msl109

Hello folks - building what's not my first set of wheels for a friend and have reached my frustration breaking point.

Rims are 28H Open Pros, 3X lacing

Hubs are Dura Ace 6sp freewheel hubs HB 7400

Ordered spokes for the front after calculating 299 mm length +/- 1mm with more than one online spoke calculator, using my own measurements which seemed to support published data.

Mavic's spoke calculator, which only asks for what you see in the pic (I assume they have ERD in the database) supports the same number.

When built, the spokes were only tensioned enough to basicaly make them straight - way too little tension.

I've read multiple things about Open Pros being inconsistent in ERD and am wary to trust published data, so I was hoping someone has had experience building a wheelset with these hubs and rims and could advise.

- or at least someone who could give me certain data on the rims and hubs. at least a certain ERD! Tried measuring this myself and have had trouble being precise. Don't care if a thread or two is showing when wheels are built but the guy I bought the spokes from is willing to modify the length for a 20% charge and I don't want to pay that twice.

Going nuts - I've built several sets of wheels and was never off unless it was my own mistake in measurement. I've triple checked. Don't want to even begin with the rear measurements until I know what's going on here.

All and any help appreciated!

maddog34

here... try this calculator... Spoke Calculator for bicycle wheels | Prowheelbuilder.com

i choose to round the length to the short side of the numbers generated, up to 1mm shorter than the recommendations...... seems to work every time.

msl109

Thanks - tried that. Maybe I missed something, but the hubs aren't listed - I chose the 7800 model, which I think is the closest. It only seemed to shave the length down to 298.4, and I'm not sure the front hub is the same as the 7400. I imagine the rear is definitely different, being a freehub.

maddog34

i re-read your top post... so... thespokes were tightened until the ran out of threads, and were still way too loose, right?

are you certain you laced the wheel correctly? check again, even if "certain", ok?

i just measured a A 7400 front, with open pro rim... came up with @ 299 actually tensioned length... so it might have stretched a bit under tension... and no, i'm not going to remove a spoke to check untensioned length, since the wheel is dead on right now... ;-) it's a 36 spoke, but that won't vary the length much, i'd think....

does anyone know how much, if any, spoke lengths change when lacing less spokes?

msl109

No - wouldn't expect you to delace your wheel Yes, till I ran out of thread. Length changes quite a bit from 36 to 28 - since the spokes are spaced out more, they have to be longer to reach. I would think if anything yours would be shorter. Bike shop checked my lacing after and agreed they were too long. But they came up with the same thing for spoke length! Measured the spokes as well. Now I'm even more puzzled. Are yours 3 cross laced?

Bill Kapaun

Yes
Everyone that ever wrote a or used a spoke length calculator app.
just change the number of spokes and magic.

Maybe the OP forgot to change ?? to 28?

maddog34

go for nice long ride, Bill.... you've turned into a grumpy troll lately.

and since you allude that you know an answer, please provide it.

quite frankly, i look up the spoke length for the combo i'm lacing, then get those spokes cut a millimeter shorter... never taken the time to compare lengths specified for a different pattern or count of spokes... have you?

Jiggle

I use double square nipples. Not sure if they will clear Open Pros, but they allow a +4.5mm spoke length margin for error. I can build most wheelsets with one spoke length.

msl109

Check the pic I posted - I put in the correct spoke count.

msl109

I'm sure the calculator is wrong. Just hoping someone familiar with this combination can report on the correct length.

maddog34

oooh-kay... at this point... i'd assume that SOMETHING was miscalculated or mis-measured... and Open pro rims have changed designs over the years... the fact that the Mavic calculator does not call for a specific ERD has me doubting it's validity.

take the hubs and rims to a wheel builder's shop, and see what spoke length they prescribe, then show them what you've already tried to build the wheel with...

and when i punched in the 7800 front hub, and open pro 622(700c) rim, at 28 holes, i came up with 297mm or so spokes... once again pointing to ambiguity in open pro rim ERD and designs, i'd guess...

go to a wheel builder shop...?

and i feel your pain... my first wheel build was a Bultaco Lobito 100 M/C rear rim/hub...it took 4 tries to get it right..... sigh... no books, no internet to scour for vids... just me, a new hub to replace the cracked one, and... gasp... a crescent wrench... i was ten years old, and had a very limited budget.... i bought my first tool kit to fix that goofy bike.... and worked on it MUCH more than i rode it... i swapped it for a Honda Z-50 mini trail... best trade i'd ever made! 1969............. i still remember that crazy Bultaco starting BACKWARDS occasionally... that was always amusing! you never knew until you let the clutch out.... the rear hub was cracking again when i swapped it for the Z-50... The Bultaco dealer had them in stock... they ordered 5 at once... think they KNEW, or what?

3alarmer 

.
...I have used the calculator online at Wheelpro for many years, and had pretty good results with it.

