Your theories on why spokes break.

[tri boy]

Broke a spoke on the Scrambler today. (rear 17" wheel), and although it used to be a common thing years back, with so many laced wheels about, I got to wondering way such short stout spokes on a 17" wheel would suffer such a failure.
They are not long thin types like on 21" wheels. And are only 10000km old. Corrosion isn't an issue, and they are checked regularly for tightness.(maybe it was too tight).
1. Just a random failure? (although I have heard of other new Bonnies doing the same).

2. Poor quality? (maybe, but I would expect early failure in that case).

3. High load while surrounding components cold. (I did "give it some" after refitting the wheel to settle the chain prior to chain slack check).

Its a minor breakage, but the failure is intriguing.
The spoke "popped" its head near the Hub hole.

[Conquiztador]

Broke a spoke on the Scrambler today. (rear 17" wheel), and although it used to be a common thing years back, with so many laced wheels about, I got to wondering way such short stout spokes on a 17" wheel would suffer such a failure.
They are not long thin types like on 21" wheels. And are only 10000km old. Corrosion isn't an issue, and they are checked regularly for tightness.(maybe it was too tight).
1. Just a random failure? (although I have heard of other new Bonnies doing the same).

2. Poor quality? (maybe, but I would expect early failure in that case).

3. High load while surrounding components cold. (I did "give it some" after refitting the wheel to settle the chain prior to chain slack check).

Its a minor breakage, but the failure is intriguing.
The spoke "popped" its head near the Hub hole.

There is two ways of getting a thread on to the spoke: Cutting it and rolling it.

When cutting it you will take away material and the 7 gauge spoke becomes in real a 8 or 9 gauge. When rolling the thread on there is no material taken off, the thread is pushed in to the spoke and the material is squeezed out to make the thread. The rolling is the much better way. But the cutting is the faster way...

If your spokes are chromed and not heat treated then they will snap very easily. (Heat treating: in household oven for 30 minutes on 200 degrees C) I did a bend test with a heat treated and non treated chromed spoke. The treated one would take approx 8 bendings but the non-treated only two bendings before snapping...

If you have non-chromed spokes (or properly heat treated) and roll on thread and they brake early, then you might have an un-eaven tightness on spokes. This is common if the rim is not true and it has been compensated by pulling the rim straight with tightening the spokes.

Common option on Harleys that have been stroked, and will rip spokes, is to fit heavier gauge spokes.

EDIT: One more thing: Good quality spokes when older and heaps of hard use will commonly break at the 90 degree bend in the hub, not at the nipple. If your spokes are not 90 degree but 45 degree or even straight, then they become even stronger.

[merv]

2. Poor quality? (maybe, but I would expect early failure in that case).

Its a minor breakage, but the failure is intriguing.
The spoke "popped" its head near the Hub hole.

You've answered it yourself - option 2 - for the head to pop off the quality just aint flash. With that and your rusty rims you haven't given me any confidence in the product.

[tri boy]

I think you may be right Merv. Had a look at the fracture area of the spoke, and I think the quality of the metal is suspect. (a bit powdery looking).
its been years since I laced wheels but I remember a simple grinding check, showing the sparks was a good indication of the quality of the spoke metal.
(Bright sparks=Good?)
Will try it tomorrow.
Ahh well, these things happen.

Quick measure of the spoke. 5mm thick at the hub end, and only 120mm long. That should be a bloody strong spoke.

[Motu]

Well theoretically spokes work in tension,so it doesn't matter how long or short it is.But if it moves from tension to compression as it rotates,then you have stress reversal....and yes it will break.Which means it's incorrectly adjusted.

But hey - in the real world how many wheels do you see with spoke tension completely out of whack.Come and see my bikes if you need a visual demonstration.For such a weak looking design they can sure take some abuse and still do the job.....and that's why they are still used on dirt bikes.

The modern bike is not designed with a slide rule and a thumb as back up.....the computer knows just how strong it needs to be these days.I reckon your tough looking Triumph spoke is no such thing.

[Max Preload]

I think you may be right Merv. Had a look at the fracture area of the spoke, and I think the quality of the metal is suspect. (a bit powdery looking).

You mean like speckles on the break? Sounds like a brittle fracture - possibly from lack of normalising/annealing after cold working. Goes well with the shitty corrosion protection on the rims I've been hearing so much about.

[tri boy]

You mean like speckles on the break? Sounds like a brittle fracture - possibly from lack of normalising/annealing after cold working. Goes well with the shitty corrosion protection on the rims I've been hearing so much about.

Yep. Looks just like that.
To be fair, 99% of my drama's with the Scrambler has been based around the wheel assy's.
Should have the new one and another, (replacing an inner, and outer) by Thurs, so I should be rolling for the WWW ride. If more of them start to fail, then I'll chat to Triumph.
Character was liberally splashed around this bike during construction.
But besides it's faults, I'm honestly enjoying it.
And if I wanted a boringly reliable, uninspired scoot, I would have walked through an Honda Dealership's front door.

[Bonez]

Yep. Looks just like that.
To be fair, 99% of my drama's with the Scrambler has been based around the wheel assy's.
Should have the new one and another, (replacing an inner, and outer) by Thurs, so I should be rolling for the WWW ride. If more of them start to fail, then I'll chat to Triumph.
Character was liberally splashed around this bike during construction.
But besides it's faults, I'm honestly enjoying it.
And if I wanted a boringly reliable, uninspired scoot, I would have walked through an Honda Dealership's front door.

I knew you'd throw that in If Triumph need reminding on how long spokes should last they're quite welcome to have a gander at my 550f or the GBs ones

Have a good ride home on Sunday?

[Motu]

If Triumph need reminding on how long spokes should last they're quite welcome to have a gander at my 550f or the GBs ones

Or they could check out some perfectly good spokes they made over 50 years ago.

[merv]

Or they could check out some perfectly good spokes they made over 50 years ago.

Exactly

[tri boy]

Broken spoke replaced, wheel back in, and ready for the WWW ride.
Should I open a betting book to see how long it takes till the next one goes "ping"?
At $8.50 a spoke, I hope its rare. Works out to be about $300 a wheel.

[Conquiztador]

Broken spoke replaced, wheel back in, and ready for the WWW ride.
Should I open a betting book to see how long it takes till the next one goes "ping"?
At $8.50 a spoke, I hope its rare. Works out to be about $300 a wheel.

In the days I used to do wheels I charged $2.50/spoke nickle plated. Custom made to fit your wheel. Chromed or SS $3.50. $50 to respoke wheel + spokes. 10 years ago now. But can't see the inflation having had that much of an effect.

[McJim]

I remember folks having problems with spokes snapping regardless of quality. The culprit was the hub where the holes for the spokes have worn causing play in the spoke - as Moto pointed out the spoke experiences compression/tension in quick siccession - if there's some play it'll snap the head off the spoke.

Of course my memories are from pushies but the principle is the same.

[Conquiztador]

I remember folks having problems with spokes snapping regardless of quality. The culprit was the hub where the holes for the spokes have worn causing play in the spoke - as Moto pointed out the spoke experiences compression/tension in quick siccession - if there's some play it'll snap the head off the spoke.

Of course my memories are from pushies but the principle is the same.

U are right. But where they brake is at the bend in the hub, not at the nipple.

[tri boy]

U are right. But where they brake is at the bend in the hub, not at the nipple.

Would that therefore mean BMW has got the ultimate spoke now? (those straight ones fitted on their big RGS's).
Or is there still a good engineering principle to use the bent ones. (some form of lateral shock absorption maybe).