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

[PatrickPending]

Whenever this happened to me (and its not for a long time) I've had a wheelbuilder re-build the wheel using good quality stainless steel spokes. I'd advise this course rather than a new machine built wheel....

 

[DCBassman]

I love these threads. I never understand them, and they just serve to persuade me that I should never, ever, touch a spoke nipple because my wheel will probably come unravelled, like a piece of knitting.

But I love them nonetheless.

Agreed!

 

[mickle]

"Loose spokes move microscopically with every pedal stroke."


There is no relative movement between spokes and spokes, or spokes and other wheel parts during a wheel's use that's enough to cause any sort of wear in a spoke. Wear implies loss of material, of which there is none.

Spokes do flex during wheel revolutions and they flex at the bottom of the wheel in the load affected zone, which is an area between the road and rim. Even then, they only flex by becoming SHORTER. This change in length is indeed microscopic and it is taken up evenly along the entire length of the spoke. Therefore the change in length per mm of spoke length is indeed microscopic. For all intends and purposes the relative movement between hub and spoke, where the spoke touches the hub flange is thus zero. We know this because there is no visible rouge in that area when you disassemble a wheel, nor is there ever any signs of material loss on the spoke itself. The hub flexes with the spoke, reducing relative movement between the two.

Steel hubs with sharp edges do indeed cause more spoke breakages but this is not because of wear, but because of conditions that exacerbates metal fatigue. An aluminium hub, being softer than the spoke, changes shape to accommodate the spoke elbow. This smearing creates a larger contact area and softer bend in the spoke, which reduces work hardening and subsequently, metal fatigue in the spoke. Steel hubs don't, causing a very sharp transition at the spoke elbow.

Flex is not movement.

Spokes flex when just rolling along and additionally with each pedal stroke. Flex from the latter is confided to rear wheels and mostly the right hand spokes only and, only every second one on the right hand side. This flex is small since even a strong launch to sprint only increases the spoke tension in those spokes by less than 5% It is less than the flex induced by mere rolling.

"Moving spokes fatigue or literally wear out if the movement is between, for example, a steel hub flange and a spoke elbow"

Movement is not flex. A spoke that's moving is one that's rattling away in the wheel because it has no tension. Such a spoke experiences no forces which cause metal fatigue. As soon as a spoke has some modicum of tension, it will flex as described above and start to fatigue. The rate of fatigue is irrelevant whether the spokes are high tensioned or just-just tensioned enough to keep the wheel together.

I've dealt with the topic of wear. It implies relative movement, abrasion and loss of material. That doesn't happen.

"Cheap spokes tend to die sooner."

Price is not a reliable indicated of poor spokes, but it has bearing. It costs more to make a double-butted spoke than a straight-gauge spoke and therefore it should cost more. But a double-butted spoke from a SAPIM box or a double-butted spoke from a Campagnolo catalogue are exactly the same spoke, although the price is hugely different. I'm sure you know this but someone else may not.

Good spokes are made from de-gassed, 18/8, cold-forged stainless steel. Rubbish spokes are made from high-tensile steel and then galvanised.
Budget spokes are made from the same material as above, but not butted (drawn) and thus don't receive as much of the benefitical cold forging. But non-butted bit of the spoke, i.e. the thin shank, flexes more than the thick bits at either end. This reduces the strain on those bits and hence, inputs less metal fatigue in those critical parts. Metal fatigue is not as prevalent in the thin, flexible central shank even though it flexes more than the thicker parts.

However, an expensive spoke still needs one bit of material treatment to make it long-lasting and that treatment is stress relieving. It is done by the builder after the build. Unfortunately the spoke company can't build it in, it has to be done after the wheel is finished. Most builders don't have a clue about this, hence the poor wheels we see.

"So run tight wheels, or use decent spokes. Or both."

For long spoke life the wheels don't have to be tight. I explained why in the text above. Tight is good however, because it prevents nipples coming loose and wheels losing trueness. Very tight is even better, but it has nothing to do with spoke life. Uneven tension also has nothing to do with spoke life but you don't want such a wheel 'cause it wobbles.

I love you Yellow Saddle.

 

[Old jon]

Flex, English word, US dictionary definition. Dunno how Newton would have defined it.

Definition of flex
(Entry 1 of 3)

transitive verb

1 : to bend especially repeatedly

2a : to move muscles so as to cause flexion of (a joint)

b : to move or tense (a muscle) by contraction

3 : use, demonstrate flexing her skills as a singer

intransitive verb

: bend

 

[Yellow Saddle]

Flex, English word, US dictionary definition. Dunno how Newton would have defined it.

