Spoke Length

src1

Hi,

I'm planning a build of Pacenti SL23 rims (ERD=588) on Dura Ace 9000 hubs (28h rear, 24h front) using CX-Ray spokes.

Using a spoke calculator the spoke lengths I get are:
Front = 275.9mm (radial)
NDS = 281.7mm (2x)
DS = 279.5mm (2x)

I've been told that CX-ray spokes stretch and I should round down giving me:
Front = 274mm
NDS = 280mm
DS = 278mm

Does this look ok?

Thanks,
Rob.

ugo.santalucia

I would stick to the measurements you have... CX Ray behave just like any other spoke

src1

Thanks, do you mean the 2nd set of measurements above or round to the nearest, i.e.,

front = 276mm
NDS = 282mm
DS = 280mm

ugo.santalucia

nearest

src1

Thanks.

cycleclinic

Yes a CX-ray is like any other spoke and all spoke permently stretch in the building process!

If your ERD is measured with the 200mmm spokes you use threaded onto the nipple until the top of the spoke is flush with the bottom of the nipple driver flat you have the largest ERD measurement permissible. In this case do not round up as CX-ray's do stretch a bit when fully tensioned and after stress relieving (watch the tension drop after stress relieving with a spoke tension gauge) other wise you may find you run out of spoke thread on some spokes and the nipple may start to bind. If you have measured the ERD to be shorter than the maximum then rounding up may work O.K. I use sapim's all the time I know how they behave.

ugo.santalucia

thecycleclinic wrote:

I use sapim's all the time I know how they behave.

Sapim's stainless steel is no different from other brands stainless steel and will behave in exactly the same way. As for permanent deformation... have you actually measured it? For the spoke to permanently stretch, you need a tension which exceeds the yield point, which I am not sure how many N corresponds to, but I would say it's higher than the ones we use when building. Once released from the tension they should go back to their original length...
Also, in metallurgy a stretch of 2 mm over a length of 300 mm is huge and does not belong to a sample of steel... something like 0.1-0.3 mm is more likely and irrelevant when it comes to calculating spoke length

cycleclinic

A CX-ray under 1200N of tension will elongate by 0.47mm. Whe stress reliveing by grasping pairs of spokes tension is increase by ~50% so that will an elongation of maybe 0.7mm in total. The tension drop indicates permant deformation. The tension drop after proper stress reliving is very noticable and can be 200N or so. I therefore have no toruble building sometimes with spokes that maybe considered on the short side as by the time I am finished with them they won't too short at all. I tend to find rounding up to be dangerous a whenI have done I find nipples binding on the threads when I reach full tension making the final truing difficult and some time I have to pull the offending spoke out and replace with another of the same length. Sapim spokes can vary in length by 0.5mm or so which creates the problem.

I always round down but I always measure the ERD of the rim myself so I know how much rounding I can get away with.

src1

Thanks both. I was just going off the published ERD, I'll wait until the rims arrive and measure them to be sure.

So assuming I've measured the ERD accurately, what's the safe limit when rounding down? For example, if the calculated length is 275.9mm, can I safely go with a 274mm with a CX-ray?

ugo.santalucia

SRC1 wrote:

Thanks both. I was just going off the published ERD, I'll wait until the rims arrive and measure them to be sure.

So assuming I've measured the ERD accurately, what's the safe limit when rounding down? For example, if the calculated length is 275.9mm, can I safely go with a 274mm with a CX-ray?

When I tried to use spokes which were 2 or 3 mm too short, it was sometimes difficult to even lace the wheel without using longer nipples... 1 mm each way is where I would draw the line

cycleclinic

Which is why I am saying measure the ERD and be sure (which the OP is now). 2-3 mm shorter than correct is too short but maybe the round down will not be too short. Until I get my first SL23 rim next week I will not know. I always find it wise to never assume anything.

Take the archetype rim for example. The published ERD is 595mm but when I have tried building with lengths calculated on that I find the spokes are too long particualy if I round up up 0.2 or 0.3mm. Using an ERD 593mm, which is how I meausre it, the spoke lengths are always right but then I only round down by 1mm at the most. Mot published ERD are on the high end of the range and therefore rounding down is needed from my experience anyway.

So I think we agree here. Anyway with sapim nipples the thread is just as long in the 14mm ones as in the 12mm nipples so going using longer nipples for spoke that are two short does not help.