Wheelbuilding and Spokes: To aero or not to aero

[Tonger]

I tried searching this both on the forum and at large and haven't had much luck.

I am in the process of building some budget 24/28 Velocity A23 Campy clinchers and weigh in the low 180's. My question is with regard to spokes. I know there's a lot of love for the Sapim Cx rays but the things are $3 each as a forged/flattened version of the lasers. The Sapim Lasers are 1/3 the cost and are touted as being functionally similar to the Cx rays except for the aero part.

Here are my questions:

1. If the Lasers and the Cx rays are functionally similar, why does the Cx ray have a higher fatigue rating?

http://www.sapim.be/index.php?st=products&sub=spokes&detail=fatiguetest

2. Is there really any discernible aero difference to otherwise justify the extra 100 bucks? Is it just because the Cx rays are the "best". I can't seem to find any data.

Thanks!

Tonger

[echappist]

I tried searching this both on the forum and at large and haven't had much luck.

I am in the process of building some budget 24/28 Velocity A23 Campy clinchers and weigh in the low 180's. My question is with regard to spokes. I know there's a lot of love for the Sapim Cx rays but the things are $3 each as a forged/flattened version of the lasers. The Sapim Lasers are 1/3 the cost and are touted as being functionally similar to the Cx rays except for the aero part.

Here are my questions:

1. If the Lasers and the Cx rays are functionally similar, why does the Cx ray have a higher fatigue rating?

http://www.sapim.be/index.php?st=products&sub=spokes&detail=fatiguetest

2. Is there really any discernible aero difference to otherwise justify the extra 100 bucks? Is it just because the Cx rays are the "best". I can't seem to find any data.

Thanks!

Tonger

drop the spoke count to 20 if you want really "aero"

the lasers really wind up, and if you are building this up yourself, you may be in for some surprises

[tv_vt]

I've been wondering about this myself as I consider re-building a set of 32h hubs with HED Belgian rims -

- DT Revolutions or CX-Rays 3x in front?
- mix of DT Revolutions and 14/15 butted or CX-Ray 3x in back?

[Peter P.]

Clue me in; do these aero spokes fit through conventional, round spoke holes? If so, there's material missing on these aero spokes vs. round spokes. Less steel = lower fatigue limit and lower lateral stiffness. Rear wheels have enough problems with durability without reducing their existing lateral strength.

I'm inclined to decline (pun intended) the use of aero spokes because I'd expect them to result in a wheel with less lateral stiffness than a wheel build with round spokes.

Any fatigue limit differences must be due to the alloy.

[merckx]

I've been wondering about this myself as I consider re-building a set of 32h hubs with HED Belgian rims -

- DT Revolutions or CX-Rays 3x in front?
- mix of DT Revolutions and 14/15 butted or CX-Ray 3x in back?
I built a set of HED C2 wheels this fall. They are 32 hole with Campy hubs. Rev on the front and Rev NDS/14db DS with brass nipples. 1635 grams for the pair and good enough for me.

[cmg]

I've been wondering about this myself as I consider re-building a set of 32h hubs with HED Belgian rims -

- DT Revolutions or CX-Rays 3x in front?
- mix of DT Revolutions and 14/15 butted or CX-Ray 3x in back?




i have a set of AC sprint 350S that were built up with the Revos in the front, NDside and Db 14s on the drive side, love em still going strong after 4 years. if your around 180lbs or above this might be the way to go, that and 28 spokes min in the rear.

[Mark McM]

Clue me in; do these aero spokes fit through conventional, round spoke holes? If so, there's material missing on these aero spokes vs. round spokes. Less steel = lower fatigue limit and lower lateral stiffness. Rear wheels have enough problems with durability without reducing their existing lateral strength.

The modern crop of aero spokes (sometimes called 'ovalized' vs. the previous 'bladed' spokes) have become popular largely because they fit standard spoke holes.

