirming the 605mm ERD was "eyeballed".. I've since used the two 200mm spokes with the spokes threaded into nipples flush with the screwdriver slot, and measured according to Musson's method, the ERD is 602mm. The only way I can explain the 3mm discrepancy is if the spokes were screwed in flush with the top of the nipple instead of the screwdriver slot, it would be 205mm.

The nipples themselves are ~10mm long, which is the reason I suggest rounding down instead of up, especially since double-butted spokes can stretch a little under tension. What you don't want to happen is to bottom out the nipple on the spoke threads before you achieve the necessary spoke tension. If the spoke is just a fraction of a mm too long, that can easily happen.

Here are some photos of the 200mm measuring spokes and the distance between the spoke ends (202mm) with the spokes inserted in opposite holes in the brand new 700c Open Pro rims.

DannoXYZ 

That's never a problem because the nipples are only threaded about 1/2 their length anyway. Take those nipples you have on the cut-off spokes and spin them down until they bottom, you'll find that the spokes will be sticking out about 5mm above the nipple heads.

I usually err on the side of too-long rather than two-short by 1-2mm. I've seen quite a few nipples break because the spokes weren't fully threaded up to the top. Brass is a lot weaker than steel...

Scooper

Hmm. The DT Swiss nipples are 12mm long, including the head, and the threads start 3.5mm from the bottom. That's a little over 2/3 of the nipple length that's threaded. The problem I've had, mentioned earlier in the thread, is that rounding up to 288mm from the 287.1mm spit out by spocalc, I ran out of threads on the 288mm spokes before I could get close to the proper tension.

I'm glad you've had better luck rounding up.

Soil_Sampler

I used 603 ERD on my open pro's and DT Revo's and nipples.
https://www.geocities.com/spokeanwheel/measure.htm#rm
https://www.geocities.com/d_halem/wheel/wheel.html

halfspeed

I've had the same problem and also always round down. It doesn't take much extra length to run out of threads.

LóFarkas

Yeah, but on one of my wheels the thread is showing at the bottom of the nipple. That's not nice, either. This is why is hate spoke calculation with a vengeance.

Thx for the measuring, Scooper. The diagram also talks about screwing the spoke in until it's at the bottom of the screwdriver slot, so I dunno. I think Sheldon recommends measuring the points of the rim where the spoke head sits, which would be a further 1mm inboard... 600 then? I'll be damned if I understand.


I guess I'll go 290 on the drive side. BTW, still nobody told us what the heck the +3mm is... that was the point of the thread