Where's the proof DB spokes more durable?
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The ultimate tensile strength is a good indicator of fatigue life and elasticity. Fatigue life is difficult to measure and the modulus of elasticity can (and is) measured during the tensile strength test.
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This makes no sense. What you are saying is that less material is stronger. A 2 mm spoke has a cross section of 3.24 mm^2, a 1.8 mm spoke has a cross section of 2.54 mm^2. So the smaller cross section is stronger? Perhaps the tensile strength numbers are really given in kgf/mm^2 and the butted spokes are stronger per mm^2 due to cold working.
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Let’s start with tensile strength. The tensile strength being measured isn’t the wheel but the resistance of the spoke to breakage. Tensile strength measurements of the spoke gives you insight into how much it takes to fatigue the spoke to the point where it will break. Higher tensile strength (or resistance to breakage) will result in a strong, more durable wheel.
Pillar spokes is about the only place where I’ve seen actual measurements of tensile strength of various types of spokes. You have to look at multiple graphs to see what effect butting has on strength but it’s pretty clear when you compare them. For example a 2.0mm (14ga) spoke breaks at 270 kgf (kilograms force which is a really dumb unit). A 2.0/1.8/2.0mm breaks at 290 kgf (about) and a 2.2/1.8/2.0mm spoke breaks at about 330 kgf. They have a 2.3/1.8/2.0mm triple butted spoke that breaks at about 420 kgf. For comparison, their 2.3mm straight spoke breaks at about 360 kgf.
Their charts show pretty conclusively that butting the spokes increases their strength.
Pillar spokes is about the only place where I’ve seen actual measurements of tensile strength of various types of spokes. You have to look at multiple graphs to see what effect butting has on strength but it’s pretty clear when you compare them. For example a 2.0mm (14ga) spoke breaks at 270 kgf (kilograms force which is a really dumb unit). A 2.0/1.8/2.0mm breaks at 290 kgf (about) and a 2.2/1.8/2.0mm spoke breaks at about 330 kgf. They have a 2.3/1.8/2.0mm triple butted spoke that breaks at about 420 kgf. For comparison, their 2.3mm straight spoke breaks at about 360 kgf.
Their charts show pretty conclusively that butting the spokes increases their strength.
Brandt's overall conclusion was that spokes break because of excessive cycling into inelastic deformation. A thick spoke has a stiffer shaft which elongates less with an increase in tension, compared to a more noodly thin spoke shaft. If strain is common between the thick and thin spokes, then more strain occurs at the nipple and the elbow of a thick spoke, than with a thin spoke. The accumulation of fatigue damage should be faster for a thick-spoke wheel than for a thin-spoke wheel. At least this how I understand Jobst. But again, this is just a suggestion.
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I get the physics argument for DB spokes over straight gauge, but what I'm wondering is if the difference is negligible, or if it's significant and tangible?
These days, my bikes get very low-stress use, just steady paced exercise, on good roads. No off-road, big-air thrashing, and no more road racing with heavy training, interval sprints, etc.
All my wheels are currently straight gauge Swiss DT's. Would I expect to get measurably longer service with DB spokes, or is this basically just an academic/technical argument, at least in my case?
These days, my bikes get very low-stress use, just steady paced exercise, on good roads. No off-road, big-air thrashing, and no more road racing with heavy training, interval sprints, etc.
All my wheels are currently straight gauge Swiss DT's. Would I expect to get measurably longer service with DB spokes, or is this basically just an academic/technical argument, at least in my case?
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Not Park Tool. I think they [twist-resist tools] were made by Competition Cycles Services by modifying a fourth hand brake cable tool.
Sadly, I gave mine away years ago when I stopped building wheels.
See the comments on this amazon page.
Sadly, I gave mine away years ago when I stopped building wheels.
See the comments on this amazon page.
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Elasticity of a spoke is like that of a frame tube, where an increase in OD has a lot of leverage in the equation for stiffness. For frame tubes the stiffness increases with the square of the OD. If the diameter increases by 10%, the stiffness increases by 1.1*1.1 = 1.21 = 21%!
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The problem with carbon fiber for spokes in a conventional wheel is that it's too rigid.
A current effort at fiber spokes is Berd, using Dyneema cord, that's been bury spliced at the hub end, and spliced onto a short length of spoke at the wheel end, and treated for abrasion resistance. There was a neato thread (haha) on MTBR about trying to recreate them hobby style, but Berd sent the ringleader a "please stop" message.
A current effort at fiber spokes is Berd, using Dyneema cord, that's been bury spliced at the hub end, and spliced onto a short length of spoke at the wheel end, and treated for abrasion resistance. There was a neato thread (haha) on MTBR about trying to recreate them hobby style, but Berd sent the ringleader a "please stop" message.