They are only missing as much material as the butted spokes they started as. In the case of Sapim CX-Ray spokes, they start life as 2.0/1.5/2.0 super-butted Laser spokes, but the 1.5 mm center section is flattened out to a 2.3 x 0.9 profile - same cross-sectional area, just a different shape. This will result in a spoke with the same structural properties as the original spoke (weight, longitudinal rigidity).

I'm inclined to decline (pun intended) the use of aero spokes because I'd expect them to result in a wheel with less lateral stiffness than a wheel build with round spokes..

Wheels with these new aero spokes will have the same rigidity as wheels built with Laser (or DT Revolution) spokes, which have the same cross-sectional areas. Spokes are only loaded longitudinally in a wheel, so although the aero spokes will have less bending and torsional stiffness, it won't change the wheel stiffness (laterally or otherwise).

Any fatigue limit differences must be due to the alloy.

CX-Ray spokes are made from the same alloy as their other spokes. Since there are no details of any kind to explain or support the graph, I think we can chalk it up to marketting puffery to justify the higher cost of the spokes. (Well, it's not complete puffery, as the more highly butted a spoke is, the greater its fatigue durability in a built wheel.)

[Tonger]

So it seems that the flattening process to create the cx ray somehow improves the fatigue strength of the spoke. This raises a couple of questions for me:

1. Does this stronger spoke allow the builder to run more spoke tension and thus create a "stiffer" wheel? Is this even desirable?

2. How much real world decrease in the aero resistance is there to be had by running bladed spokes like the cxray, is it real or mainly theoretical?

If there isn't much measurable aero or strength/stiffness benefit, I'm inclined to use the cheaper lasers.

Thanks so much to those who have weighed in!

[giverdada]

i actually just built the exact same wheel, er, almost.

and, i'm completely stoked about it.

my former wheel: tune hub, 32 cx-ray spokes, mavic reflex tubular. the thing barely weighs as much as the tubular i put on it. amazing. the verdict, however, was not the gloriousness i was expecting. i think the wheel is too light. crosswinds were terrible. descents were jittery and a bit sketchy, kind of overly responsive. every twitch turned into a move. i was just trying to steer.

so i figured i'd try a heavier wheel. been curious about this whole wide rim clincher thing, so went ahead and picked up a $50 velocity A23 on flea bay, along with a $20 ringle hub, NOS, all in 28 hole. got some sapim lasers, and built the thing up. best wheel i've ever built, in terms of patience and care and quality (super true and round and even-tensioned), and i've been riding it for a week to check it out and see what the verdict is.

fantastic.

i finally got to bomb down a short but technical descent today, the one that used to sketch me out on the super light tune wheel, and it was rock solid. makes my frame ride like the dream it is. makes me ride with a lot more confidence, particularly in cornering. not a bunch of marketing racket. i gave it a go, and the difference was drastic. i recommend the whole build. the front hub is certainly not fancy, and costs less than a medium rare slice of your favourite animal, but damn, the wheel is solid!

my lady friend has sapim race laced to the hed belgium rims, and they're pretty and light and solid as well, though the rims are twice the price and twice as nice (finish and looks-wise).

give'r.

and post 'em when you're done!

[firerescuefin]

Interesting timing on the thread. I asked Eric to build some Ambrosio Nemesis wheels with King Campy Hubs and Sapim cx-ray spokes. He asked me my weight, type of riding I expected to do, and what characteristics I was looking for in the wheel and recommended some different spokes (Wheelsmith DB 14s). I'll let him chime in on the why, but I decided to "trust my builder". :)

[echappist]

32 spokes is not going to be aero no matter what spokes you use. It's blingy, but offers no real aerodynamic advantage.

True aero front wheels should have 20 spokes max (and shouldn't be your everyday training wheel). That's the reason why some of the POS shimano wheels (16H front) actually did better than some of the 35mm deep wheels in the Roue Artisan wheel test.

