Why do my spokes keep breaking? - Bike wheel science.

[silva]

New stainless steel spoke. New brass nipple.
[Take care when googling.]
See this thread for more: https://www.cyclechat.net/threads/rusty-nipples-ok-corroded-but-wheres-the-drama-in-that.198848/
I'd be rather surprised if the eyelets on your rim were steel. So the 'rust' you report is not from those.
A magnet is your friend.
Consider replacing all the nipples with brass ones, while you have the tyre off.

https://www.customcutspokes.co.uk/product-category/bicycle-spoke-nipples/14g-spoke-nipples/
https://sheldonbrown.com/wheelbuild.html
"Many good quality rims have "eyelets" or "ferrules" to reinforce the spoke holes. The best double-wall rims have sockets which spread the load to both layers, allowing these rims to be lighter and/or stronger."

Rim without eyelets	Rim with eyelets	Rim with recessed spoke holes and no eyelets	Rim with recessed spoke holes and eyelets	Socketed rim with eyelets
Rim cross-sections
View attachment 730327	View attachment 730328	View attachment 730329	View attachment 730330	View attachment 730331

That magnet is an idea, chosed a small one, less than 1 cm diameter.
What I named "inserts" and you name "eyelets" are confirmed as non stainless steel: the magnet sticks, even when upside down = hanging and shaking. It is indeed rust, no dust or so, and it also showed that typical expansive deformation of rust hence I took it for non stainless without a magnet test.
You say that that would surprise you, it is the case nevertheless, what was your reason to not expect it?

What I also discovered with the magnet, to my surprise then, that the spokes differ in material. Some are stainless (A2 grade, magnet hangs very slightly, easy to shake off) and others steel, magnet sticks firm.
About a quarter are stainless, the others not.
And that also confirms their different looks, when I cleaned the spokes, some started blinking, others not. I assumed it was due to different ages, but it's due to different materials.
The one that sat in the broken nipple is a stainless one.

Apparently the dealer originally sold the wheel with steel spokes, to afterwards replace broken or suspected ones with stainless ones. The distribution stainless/non stainless appears random.
Stainless steel has quite different mechanical properties than steel, and getting a wheel straight is all about balancing tensions. How does that work out when the materials differ?

 

[silva]

About the broken nipple, I have its two pieces in front of me, externally I don't see any damage or sign of wear. It's broken a couple mm inwards from the "flange" that acts as support against the tensioning.
It's not magnetic, not the slight of A2 / 304 grade stainless. A4 / 316 is totally not magnetic. Certain treatments of A2 / 304 can cause it to loose its diamagnetism. So I don't know. On sight there is like a yellowish but hard to be sure, which maybe indicates brass. Maybe a magnifying glass on the break surfaces tell something.

The spokes and nipples on my travelbike rear wheel I do know since I asked and got a parts list / price offer for my spare rear wheel.
The spokes are named as "Sapim DB Race", which duckduckgoes to "double butted stainless steel"
The nipples as "Nippel Sapim Secure" with an addition that is translated to "locking agent ; sealing agent ; lock compound ; locking compound ; locking device"
Duckduckgo shows

Sapim Polyax Aluminium Nipple 2mm - Secure Lock. Sapim's anodized Polyax nipples are treated with a special coating to increase durability and allow for easier wheel building. The nipples have a 2 mm thread for rims with Ø 3.5 mm holes. Aluminium saves weight, compared to other materials like brass.

During the 7 years I used the travelbike, I have had not a single spoke break/losening/whatever, despite the first Ryde Yura rims outer wall broke in the middle all around, and the inner and sidewall also started cracking, rim replaced by an EXAL like this older/current bike now, didn't even know upto this story it was an EXAL.
Based on this experience, I would say the Sapim spokes and nipples did a quite good job.
The comparison is not fair though, since a 62 mm wide tyre has much more air in it, so absorbs shocks more thereby saving the spokes that. That was one of the reasons to want 62 mm on my travelbike, whose frame and setup allows two different tyre widths so I could have chosen in a range 42-55 mm instead.

The currently mounted rear wheel in the previous/older bike also has a mix of stainless and non stainless spokes, tested 8 sequential and found 2 stainless so it looks like a same story.

 

[rydabent]

3 in 3 months! Ridden for 3 years prior with no broken spokes. I get them fixed by lbs within a few days after each one has gone.

Shizuoka hoy 1 bike, Commuting down canal towpath 8 miles each way with paneers with clothes and lunch in them. i weigh 87kgs and haven't put on weight recently.

Anything I should be looking for before just replacing another spoke? is it easy to do for a fairly hands off cyclist who usually uses lbs for much more than puncture or minor tweaks? Should I look at new wheel?

I have never broken a spoke. In the last 16 years sicne I have retired, I have ridden over 25,000 miles on a recumbent or a trike that you cant "unload" going over a bump or pot hole. Im sure the 5 wheels I was riding on were computer built. In all those miles, I have never once had to put a spoke wrench on one, and they spin absolutely true.