Definition of flex
(Entry 1 of 3)

transitive verb

1 : to bend especially repeatedly

2a : to move muscles so as to cause flexion of (a joint)

b : to move or tense (a muscle) by contraction

3 : use, demonstrate flexing her skills as a singer

intransitive verb

: bend

I agree with you but sometimes I have to simplify in order to keep my audience, and there's no slight intended to my audience. They are a mix and I want them all to have a better understanding of this complex but fascinating topic. The flex that happens with wheel use is elastic deformation aka stretching and compression of the spokes and true flex in the rims. It just isn't as elegant, but, you are of course right. As soon as I use the C word (compressing) in these explanations, it opens a can of worms and starts WW III. If you search under my name for "in compression" and "in tension" you'll see what I mean. This topic repeats every 100 days or so and I prune a bit here and shortcut a bit there each time.

 

[Ajax Bay]

The flex that happens with wheel use is elastic deformation aka stretching and compression of the spokes and true flex in the rims. It just isn't as elegant, but, you are of course right. As soon as I use the C word (compressing) in these explanations, it opens a can of worms and starts WW III. If you search under my name for "in compression" and "in tension" you'll see what I mean. This topic repeats every 100 days or so

Well don't use the 'C' word then. Spokes remain in tension unless they are loose (tension = 0N) so they cycle from high tension (normal static ?1000+N) to a lower tension (ie they are stretched less (the lowest spokes in a weight-bearing rolling wheel)). "elastic deformation aka change in tension of each spoke, moderated by true flex in the rims"? A spoke in compression would poke its nipple up and through the rim tape rather quickly, probably with flattening results. I have not experimented to see whether a puncture results - to do so would be nugatory as there is no mechanism other than complete wheel/rim failure which would force a nipple thus. Single rifleman minor skirmish, not WWIII, to deter casual use of the 'C' word.

 

[rogerzilla]

Jobst Brandt also talked about the wheel "standing" on its lowest spokes (equating a reduction in tension with compression). It's annoyed a lot of people over the years. He was correct but his choice of words didn't help much.

 

[Yellow Saddle]

Well don't use the 'C' word then. Spokes remain in tension unless they are loose (tension = 0N) so they cycle from high tension (normal static ?1000+N) to a lower tension (ie they are stretched less (the lowest spokes in a weight-bearing rolling wheel)). "elastic deformation aka change in tension of each spoke, moderated by true flex in the rims"? A spoke in compression would poke its nipple up and through the rim tape rather quickly, probably with flattening results. I have not experimented to see whether a puncture results - to do so would be nugatory as there is no mechanism other than complete wheel/rim failure which would force a nipple thus. Single rifleman minor skirmish, not WWIII, to deter casual use of the 'C' word.

Compression as a concept is important and I'll continue using it. When a wheel cycles, the load affected zone undergoes compression, which leads to a reduction of tension in the spoke. I don't think I've ever explained it to mean anything other that what I've just said. Seen from another angle. When a bike is loaded, there is a compressive force between the hub and road and thus the hub stands on the bottom spokes. There is no other way to state that fact. Compression is important.

 

[Levo-Lon]

I usually find the word Alex and Superstar and wheel in the same sentence = a odd snapped spoke.

All my quality wheels seem to stay straight and true.
Capagnolo Mavic and DT Swiss ..DT suffer with shyte bearings but fine when you bin the plastic ones and fit good uns

 

[Ajax Bay]

Seen from another angle. When a bike is loaded, there is a compressive force between the hub and road and thus the hub stands on the bottom spokes.

With all due respect, no there isn't and no it doesn't. Let me explain (to use your vernacular) and offer you a view "from another angle".
When a bike is loaded (front wheel for ease of visualisation) a downwards force is exerted on the fork (which is in compression). That downwards force is balanced by an upwards force from the hub. The latter force is exerted on the hub by a balance of the radial positive forces from all the spokes. All the spokes are in tension and the force they're exerting is radial in all cases (they 'pull' away from the hub). At the outer end of every spoke is the rim and the rim exerts a force on every spoke equal (obv) to the tension in the spoke. The rim is in compression, all the time. The contact area of the tyre experiences a 'normal' (ie vertical) force from the road - this force is equal to the load borne by the wheel (referred to in Line 1 above). As you sometimes suggest to others, draw yourself a diagram and mark in the forces and why not label each item which is 'in compression'.
Besides the rim (and the axle with a QR providing the compression) I cannot think of an element of the wheel in compression. Maybe you can (leave the tyre out of it, I suggest).
Isn't the wheel a marvelous invention?