[Louis]

That's the reason why some of the POS shimano wheels (16H front) actually did better than some of the 35mm deep wheels in the Roue Artisan wheel test.

Hey, I had one of those Shimano wheels on my daily driver front for a while. :)

And yes, you do notice the difference, especially in high speed descents. After a month or so I got scared, since the spoke count is so low, and took it off. Given how inexpensive it was, I was impressed. I would not call them POS - I would call them high value and perfectly adequate for a TT with little climbing. But probably not an everyday wheel.

[kramnnim]

A mechanic friend of mine is a fan of CX Rays...not because of the strength or aero advantage, but because it's easy to prevent spoke wind up when tensioning/truing.

[Louis]

I think Old P said somewhere that he thought that the X-rays weren't worth the mark-up. You may want to search for that.

[echappist]

Hey, I had one of those Shimano wheels on my daily driver front for a while. :)

And yes, you do notice the difference, especially in high speed descents. After a month or so I got scared, since the spoke count is so low, and took it off. Given how inexpensive it was, I was impressed. I would not call them POS - I would call them high value and perfectly adequate for a TT with little climbing. But probably not an everyday wheel.
and i had one of them too. The rims were damned heavy. If i get new wheels i'm going 20F 28R with CX-Rays up front and Sapim Race DS and CX-Rays NDS.
Durability won't be too big an issue as i'll be beating the crap out of my 32F/R box sections.
I think Old P said somewhere that he thought that the X-rays weren't worth the mark-up. You may want to search for that.
not that i don't disagree, but it's all in the eye of the beholder

[Kontact]

We were talking about this at work today. My boss is endlessly researching this sort of thing, and he pointed out that he has never seen an actual comparative wind tunnel test of all the different spoke types. He went on to say that a round spoke of minimal diameter, like Revolution (2.0/1.5), might actually be more aero overall because it provides a small profile in all wind conditions.

I was holding an older Kysrium with flat bladed metal spokes at the time, and we agreed that these rectangular spokes were probably the least aerodynamic spoke, since the sharp edges would produce so much more drag than any round spoke.

Zipp published this, but there are so few spoke choices in the test, that there isn't a big conclusion to draw. They do explain why they believe the CX-Ray is so strong.
http://www.zipp.com/_media/pdfs/technology/spokeshape.pdf


I think ovals are your best bet, follow by 1.5mm round.

[Chance]

1. Does this stronger spoke allow the builder to run more spoke tension and thus create a "stiffer" wheel? Is this even desirable?

This is a bad assumption. You should do a little research to confirm but it's almost certain that adding additional tension beyond what is normally required doesn't make a wheel stiffer. As long as all spokes remain in tension at all times it won't affect stiffness significantly. Some published test data actually shows that wheel lateral stiffness goes down if spoke tension is increased too much.

[fogrider]

its not that the wheel is too light, its because the wheel is not stiff enough. more spokes will make it stiffer and better handling. or a stronger rim will do the same. but my carbon rim is plenty strong and plenty light!

i actually just built the exact same wheel, er, almost.

and, i'm completely stoked about it.

my former wheel: tune hub, 32 cx-ray spokes, mavic reflex tubular. the thing barely weighs as much as the tubular i put on it. amazing. the verdict, however, was not the gloriousness i was expecting. i think the wheel is too light. crosswinds were terrible. descents were jittery and a bit sketchy, kind of overly responsive. every twitch turned into a move. i was just trying to steer.

so i figured i'd try a heavier wheel. been curious about this whole wide rim clincher thing, so went ahead and picked up a $50 velocity A23 on flea bay, along with a $20 ringle hub, NOS, all in 28 hole. got some sapim lasers, and built the thing up. best wheel i've ever built, in terms of patience and care and quality (super true and round and even-tensioned), and i've been riding it for a week to check it out and see what the verdict is.

fantastic.

i finally got to bomb down a short but technical descent today, the one that used to sketch me out on the super light tune wheel, and it was rock solid. makes my frame ride like the dream it is. makes me ride with a lot more confidence, particularly in cornering. not a bunch of marketing racket. i gave it a go, and the difference was drastic. i recommend the whole build. the front hub is certainly not fancy, and costs less than a medium rare slice of your favourite animal, but damn, the wheel is solid!

my lady friend has sapim race laced to the hed belgium rims, and they're pretty and light and solid as well, though the rims are twice the price and twice as nice (finish and looks-wise).

give'r.

and post 'em when you're done!

