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  1. #1
    i ride a bicycle
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    spoke misconception

    I just wanted to try and clear up one of the most common misconceptions I see on these forums. I'm sure someone else has made this point before, but it bugs me every time I see this bit of mis-information, so here it is again, with as little "engineer-speak" as possible.

    Wheels built with straight gage spokes are not "stronger" than double butted spokes.
    (using the common mistake of replacing "durable" with "strong")

    Yes, in a uniaxial tensile test, if you were to pull on the ends of the two spokes, the straight gage one would be stronger (you would have to pull harder to snap it).

    BUT, the most common failure mode for a spoke is fatigue (at the head or threads), NOT tensile overload. The double butted spoke resists this failure by stretching in the thinner center section. Straight gage spokes will build a stiffer wheel (because of their relative lack of elasticity), but butted spokes will build a more durable one.

    Of course, the easiet way to avoid spoke failure is to ride a wheel built with proper, even spoke tension, regardless of spoke type. Straight gage spokes should be plenty reliable in a well built, well maintained wheel.

    I hope this post isn't received as a troll, I just want to share a little bit of "how things work" knowledge today.

    Mac

  2. #2
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    I agree with you statements regarding fatigue.

    Now, would you please elaborate on why the double-butted spoked wheel is more durable than the straight-gage spoked wheel?

  3. #3
    dances with bicycle 46x17's Avatar
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    Quote Originally Posted by lunacycle
    I agree with you statements regarding fatigue.

    Now, would you please elaborate on why the double-butted spoked wheel is more durable than the straight-gage spoked wheel?
    Because stress will not be as concentrated at the ends (elbow, nipple) where spokes fail, due to the more flexible thinner midsection.
    Life can only be understood backwards; but it must be lived forwards.
    -- Soren Kierkegaard

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    Quote Originally Posted by 46x17
    Because stress will not be as concentrated at the ends (elbow, nipple) where spokes fail, due to the more flexible thinner midsection.
    That seems plausible, although I don't quite understand why.

    All things being equal, the ends of the spokes will experience identical tensile forces under ideal conditions. Does the concept of hysteresis somehow work into the equation?

  5. #5
    aka mattio queerpunk's Avatar
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    Quote Originally Posted by lunacycle
    I agree with you statements regarding fatigue.

    Now, would you please elaborate on why the double-butted spoked wheel is more durable than the straight-gage spoked wheel?
    hooray, for the second time today, i get to do this:
    "building and bridges were made to bend in the wind
    to withstand the world,
    that's what it takes.
    all that steel and stone is no match for the air my friend
    what doesn't bend breaks, what doesn't bend breaks."

    ani difranco knows her wheelbuilding. she knows that doublebutted buildings and bridges must bend in order to withstand the world (that's what it takes!). same goes for wheels.
    the hipster myth.

    i practice vagabondery.

  6. #6
    Strange As Angels Fixxxie's Avatar
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    Quote Originally Posted by sac02
    I just wanted to try and clear up one of the most common misconceptions I see on these forums. I'm sure someone else has made this point before, but it bugs me every time I see this bit of mis-information, so here it is again, with as little "engineer-speak" as possible.

    Wheels built with straight gage spokes are not "stronger" than double butted spokes.
    (using the common mistake of replacing "durable" with "strong")

    Yes, in a uniaxial tensile test, if you were to pull on the ends of the two spokes, the straight gage one would be stronger (you would have to pull harder to snap it).

    BUT, the most common failure mode for a spoke is fatigue (at the head or threads), NOT tensile overload. The double butted spoke resists this failure by stretching in the thinner center section. Straight gage spokes will build a stiffer wheel (because of their relative lack of elasticity), but butted spokes will build a more durable one.

    Of course, the easiet way to avoid spoke failure is to ride a wheel built with proper, even spoke tension, regardless of spoke type. Straight gage spokes should be plenty reliable in a well built, well maintained wheel.

    I hope this post isn't received as a troll, I just want to share a little bit of "how things work" knowledge today.

    Mac
    +1
    Quote Originally Posted by sefb222 View Post
    a good reason to form a demolition derby, for fixed gear bikes and the fools who love them.