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Let’s start with tensile strength. The tensile strength being measured isn’t the wheel but the resistance of the spoke to breakage. Tensile strength measurements of the spoke gives you insight into how much it takes to fatigue the spoke to the point where it will break. Higher tensile strength (or resistance to breakage) will result in a strong, more durable wheel.
Pillar spokes is about the only place where I’ve seen actual measurements of tensile strength of various types of spokes. You have to look at multiple graphs to see what effect butting has on strength but it’s pretty clear when you compare them. For example a 2.0mm (14ga) spoke breaks at 270 kgf (kilograms force which is a really dumb unit). A 2.0/1.8/2.0mm breaks at 290 kgf (about) and a 2.2/1.8/2.0mm spoke breaks at about 330 kgf. They have a 2.3/1.8/2.0mm triple butted spoke that breaks at about 420 kgf. For comparison, their 2.3mm straight spoke breaks at about 360 kgf.
Their charts show pretty conclusively that butting the spokes increases their strength.
Pillar spokes is about the only place where I’ve seen actual measurements of tensile strength of various types of spokes. You have to look at multiple graphs to see what effect butting has on strength but it’s pretty clear when you compare them. For example a 2.0mm (14ga) spoke breaks at 270 kgf (kilograms force which is a really dumb unit). A 2.0/1.8/2.0mm breaks at 290 kgf (about) and a 2.2/1.8/2.0mm spoke breaks at about 330 kgf. They have a 2.3/1.8/2.0mm triple butted spoke that breaks at about 420 kgf. For comparison, their 2.3mm straight spoke breaks at about 360 kgf.
Their charts show pretty conclusively that butting the spokes increases their strength.
Clue: Fatigue life, yield strength, and modulus of elasticity are all measured independently of ultimate tensile strength at breakage. Keep going down your rabbit hole and you’ll soon post something like “Tighten the nipple until the spoke breaks then back off x turns.” HAW!
Last edited by AnkleWork; 08-04-19 at 02:03 PM.
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Moment of inertia is only relevant for bending and twisting (torsion), not elongation (strain).
ε (strain) = σ (stress) / E (Young's modulus)
If you double the cross sectional area, you half the stress, and thus the strain.
ε (strain) = σ (stress) / E (Young's modulus)
If you double the cross sectional area, you half the stress, and thus the strain.
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Does anybody care about the tensile strength of a spoke at breaking? It's the usual shuck and jive attempt to cover up erroneous pontifications. Modulus of elasticity, yield strength, and fatigue life are independent of breaking strength and are investigated, specified and published for structural metals and each is used to inform the design process. Your “breakage” force is only meaningful at catastrophic failure for determining safety factors.
If you can get them all off the same curve...like those presented by Pilar Spokes...they are related by definition.
Clue: Fatigue life, yield strength, and modulus of elasticity are all measured independently of ultimate tensile strength at breakage.
Keep going down your rabbit hole and you’ll soon post something like “Tighten the nipple until the spoke breaks then back off x turns.” HAW!
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...if a spoke breaks in the forest, and there's no one around to hear it, does it still make a sound ?
...if a spoke breaks in the forest, and there's no one around to hear it, does it still make a sound ?
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Micro examination of spoke failure always shows the same thing. We know why and where spokes fail but cannot answer the “when.” The measure of science is not hindsight ability to explain but predictive power. By this standard, the science of spoke failure is as weak as most macroeconomics.
If breakage is wire material based, then consistency with adequate strength, is paramount. You bet. Wire consistency is an obsession of spoke makers and they have as little control as you once they accept the wire. There are too many variables over too many miles of wire. Variables include annealed state, surface finish, micro-hardness variability, occlusions and contaminants, crystal structure uniformity, and more.
Making strong wire (high tensile strength) is straight forward. Making it über consistent is very hard. Stainless wire for high end spokes is so extreme and unique in this parameter that it sees no other industrial use. No one would pay so much for consistency outside of medicine and there aren’t many medical uses for 14g wire.
If breakage is wire material based, then consistency with adequate strength, is paramount. You bet. Wire consistency is an obsession of spoke makers and they have as little control as you once they accept the wire. There are too many variables over too many miles of wire. Variables include annealed state, surface finish, micro-hardness variability, occlusions and contaminants, crystal structure uniformity, and more.
Making strong wire (high tensile strength) is straight forward. Making it über consistent is very hard. Stainless wire for high end spokes is so extreme and unique in this parameter that it sees no other industrial use. No one would pay so much for consistency outside of medicine and there aren’t many medical uses for 14g wire.