[Peter P.]

The latest information I could find concludes that using an aero specific wheel i.e., a disk or a 2-4 spoke wheel, will save roughly 10-15 seconds at 30mph on a 25mi. TT versus an 18 aero-spoke conventional rimmed aluminum wheel.

You should be able to draw some reasonable conclusions about how that would translate into any benefit you'd receive from aero spokes vs. round spokes, and whatever other combinations you have.

My opinion; if you're not 30 seconds away from the podium already, fancy wheels aren't going to put you there. You can't buy your victory unless your name is Alexander Vinokourov.

[oldpotatoe]

I tried searching this both on the forum and at large and haven't had much luck.

I am in the process of building some budget 24/28 Velocity A23 Campy clinchers and weigh in the low 180's. My question is with regard to spokes. I know there's a lot of love for the Sapim Cx rays but the things are $3 each as a forged/flattened version of the lasers. The Sapim Lasers are 1/3 the cost and are touted as being functionally similar to the Cx rays except for the aero part.

Here are my questions:

1. If the Lasers and the Cx rays are functionally similar, why does the Cx ray have a higher fatigue rating?

http://www.sapim.be/index.php?st=products&sub=spokes&detail=fatiguetest

2. Is there really any discernible aero difference to otherwise justify the extra 100 bucks? Is it just because the Cx rays are the "best". I can't seem to find any data.

Thanks!

Tonger

An aside...A23 aren't the heaviest rims , and at 24/28 and 180 pounds, use DT Comps(or Sapim Race spokes). Oval spokes don't mean a lot in terms of aero-ness but thin spokes they are, along with lasers(or DT Revs). Don't make the 3 strikes and you are out 'mistake'...thin spokes, low spoke count, lightish rims.....

[oldpotatoe]

So it seems that the flattening process to create the cx ray somehow improves the fatigue strength of the spoke. This raises a couple of questions for me:

1. Does this stronger spoke allow the builder to run more spoke tension and thus create a "stiffer" wheel? Is this even desirable?

2. How much real world decrease in the aero resistance is there to be had by running bladed spokes like the cxray, is it real or mainly theoretical?

If there isn't much measurable aero or strength/stiffness benefit, I'm inclined to use the cheaper lasers.

Thanks so much to those who have weighed in!

1. Depends on the rim..may fail a rim

2. Almost not measurable,their marketing is about their durability, but they start life as a thin spoke(Laser).

3. For a rear wheel for just about anybody unless they are about a buck-thirty-five, I'd say use Sapim Race or DT Comps, at least on the drive side rear.

[Tonger]

Old Potatoe,

I absolutely respect your experience in designing and building wheels and appreciate your advice - thanks! There's just something that I don't understand. According to the Sapim website, the Race is the largest spoke but has the weakest middle section and fatigue strength.

Strength of Middle Section:
Race 1350 N/m2
Laser 1500 N/m2
Cx Ray 1600 N/m2

Fatigue Test/Wheel Revolutions:
Race 980,000 revolutions
Laser 1,250,000 revolutions
Cx Ray 3,500,000 revolutions

http://www.sapim.be/index.php?st=products&sub=spokes&category=3960&id=3385&detail=butted

As a fat guy armchair consumer with no expertise, it seems like the Race would be larger but result in a weaker wheel, and that one made with the Cx Ray would be strongest. I don't know if this type of data is marketing driven or doesn't measure up in the real world but I'd really value your opinion. It could also be that these differences in spoke size result in different building methods that the consumer doesn't understand or appreciate.

