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Discussion Starter · #1 · (Edited)
I'm in learning mode, and have read a number of sources (mostly internet) that "double butted" spokes are:

  • thinner in the middle than at ends
  • lighter than equivalent straight guage (i.e., a 14/15 db is lighter than a straight 14 guage)
  • more vertically compliant (i.e., flexes more in the middle of the spoke under weight), and
  • (most imporant for this post) just as strong as straight gauge, since spokes tend to stress more/break at the ends, not the middle.
If this list is accurate, then the only reason to use straight guage spokes (harsher, heavier) would appear to be to save a little $ on the cost (db=more expensive).

Question: Is this list accurate? In other words, are double-butted spokes really as strong as straight guage? Anything more than word-of-mouth to verify (wheelbuilder direct experience; published internet tests?).

By the same token, can anyone point me to a source that verifies the conventional wisdom that db spokes are less harsh? It occurs to me, for example, that looking at the spoke angles on a 32-spoke 3x laced wheel, that much of the verticle "give" would be a function of the spoke angle vs. the butting of the spoke, but this is my speculation only. Would butting make more of a difference for harshness in a 2x laced wheel?

Finally, when a spoke (double butted or otherwise) is ovalized or flattened to become more aero, does all of this go out the door? Any links to web sites with good info on these points is welcome. Thanks.
 

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drewmcg said:
I'm in learning mode, and have read a number of sources (mostly internet) that "double butted" spokes are: just as strong as straight gauge, since spokes tend to stress more/break at the ends, not the middle.
I agree with your other points but not this one. How could less metal leave a spoke just as strong as one with more metal? I would think that in a true tension contest the thicker spoke would be stronger and that both spokes would break in the middle. But - as spokes, within a wheel, are rarely tested to their maximum tensile strength, butted spokes are strong enough.

Spokes in a bike wheel suffer fatigue from constant tension increase/decrease which manifests at the j-bend.

then the only reason to use straight guage spokes (harsher, heavier) would appear to be to save a little $ on the cost
That I agree with.

are double-butted spokes really as strong as straight guage?
Strong enough as a true tension-to-failure test never happens within a bicycle wheel.
 

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I have tested the tensile strength of DT 14g and 14/15g spokes using a tensile testing machine. The results I got after pulling multiple spokes of each gauge were that 14g broke consistantly at the bend. This is because the diameter of the spoke necks down to about 1.9mm as a result of the bending process. 14/15g broke in the middle of the spoke. The 14g spokes had an average tensile strength of 740lbs. The 14/15 gauge spokes had an average tensile strength of 715lbs. As Mike T. mentioned you will never reach these loads while riding so you shouldn't have to worry about it. The only advantage 14/15 has in terms of fatigue strength over 14g is that it is slightly more elastic. This helps prevent the spokes from completly loosing tension while riding which would accelerate fatigue. The other advantage is that if you took two wheels, one with 14g and one with 14/15g spokes, and applied the same force to each wheel, the wheel with the 14/15g spokes would deflect more. This allows the rim, in theory, to be able to distribute the load to neighboring spokes thus allowing each spoke to experience a lower stress level.
The only time I would use straight gage is if the person needed a stiff wheel or the wheel is going to see lots of high load impacts like a bike used for jumping.
In terms of bladed or oval spokes the same rules apply.

Steve
 

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ssauter said:
I have tested the tensile strength of DT 14g and 14/15g spokes using a tensile testing machine. The results I got after pulling multiple spokes of each gauge were that 14g broke consistently at the bend. This is because the diameter of the spoke necks down to about 1.9mm as a result of the bending process. 14/15g broke in the middle of the spoke. The 14g spokes had an average tensile strength of 740lbs. The 14/15 gauge spokes had an average tensile strength of 715lbs.
Which proves the theory that "One measurement is worth 50 expert opinions".
 

