16

I'm building a rear wheel for the first time. I have purchased the rim and hub and am now sourcing the spokes. I put the measurements into an online spoke calculator (on wheelpro.co.uk) and it estimated 295mm and 293mm for the right and left side respectively.

Now I can find 295mm spokes but, so far, I haven't been able to find a 293. I will keep looking, but I have two questions:

1) How precise do the spoke lengths need to be. If they are out by a millimetre is it likely to cause a significant problem?

2) If they are out by 1-2mms, is it better if they are too long or too short. So if it turns out to be impossible to get a 293mm, would a 294mm or a 292mm be a better alternative.

Many thanks in advance.

Chris

  • It should be pointed out that one can purchase a spoke threader/cutter to allow over-long spokes to be threaded and cut to the desired length. – Daniel R Hicks Mar 26 '18 at 20:56
  • 1
    Note, though, that you cannot / should not thread over existing threads. So, besides being way over the top for your single wheel, this would only be of help if the spokes were a substantial amount too long. – aspseka Mar 26 '18 at 21:43
9

A typical spoke has around 8-10 mm of threading on it, and you don't necessarily have to engage all of it with the nipple. So it's not super sensitive to small variations in length.

For most folks, too long or two short really doesn't matter as long as it's within about 2 mm, so just round to whichever stock size is closest to ideal.

If you go beyond the 2 mm, it is better to be a little short than long. A spoke has quite a bit of thread on it, and you don't need all of the nipple engaged to the thread (though more is better), so you have several mm of room to work with. However, if the spoke is too long, and the spoke pokes out of the nipple, that may be trouble as the rim strip may not be able to protect the tube from the spoke. Additionally, you may bottom out the spoke nipple, and be unable to tension any further.

So if I were in your position, while the 292 would be your best choice, I'd be tempted to just get all 295 so I don't have to keep track of the two different sizes if I had a good double wall rim, but it's not a ton of work either to just have two different stacks.

  • I have seen factory-built wheels where the spoke ends were ground off flush with the nipple heads. – Daniel R Hicks Mar 25 '18 at 21:23
  • 1
    While I agree that there is a good amount of room in either direction, I'm more likely to go long than short. If the rim is double walled, you're unlikely to have a spoke long enough to cause issues with the rim tape. If you're using aluminum nipples (not that i recommend them), short spokes make the nipples more likely to break. Since a lot of people building wheels are using these higher end parts, I'll usually recommend long over short. – ankh-morpork Mar 25 '18 at 21:56
8

This topic is really poorly understood. Anyone who ever who tells you it's fine to just straight round up or round off when choosing spoke lengths just hasn't been through the disaster scenarios that can be caused by it, or perhaps doesn't remember because the experience is too painful and embarrassing. Always round down unless you can test and measure the spokes in question before building to avoid running out of threaded length or poking the tube. If you have to order spokes, just round down. If your calculated optimal length is accurate (and there's no reason it shouldn't be) and you must round down to the next even number from there (as in the 292 in your example), nothing meaningful will ever be lost in terms of durability from that extra 1.99mm max.

Some people point out that having al nipples miss this much engagement introduces some kind of non-zero additional risk of the nipple breaking. That's probably true. However, you'll never know if that's what caused it, because al nipples break anyway, especially for 2mm spoke ends. (1.8s are less prone to breaking because of the extra material.)

Note that your measurements and your calculation must be accurate for the above to work out. You don't want to miss more thread engagement than around this amount. That would be poor form. Never use a calculator where you can't see that the math it's doing is nothing other than the pure classic spoke length formula. Some calculators try to invisibly factor in estimates of spoke stretch and rim compression, which you should do manually based on the parts you're using. (Spoke gauges differ in stretch, the amount of tension also causes it to differ, and the stoutness/cross-sectional area of rims make them differ in compression.). Spocalc is a good one that doesn't do this. And always measure the hub and rim yourself. It seems pedantic, but I promise that if you build enough wheels, or are ever working for a wheel producer and are tasked with doing spoke calculations and there's never time to do test runs of anything (this was my situation that taught me good spoke calculation habits), you'll find all this necessary to have consistently good results.

If you have to order the hub/rim along with the spokes (particularly the rim, because manufacturer list measurements for hubs are usually very accurate), just take vendor/manufacturer listed measurements on faith for calculation purposes, but measure yourself once you do have everything to make sure what you're about to do is going to work. Second-guess published ERDs early and often.