Thanks so much,

Tonger

[Chance]

To my prior point in previous post, this data from Sheldon Brown (other sources available too) shows that adding more tension doesn't necessarily add lateral stiffness. In simple terms a spoke's stiffness doesn't change when pulled. You just can't have them go slack. Proper tension may help reduce fatigue but it doesn't make a wheel significantly stiffer if at all.

http://sheldonbrown.com/rinard/wheel/tension.gif

http://sheldonbrown.com/rinard/wheel/index.htm

[oldpotatoe]

Old Potatoe,

I absolutely respect your experience in designing and building wheels and appreciate your advice - thanks! There's just something that I don't understand. According to the Sapim website, the Race is the largest spoke but has the weakest middle section and fatigue strength.

Strength of Middle Section:
Race 1350 N/m2
Laser 1500 N/m2
Cx Ray 1600 N/m2

Fatigue Test/Wheel Revolutions:
Race 980,000 revolutions
Laser 1,250,000 revolutions
Cx Ray 3,500,000 revolutions

http://www.sapim.be/index.php?st=products&sub=spokes&category=3960&id=3385&detail=butted

As a fat guy armchair consumer with no expertise, it seems like the Race would be larger but result in a weaker wheel, and that one made with the Cx Ray would be strongest. I don't know if this type of data is marketing driven or doesn't measure up in the real world but I'd really value your opinion. It could also be that these differences in spoke size result in different building methods that the consumer doesn't understand or appreciate.

Thanks so much,

Tonger

Since the wheel is the sum of it's parts and the spokes must 'support' the rim, a thin spoke may make for a weaker wheel, one that needs to be trued more often.

I think the marketing of CX-Rays means it's a very strong spoke but I think must be used in a properly designed wheel. For instance, a 32 spoke wheel on a 420 gram rim(about what a OpenPro is)...DT Comp or Sapim Race vs DT Aerolights or Sapim CX-rays, I don't think the CX-Ray wheel is automatically a longer lasting, stiffer, stronger wheel.

Their numbers mean that one should use a Laser vs Race if you don't want to spend the $ for Cx-Rays, but with a 420 gram rim, IMHO, these and Revolution spokes are too thin, result in a wheel that 'may' need to be trued quite often. Same for CX-Rays.

[Chance]

Old Potatoe,

I absolutely respect your experience in designing and building wheels and appreciate your advice - thanks! There's just something that I don't understand. According to the Sapim website, the Race is the largest spoke but has the weakest middle section and fatigue strength.

Strength of Middle Section:
Race 1350 N/m2
Laser 1500 N/m2
Cx Ray 1600 N/m2

Fatigue Test/Wheel Revolutions:
Race 980,000 revolutions
Laser 1,250,000 revolutions
Cx Ray 3,500,000 revolutions

http://www.sapim.be/index.php?st=products&sub=spokes&category=3960&id=3385&detail=butted

As a fat guy armchair consumer with no expertise, it seems like the Race would be larger but result in a weaker wheel, and that one made with the Cx Ray would be strongest. I don't know if this type of data is marketing driven or doesn't measure up in the real world but I'd really value your opinion. It could also be that these differences in spoke size result in different building methods that the consumer doesn't understand or appreciate.

Thanks so much,

Tonger
Tonger, the way the data is expressed may be confusing you a little. And it's no surprise because it isn't very clear in my opinion.

Note that the strength is listed as N/mm^2. That's newtons of force per millimeter square of cross sectional area. It's like saying the metal's strength in pounds per square inch.

On that basis the stronger metal has greater fatigue life as it should. However, the stress on the spokes can be changed dramatically by changing the cross section by making the spokes larger in diameter. What seems to be missing is a good explanation of how they did the fatigue test so the data can be clearer to use. At least from my perspective.