  7. #7
    dances with bicycle 46x17's Avatar
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    Quote Originally Posted by lunacycle
    That seems plausible, although I don't quite understand why.

    All things being equal, the ends of the spokes will experience identical tensile forces under ideal conditions. Does the concept of hysteresis somehow work into the equation?
    THink of force being applied to spoke tugging on both ends.

    Think of straight spokes vs. DB ones.

    Which one will stretch more and where?

    Right the DB one will stretch more! Where? In the skinny section!

    Does that equal less stress at the ends?

    Yes, because the skinny middle section absorbs some of it.
    Life can only be understood backwards; but it must be lived forwards.
    -- Soren Kierkegaard

  8. #8
    na975
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    this is a job for myth busters!

  9. #9
    i ride a bicycle
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    Quote Originally Posted by na975
    this is a job for myth busters!
    or just an engineer

  10. #10
    otherwiseordinary lymbzero's Avatar
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    How much weight is saved?

    what's is the difference in weight between 36 DB spokes and 36 SG spokes?

    I build my wheel with straight gauge spokes and now I just feel inferior.

  11. #11
    Cornucopia of Awesomeness baxtefer's Avatar
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    going from 14 SG to 14/15 DB spokes will save you about 1g/spoke (DT champs->comps)

    so you could have saved ~2.5 ounces.
    {o,o**
    |)__)
    -"-"-

    O RLY?

  12. #12
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    Quote Originally Posted by 46x17
    THink of force being applied to spoke tugging on both ends.

    Think of straight spokes vs. DB ones.

    Which one will stretch more and where?

    Right the DB one will stretch more! Where? In the skinny section!

    Does that equal less stress at the ends?

    Yes, because the skinny middle section absorbs some of it.
    I have to disagree with that last sentence. Assuming that the cross-sectional areas at the ends of both spokes are identical, the stresses at the ends must also be identical when subjected to the same forces. Actually, the tensile stress in the middle portion of a 14/15 ga spoke will be about 23% higher than that of the 14 ga spoke, given the same forces at the ends.

    I think we need to forget about the spoke as an individual element, and look at the wheel as a system. I believe that the enhanced durability of the DB spoked wheel is due to its inherent ability to transfer loads to adjacent spokes better than the SG spoked wheel, thus preventing any one spoke from being overloaded repeatedly.

  13. #13
    No cud for foil. DasProfezzional's Avatar
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    Put big ol' straight-gauge on mountain bikes. They look dopeness.

  14. #14
    raodmaster shaman
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    Quote Originally Posted by sac02
    I just wanted to try and clear up one of the most common misconceptions I see on these forums. I'm sure someone else has made this point before, but it bugs me every time I see this bit of mis-information, so here it is again, with as little "engineer-speak" as possible.

    Wheels built with straight gage spokes are not "stronger" than double butted spokes.
    (using the common mistake of replacing "durable" with "strong")

    Yes, in a uniaxial tensile test, if you were to pull on the ends of the two spokes, the straight gage one would be stronger (you would have to pull harder to snap it).

    BUT, the most common failure mode for a spoke is fatigue (at the head or threads), NOT tensile overload. The double butted spoke resists this failure by stretching in the thinner center section. Straight gage spokes will build a stiffer wheel (because of their relative lack of elasticity), but butted spokes will build a more durable one.

    Of course, the easiet way to avoid spoke failure is to ride a wheel built with proper, even spoke tension, regardless of spoke type. Straight gage spokes should be plenty reliable in a well built, well maintained wheel.

    I hope this post isn't received as a troll, I just want to share a little bit of "how things work" knowledge today.

    Mac
    some things to clarify/add here.

    one thing everyone should understand is that DB spokes are made through the process of drawing (putting the wire in a die with a smaller diameter and pulling it through the hole). in addition to reducing the diameter it changes the metal properties. in the simplest terms it makes the metal more brittle. it will take more force to permanently stretch it again, but it will stretch less before snapping. IT DOES NOT CHANGE THE ELASTICITY. the stress/strain line for a cold worked (drawn) steel follows that for non-cold worked, it just goes further up before arcing over into the plastic range.

    while the larger cross section of a SG spoke might make for a minuscule increase in bending moment of the spoke, wheel stiffness is also hugely dependent on spoke tension.