 

[Yellow Saddle]

With all due respect, no there isn't and no it doesn't. Let me explain (to use your vernacular) and offer you a view "from another angle".
When a bike is loaded (front wheel for ease of visualisation) a downwards force is exerted on the fork (which is in compression). That downwards force is balanced by an upwards force from the hub. The latter force is exerted on the hub by a balance of the radial positive forces from all the spokes. All the spokes are in tension and the force they're exerting is radial in all cases (they 'pull' away from the hub). At the outer end of every spoke is the rim and the rim exerts a force on every spoke equal (obv) to the tension in the spoke. The rim is in compression, all the time. The contact area of the tyre experiences a 'normal' (ie vertical) force from the road - this force is equal to the load borne by the wheel (referred to in Line 1 above). As you sometimes suggest to others, draw yourself a diagram and mark in the forces and why not label each item which is 'in compression'.
Besides the rim (and the axle with a QR providing the compression) I cannot think of an element of the wheel in compression. Maybe you can (leave the tyre out of it, I suggest).
Isn't the wheel a marvelous invention?

Every ounce you put onto the bike is felt by the road. There is a compressive force between road and bike. Put a bathroom scale under each wheel and you'll see.

The only spokes that interact to transmit that net downward force are the ones in the load affected zone, i.e the two or three right at the bottom. The other radial forces in the other spokes play no role in transmitting the bike's weight down to the road. None. They do play a role in keeping the wheel in one piece, but that they do equally when the wheel is held in your hand and touches nothing.

None of the spokes go into compression and no matter how many times you say I said it, I don't and have not. Those spokes receive a compressive force that does not exceed their tension.

 

[Ajax Bay]

"Every ounce you put onto the bike is felt by the road." Yes
"There is a compressive force between road and bike." Like the compressive force in the sole of a worn shoe (ETA: ie a shoe which is being worn). Difference is that the wheel is a structure so saying that "there is a compressive force between road and bike" lacks meaning/utility.

"The only spokes that interact to transmit that net downward force are the ones in the load affected zone, i.e the two or three right at the bottom." Depends what you want "interact" to mean. I agree that they're the ones in which tension reduces most, which means they pull down on the hub less than when the wheel is unloaded.
"The other radial forces in the other spokes play no role in transmitting the bike's weight down to the road. None."
Sorry, I have difficulty with that. The hub is 'hanging' on the spokes above it and the upwards force it has to exert on the fork drop outs is because the lower spokes have reduced the amount they are pulling down.
"They [the other spokes] do play a role in keeping the wheel in one piece, but that they do equally when the wheel is held in your hand and touches nothing."
Agreed with the observation that all the spokes (including the lower ones) "play a role in keeping the wheel in one piece" both unloaded and loaded.
"None of the spokes go into compression and no matter how many times you say I said it, I don't and have not."
Apologies if I have suggested that you have said "spokes go into compression". I have tried to find where I have put those words 'into your mouth' but with no success.
"Those spokes receive a compressive force that does not exceed their tension." The lower spokes just experience a cyclical lower tension - the force exerted on a lower spoke by the rim lessens and results in a lower tensile force. This means that the resultant force (the result of all the spokes pulling on the hub at various tensions) on the hub is up (ie operates vertically upwards through the drop outs and is equal to the weight on the wheel).
I think we agree what happens but I think your use of certain terms (like compression, compressive and 'stands') clouds rather than provides the clarity needed for "a better understanding of this complex but fascinating topic".
Draw a diagram.
And a few quotes which show you're keen that others draw diagrams:

A force diagram will take you 30 seconds to do. I'll wait until after the weekend. Pardon my cynicism but I've been here before.

Draw a force diagram and you'll see that your scenario is implausible.

 

[Yellow Saddle]

OK, my force diagram is on its way. I'd like you to do one too please. Then we can compare. Hang in there.

 

[Ajax Bay]

Hang in there.

Will do. That's what my load bearing hubs are doing.

 

[Yellow Saddle]

Here is the data:
1) The fork is loaded with a force of 60. Ignore units, I've kept all units the same, so it doesn't matter.
2) This wheel has 11 spokes. When the wheel is unloaded, each spoke has a tension of 100.
3) The flat bit at the bottom is the Load Affected Zone (LAZ henceforth). There are three spokes in the load affected zone. Because of the load on the hub, the tension in those thee spokes is reduced. The total reduction equals the load on the hub. Therefore the tension in each one of those is now only 80. 100-80 =20. 3 x 20 = 60, which equals the load.

Ignore the fact that the three bottom spokes are all in equal tension. In reality it the middle one would have the lowest tension and then those straddling it a bit more etc etc. It complicates the basic explanation. We can go into how that varies at a later stage.

The system is in equilibrium and conserves Newton's laws.