Spoke length advice.

[andrewc]

Several years ago I bought a SON Dynohub (6 bolt disc fitting)  with the intention of building it into a wheel.  I'd already done this with another SON and that is still fine.


For this one I intended to use it with a Mavic XM719 rim (rim brake version) so bought what I thought were appropriate spokes.  I then hit a problem in the build & shoved it in the back of the cupboard.   


I've just decided to revisit this and think the problem is caused by my having all of the spokes the same length.  For some reason I ordered 4 packs of  DT Swiss double butted in 258mm. 


When I rotate the hub to put the 3rd set of spokes in the first one barely reaches the rim.




I'm having difficulty working out what length I should be using. 


SON's own calculator recommends 260 / 259 on the right side and 259/258 on the left.  However this uses an ERD of 536.2 for the rim, where most other calculators use 539 (I think Mavic give 536 as the SSD).     Other calculators suggest that 258 would probably be OK on both sides, which doesn't seem to work.


Can someone cleverer than me tell me what I'm doing wrong ?






 

[D.A.L.E.]

Were Mavic using 16mm nipples for that calculation of ERD?

From memory, every SON disc hub I've built have been same size on both sides, so I don't think that's the problem. I'd be measuring the ERD properly, that sounds like the issue.

[citoyen]

From memory, every SON disc hub I've built have been same size on both sides

Unless it’s a disc brake model?

I would check you’ve not got the different length spokes mixed up for a start.

[citoyen]

I'd be measuring the ERD properly.

That should go without saying. Don’t trust someone else’s measurements.

[D.A.L.E.]

From memory, every SON disc hub I've built have been same size on both sides

Unless it’s a disc brake model?

Huh?

[andrewc]

It's a disc hub, though I'll be using rim brakes.  https://nabendynamo.de/en/service/spoke-length/


Using the ERD from SON's calculator.



Using ERD of 539.



I'll have a go at measuring the rim myself later. 

[Lightning Phil]

Yep spokes same length for a disc SON.  Never been a problem doing that on my builds anyway. Have you put the right number of crosses into the calc and did you actually measure the rim for ERD?

[vorsprung]

edd spoke calc https://leonard.io/edd/with the rim you said in 32h and a "Son centre lock disk" hub gives

Crosses   3
Spoke length left   259.7 mm
Spoke length right   259.9 mm

So if the problem seemed like the spokes were a bit short, maybe they were

[JefO]

Luckily I always measure everything, I recently bought a rim which clearly stated the ERD on it, but which actually measured 2mm less than the stated ERD.

Also, only put the nipples on 3 turns when you are lacing, and start tensioning when all spokes are in place.

Good luck   

[andrewc]

I'll try again tomorrow.   It's Prom's season, so I'm spending the evening with R3 and a glass or two of wine !

[vorsprung]

I'll try again tomorrow.   It's Prom's season, so I'm spending the evening with R3 and a glass or two of wine !

good tactic

[citoyen]

From memory, every SON disc hub I've built have been same size on both sides

Unless it’s a disc brake model?

Huh?

Just speculating as to a possible reason why it might need different spoke lengths, but I’ll take Phil’s word for it that it’s not the case.

[andrew_s]

ERD and SSD are different dimensions, which normally differ by close to 3 mm.

ERD is the diameter at the spoke ends, normally at the base of the screwdriver slot in the nipple head.

SSD (Spoke seat diameter) is the diameter of the point where the nipple contacts the rim (or eyelet, if there is one).

If a rim has a quoted SSD, you should add 3 mm when calculating the spoke length. Some length calculators list this explicitly in the rim description (e.g. Damon Rinard's Spocalc spreadsheet)

[andrew_s]

From memory, every SON disc hub I've built have been same size on both sides

Unless it’s a disc brake model?

Huh?

Just speculating as to a possible reason why it might need different spoke lengths, but I’ll take Phil’s word for it that it’s not the case.

The closer the hub flange is to the centreline of the bike, the shorter the spoke will have to be.

For a non-disc rear hub, the amount that the driveside flange is moved inwards to make room for the cassette means that the spokes are typically 2 mm shorter than those on the other side.

With a front disc hub, the disc side flange also has to be moved inwards to allow space for the disc and its fittings. Note that the disc position is dictated by the requirement that the outer edge of the disc remains clear of the fork blade (usually a disc of ~205 mm whould be allowed for).

The distance for a disc is less than that for a cassette, so the nominal L & R spoke lengths would differ by maybe 0.7 mm (guess). You normally buy spokes in increments of 2 mm, so it's normal to use the same length spoke both sides.

[Lightning Phil]

Less than that because right hand connector also brings spokes towards centre line.  Data from Son 28 with a DT Swiss hub selected. Just 0.1mm difference

[JonBuoy]

Less than that because right hand connector also brings spokes towards centre line.  Data from Son 28 with a DT Swiss hub selected. Just 0.1mm difference

Two hubs.  That is a really unusual wheel.

I think that the different PCDs on the RH and LH flanges on the OP's hub has more effect than the different offsets.