Butted vs. straight-gauge spokes

[Sheldon Brown]

Jobst referenced:

http://www.efunda.com/DesignStandards/springs/calc_comp_fatigue_eqn.cfm
http://www.grc.nasa.gov/WWW/RT1996/5000/5220l.htm

I observed:

>In the second of these references, I found:

> "In monolithic materials, it has been observed that tensile mean stresses are detrimental and
> compressive mean stresses are beneficial to fatigue life in comparison to a base of zero mean
> stress."

> There has been some controversy on this list about the observed greater durability of quick
> release axles vis-a-vis solid axles, and this would seem to support those of us who maintain that
> QR axles are less prone to breakage than solid ones, despite having less material.

Jobst asserted:

>That doesn't apply to axle steel and the difference between QR axles and solid ones is
>structurally minimal, the material (bore) lying in the neutral axis. I am fairly sure that the QR
>axles are better material. My rear axle failures resulted from horizontal slot dropouts which
>give no fore and aft bending support to the axle. "Vertical" dropouts support the forward
>circumference of the jam nut and therefore, support most of the bending load from chain tension,
>the one that generally causes fatigue failures. That is why these dropouts occasionally fail on
>the right* side.

There has been a lot of verbiage spilt since then and lots of arcane references, but none of them
has explained what property makes "axle steel" not subject to the same physical laws that govern
other "monolithic materials."

I remain unconvinced that the NASA quote doesn't apply, and indeed explain the observed greater
reliability of quick-release axles.

Jobst:

> Unloaded, the QR axle is in compression, with chain load and some road shock, depending on how
> tightly the QR is stressed, the axle deflects into tension and then back into compression. This
> is a stress reversal.

certainly there is occasional tension on parts of a quick-release axle, but the greatly
predominant mode is compression, as is evidenced by the many riders who've ridden long distances
on broke QR axles held together by their skewers, and only noticed the break after removing the
wheel for some reason.

The compression of the skewer creates a bias that prevents the axle from experiencing any tensile
load except at times of unusual stress, as when hitting a pothole. Even when this happens, the
magnitude of the tensile stress is greatly decreased by the countervailing compressive force from
the skewer. Since it is clearly the tensile forces that cause the axle to break, I find the NASA
hypothesis quite convincing.

Sheldon "Sometimes NASA Is Right" Brown Newtonville, Massachusetts
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http://captainbike.com http://sheldonbrown.com

 

[Jobst Brandt]

Sheldon Brown writes:

http://www.efunda.com/DesignStandards/springs/calc_comp_fatigue_eqn.cfm
http://www.grc.nasa.gov/WWW/RT1996/5000/5220l.htm

>>> In the second of these references, I found:

>>> "In monolithic materials, it has been observed that tensile mean stresses are detrimental and
>>> compressive mean stresses are beneficial to fatigue life in comparison to a base of zero mean
>>> stress."

>>> There has been some controversy on this list about the observed greater durability of quick
>>> release axles vis-a-vis solid axles, and this would seem to support those of us who maintain
>>> that QR axles are less prone to breakage than solid ones, despite having less material.

>> That doesn't apply to axle steel and the difference between QR axles and solid ones is
>> structurally minimal, the material (bore) lying in the neutral axis. I am fairly sure that the QR
>> axles are better material. My rear axle failures resulted from horizontal slot dropouts which
>> give no fore and aft bending support to the axle. "Vertical" dropouts support the forward
>> circumference of the jam nut and therefore, support most of the bending load from chain tension,
>> the one that generally causes fatigue failures. That is why these dropouts occasionally fail on
>> the right* side.

> There has been a lot of verbiage spilt since then and lots of arcane references, but none of them
> has explained what property makes "axle steel" not subject to the same physical laws that govern
> other "monolithic materials."

> I remain unconvinced that the NASA quote doesn't apply, and indeed explain the observed greater
> reliability of quick-release axles.

The QR rear axle operates like the rotating wheel axle in the example given earlier. That is,
although it has a relatively light compression it has large tension caused mainly by chain pull.
Considering that a crank is about 170mm and a chainwheel is about 100mm radius, then a 160lb rider
outs a 270lb chain force on the axle to which at least half the rider weight bears on the rear axle
vertically. When (not) supported by a horizontal dropout slot, this is a large bending load to which
the QR tension adds practically nothing. The bending load causes about 500lbs tension and
compression in the forward and trailing part of the axle, the ball bearing being about an inch away
from the face of the jam nut and having a diameter of somewhat less than 0.2".

>>> Unloaded, the QR axle is in compression, with chain load and some road shock, depending on how
>>> tightly the QR is stressed, the axle deflects into tension and then back into compression. This
>>> is a stress reversal.

> Certainly there is occasional tension on parts of a quick-release axle, but the greatly
> predominant mode is compression, as is evidenced by the many riders who've ridden long distances
> on broke QR axles held together by their skewers, and only noticed the break after removing the
> wheel for some reason.

The compression from the QR is piffles compared to the dynamic bending loads the axle supports. I
think the stresses caused by bending are grossly underestimated by most observers.

> The compression of the skewer creates a bias that prevents the axle from experiencing any tensile
> load except at times of unusual stress, as when hitting a pothole. Even when this happens, the
> magnitude of the tensile stress is greatly decreased by the countervailing compressive force from
> the skewer. Since it is clearly the tensile forces that cause the axle to break, I find the NASA
> hypothesis quite convincing.

I think that a few simple calculations will show that the QR is not producing much compression in
comparison to the tension and compression caused by chain pull and rider load.

Jobst Brandt [email protected] Palo Alto CA