    A DB spoke can handle no more tension than a SG one because the head and the first inch or so do not recive the additional cold working (hence double, not "completely different" butting). both spokes have the same achileis heel for fatigue at the head.

    butting simply strengthens the thiner section so that it can handle the same force (but higher stress, F/A) that the larger gauge section does. the force will also be the same throughout the entire spoke, regardelss of which section elongates more proportionaly.

    DB makes a lighter wheel, thats about it. it wont make it stronger, it wont make it more durable. but it also wont make it less durable, since the part thats gonna fail is the same on both.

  15. #15
    Taking "s" outta "Fast" AfterThisNap's Avatar
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    Most spokes are close to free for me, but I still prefer wheelsmith DB spokes because they're damn purty.
    Carries suspicious allegiance to Brooklyn Machine Works.

  16. #16
    i ride a bicycle
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    As I mentioned, I really was trying to avoid 'engineer speak' and just provide a simplified explaination that could be used by someone without an engineering degree...

    Quote Originally Posted by roadgator
    some things to clarify/add here.
    one thing everyone should understand is that DB spokes are made through the process of drawing (putting the wire in a die with a smaller diameter and pulling it through the hole).
    not exactly. they are drawn and then swaged.

    Quote Originally Posted by roadgator
    ...in addition to reducing the diameter it changes the metal properties. in the simplest terms it makes the metal more brittle. it will take more force to permanently stretch it again, but it will stretch less before snapping. IT DOES NOT CHANGE THE ELASTICITY. the stress/strain line for a cold worked (drawn) steel follows that for non-cold worked, it just goes further up before arcing over into the plastic range.
    I agree with most of this statement. But your mistake is that you are only looking at the steel, not as the spoke as a functional part. Cold working a metal raises the tensile and yeild strength, lowers the ductility, and does not effect the Young's modulus. BUT, now that the cross section is reduced, the center of the spoke will see higher stress. (Don't worry about this; a spoke will only ever see a fraction of the tensile load needed to fail it in tension) And as you noted, since Young's modulus is unchanged, and stress=E*strain, strain increases too. And what is strain? Elongation. So even though the property of the STEEL (Youn'g modulus of elasticity) has not changed, the SPOKE acts in a more elastic manner. The steel and the spoke are two different concepts.

    Quote Originally Posted by roadgator
    while the larger cross section of a SG spoke might make for a minuscule increase in bending moment of the spoke, wheel stiffness is also hugely dependent on spoke tension.
    I was in no way discussing bending in a spoke. And you are mistaken in noting the "increased bending moment of the spoke". I believe what you are actually meaning to refer to is the second moment of area, which also commonly (though technically improperly) called moment of inertia.

    Quote Originally Posted by roadgator
    A DB spoke can handle no more tension than a SG one because...
    Again, the ultimate strength in tension of the spoke is completely irrelevant. (assuming we are not discussing spokes of inferior quality) No spoke will ever see tensile forces nearing its failure point

    Quote Originally Posted by roadgator
    butting simply strengthens the thiner section so that it can handle the same force (but higher stress, F/A) that the larger gauge section does.
    and simultaneously makes it act more elastic

    Quote Originally Posted by roadgator
    DB makes a lighter wheel, thats about it. it wont make it stronger, it wont make it more durable.
    wrong, wrong, and wrong
    Last edited by sac02; 02-02-07 at 09:59 AM.

  17. #17
    jack of one or two trades Aeroplane's Avatar
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    Quote Originally Posted by DasProfezzional
    Put big ol' straight-gauge on mountain bikes. They look dopeness.
    Definitely true. On a road bike, the DB spokes (or bladed spokes, yum!) look better.
    Quote Originally Posted by Dr Irwin Goldstein
    Men should never ride bicycles. Riding should be banned and outlawed. It is
    the most irrational form of exercise I could ever bring to discussion.

  18. #18
    cab horn
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    Nipples and spokes explained, by Sheldon Brown;

    Spokes
    The material of choice for spokes is stainless steel. Stainless is strong and will not rust. Cheap wheels are built with chrome-plated ("UCP")or zinc-plated ("galvanized") carbon-steel spokes, which are not as strong, and are prone to rust.
    The leading brands of spokes available in the U.S. market are DT and Wheelsmith.

