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Question: Bladed spokes and stiffness...(16 posts)

Question: Bladed spokes and stiffness...5ive
Dec 18, 2002 9:30 AM
I'm wondering not about the aerodynamic benefits of bladed spokes, but rather IF they contribute to the stiffness of a wheelset. I've been riding Campy Nucleons with bladed spokes and I find them to be quite stiff when I climb or sprint despite their low spoke count. But I'm not sure how much of this is due to their bladed spokes vs. high tension on the spokes. I've also noticed that a company like American Classic recommends 16/20 bladed spokes for heavier riders who want to purchase their 420 wheelset despite the fact that they offer the same wheelset with more (but non-bladed) spokes. Is there a proven correlation between bladed spokes and lateral stiffness? If so, why are they stiffer (again, laterally) than regular spokes? Thanks in advance for any info.
hmmDougSloan
Dec 18, 2002 10:08 AM
From what I know, wheel lateral stiffness is largely related to the rim itself as well as spoke tension. Typically, low spoke count wheels have stronger rims, which probably contributes to stiffness.

I can't see how the bladed nature of the spokes would make any difference. The only way spokes can matter here is they dynamic stretching under loads, and I've read that spokes stretch very, very little once properly tensioned.

There is some dispute of the issue, but I understand that spokes support the rim under tension, not compression. The only property of the spoke what should matter, then, is it's elasticity beyond where it's already tensioned statically. I would expect, but I'm no engineer, that a flat spoke would actually be more elastic than a round one for a given mass of a spoke. That's merely intuitive, though.

I agree that the Nucleons are very solid wheels.

Doug
hmm5ive
Dec 18, 2002 12:27 PM
I agree with you. I always suspected that the bladed spokes per se add nothing to the wheel stiffness on their own. But I never understood why a wheelset like Nucleons (with their low spoke count) felt stiff. I understand that you own lots of wheelsets. Do you happen to know if Campy happens to tension the spokes on their Nucleons higher than other wheelsets? Thanks.
still no effectthe other Tim
Dec 18, 2002 1:35 PM
Spoke tension has no effect on wheel stiffness (as long as it is sufficient to keep the spoke from going slack). Spokes obey Hooke's law. It doesn't matter what the initial tension is; they will respond with the same change in elongation for a given change in tension.
no effectthe other Tim
Dec 18, 2002 12:02 PM
Only the cross-sectional area and length of each spoke affect the wheel's stiffness (lateral and torsional). Other things that affect stiffness are number of spokes, lacing pattern, flange spacing, flange diameter and rim stiffness.
no effect...NOT TRUE!fredthebiker
Dec 18, 2002 4:07 PM
A bladed spoke DOES add to stiffness in a wheel. If you take a 20 mm deep rim and compare it to a 30mm deep rim, both with bladed spokes and the deep one having fewer spokes, the 20mm will be AS stiff as the 30mm. I know this from experience.

I have built many a wheel in the last 15 plus years.
I will tell you that tension does also play a big part in the stiffness of a wheel.

A bladed spoke is stiffer then it's original counterpart, a 14 gauge straight round spoke. The reason is because the bladed spoke has been work hardened more then the round spokes. Also, the bladed spokes stretch less then the round. If you look at DT's tension meter chart, it will show you that the bladed have a faster tension rate and a stiffer rating then the round counterpart.

The reason American Classic can build a wheel with 16 spokes front and 20 spokes rear to fit a 250 lb rider is because of the lack of flex and the increased stiffness of the spokes.

I am not calling anyone a liar or anything, but, if you compare the facts(the wheels that are out there) and the facts of the spokes themselves, you will find that a bladed spoked wheel IS STIFFER and STRONGER.

Fred
Confusing "facts"Kerry
Dec 18, 2002 5:39 PM
The work hardening of a bladed spoke would make it stiffer in bending, but not in tension. Your example of the two wheels does nothing to support your argument. Is it news that a 20 mm rim with more spokes will be as stiff as a 30 mm rim with fewer? Tension is what we are talking about, not spoke cross section pattern. Why would a bladed spoke stretch less than a round spoke with the same amount of material (same cross sectional area)? No reason supported by metallurgy. Spokes don't stretch appreciably anyway - it is the deformation of the rim that causes spoke tension to go up and down as the wheel rotates under load. DT's tension chart is reflecting the geometry of the spoke and it's resistance to sideways deformation, which has nothing to do with the stiffness of the wheel. Do you have any "facts" that actually support your argument?
Fred, my good friend, your logic defies itself.the other Tim
Dec 18, 2002 6:07 PM
Two wheels with different rims and a different number of spokes have the same stiffness because of the spoke shape? Sorry, I need a little help with that. Did I mention that the number of spokes and the stiffness of the rim affect the stiffness of the wheel?

Interestingly, work-hardening changes the ductility of a metal, and not it's elastic properties. If by stretching you mean inelastic elongation, then you are correct; a work-hardened spoke will stretch less, but that has nothing to do with the wheel's stiffness.

