Exploding Spokes?
Wheel builders have an intense curiosity about the small details. Riders are concerned with the entire wheel and its performance. We builders want to know every “why” about the structure.
Steve Liming of SRLPE Wheel Works in Scottsdale noticed something unusual the other day. Steve is a professional engineer and well regarded builder, I’m eager to hear his observations. Thanks to the network of curious wheel builders, the subject is expanding. I will follow this post with more detail and data.
For the time being, remember that we don’t know all there is to the complexity of the structure and the materials used in our wheels. Sometimes it is all within expectations. But we’re always challenging the status quo and new products are raining down. One rider or builder’s weird or unpleasant experience might bring valuable insights available no other way.
Pin holes, X-fractures, material mayhem! © Steve Liming
Here are spoke ends. These were ridden hard off road, the wheel unbuilt, spokes shortened a couple millimeters, and threads rolled. Wow, in my 40 years of staring at spokes I never saw something like this.
Side view. © Steve Liming
Another view, here from the side. Yup, the spoke is exploding.
What we want to know is:
(1) Are these spokes exhausted from hard riding?
(2) Is trimming and rethreading spokes too much to ask of the material (stainless)?
(3) Are the spokes defective, brittle, weak, contaminated or what?
(4) What happens to cause reliable stainless to fracture, crumble, and develop lengthwise holes?
Good news, there’s no epidemic, this is rare. But we need to understand the causes. Check for a followup post with answers to the above questions. I have some guesses but I think we’re in for some surprise, too. If you have personal experience like this or special insight, now’s the time to share.
From a former boss during my metal-working years: “Two things you need to know about stainless steel: it aint stainless and it aint steel.”
I’m sure this helps…
Seems like a bad idea to roll threads in a well used spoke. Spokes are pretty cheap, use new ones.
Understood. A highest end spoke retails for as low as $1.25 each. For this MTB wheel, that is $41.58. Cheap is relative. Also part of the decision is availability of a spoke cutter and that about 100% of cutter users rethread used spokes without the event we’re examining. Taken together, I can see why this rethread was attempted. The rarity of this event is what makes it intriguing to discuss and understand. Still, you have a point.
An interesting point is that I also shortened two new unused identical spokes, and one fractured in the same manner. The second spoke’s tip expanded such that it was not possible to thread a nipple on the end.
I wonder if this has anything to do with it?
https://en.m.wikipedia.org/wiki/Rotary_piercing
Another article on the rotary piercing process,second paragraph is illustrating
the possible action that’s happening to the spokes.
http://www.thefabricator.com/article/tubepipeproduction/making-seamless-tubing-with-a-floating-mandrel-mill
Same metallurgy, external pressure drives work hardening and when extreme can generate cracks. The processes you reference use the approach to pierce metal bars to make tubes. Flat die thread rolling, if die distance is too small for the work, does the same. Stainless will explode with radial cracks and a central sort of void. Pressure damages the dies as it destroys the spoke area. Isn’t metal forging intriguing? From blacksmithing to sheet metal, almost magical transformations can be arranged. Of course, it’s all down to microstructure and user experience.