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

Location
Loch side.
The more spokes in a wheel the less reduction (%) the lowest spoke experiences. Will the magnitude of the cyclical range of tensile force in a spoke be related to fatique life? Probably (intuitively), but I don't know.
The spokes fatique and eventually, when @hobo gives that last push, the rear wheel spoke that happens to be weakest parts. On my ride last week each of my spokes (36 spoke front wheel) went through this cycle (1000N to >800N) over 100,000 times. Last year the spokes in my front wheel which I built it in Jan 18, completed this cycle over 5 million times. I used the original spokes from the fubarred wheel (rim) so I estimate these spokes have completed 20 million cycles with me aboard (plus no idea how many with the first owner on board). Pleased to say no front spokes broken yet.
This quote is a year old, forgive me for re-hashing it. I think it bears relevance to the entire discussion. I've put the important question in bold.

The answer is yes, the cyclical range does have an effect on fatigue life (sometimes called Endurance Limit or even Fatigue Strength). Steel has a fatigue limit, which means that cyclical changes in stress have no effect on the fatigue life of the piece. In short, it means that if the changes are kept below a certain level, then the material effectively an infinite life. This limit ranges a bit and can be as high as 35% of tensile strength or as low as 65% of tensile strength.

We can relate back to spokes. A 1.8mm spoke has a tensile strength of about 2400N. Typically, we build to no more than 1500N and the cycles will be (big approximation here) about 200N. Even with a high limit of 35% you can see that a wheel can easily be built with infinite spoke life.

The important factors are: enough spokes; stiff enough rim and proper stress relieving.

Unfortunately, rims, made from aluminium, has a fatigue limit and it is pretty low too. Rims therefore crack long before spokes break, if the wheelbuilder knows his onions.

Edit: An error was pointed out to me and I've changed the magnitude of cyclical changes from 20N to 200N. The demonstration remains valid.
 
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Dogtrousers

Kilometre nibbler
Unfortunately, rims, made from aluminium, has a fatigue limit and it is pretty low too. Rims therefore crack long before spokes break, if the wheelbuilder knows his onions.
In my experience neither happens. Rather the braking surface wears out before the rims crack or the spokes break.

This may be because I've been lucky and/or my wheels have been built by an alliumologist.

Or maybe it's because I and my bike are such dainty featherweights we don't stress the wheels. ;)
 

Ian H

I am an ancient randonneur, & I stop often for tea
Location
East Devon
In my experience neither happens. Rather the braking surface wears out before the rims crack or the spokes break.

This may be because I've been lucky and/or my wheels have been built by an alliumologist.

Or maybe it's because I and my bike are such dainty featherweights we don't stress the wheels. ;)
It's cos you brake too much. To quote a very eminent green-clad cyclist: "Brakes are for emergencies only!"
 
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Location
Loch side.
In my experience neither happens. Rather the braking surface wears out before the rims crack or the spokes break.

This may be because I've been lucky and/or my wheels have been built by an alliumologist.

Or maybe it's because I and my bike are such dainty featherweights we don't stress the wheels. ;)
Of course it is perfectly plausible that the brake surface wears out before the alu cracks. In dry countries, wheels fail from aluminium fatigue. In wet countries, from rim wear. There are plenty of variations in-between, including terrain and traffic, all which has bearing on how much you brake.
 
I've just broken a spoke on the track bike. 16 years on the road and occasionally the velodrome, including my entire racing career, and it was a crap unbranded set that came with a Fuji Track and which I reused, Scrooge-like, when I swapped the original hub for a Goldtec in...er...2004. I'm not too ashamed, especially as it broke after this year's Dunwich Dynamo (which no doubt finished it off).

I'll rebuild the wheel with DT Competition spokes. The rim is, of course, as new since it's never known the kiss of a rear brake.
 

Ian H

I am an ancient randonneur, & I stop often for tea
Location
East Devon
If you're racing for a green jersey, maybe. If you're riding in London, emergencies tend to crop up every 100m or so.
Nev didn't race much. Long-distance touring was his forté. But always in Evesham green.
Emergencies? Sounds like a lack of anticipation.
 

Ian H

I am an ancient randonneur, & I stop often for tea
Location
East Devon
In London, it's impossible to anticipate the idiots around you. There's too many of them.
That hasn't been my experience over several decades of urban riding in London and other cities.
 
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