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I noticed that (most) spoke nipples have a thread which does not run through the entire length of the nipple. As can be seen in the image below some of the length of the nipple has no threads (and is called a "bore" apparently).

enter image description here

This left me with some questions:

  • What is the purpose of the bore?
  • Wouldn't the nipple be stronger with longer threads? Are there disadvantages to using longer threads (/sacrificing some of the length of the bore to add more thread length)?
  • Is the amount of thread the same on longer/shorter nipples and is the difference between these different lengths of nipples just in the length of the bore? If so (if thread length is the same)...what is the advantage of using longer nipples?
  • Are there spoke nipples where the thread does have a longer length (thread across the entire length of the nipple)?

Thank you for any input, it's greatly appreciated :)

  • 2
    I’m waiting for an ultralight wheel build where somebody cuts off the bore to reduce nipple weight by 15% ;) – Michael Jan 31 at 20:24
  • Is this for a specific wheel build or a general question? And is there a difference in bores between normal brass nipples and superlight aluminium or titanium nipples? – Criggie Feb 2 at 1:22
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    @Criggie I am currently building a wheel for an e-bike (spokes on original wheel kept breaking) so that got me wondering why the threads weren't made longer on the longer nipples (since that might create a stronger wheel) . But I guess it's more of a general question (out of curiosity). I have decided to use steel nipples on the wheel built so they should be plenty strong (even with the bore) I think :) – Maarten -Monica for president Feb 2 at 1:28
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    I've never seen steel nipples - perhaps they're chromed brass ? Which would be ideal for an ebike wheel where torque is pointier. – Criggie Feb 2 at 1:32
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    @Criggie the box says they are indeed made of steel, also they appear to be a lot stronger (you can tighten them more without the spoke tool interface (the flat sides on the nipple on which you place the spoke tool) getting damaged. Here is an example (these are steel, zinc-plated: internet-bikes.com/en/… ) Just tested: They stick to a magnet so most likely indeed steel – Maarten -Monica for president Feb 2 at 1:35
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Adding threads to any sort of bolt under tension doesn't actually make it much stronger, beyond the first ≈5 threads. Adding more basically just adds dead mass; either way the whole thing will generally fail somewhere close to the first thread. So, filling the bore with threads wouldn't have any benefit. What it would do however is move the expected point of failure out of the relative safety of the unthreaded bore, which particularly protects the spoke from being bent at that point, which would further diminish its strength. The bore makes it so that if there's some bending, it will be on the unthreaded part of the spoke, which is stronger to begin with, so not as worrisome.

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    Although I suspect a brass or aluminum nipple needs more than 5 or so threads to max out in strength - those metals are pretty soft. – Andrew Henle Feb 1 at 2:01
  • @AndrewHenle very roughly aluminum shear strength is >200MPa, steel tensile strength is <400MPa so (ignoring that the first thread takes more load) you need twice the area for the steel spoke to break before the aluminium thread shears, so for a 2mm spoke pi R squared vs 2 pi R h you need 2mm of thread or about four threads. – Pete Kirkham Feb 1 at 2:37
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    @PeteKirkham How could that possibly be true? More threads = more surface area, and thus lower pressure on the threads for a given force. – nick012000 Feb 2 at 11:55
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    @nick012000 that's basically what Pete said! Yes, more threads means you reduce the shear stress at the threads themselves, but that doesn't buy you anything if the bolt itself rips apart because the total normal stress (tension) is too much for it to handle. And specifically, at the inward edges of the first thread you'll get “hot spots” of tension because of elastic deformation, and these cause the bolt to snap before the threads further back get anywhere near their shear limitation. – leftaroundabout Feb 2 at 14:38
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    @nick012000 The problem here is that the bolt does not exactly fit to the nut - there must be some gap so one can even screw the bolt on the nut anyway. When one pull or push the screw the first surface in the contact bears all the load and deforms. Then more surface is engaged until the deformation stops. This usually happen within first three to five screws. The rest is unloaded because of the manufacture tolerances. – Crowley Feb 3 at 10:48
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  • The bore guides the thread of the spoke to the threading of the nipple allowing easier spoking with less risk of cross-threading.
  • The bore also allows the spoke to be threaded deeper w/o adding more threading to the spoke. Imagine you want to thread the spoke all the way into the nipple. If there wasn't a bore, but the inside thread of the nipple would reach all the way up to the spoke side then the spoke would have to have threading there, too. This would also be unnecessary, as I cannot imagine that you would need more than 10 threads for the spoke to securely hold the nipple.
  • Another benefit is optical, hiding the spoke's threading in most cases.
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  • Could you please elaborate on what you mean by "deaper threading" of the spoke? Thank you! – Maarten -Monica for president Feb 1 at 9:22
  • Yeah, I'm sceptical about the second point as well, my experience is that you can generally screw the nipple all across the spoke's thread because the spoke's thread is significantly wider than the basic spoke diameter. -- The first point, however, is spot on, you got my upvote for that. – cmaster - reinstate monica Feb 1 at 12:04
  • The inner diameter is smaller than the unthreaded part of the spoke, so it won't go inside the threads in the nipple. – ojs Feb 1 at 12:12
  • I like the first point but I don't quite follow the second point, can you rephrase it so that it's a bit clearer? (imagine i'm an idiot ><) – Swifty Feb 1 at 15:33
  • Imagine you want to thread the spoke all the way into the nipple. If there wasn't a bore, but the inside thread of the nipple would reach all the way up to the spoke side then the spoke would have to have threading there, too. This would also be unnecessary, as I cannot imagine that you would need more than 10 threads for the spoke to securely hold the nipple. – nine stones Feb 1 at 16:44
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When inserting a spoke in the nipple, the spoke typically has to be bent quite a bit before making it into the nipple.

