spoke length
 

I intend to build a pair of wheels. Never done it before but I fancy having a go. I've read several articles and for simplicity Sheldon's page is the one I'll probably follow. I've looked at the spoke calculators and feel happy to use them. But, after using these to determine the correct length it occurred to me that I'm unsure where the spoke should end at the rim. Is it flush with the inside of the rim or should it enter the rim slightly? Is there a rule for this? I feel daft asking because I've replaced plenty of spokes in the past on different wheels and none are consistent - as far as I can remember. I really don't want to make a mistake. Or does everyone use a calculator and just go on trust?

Yes, there's a rule (sort of). It's just good engineering practice. The spoke must reach into the head of the nipple which acts as the nut that holds against the rim. Most consider a spoke reaching between the bottom of the screwdriver slot and the top of the nipple to be ideal.

Here we get into a bit of confusion. Most calculators use a formula which has the spoke reach up the the diameter you entered as the ERD (effective rim diameter). Most rim makers allow for the height of the nipple when publishing the ERD for their rims, adding 4-5mm to the actual diameter. Others don't and you have to either add 2-3mm to the resulting calculation, or 5mm to your measured rim diameter at the spoke hole where the nipple seats.

This 5mm difference between the so called effective rim diameter, and the actual diameter is the cause of many spoke length calculation errors, and for my part, I'd rather that folks published actual specs which can be measured and verified, rather than guess at the nipples I plan on using and "correcting" for me accordingly.

This is where ERD comes into play. Its the diameter as measured from the ends of the spokes. Ideally, this corresponds to the ends of the nipples, but due to the vagaries of fortune, it doesn't always work out just the way we want. If the spokes stick out of the nipples a bit, it isn't a problem, especially if you have a double-walled rim.

As for calculators, most of them use the same math, so most of them end up with the same length. As long as you have measured your ERD correctly, you'll be ok. You did measure your ERD, right? :-).

(As usual, FBinNY beat me to the punch, and with a better answer).

An addendum: you can make ERD measuring devices by cutting two spokes to a known length (200mm works well) and screwing them into the nipples that you are going to use. Measure the gap (or overlap) between the spokes, add (or subtract) from the sum of the spokes, and you'll have an ERD that should be accurate for your nipples.

Is ERD calculated as a function of spoke tension? In other words, is ERD the figure before the wheel is built, or after?

It's moot. the dimensions don't change enough during the build to make a difference. The circle circumscribing the tops of the nipples is the same before and after building, regardless of the spoke length.

Ther are a number of small variables involved anyway. For example we use the centers of the spoke holes in the calculations, but spokes are measured to the inside of the elbow, so there's 1mm. Also the spokes don't assume a straight line form hub to rim, taking a slightly longer route because of the interlace. The spoke does elongate slightly under tension. In the end all these small variables are factored into the different formulas one way or another, or maybe not at all, which may account for why different calculators may come up with slightly different answers for identical inputs.

In any case, the best practice is to pick a calculator, use a consistent way of measuring everything, and generate a spoke length. Build the wheels, and make a note of how much the spoke ends up off target (just sub flush of the nipple head) and retain that as a correction factor for all future calculations. After a few builds the results will be so predictable that you can calculate lengths and buy spokes with absolute assurance. (provide you're consistent in your methods)

I suppose what I was really concerned about was if the spoke was a tad too long the nipple could run out of thread when tightening. Is the thread length standard with all manufacturers (I sense a hollow laugh)?

The only standard is that there is no standard. I carefully select nipples with threads short enough that I can thread spokes up to 2-3mm beyond the top of the head. When I couldn't find these, there were times I'd use a 2mm drill to deepen the skirt (hub end) a bit to get that margin of error.

A perfect build would have the spoke end flush with the edge of the nipple with the right tension and and runout. Below that is fine for government work.

http://www.youtube.com/watch?v=gCXFKN9JvCQ

=8-)

Thanks all for the replies. My hubs arrived today so I'll play around with the calculators until I pick the optimum length - I'll tape my new rim to my old wheel to see if the calculations look correct (!). I'm getting quite excited, but then I'm a bit sad .....