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I work in a field where we use some of these machines and the ones for fatigue are different. The frequency and amplitude that they pull is dictated by the test, and they need an actuator and control system to do that. A test for modulus and yield is much simpler. It just pulls with increasing force and measures the strain. There are other types too. We make rubber stuff and the strain rate is important for us because it's viscoelastic. Our machines are Instron brand.
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If the spokes are more elastic, doesn't the rim deform more, thus break sooner?
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I work in a field where we use some of these machines and the ones for fatigue are different. The frequency and amplitude that they pull is dictated by the test, and they need an actuator and control system to do that. A test for modulus and yield is much simpler. It just pulls with increasing force and measures the strain. There are other types too. We make rubber stuff and the strain rate is important for us because it's viscoelastic. Our machines are Instron brand.
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Double-butted spokes do more than save weight. The thick ends make them as strong in the highly-stressed areas as straight-gauge spokes of the same thickness, but the thinner middle sections make the spokes effectively more elastic, allowing them to stretch (temporarily) more than thicker spokes.
As a result, when the wheel is subjected to sharp localized stresses, the most heavily-stressed spokes can elongate enough to shift some of the stress to adjoining spokes. This is particularly desirable when the limiting factor is how much stress the rim can withstand without cracking around the spoke holes.
As a result, when the wheel is subjected to sharp localized stresses, the most heavily-stressed spokes can elongate enough to shift some of the stress to adjoining spokes. This is particularly desirable when the limiting factor is how much stress the rim can withstand without cracking around the spoke holes.
Single- and triple-butted spokes solve one of the great problems of wheel design: Since spokes use rolled, not cut threads, the outside diameter of the threads is larger than the base diameter of the spoke wire. Since the holes in the hub flanges must be large enough for the threads to fit through, the holes, in turn, are larger than the wire requires. This is undesirable, because a tight match between the spoke diameter at the elbow and the diameter of the flange hole is crucial to resisting fatigue-related breakage.
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Shelbyfv, I have posted a few times on this subject and have yet received an answer. What brought it up this time was a discussion with the service manager at the shop where I work weekends. He insists DB spokes are more durable and resist breaking better than straight gauge. I asked him for proof, which of course he did not have. I suppose this has to be laid to rest, and I will have to simply turn the other cheek when I hear the proclamation of the virtues of DB spokes.
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Shelbyfv, I have posted a few times on this subject and have yet received an answer. What brought it up this time was a discussion with the service manager at the shop where I work weekends. He insists DB spokes are more durable and resist breaking better than straight gauge. I asked him for proof, which of course he did not have. I suppose this has to be laid to rest, and I will have to simply turn the other cheek when I hear the proclamation of the virtues of DB spokes.
I suspect there are a lot of choices in bike design that are not backed up by detailed analysis or testing. There aren't enough bikes in the world to test every possible configuration and riding condition to destruction with enough data for good statistics. As a result, we're often faced with gathering the best advice that we can get, and making our own choices. Best of luck!
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This sounds like a rational plan if the idea still bothers you. On the other hand, there seems to be no "proof" that DB spokes are less durable. Frankly, I lack the educational accomplishment to evaluate any such proof absent someone to interpret it for me. I'm confident DT, Mavic and the other manufacturers have employees who are familiar with whatever data exists and are able to use it to the benefit of their products. They have much more "skin in the game" than any of us. Durability may be only one of a number of qualities, but clearly they have decided that DB spokes are appropriate in better wheels.
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Shelbyfv, I have posted a few times on this subject and have yet received an answer. What brought it up this time was a discussion with the service manager at the shop where I work weekends. He insists DB spokes are more durable and resist breaking better than straight gauge. I asked him for proof, which of course he did not have. I suppose this has to be laid to rest, and I will have to simply turn the other cheek when I hear the proclamation of the virtues of DB spokes.
But with bikes, we like to optimize things. Straight-gauge spokes tend to break (if/when they do) at the ends, meaning that there's more material in the center than necessary; the ends are the weak links. So it's logical to think that if you put extra material on the ends and took it away from the middle in just the right amount, you'd end up with a spoke that is equally durable from end to end. From there, you'd scale the whole thing up or down according to your needs. (My rando bike wheelset is an experiment in scaling down -- 4300 miles so far on 32/3x 1.8/1.6/1.8 and I'm wondering if I could have pushed it further!)
Given the question of how straight-gauge spokes don't seem to break a lot more even though they are more likely dip to zero tension at the bottom or when hitting potholes, I think the extra material compensates to some degree. Plus (per Jobst), spokes carry load in proportion to their tension, so I think that's another way that life isn't as hard for NDS spokes as one would imagine.
So if the extra weight of straight-gauge spokes doesn't bother you or cause problems, the world is your oyster. They're cheaper and easier to build into wheels. People ride many thousands of happy miles on straight-gauge spokes without being aware of it.