The bore decouples the acts of bending the spoke to fit the nipple, and turning the nipple to make the threadings connect.

That way, the threads of spoke and nipple are aligned before the threads catch even with the spoke still being bent somewhat, basically eliminating the otherwise imminent danger of cross-threading. It also prevents the end of the tensioned spoke from unleashing and ripping through your hand while you try making the first turn of the nipple.

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The "bore" allows the unthreaded portion of the spoke to enter the nipple body, eliminating the need to thread the spoke more or use a shorter nipple. This is especially useful in some wheel-building factories where, to simplify operations, the same length spoke is used on both sides of a wheel, and any protrusion beyond the nipple end is ground off.

Note that the "bore" end of the nipple has its length determined in part by how thick the rim body is. Some rims have a thicker cross-section as measured from spoke side to tube side, and hence they need longer nipples. But they don't need more threads.

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Threads corrode. The longer the threaded piece, the harder to loosen corroded threads. Applying a large amount of force with a nipple key can deform the nipple and damage the threads underneath. So not having threads underneath prevents damage to nipple and spoke.

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  • The corrosion is usually addressed by applying some lube before screwing the nipple onto the spoke. – cmaster - reinstate monica Feb 1 at 11:59
  • Or, using materials like stainless steel and brass – ojs Feb 1 at 12:13
1

What is the purpose of the bore?

Perhaps so that the threaded part of the spoke is covered by the nipple?

Wouldn't the nipple be stronger with longer threads?

Yes.

Are there disadvantages to using longer threads /sacrificing some of the length of the bore to add more thread length)?

Apparently not, as you don't hear people complaining about spokes ripping out of nipples.

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    It looks cleaner, doesn't catch grit and is easier to clean. Visible thread on the spoke is considered unsightly and should be hidden. The correct spoke length is chosen in that regard. – Carel Jan 31 at 17:30
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A thicker ring around a rod, counterintuitively, doesn't strengthen it, especially if there is a sharp transition from thin to thick. It weakens it, by creating what is called a stress riser - think of clamping a square metal rod in a vise and gripping it with pliers, you will have an easier not harder time breaking it, since the clamped portions can NOT deform to deal with the applied force. Sharply cut threads are likely to just achieve the same...

And while this is more of a problem with bending than tensile/compressive loading, if a thinner rod (equivalent to the thinnest cross section in the threaded version) was sufficient, a thinner rod would have been used in the first place....

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    Question is about the hole (bore) in the nipple before the threads. Also, the threads on a spoke are rolled on (like a form tool like knurling) and are not cut in like a screw-cutter. So you're saying the bore allows an easier transition in angle from the threadded section to the free unsupported length of spoke ? – Criggie Feb 2 at 20:49
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another reason for using longer spoke nipples (which providing they have approximately equal thread length, which appears to be quite common) is to allow for the tool interface of the nipple to reach far enough through the rim in order to be able to tighten the nipples. Longer spoke nipples will (with the same thread length) have a longer bore length.

On some (especially deeper) rims the spoke nipple will have to be longer in order for the tool interface (flat sides) to properly extend past the rim itself .

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  • I believe the question was about why threads don't extend all the way through the nipple, but there is a smooth bore instead. – ojs Feb 9 at 14:26
  • @ojs true. But one of the questions is also: what is the purpose of longer spoke nipples if the threaded length on all spoke nipples is approximately the same. My answer lists one of the scenarios where a longer spoke nipple might be useful (thus shining light on one of the purposes of a longer spoke nipple ). – Maarten -Monica for president Feb 9 at 23:41

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