[ergott]

Interesting timing on the thread. I asked Eric to build some Ambrosio Nemesis wheels with King Campy Hubs and Sapim cx-ray spokes. He asked me my weight, type of riding I expected to do, and what characteristics I was looking for in the wheel and recommended some different spokes (Wheelsmith DB 14s). I'll let him chime in on the why, but I decided to "trust my builder". :)

CX-Rays are a nice upgrade for wheels, but in a 32 spoke, box section wheel, aerodynamics aren't really a concern. That would bring you to the Sapim Laser (DT Revolution). I've used those spokes for that type of wheel before.

I don't recommend using light gauge spokes for the rear, drive side of any geared wheelset no matter how many spokes. During my conversation with firerescuefin he mentioned that he wants a care free set of wheels for everyday riding. With that being a priority and knowing his approximate weight I suggested the Wheelsmith DB14s.

The Wheelsmith DB14 dimensions are 2.0/1.7/2.0mm (threaded end/center/elbow). For comparison, the Sapim Race and DT Competitions are 2.0/1.8/2.0mm and Sapim Lasers along with DT Revolutions are 2.0/1.5/2.0mm. I like the Wheelsmith DB14s as a good middle ground where the wheels won't be to springy and they won't be too stiff either. It's a good ride for this choice.

[ergott]

I am in the process of building some budget 24/28 Velocity A23 Campy clinchers and weigh in the low 180's.

Tonger

What hubs are you going to use?

[Mark McM]

Old Potatoe,

I absolutely respect your experience in designing and building wheels and appreciate your advice - thanks! There's just something that I don't understand. According to the Sapim website, the Race is the largest spoke but has the weakest middle section and fatigue strength.

Strength of Middle Section:
Race 1350 N/m2
Laser 1500 N/m2
Cx Ray 1600 N/m2

Fatigue Test/Wheel Revolutions:
Race 980,000 revolutions
Laser 1,250,000 revolutions
Cx Ray 3,500,000 revolutions

http://www.sapim.be/index.php?st=products&sub=spokes&category=3960&id=3385&detail=butted

As a fat guy armchair consumer with no expertise, it seems like the Race would be larger but result in a weaker wheel, and that one made with the Cx Ray would be strongest. I don't know if this type of data is marketing driven or doesn't measure up in the real world but I'd really value your opinion. It could also be that these differences in spoke size result in different building methods that the consumer doesn't understand or appreciate.

Thanks so much,

Tonger

The Sapim website is tossing arond terms like stress, strength and durability like they are equivalent properties but they are really just attempting to confuse the consumer in an attempt to sell their spokes.

Although labeled 'strength', what Sapim is actually reporting is the failure stress of the (thin) middle section of the spokes. Stress is load per unit area, so to find that actual strength, you have to multiply the load by the cross sectional area. The area for round spokes is pi x diameter^2 / 4, and for oval spokes is pi x maj. diam. x min. diam. / 4 (The area of the CX-Ray spokes is approx. the same as the 1.5mm spokes, because that is what they started as, and confirmed because they weigh the same). This gives:

Strength of Middle Section:
Race (1350 N/m2)(pi x 1.8mm^2 / 4 ) = 3435 N
Laser (1500 N/m2)(pi x 1.5mm^2 / 4 ) = 2650 N
CX-Ray (1600 N/m2)(pi x 2.3mm x 0.9mm / 4 ) = 2600 N

Not suprisingly, these tells us that the thicker spokes are ultimately stronger - so why do the CX-Ray spokes have the longest fatigue life? Well, for one thing, the strength of the middle of the spoke is irrelevant - spokes don't break in the middle, they break at the ends. (Sapim no doubt knows this, but don't want to tell us the ends of all three spokes are identical, since they start from the same 2.0mm wire stock.)