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ssauter said:
I have tested the tensile strength of DT 14g and 14/15g spokes using a tensile testing machine. The results I got after pulling multiple spokes of each gauge were that 14g broke consistantly at the bend. This is because the diameter of the spoke necks down to about 1.9mm as a result of the bending process. 14/15g broke in the middle of the spoke. The 14g spokes had an average tensile strength of 740lbs. The 14/15 gauge spokes had an average tensile strength of 715lbs. As Mike T. mentioned you will never reach these loads while riding so you shouldn't have to worry about it. The only advantage 14/15 has in terms of fatigue strength over 14g is that it is slightly more elastic. This helps prevent the spokes from completly loosing tension while riding which would accelerate fatigue. The other advantage is that if you took two wheels, one with 14g and one with 14/15g spokes, and applied the same force to each wheel, the wheel with the 14/15g spokes would deflect more. This allows the rim, in theory, to be able to distribute the load to neighboring spokes thus allowing each spoke to experience a lower stress level.
The only time I would use straight gage is if the person needed a stiff wheel or the wheel is going to see lots of high load impacts like a bike used for jumping.
In terms of bladed or oval spokes the same rules apply.

Steve
The issue is not which spoke is stronger, but which spokes build the stronger wheel. In realty, a wheel with straight gauge spokes will be just as strong as one with double butted spokes, and quiet a bit more rigid laterally, which may or may not be important depending upon a number of factors.
Double butted spokes typically last longer, because the elbows are less likely to fatigue, mostly because most of the cyclic change in strain occurs in the narrow section, but also because some of the change in load is distributed o other spokes, as you noted.
Double butted spokes may be more elastic, but they will still go slack at about same change in tension as straight spokes. If the low tension spokes in a wheel are going slack, the solution is to equalize strain by mixing light gauge with heavy gauge spokes.
Straight gauge spokes are easier to build with, because they resist twisting better. That is important for machine built wheels, and for low-skill builders.

em
 

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Discussion Starter · #6 · (Edited)
eddie m said:
The issue is not which spoke is stronger, but which spokes build the stronger wheel. In realty, a wheel with straight gauge spokes will be just as strong as one with double butted spokes, and quiet a bit more rigid laterally, which may or may not be important depending upon a number of factors.
. . . em
I guess I don't quite see why straight-gauge spokes are more rigid laterally.

When there is a lateral force on the rim (say, because bike is in a steep turn, or heavy force applied to the pedals in a sprint or climbing a hill), in order for the rim to deflect from the verticle plane of the wheel it would have to "pull" the spokes from the opposite flange so as to "stretch" them longer than their normally length. I guess I though a db spoke would flex more (in the middle), but it is not obvious to me that it will also stretch more. Is this true? I mean, how much stretch can there be in a spoke (as apposed to hub flange flex, spoke nipples pulling into the eyelet, etc.)? It seem to me most of the stretch would be at the j-bend (where the db spokes and straight gauge spokes are of equal diameter).

Can you (or others) confirm the increased lateral stiffness of straight guage spokes empirically?

Assuming this is true, and taking the "strong enough" point from the very helpful responses above, I guess I'm a little unsure what to do with this. It sounds like one might wisely apply the "enough" principle to the lateral stiffness issue as well: straight guage spokes make stiffer (laterally, as well as vertically) wheels, but db spokes make "stiff enough" wheels, depending on multiple factors (rider weight, riding style, spoke count, lacing, rim selection, etc.).
 

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Discussion Starter · #7 ·
eddie m said:
The issue is not which spoke is stronger, but which spokes build the stronger wheel. In realty, a wheel with straight gauge spokes will be just as strong as one with double butted spokes, and quiet a bit more rigid laterally, which may or may not be important depending upon a number of factors.
Double butted spokes typically last longer, because the elbows are less likely to fatigue, mostly because most of the cyclic change in strain occurs in the narrow section, but also because some of the change in load is distributed o other spokes, as you noted.
Well, I'm now confused about what we mean by "strength". I guess I equate "strenght" with a spoke not breaking. Under this definition, per your post double-butted spokes are stronger because they "typically last longer". Or am I missing something?

I suppose one could define "strength" of the wheel with a focus on the rim instead of spoke breakage. Not sure if straight vs. db spokes are ever associated with more/less damage to rims so as to make them "stronger" in that way.
 

