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

[Yellow Saddle]

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.

 

[Dogtrousers]

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]

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!"

 

[Yellow Saddle]

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.

 

[rogerzilla]

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.

 

[Dogtrousers]

It's cos you brake too much. To quote a very eminent green-clad cyclist: "Brakes are for emergencies only!"

If you're racing for a green jersey, maybe. If you're riding in London, emergencies tend to crop up every 100m or so.

 

[Ian H]

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.

 

[Flying Dodo]

Nev didn't race much. Long-distance touring was his forté. But always in Evesham green.
Emergencies? Sounds like a lack of anticipation.

In London, it's impossible to anticipate the idiots around you. There's too many of them.

 

[Ian H]

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.

 

[Nigelnightmare]

Assume that every other road user is a complete idiot & out to get you and you won't be far wrong + you'll probably live longer too.

 

[preventec47]

This is an old thread so i wont go into great detail but everyone interested should google Metal Fatigue and Endurance Limit etc.
This spoke situation is EXACTLY the same as rod bolts in a piston rod in a motor. If the bolts are not torqued to a sufficiently tight
"stretch" beyond the endurance limit then with every rotation of the motor, one more stress cycle is added to the bolt until
it reaches its fatigue life and fails. however, it the bolt is stretched to beyond the endurance limit then each engine cycle does
NOT cause the rod bolt to cycle above and below the fatigue limit with each engine rotation which adds to the cycles leading to failure.
SO, loose or non sufficiently tight spokes DO fatigue but sufficiently tight spokes will NOT accumulate fatigue cycles toward failure.
To clarify further, NO movement is needed to create fatigue. ONLY stress and de stress to and past the endurance PSI limit and back under is needed to cause weakening of metal until a failure occurs at a MUCH less tensile strength than what the original metal was
rated for.

 

[raleighnut]

This is an old thread so i wont go into great detail but everyone interested should google Metal Fatigue and Endurance Limit etc.
This spoke situation is EXACTLY the same as rod bolts in a piston rod in a motor. If the bolts are not torqued to a sufficiently tight
"stretch" beyond the endurance limit then with every rotation of the motor, one more stress cycle is added to the bolt until
it reaches its fatigue life and fails. however, it the bolt is stretched to beyond the endurance limit then each engine cycle does
NOT cause the rod bolt to cycle above and below the fatigue limit with each engine rotation which adds to the cycles leading to failure.
SO, loose or non sufficiently tight spokes DO fatigue but sufficiently tight spokes will NOT accumulate fatigue cycles toward failure.
To clarify further, NO movement is needed to create fatigue. ONLY stress and de stress to and past the endurance PSI limit and back under is needed to cause weakening of metal until a failure occurs at a MUCH less tensile strength than what the original metal was
rated for.

Billhooks, I've got wheels 30 odd years old,

 

[Ajax Bay]

@preventec47
I've tried to recast your contribution - hopefully I've got it right enough.

"i wont go into great detail
google Metal Fatigue and Endurance Limit etc.
If spokes . . . are not tensioned/stressed to cause a "stretch" beyond the endurance limit then with every rotation of the wheel is one stress cycle (for each spoke in turn) until it reaches its fatigue life and fails.
If the spoke is stretched to beyond the endurance limit then each wheel cycle does NOT cause the spoke tension to cycle above and below the fatigue limit so this doesn't add to the cycles leading to [fatigue] failure.
SO, insufficiently tight spokes DO fatigue but sufficiently tight spokes will NOT accumulate fatigue cycles toward failure.
To clarify further, NO movement is needed to create fatigue. ONLY stress and de stress to and past the endurance limit and back under is needed to cause weakening of metal until a failure occurs"

Perhaps you could jump to the 'so what'? What are you suggesting will happen in practice?
You say "sufficiently tight spokes will NOT accumulate fatigue cycles toward failure" That a properly tensioned/tight spoke never fatigues? Or what?
A spoke (say plain gauge 2mm diameter) has a cross section of 'Pi'mm2. It's under tension of (say) 1000N so about 318N/mm2. "A simple rule of thumb calculation for the fatigue limit is one-half of the ultimate tensile strength " which for stainless steel (material of spokes) seems to be in the region of 500N/mm2. Half that is 250N/mm2. So properly tensioned spokes are above the fatigue limit of their material.
[A MPa = 1N/mm2]
On my long ride last week each spoke was cycled from its normal 'rest' (stationary) stress to maybe 200N less (32 spoke wheel), and back to normal more than a quarter of a million times.
After many millions of cycles I expect the first spoke (rear wheel, right side) to fail through fatigue. Before then it's highly likely I'll have replaced the rim (caliper brakes) and likely used a new set of spokes to lace the new rim up.
Do you think a bicycle's spoked wheel rim moves radially as it rolls, with a (rider) load?

 

[rogerzilla]

The left spokes usually go first on a rear wheel, because they can't be tightened enough without massively overtightening the right spokes (and either splitting the nipples or damaging the rim).

 

[Ajax Bay]

The left spokes usually go first on a rear wheel, because they can't be tightened enough without massively overtightening the right spokes (and either splitting the nipples or damaging the rim).

Left spokes can be tightened "enough" . . . "without massively overtightening the right spokes". Every properly tensioned rear wheel has spokes like this. Every rear wheel with under tensioned spokes can be remediated so the spokes are tensioned and the rim is true, without overtightening the right spokes. Sorry I either don't understand what you are trying to say, or I don't agree, but persuade me.
Why would rear wheel left hand spokes (at a lower tension than the right hand spokes) fail (presume though fatigue) "first"? Both sets of spokes are going though the cycle of 'lower tension' as their bit of rim is at 6 o'clock but every spoke stays in tension throughout (see upthread).
I think we had established upthread (a few pages for you to check) that the spokes that have to work hardest (and tend to be the earliest to fail if the rim doesn't go first) are the trailing spokes on the right hand side, because they are at a higher tension and they have additional stress from the torsional load from the drive train. I remember asking this specific question - was it in 2019?