On Thu, 08 Feb 2007 11:50:03 -0600, A Muzi <
[email protected]>
wrote:
>>>>>> 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.
>
>>>>> I have no doubt that it is an exaggerated test cycle. But don't they
>>>>> know in marketing hype one doesn't cite exaggerated examples, but best
>>>>> case scenarios. I have SAPIM-built wheels which have done closer to 40
>>>>> 000 kms. Why would a company misrepresent itself so much? I don't
>>>>> understand and I don't suppose there is a rational answer for this.
>
>jim beam wrote:
>>>> there /is/ a rational answer for this - almost all fatigue testing is
>>>> done on an accelerated basis. since that's standard practice, their
>>>> reporting is simply consistent with standard practice.-
>
>> Johan Bornman wrote:
>>> You miss the point, being scientifically rational doesn't make sense
>>> in this brochure.
>>
>jim beam wrote:
>> it makes more sense than being scientifically irrational or making
>> claims like "our spokes last forever if you don't use them", which is
>> basically where you end up if you exclude presentation of normal data.
>
>> Johan Bornman wrote:
>>> It is intended for consumers who can quickly do the
>>> sums and discover that SAPIM kind of suggests a useful life of 1 000
>>> kms for wheels built with its spokes.
>
>> Johan Bornman wrote:
>>> A good copywriter with the
>>> company's interest at heart would have phrased the exercise completely
>>> differently.
>
>jim beam wrote:
>> bottom line, while /you/ may not appreciate the format, the fact is,
>> sapim are the only ones that dare to present fatigue data. that should
>> tell you more than anything else.
>
>Maybe.
>Or were those numbers pulled out of the air? "2000k lifetime" is
>nonsensical to anyone with familiarity. But any ad writer would eagerly
>write "one million cycles" for any product - especially if he weren't
>burdened by knowledge of bikes or spokes.
>
>If they meant 1MM cycles at an extreme load, why no mention of that? If
>that was intended, it's simple in English:
>"Our product will bear up to 1MM cycles of extreme load"
>"Our product sustained 1MM cycles of industry standard fatigue testing"
> "Our competitor's product failed at merely 985,00 cycles"
>
>That is not what they wrote.
>
>I was also curious about the smaller-section aero spoke showing 2.3x
>longevity. That does not make any sense at all as _less_ material just
>couldn't bear _2.3x_ the load cycles. And the values are nice and round,
>too, 1,000,000 and 2,300,000. Convenient numbers, those, as they just
>happen to fit the price curve but do they relate to the actual product?
>
>It is not at all clear to me.
Dear Andrew,
Here's the data, culled from the Sapim site's individual pages,
ordered by cycles to failure:
per n/mm^2
64 x middle test cycles
model 260mm hub x middle x rim strength to failure
----- ---- --- --------- --- -------- ---------
cxray 278g 2.0 x (0.9 x 2.3) x 2.0 1600 3,500,000
strong 430g 2.3 x 2.0 x 2.0 1400 1,600,000
laser 279g 2.0 x 1.5 x 2.0 1500 1,250,000
cx 423g 2.0 x (1.3 x 2.8) x 2.0 1200 1,220,000
race 360g 2.0 x 1.8 x 2.0 1350 980,000
leader 424g 2.0 x 2.0 x 2.0 1080 870,000
Note that the cycles to failure aren't all that rounded.
Judging by weight, the cxray aero spokes (278 grams) are probably
flattened laser butted spokes (279 grams).
The cx aero spokes (423 grams) are probably flattened straight leader
spokes (424 grams).
Since the round spokes are drawn to thin their midsections, their
midsections work harden and strengthen, with the n/mm^2 strength
rising (1080, 1350, 1500) as they grow thinner (2.0, 1.8, 1.5).
The exception is the "strong," which has a 2 mm middle and rim end
(single-butted like a motorcycle spoke). The "strong" probably lasts
about twice as long as the straight 2 mm "leader" because it's got a
much thicker 2.3 mm elbow, which accounts for the extra 6 grams of
weight over the straight 2 mm spoke. It's probably a thinned 2.3 spoke
blank, which accounts for its higher 1400 n/mm^2 midsection strength
The aero spokes also show greater midsection strength than the round
spokes from which they were probably stamped, again due to work
hardening, but this doesn't affect the unworked elbows where they
probably fail.
Jim Beam's explanation for why the flattened aero spokes last longer
makes sense--their angle to the elbow lets them put less bending
strain on the elbow. That is, when an aero spoke bends at the elbow,
it probably does less bending at the elbow and more bending at the
thin section than a straight-section spoke, spreading the stress out.
The round spokes (leader 2.0, race 1.8, laser 1.5) probably last
longer in testing as their midsections grow thinner for the same
reason--as the thinner midsection bends a little more, it puts a
little less bending strain on the elbow, spreading the bending stress
over more spoke.
Cheers,
Carl Fogel