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eddie m said:
The issue is not which spoke is stronger, but which spokes build the stronger wheel. In realty, a wheel with straight gauge spokes will be just as strong as one with double butted spokes, and quiet a bit more rigid laterally, which may or may not be important depending upon a number of factors.
Double butted spokes typically last longer, because the elbows are less likely to fatigue, mostly because most of the cyclic change in strain occurs in the narrow section, but also because some of the change in load is distributed o other spokes, as you noted.
Double butted spokes may be more elastic, but they will still go slack at about same change in tension as straight spokes. If the low tension spokes in a wheel are going slack, the solution is to equalize strain by mixing light gauge with heavy gauge spokes.
Straight gauge spokes are easier to build with, because they resist twisting better. That is important for machine built wheels, and for low-skill builders.

em
I agree that most of the strain will occur in the middle of a db spoke, but I would argue that the spoke elbow will see the same amount of stress whether it is db or straight gauge. When a spoke is in tension the whole length of the spoke will be at the same tension. So if there where two spokes, one db and the other straight gauge, and each was in tension and both spokes had the same load applied to them, they would each decrease in tension the same amount. The only difference would be that the db spoke would change in length more that the straight gauge spoke. Just because the mid-section of the db spoke changes more in length does not necessarily mean that the elbow is recieving less stress. If both spokes see the same tension change then since the elbow of a 14/15 and 14g have the same cross sectional area the elbow would also see the same stress.

I don't quite understand what you mean when you say "Double butted spokes may be more elastic, but they will still go slack at about same change in tension as straight spokes." All spokes will go slack with the same amount of tension change.

Steve
 

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What do you mean by spokes flexing versus stretching? Since a spoke is in tension it does very little bending. Most of what a spoke does is increase or decrease in length when a lateral or radial load is applied. Only when a spoke looses tension does it do a significant amount of bending.
In terms of spoke stretch versus hub flange and rim eyelet movement, the spoke will by far have the most movement in it.
Db spokes do make "stiff enough wheels" in most cases, but the big advantage with them is the increase in fatigue life of the spoke. This is why I generally use db spokes for most of the wheels I biuld. Hope this helps clear some things up.

Steve
 

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Discussion Starter · #10 ·
ssauter said:
What do you mean by spokes flexing versus stretching? Since a spoke is in tension it does very little bending. Most of what a spoke does is increase or decrease in length when a lateral or radial load is applied. Only when a spoke looses tension does it do a significant amount of bending.
In terms of spoke stretch versus hub flange and rim eyelet movement, the spoke will by far have the most movement in it.
Db spokes do make "stiff enough wheels" in most cases, but the big advantage with them is the increase in fatigue life of the spoke. This is why I generally use db spokes for most of the wheels I biuld. Hope this helps clear some things up.

Steve
This is helpful. So the "stiffness" issue (laterally and vertically) comes down to spoke stretch.

Has anyone published a comparison of the qualities of two wheels with identical rims built up with, say, 28 3x 14g spokes vs. 32 3x 14/15g db spokes (or 32 vs. 36)? Which wheel is stronger--either in terms of reducing risk of spoke breakage or rim damage? Which holds up to a 200lb + rider better? Is one more comfortable than the other for long rides? Does one accellerate/climb better? Does one suffer from too much lateral flex while the other does not? If the rear rim is offset (think Velocity Aerohead OC), does that change the equation somehow?

On of the tire manufacturers (Michelin, I think, but maybe Continental) used to publish on its boxes a sort of spiderweb-looking graphic that depicted the relative strengths of its tires for various qualities (e.g., wear, puncuture resistance, ride quality, rolling resistance, grip). Such a graphic would be enormously useful for wheel builds, non?
 