I measure the hubs myself, using the method as illustrated here to input the hub data.

I usually do a measurement of ERD myself in a couple of spots, then compare it to what's listed for the rim by the maker.
It's never far off, but I'm not afraid to use either measurement, because a few mm of ERD don't seem to affect the end result that much.

I've built at least one set of wheels using those hubs, but not with those rims, so i don't have anything else practical to offer.

msl109

Funny story. First wheel build for me was along time ago and a complete disaster. Few years ago tried again with the help of a great guy at my LBS. Stepped me through the whole thing - and helped me out with the measurements. I used Open Pros for that build, and as I recall, he miscalculated them the first time around. He was really thrown by it - the guy had built thousands of wheels, really knew his stuff. He adjusted by feel a few mm and all was well. I'm wondering now if he miscalculated at all - he may have gone through what I am now. In any case, this wheelset is about my fourth or fifth since that training and this is the first time I've used OPs again and the first time I've been off ...

I did go to one bike shop - they calculated 300mm, which made me feel better but didn't solve my problem ... maybe I'll try to find a shop that deals primarily with wheelbuilding and inquire.

msl109

I'll run them through that calculator as well and check the method you're talking about. I used a spoke and a shifter cable made taut to determine ERD - I was off what was listed by a couple mm but the difference, as you say, didn't make that much difference..

Thanks for the help

msl109

Hmm. Now that's interesting - by a +4.5mm margin for error, you mean when the spokes are too short? I can't visualize how they would compensate for too long a spoke but maybe I should use them when I rebuild and go a little short if that's how they compensate.

dabac

It's rare for a calculator to get the maths wrong.


However, GIGO applies. Garbage in, garbage out.


ERD has two interpretations.
Either where nipple meets rim, or where you want the spoke end.
First wheels I built, Mavic's published ERD numbers referred to where nipple met rim, so you had to add 4-5 mm to get the spoke to reach the flange of the nipple.
Last wheels I built with Stan's rims, Stan's too used this as their published number.


If you run out of thread Before reaching useful tension, this ERD confusion could easily explain that.
Maybe MAVIC still consider nipple/rim as the ERD, and then has the calculator add a Little to compensate.
Then you come along and input spoke end as ERD and end up too long.
Double compensation.


There are two cheap, but a Little time consuming ways out of your current situation:
1) add two washers with 4 mm ID under each nipple
2) drill out 2-3 mm of thread from the hubside end of the nipple.

dabac

Too short can kinda-sorta be compensated for by using longer nipples.


That concept is a bit intriguing.
DT sells 16 mm nipples, where you'd need to be really, really spot on to get the spoke fully into the flange of the nipple - with Little to no room for further trueing.
And DT doesn't post any disclaimers about the use of these nipples.


OTOH, it's quite well accepted by wheel builders only to use spokes that do reach fully into the flange portion of the nipples.


So either builders are overly cautious, or DT are a bit daring.
Take your pick.





That part is easy.


The threaded length in a (DT) spoke is something like 3-4 times longer than needed WRT strength(but it can make build easier). By leaving more of the nipple "empty" at the hub end, the nipple can thread on longer. And since overshoot is rarely a problem on double-wall rims, it makes for extra adjustability.

msl109

Thanks - as things are, the ERD being slightly off could make the difference. 1.5. -
2mm difference in length might be enough to get proper tension. Thanks for the two final suggestions - that's actually very helpful. I may go one of those routes as I continue to probe this mystery.

davidad

I found that the width on a DA front was 37mm and the hole circle dia. is 38mm. Using 604 for the erd I come up with 296mm for the spokes using Damon Renard's Spoke Calc.

Jiggle

No, you can only go +4.5mm over the minimum length. Obviously the spoke head needs to meet the nipple head at the top edge of the rim bed to support it. Traditional nipples will have the spoke protrude through the top of the nipple very quickly so your spoke driver will no longer engage. With the double square, you can use the Park SW-16. Engagement is 1/8 of a turn instead of the usual 1/4 of a turn, making wheel building faster.

msl109

That makes more sense - maybe I'll go that route and not bother changing spoke size. Thanks - great tip.

msl109

296 was the "don't quote me" number my LBS guy recalled from "back in the day" when he used to build this wheel all the time. I measured 603 ERD and 37.5 diameter, (he said 604) width 35.5 (center of flange to center) but it's easy to be off a mm. We're both in the same ballpark. So the fact that Mavic is off by 3mm is surprising to say the least.

Soil_Sampler

Measure ERD yourself. Don't trust anybodies numbers.

Dan Burkhart 

I don't trust manufacturers data, Mavic least of all. They list their rim measurements under something called spoke support diameter which is something different from a usable ERD.
This may be old outdated info I just threw out there, because I haven't bothered consulting their web site in a long time due to the futility of it.

msl109

My feelings exactly after this experience.