What gets overlooked in this discussion is that there are whole brands/models of spoke, or spoke/nipple combinations, that offer zero ability for the spoke to protrude past the top of the nipple. They don't have the threaded length to do it. So rounding up walks you right into the failure situation of running out of thread and having to either start over or accept an untrueable wheel with bad tension. Various Wheelsmith bags I've used, for example, have been like this, with Wheelsmith nipples.

Contemporary DT Champions and Competitions, the most commonly encountered premium spokes, avoid this occurrence by having some superfluous/generous threaded length. Some other spoke brands and models do not, and give you no leeway for the spoke to be long. And single wall builds naturally have no such leeway.

Wheelbuilding texts almost never remember to tell you that longer length nipples usually have more threads, and you need to compensate for this to avoid bottoming out the threads on the spoke. Most handbuilt wheels are done with 12mm nipples, but if you ever use 14s or especially 16s, you'll find the rim's ERD is effectively a few millimeters smaller. Or, in other words, ERD is usually calculated as the diameter of the circle formed by the tip of the spokes in a state of optimal thread engagement. Usually that's synonymous with the tip of the spoke being just about flush with the top of the nipple, somewhere between the bottom of the screwdriver slot and the top. 12mm nipples can all handle that, but most of the time if you thread an aftermarket/handbuilding-worthy/premium (however you want to say it) spoke into a 16, you'll find it runs out of thread and bottoms out before it reaches the top.

If you do need to round up, or insist on getting full thread engagement, always test it out first by taking a nipple and spoke that you're about to use and see how far the nipple screws on before it bottoms out, and then measure the amount of protruding spoke you have to work with to make sure it will be acceptable with whatever your calculated length was. Also make sure the rim's depth is such that it won't cause spoke to protrude up and poke the rim strip/tube. If the spoke meets these criteria it's fine, but it can't just be assumed that it will until you test. Do that test on enough spokes and you'll see.

6

A spoke of ideal length is such that it is flush with the head in a finished wheel. Then it engages all threads, maximising stability. However, from there you "only" have 1.5mm tolerance towards too long [for DT spokes and DT nipples, measured myself some time ago], then all the threads of the spoke is wound through the nipple and it binds, you cannot tension more. This is bad, insufficiently tensioned wheels are bound to fail.

For this reason, spoke lengths are calculated with respect to the bottom of the slot in the head of the nipple, that is 1mm below to top of the head [DT standard]. So from there you get 2.5mm tolerance toward too long... if the calculator has the correct length. Try it, ask 5 different calculators and you will get 3 different answers.

Too short is risky as well, but less easy to quantify. The threads in your average nipple are 4mm long, engaging just 3mm is ok, 2mm might also... That would be 1mm short of the ideal length. If you use aluminium nipples, however, 2mm will most likely not suffice.

A frequent advise given is to round up to the next even number unless the difference to the lower even number is 0.5mm or less. So with 292.4 get 292mm spokes, with 292.8mm get 294mm.

In essence, you will want to be pretty precise. 2mm too long might just be ok, 2mm short might be too short. Be sure to measure the ERD yourself (!) and as precise as you are able to.

1

Some things that influence how much off (+/-) you can be from the optimal length:

  • Length of threaded area on spoke
  • Length of threaded area on nipple
  • Spoke count

The spokes on my last build had 10mm thread, and the nipples, despite being 12mm long, only had 7mm thread. If, for example, I had used 3mm longer spokes, there would have still been full 7mm thread engagement ✓; because of the longer thread on the spoke. If, on the other hand, I had used 3mm shorter spokes, there would have only been 4mm thread engagement; perhaps not safe enough.

  • Longer thread on spokes allows longer spokes to be used.
  • Longer thread on nipples allows shorter spokes to be used.

With lower spoke count, thread engagement is more important, because stress is distributed among fewer spokes.

Trick: You can use nipple washers to effectively increase the ERD, better suiting longer spokes. Regular washers may work too, if proper in size.

  • you wouldn't want to use a spoke which is 3mm too long or too short though, so isn't this a moot point? even if spokes are available in only 2mm increments, you could choose between 1mm too long or 1mm too short, which would be preferable – Swifty Jul 14 at 5:03

protected by Community Oct 1 '18 at 14:39

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.