Now the next point of confusion - when Sapim reports fatigue life, they are referring to the fatigue life of a built wheel (not a test of spokes by themselves). The fatigue life (cyles to failure) of structural member is influenced by many factors, but one of the most important is the magnitude of the load cycles - the lower the load, the more cycles to failure. In a built wheel, the wheel load is distributed among multiple spokes, and how widely distributed the load is depends on the relative stiffness of the rim and spokes - the less stiff the spokes, the more widely distributed the load is. (Think about a sleeping on a mattress with very stiff coil springs, vs. one with more flexible springs - with the very stiff springs, the sleeper's weight would be concentrated on a few contact points, but with the more flexible springs the springs at the contact points could 'give' a little, spreading the load to adjacent springs, distributed the sleeper's weight across a larger area). With thinner spokes, the spokes flex a little more, distributing loads across more spokes, and the load experienced by any individual spoke is reduced - thus increasing fatigue cycles to failure.

As to why Sapim data on fatigue life, I think the numbers they report are meaningless. They don't report how the test was performed, but I suspect that it was an accelerated fatigue test (test with loads far larger than would be experienced in real life). A 700c wheel rotates approx. 800 times per mile, so from Sapim's data:

Fatigue Test/Miles:
Race (980,000 revolutions) / (800 rev/mile) = 1225 miles
Laser (1,250,000 revolutions) / (800 rev/mile) = 1562 miles
Cx Ray (3,500,000 revolutions) / (800 rev/mile) = 4375 miles

Is Sapim claiming that their Race spokes will only last 1225 miles, and that even their best spokes will only last 4375 miles? I hope not, because I expect all my wheels to last far longer than that. I suspect that they used an unrealistic fatigue test protocol to exaggerate the difference in spoke durability.

[ergott]

Pin this up somewhere. Gold.

The Sapim website is tossing arond terms like stress, strength and durability like they are equivalent properties but they are really just attempting to confuse the consumer in an attempt to sell their spokes.

Although labeled 'strength', what Sapim is actually reporting is the failure stress of the (thin) middle section of the spokes. Stress is load per unit area, so to find that actual strength, you have to multiply the load by the cross sectional area. The area for round spokes is pi x diameter^2 / 4, and for oval spokes is pi x maj. diam. x min. diam. / 4 (The area of the CX-Ray spokes is approx. the same as the 1.5mm spokes, because that is what they started as, and confirmed because they weigh the same). This gives:

Strength of Middle Section:
Race (1350 N/m2)(pi x 1.8mm^2 / 4 ) = 3435 N
Laser (1500 N/m2)(pi x 1.5mm^2 / 4 ) = 2650 N
CX-Ray (1600 N/m2)(pi x 2.3mm x 0.9mm / 4 ) = 2600 N

Not suprisingly, these tells us that the thicker spokes are ultimately stronger - so why do the CX-Ray spokes have the longest fatigue life? Well, for one thing, the strength of the middle of the spoke is irrelevant - spokes don't break in the middle, they break at the ends. (Sapim no doubt knows this, but don't want to tell us the ends of all three spokes are identical, since they start from the same 2.0mm wire stock.)

Now the next point of confusion - when Sapim reports fatigue life, they are referring to the fatigue life of a built wheel (not a test of spokes by themselves). The fatigue life (cyles to failure) of structural member is influenced by many factors, but one of the most important is the magnitude of the load cycles - the lower the load, the more cycles to failure. In a built wheel, the wheel load is distributed among multiple spokes, and how widely distributed the load is depends on the relative stiffness of the rim and spokes - the less stiff the spokes, the more widely distributed the load is. (Think about a sleeping on a mattress with very stiff coil springs, vs. one with more flexible springs - with the very stiff springs, the sleeper's weight would be concentrated on a few contact points, but with the more flexible springs the springs at the contact points could 'give' a little, spreading the load to adjacent springs, distributed the sleeper's weight across a larger area). With thinner spokes, the spokes flex a little more, distributing loads across more spokes, and the load experienced by any individual spoke is reduced - thus increasing fatigue cycles to failure.