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ssauter said:
I agree that most of the strain will occur in the middle of a db spoke, but I would argue that the spoke elbow will see the same amount of stress whether it is db or straight gauge. When a spoke is in tension the whole length of the spoke will be at the same tension. So if there where two spokes, one db and the other straight gauge, and each was in tension and both spokes had the same load applied to them, they would each decrease in tension the same amount. The only difference would be that the db spoke would change in length more that the straight gauge spoke. Just because the mid-section of the db spoke changes more in length does not necessarily mean that the elbow is recieving less stress. If both spokes see the same tension change then since the elbow of a 14/15 and 14g have the same cross sectional area the elbow would also see the same stress.
If you you have a straight guage spoke and a DB in the same wheel, the change in length under load is about the same, but the change in stress is greater in the straight spoke. More if the load is carried by the heavier spoke. Think of it this way: If you had 2 identical flagpoles side by side, connected by a crossmember at top, and applied enough force to deflect them both 5 feet, the stresses and strain in both would be identical. If you replaced one of the poles with a less flexible one, but applied the same force, the deflection would be less, and the strain in the original pole would be less, and the strain and bending moment would be greater in the more rigid pole. It's the same with tension spoke wheels. The heavier gauge spokes take more of the load. If you are concerned that non drive side spokes are going slack because of low tension, you can to use lighter gauge spokes on the non drive side, or heavier gauge on the drive side. If you use the same gauge both sides, you can't correct the problem, regardless of how the spokes are butted.
ssauter said:
I don't quite understand what you mean when you say "Double butted spokes may be more elastic, but they will still go slack at about same change in tension as straight spokes." All spokes will go slack with the same amount of tension change.

Steve
Exactly.

em
 

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drewmcg said:
This is helpful. So the "stiffness" issue (laterally and vertically) comes down to spoke stretch.

Has anyone published a comparison of the qualities of two wheels with identical rims built up with, say, 28 3x 14g spokes vs. 32 3x 14/15g db spokes (or 32 vs. 36)? Which wheel is stronger--either in terms of reducing risk of spoke breakage or rim damage? Which holds up to a 200lb + rider better? Is one more comfortable than the other for long rides? Does one accellerate/climb better? Does one suffer from too much lateral flex while the other does not? If the rear rim is offset (think Velocity Aerohead OC), does that change the equation somehow?
Here it is:
Bicycle Wheel Spoke Patterns and Spoke Fatigue, Henri P. Gavin

Gavin's conclusion is that the spoke pattern doesn't matter much, at least not for 36 spoke wheels.

em
 

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ssauter said:
What do you mean by spokes flexing versus stretching? Since a spoke is in tension it does very little bending. Most of what a spoke does is increase or decrease in length when a lateral or radial load is applied. Only when a spoke looses tension does it do a significant amount of bending.
In terms of spoke stretch versus hub flange and rim eyelet movement, the spoke will by far have the most movement in it.
Db spokes do make "stiff enough wheels" in most cases, but the big advantage with them is the increase in fatigue life of the spoke. This is why I generally use db spokes for most of the wheels I biuld. Hope this helps clear some things up.

Steve
When I started racing 35 years ago, we used 36 spoke wheels with light weight sew-up rims. The rims had to be replaced frequently, usually by inexperienced local wheel builders, who didn't know enough to stress relieve the spokes. The spokes available then were of poor quality compared to modern spokes. Broken spokes were common.
With better quality spokes, more skilled wheel builders, and heavier section modern clincher rims, I've only broken one spoke in the last 25 years, and that was I a wheel that was badly damaged on a pothole. Good quality straight gauge spokes should outlast several rims.

em
 

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eddie m said:
When I started racing 35 years ago, we used 36 spoke wheels with light weight sew-up rims. The rims had to be replaced frequently, usually by inexperienced local wheel builders, who didn't know enough to stress relieve the spokes. The spokes available then were of poor quality compared to modern spokes. Broken spokes were common.
My findings exactly. Except *I* was the "inexperienced local wheel builder" as I've always built my own. In my roadie racing days (early 60's to early '80s) 36 hole rims were the norm. The tourists and club riders used 40h. The early enthusiasts spokes that were common were chrome plated ones - their sparkle was proportional to their snappage. In fact it wasn't until '86 that I dared to go with the new exotic 32h narrow clinchers with stainless steel spokes. :eek:

With better quality spokes, more skilled wheel builders, and heavier section modern clincher rims, I've only broken one spoke in the last 25 years.
With the better, modern stainless spokes and the readily available knowledge of how to build better wheels, spoke breakage is just about a thing of the past. I do not remember the last spoke that I broke. With a wheel containing sufficient and equal spoke tension the breakage would have to be trauma induced.