I've already explained why tension cannot affect the stiffness of a wheel unless it is too low.
According to BrandtVVS
Dec 19, 2002 12:48 PM
It happens I was reading Jobst Brandt's book, "The Bicycle Wheel" last night. He shows that wheel stiffness is drived from rim strength, the inflated tire, spoke elasticity, spoke count and spoke tension (and a few other factors I'm forgetting). For example, he states that thinner spokes have more elasticity and stretch more than thicker spokes of the same material (making a "softer" wheel).

He described the results of his finite element analysis and shows a number of diagrams showing exaggerated wheel flex under loads and states that the amount of wheel flex is too small to be discernable to the rider under controled conditions. Interestingly, his brief discussion of bladed spokes recognized their aerodynamic benefits but, instead, focussed on installation problems, e.g. keeping them straight. The book was written in 1981, before, I believe, the proliferation of low spoke count, machine built wheels and the current emphasis on aerodynamics.

So, if I understand Mr. Brandt correctly,

(1) A bladed spoke with greater cross sectional area than the comparison spoke of the same material in any shape will stretch less and make a stiffer wheel all other factors being equal.

(2) Low spoke count wheels must have much higher spoke tensions as fewer spokes support the rider's weight at any given time. Such spokes need to be stronger (thicker, heavier) but they give aerodynamic benefits (more aerodynamic shape and fewer of them).

(3) The cross sectional shape of a spoke has nothing to do with the stiffness of the wheel.

Hope this helps....
Well ...the other Tim
Dec 19, 2002 4:11 PM
I don't have Brandt's book handy right now, but I have read it, and I'm confident he would not have said that a wheel's stiffness depends on spoke tension. If he did, he is mistaken, but I don't think he did. Other than that, I agree with everything you said.
Well ...VVS
Dec 20, 2002 9:15 AM
Quite possible, on reflection... His book, the TV and my wife were competing for my attention at the same time. It's a miracle I retained anything at all.

Best wishes for the holidays to all
According to BrandtJofa
Dec 21, 2002 1:59 PM
This is a reasonable abstract of Jobst's thesis, but he is -properly- quite particular about subjective descriptions like "softer" being applied to wheels. Such terms belong to the mythology which he has debunked. Central to his thesis is that wheel deflection is practically insignificant in use, as you mentioned.

Jobst contributes to the Usenet group rec.bicycles.tech and has commented in depth on apparently new developments such as low spoke count, wheelbuilding machines (which I believe he helped design) and aerodynamics. Nothing different enough has occured however that he feels the need to re-edition the book: everything in it is as pertinent today as it was 20 years ago.

Jofa
I wouldn't want to beat a dead horse,the other Tim
Dec 22, 2002 10:32 AM
but this one might still be kicking. Brandt indicates that wheel stiffness is orders of magnitude greater than that of an inflated tire, requiring no further consideration. However, he is clearly referring to
i radial
stiffness.

i Lateral
and
i torsional
stiffness are different matters. While he somewhat discounts those in his book, he does not go so far as to reduce them to insignificance. Differences in lateral stiffness are easy to detect when riding, and by direct measurement, and they are not overwhelmed by tire compliance. Torsional stiffness plays a role in durability, but is as easily achieved as radial stiffness.

Most of the parameters that affect lateral and torsional wheel stiffness have already been mentioned, and they are very well treated by Brandt.
good point; don't confuse types of stiffnessDougSloan
Dec 22, 2002 2:40 PM
I thought the discussion was about lateral stiffness; vertical stiffness/compliance is entirely different.

I can tell a whopping difference between some wheels in lateral stiffness, but none at all vertical. Some are rock solid laterally, some are darn right whippy, like Velomax Ascent Pros (Lew carbon rims), which are very narrow and low spoked.

Doug
weakly kicking, stillJofa
Dec 22, 2002 4:26 PM
JB certainly reduces torsional stiffness to insignificance, inasmuch as the only concern which might include it is that of durability. Lateral stiffness is something else but is included under the general banner of 'insignificant'. Wheels don't ordinarily experience a high degree of lateral deflection, otherwise we and our bicycles would fall over. The deflection they do show can be heard on hillclimbs in brake rub when we have our brake pads positioned close to the rim. This supposedly high degree of deflection is stil dwarfed by the tyres: the same point still stands, that it is the tyres which give. I'v ridden all sorts of wheels over the years but have the sense not to trust myself to make an impartial decision about their behaviour.

Jofa
Mythology at both ends of the spectrum.the other Tim
Dec 23, 2002 10:44 AM
It's agreed that radial rim flex is dwarfed by tire compliance. The same is not true of lateral flex. A simple experiment demonstrates this. Place your thumb on the tread of a mounted tire with the wheel held in a supported bicycle or truing stand. Push your thumb laterally. With very little effort, you will observe considerable rim deflection with almost undetectable tire distortion.

By performing the 'measurement' with wheels of different design, it's easy to tell the difference in lateral stiffness. Although there is probably no argument about the relative stiffness of front and rear wheels, it's illuminating to perform this comparison on a pair of wheels of the same design.

Of course, if you push your thumb toward the center of the wheel, you will see the converse: substantial tire compression with virtually no rim deflection.

Riders assign different values to this parameter. I have one set of (pretty) wheels that did little to inspire me to the top of Mt. Hamilton this summer, and did less to inspire confidence on the descent. On the basis of
i my
values, they won't be making the trip again.