As to why Sapim data on fatigue life, I think the numbers they report are meaningless. They don't report how the test was performed, but I suspect that it was an accelerated fatigue test (test with loads far larger than would be experienced in real life). A 700c wheel rotates approx. 800 times per mile, so from Sapim's data:

Fatigue Test/Miles:
Race (980,000 revolutions) / (800 rev/mile) = 1225 miles
Laser (1,250,000 revolutions) / (800 rev/mile) = 1562 miles
Cx Ray (3,500,000 revolutions) / (800 rev/mile) = 4375 miles

Is Sapim claiming that their Race spokes will only last 1225 miles, and that even their best spokes will only last 4375 miles? I hope not, because I expect all my wheels to last far longer than that. I suspect that they used an unrealistic fatigue test protocol to exaggerate the difference in spoke durability.

[oldpotatoe]

The Sapim website is tossing arond terms like stress, strength and durability like they are equivalent properties but they are really just attempting to confuse the consumer in an attempt to sell their spokes.

Although labeled 'strength', what Sapim is actually reporting is the failure stress of the (thin) middle section of the spokes. Stress is load per unit area, so to find that actual strength, you have to multiply the load by the cross sectional area. The area for round spokes is pi x diameter^2 / 4, and for oval spokes is pi x maj. diam. x min. diam. / 4 (The area of the CX-Ray spokes is approx. the same as the 1.5mm spokes, because that is what they started as, and confirmed because they weigh the same). This gives:

Strength of Middle Section:
Race (1350 N/m2)(pi x 1.8mm^2 / 4 ) = 3435 N
Laser (1500 N/m2)(pi x 1.5mm^2 / 4 ) = 2650 N
CX-Ray (1600 N/m2)(pi x 2.3mm x 0.9mm / 4 ) = 2600 N

Not suprisingly, these tells us that the thicker spokes are ultimately stronger - so why do the CX-Ray spokes have the longest fatigue life? Well, for one thing, the strength of the middle of the spoke is irrelevant - spokes don't break in the middle, they break at the ends. (Sapim no doubt knows this, but don't want to tell us the ends of all three spokes are identical, since they start from the same 2.0mm wire stock.)

Now the next point of confusion - when Sapim reports fatigue life, they are referring to the fatigue life of a built wheel (not a test of spokes by themselves). The fatigue life (cyles to failure) of structural member is influenced by many factors, but one of the most important is the magnitude of the load cycles - the lower the load, the more cycles to failure. In a built wheel, the wheel load is distributed among multiple spokes, and how widely distributed the load is depends on the relative stiffness of the rim and spokes - the less stiff the spokes, the more widely distributed the load is. (Think about a sleeping on a mattress with very stiff coil springs, vs. one with more flexible springs - with the very stiff springs, the sleeper's weight would be concentrated on a few contact points, but with the more flexible springs the springs at the contact points could 'give' a little, spreading the load to adjacent springs, distributed the sleeper's weight across a larger area). With thinner spokes, the spokes flex a little more, distributing loads across more spokes, and the load experienced by any individual spoke is reduced - thus increasing fatigue cycles to failure.

As to why Sapim data on fatigue life, I think the numbers they report are meaningless. They don't report how the test was performed, but I suspect that it was an accelerated fatigue test (test with loads far larger than would be experienced in real life). A 700c wheel rotates approx. 800 times per mile, so from Sapim's data:

Fatigue Test/Miles:
Race (980,000 revolutions) / (800 rev/mile) = 1225 miles
Laser (1,250,000 revolutions) / (800 rev/mile) = 1562 miles
Cx Ray (3,500,000 revolutions) / (800 rev/mile) = 4375 miles

Is Sapim claiming that their Race spokes will only last 1225 miles, and that even their best spokes will only last 4375 miles? I hope not, because I expect all my wheels to last far longer than that. I suspect that they used an unrealistic fatigue test protocol to exaggerate the difference in spoke durability.


I agree, great overview and how a manufacturer can use numbers to make any point, valid or not.