On 7 Feb 2007 06:07:53 -0800, "Johan Bornman" <

[email protected]>

wrote:

>In my hand I have a SAPIM brochure - Trust in SAPIM Spokes.

>

>On the second-last page of the brochure, SAPIM has a graph showing the

>estimated spoke life in wheel revolutions. The company's standard

>double-butted spoke, the Race, has an estimated life of just under 1

>million revolutions.

>

>Let's say 1 million for argument sake, and that a wheel revolution

>translates into two meters on the road. Therefore, the spoke life is

>only 2 million meters or, 2000 kms. Not exactly something to brag

>about.

>

>Interestingly, but as an aside to this inaccurate estimate by SAPIM,

>the company's CD-Ray product, a 2,3mm wide bladed spoke, has an

>estimated life of 3,5 million "Wheel Revolutions (in cycles), as SAPIM

>puts it.

>

>Why would the company think that it's bladed spoke will last longer

>than the round one? Since diameter is a difficult comparison in this

>case, perhaps weight is. The bladed spoke has a weight index of 278,

>whereas the round one 360. The index btw, is my invention, they weigh

>278g per sample length per X number of spokes.

>

>Johan Bornman
Dear Johan,

Your question has come up before--it would be nice if Sapim would

include a description of its testing.

The Sapim spoke fatigue tests must use exaggerated stress cycles

because it takes literally years to break spokes in continuous testing

under ordinary low stress cycles--no spoke manufacturer sells spokes

that are expected to fail in only 2,000 km of normal riding.

In the study below, measured road-test spoke-stress ranged from only

20 to 150 MPa. It took months to break 76 test spokes with continuous

10-cycle-per-second testing at 174 to 501 MPa:

"In 1984 and 1985, fatigue tests on stainless steel bicycle spokes

were carried out for Wheelsmith, Inc. at Stanford University. Constant

cycle tests were conducted with pre-tensioning stresses of 174 MPa,

250 MPa, 343 MPa, 347 MPa, 424 MPa, and 501 MPa. . . . In 68 spokes

the failure occurred at the cold-worked elbow; in the remaining 8

spokes the failure occurred at the threads."

"The smallest stress cycle in the fatigue tests was 174 MPa, whereas

the stress range from the road test data was 20 MPa to 150 MPa. Hence,

the fatigue data was extrapolated to the low stress range. (To have

tested a single sample at 40 MPa would have required over a year of

continuous testing at 10 cycles per second, and an unwarranted use of

facilities.)"

http://www.duke.edu/~hpgavin/papers/HPGavin-Wheel-Paper.pdf
Ten stress cycles per second for a spoke on a 700c wheel is roughly 45

mph. It's also 315,360,000 cycles per year, roughly 630,000 km or

380,000 miles.

It's likely that those 1984 spokes were significantly less durable

than modern spokes.

Cheers,

Carl Fogel