Good quality straight gauge spokes should outlast several rims.
You're probably right but I'd have to be in financial difficulties to warrant the need to save $0.25 per spoke.
 

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Mike T. said:
You're probably right but I'd have to be in financial difficulties to warrant the need to save $0.25 per spoke.
I'm riding a 28 spoke wheel with 2 mm straight gauge spokes. That makes the wheel more rigid laterally, and I'm not sure it would be better with DB spokes. straight gauge are easier to build with as well, so it's not just about the cost.

em
 

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Discussion Starter · #16 ·
. . . so if you have a problem breaking non-drive side spokes on the rear, use thinner spokes on that side?
 

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drewmcg said:
Well, I'm now confused about what we mean by "strength". I guess I equate "strenght" with a spoke not breaking. Under this definition, per your post double-butted spokes are stronger because they "typically last longer". Or am I missing something?

I suppose one could define "strength" of the wheel with a focus on the rim instead of spoke breakage. Not sure if straight vs. db spokes are ever associated with more/less damage to rims so as to make them "stronger" in that way.
Sometimes words do get in the way of communication, don't they?

Here's one way to look at the problem. Assume a decently built wheel that's adequate to the task: No 16-spoke rears under 250lb riders. Let's say 32x3 Open Pro as our reference standard.

In terms of 'can it take a bigger hit' the difference between straight-gauge and butted will be fairly small. Truth is, the rim is going to yield before the spokes do, so it hardly matters which spoke is stronger than the other. Except that the butted spokes will "stretch" slightly more under load, which may give the rim a bit more work to do. For imagery, it's not at all hard to crush an empty rim vertically, but once it's tensioned with spokes, it'll take tremendous force. Advantage in this definition: Straight gauge.

But in terms of which will last longer in normal conditions, the story changes - maybe. Any object experiences it's fatigue at it's weakest point. On a straight spoke, that's at the J. On a butted spoke, it's in the middle. But because of the way wire is drawn vs bent, that's a good thing. Repeated loadings will affect a bent wire before one being 'stretched' elastically. In essense, the butted section serves as a 'shock absorber' for the elbow. Technically, that's nowhere near correct, but the image is useful. So, failing catastrophic impact levels, the butted wheel should last longer.

At least in theory. As a practical matter, the vagarities of building real wheels mean it makes little final difference unless building wheels on the very edge of practicality for the intended purpose.
 

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eddie m said:
If you you have a straight guage spoke and a DB in the same wheel, the change in length under load is about the same, but the change in stress is greater in the straight spoke. More if the load is carried by the heavier spoke. Think of it this way: If you had 2 identical flagpoles side by side, connected by a crossmember at top, and applied enough force to deflect them both 5 feet, the stresses and strain in both would be identical. If you replaced one of the poles with a less flexible one, but applied the same force, the deflection would be less, and the strain in the original pole would be less, and the strain and bending moment would be greater in the more rigid pole. It's the same with tension spoke wheels. The heavier gauge spokes take more of the load. If you are concerned that non drive side spokes are going slack because of low tension, you can to use lighter gauge spokes on the non drive side, or heavier gauge on the drive side. If you use the same gauge both sides, you can't correct the problem, regardless of how the spokes are butted.


em
Ok, now I think I am starting to understand what you are trying to say. I was looking at just change in tension in a 14g spoke versus 14/15g. If both spokes experienced the same tension change they would also experience the same stress but in order for this senario to occur a wheel with 14/15g spokes would in theory have to have a higher load placed on it in order for the spokes to see the same tension change as a 14g spoked wheel. This is because the rim has to deflect more and therefor can distribute the load to more of the neighboring spokes.

Steve
 
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