    Titanium is also used for spokes, but, in my opinion it is a waste of money. Titanium spokes should only be used with brass nipples, which makes a combination that is not significantly lighter than stainless spokes with aluminum nipples.

    Carbon fibre spokes have been available, but turned out to be brittle and dangerous.

    Spoke Gauges
    The diameter of spokes is sometimes expressed in terms of wire gauges. There are several different national systems of gauge sizes, and this has been a great cause of confusion. A particular problem is that French gauge numbers are smaller for thinner wires, while the U.S./British gauge numbers get larger for thinner wires. The crossover point is right in the popular range of sizes used for bicycle spokes:

    U.S./British 14 gauge is the same as French 13 gauge
    U.S./British 13 gauge is the same as French 15 gauge

    Newer I.S.O. practice is to ignore gauge numbers, and refer to spokes by their diameter in millimeters:
    U.S./British 13 gauge is 2.3 mm
    U.S./British 14 gauge is 2.0 mm
    U.S./British 15 gauge is 1.8 mm
    U.S./British 16 gauge is 1.6 mm

    Spokes come in straight-gauge or swaged (butted) styles. Straight gauge spokes have the same thickness all along their length from the threads to the heads.
    Swaged spokes come in 5 varieties:


    Single-butted spokes are thicker than normal at the hub end, then taper to a thinner section all the way to the threads. Single-butted spokes are not common, but are occasionally seen in heavy-duty applications where a thicker than normal spoke is intended to be used with a rim that has normal-sized holes.

    Double-buttedspokes are thicker at the ends than in the middle. The most popular diameters are 2.0/1.8/2.0mm (also known as 14/15 gauge) and 1.8/1.6/1.8 (15/16 gauge).
    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. This allows 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 hole.


    Triple-butted spokes, such as the DT Alpine III, are the best choice when durability and reliability is the primary aim, as with tandems and bicycles for loaded touring. They share the advantages of single-butted and double-butted spokes. The DT Alpine III, for instance, is 2.34mm (13 gauge) at the head, 1.8mm (15 gauge) in the middle, and 2.0mm (14 gauge) at the threaded end.
    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 to fit the threads 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.

    Since single- and triple-butted spokes are thicker at the head end than at the thread end, they may be used with hubs that have holes just large enough to pass the thick wire at the head end.


    Æro (elliptical) spokes are a variety of double-butted spoke in which the thin part is swaged into an elliptical cross section, which makes them a bit more ærodynamic than round-section spokes. The most widely available spoke of this type is the Wheelsmith Æro. These are 1.8mm (15 gauge) at the ends, and the middles are equivalent to 16 gauge, but in the form of a 2.0 x 1.6mm ellipse. The Wheelsmith &Aelig;ro is my favorite spoke for high-performance applications, not just because of whatever ærodynamic advantage it may offer, but because the flat center section provides an excellent visual indicator to help the wheelbuilder eliminate any residual twist in the spoke. This helps build a wheel that will stay true.

    Æro (bladed) spokes have a more pronounced æro shape, flat, rather than elliptical. Although they are the most ærodynamic of spokes, they won't normally fit through the holes in a standard hub because they are too wide. To use "blades", the hub must be slotted with a file. This can weaken the flange, and will usually void the warranty of the hub. It is also a lot of trouble.
    There was a fad in the early '90s for Hoshi "blades" which had a double bend instead of a conventional head. This allowed the spokes to be inserted "head first" into the hub flange, so that they could be used with normal hubs. Unfortunately, they turned out to be prone to breakage, and I can't recommend them.

    My Bicycle Glossary has a Table of Spoke Weights, for those who care about such things.


    Nipples
    Nipples are commonly made of nickel-plated brass. This is a good material choice, because brass takes very smooth threads, and brass nipples don't get corroded into position too easily.
    For light-weight, high-performance wheels, aluminum nipples are available. Aluminum nipples do save a small amount of weight, and they can be quite reliable if used properly. They should only be used with rims that have eyelets of some material other than aluminum, because aluminum/aluminum contact between rim and nipple can result in chemical welding, immobilizing the nipples.
    ..
    Mes compaingnons cui j'amoie et cui j'aim,... Me di, chanson.

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