June 05, 2015 6 Comments
Here’s a post just for wheel builders. Some topics are best shared with those who understand (not necessarily agree) with what I am saying. If you are not a builder, proceed at your risk. If you find content disturbing, please know that it takes a builder’s perspective. If you don’t have it, don’t blame us.
I’m weary of builders on the spot for spoke breakage. That was true in my youth, an age of much greater wheel building ignorance. Such worry plagued my early career and those of peers. It’s likely some aspiring builders ditched the trade rather than face ever present guilt over spoke breakage.
Today we can be far more confident about wheel dynamics and component failure. For those of you fresher into the trade, I’m going to try and convince you that spoke breakage is a concern we share with riders but it is rarely owed to our building. This is vital if you and your customer are to build lasting trust.
Today’s scene is different
Spokes have never been better. At the same time, wheel longevity expectations have never been shorter (fashion, weight). Consequently, riders do not experience spoke breakage as much as before. However, in spite of rarity, spoke breakage today is becoming a more disturbing event. Ironic, yes? Typical perception, the rarer an undesired event becomes the freakier it seems.
The spoke world is also more complicated. There was a time when only DT, Sapim, and Wheelsmith were used for racing wheels. Today we have many brands using proprietary spokes (Mavic, Campagnolo, Shimano) and new premium spokes like Max (TW), Pillar (TW), Mach1 (FR), and others.
I am here to tell you
Very few have the opportunity to conduct extensive spoke and wheel testing and reconcile with hundreds of thousands of wheels in the field; from recreational touring to World’s, from expeditions to exhibitions.
Even fewer of those can share without reservation. I don’t make wheels, hubs, spokes, or rims, while most colleagues are unable to disclose many important facts due to employment. That’s no conspiracy and we would do the same. A small price to chase dreams.
Spoke testing is challenged to duplicate real world conditions. They break rarely and are used by the billions in vastly different ways. The biggest problem is statistical validity. You can test 12 spokes to failure over several days in dedicated fixtures. Costly but you will know a lot about these 12. Since good spokes fail less often than 1:12, you really need to test 1,000 over months. Impractical, of course.
Lab testing enables control and precision vital to analysis. To my experience, however, nothing beats a sponsored team with 100% reliable mechanics. That way you can get dependable feedback over huge mileage, multiple riders of differing styles, all within one season. Dealing directly with riders is far less productive as they are each unique and have a hard time remembering details. Good mechanics (often builders themselves) can share the facts. Teams rock for equipment (especially spoke) evaluation.
What you must know about spoke breakage
While many variables affect spoke breakage, the #1, Numero Uno, 98% reason behind unexpected spoke breakage is in the wire. You can do everything right in wheel design and building and have a spoke break in 50 or 500 miles; both premature.
Note: I am speaking of unexpected breakage. Not included: heavily used wheels with 50,000 miles; spokes broken from invading pedals, wire, or lumber; wheels purposely underbuilt (for 1 day events) but used otherwise; crashes; and so forth.
Spokes break from fatigue, repeated cycles of loading and unloading. This requires three factors:
(1) Use - spokes at rest do not fatigue.
(2) Stress risers - spokes break first where stresses are highest.
(3) An aberration in the metal micro-structure - without which (and they are inevitable) fatigue cannot start a crack in steel.
Note: the load cycles we are discussing are “within the elastic limit” of the material. With bicycles, all riding loads are such. You can exceed the elastic limit in lab testing but it does not occur during normal riding.
As builders, we do our best to minimize the fatigue risks of wheels such as:
(1) Bedding elbows into the hub flange to maximize support.
(2) Using patterns that don’t induce sharp kinks at the nipple.
(3) Setting spokes into their path so their shape is stable during riding.
(4) Avoiding excess tension, whose friction might damage threads by twisting.
(5) Stress relieving the structure so it is in a more relaxed state during use.
(6) Freeing spokes from unnecessary demands (adequate number, gauge, etc.).
Even when all of these (and other) precautions are taken, spokes can (and will) break unexpectedly. At the same time, you can ignore nearly all the rules above and still have an everlasting wheel. Spoke material (stainless in particular) can endure incredible abuse if it is perfect. This has been many times observed in lab and field study.
What to do
Design and build wheels as if spoke breakage were under your control. All those techniques have a small chance of minimizing breakage and the karma is obvious.
When a broken spoke comes your way, it is vital you:
(1) Trust what I’ve said, breakage is nearly random. Believe this. There’s more with poor spokes and less with better. But either way, breakage can barely be predicted.
(2) Express sympathy and sadness for the affected. A competitor who could have won a sprint needs sympathy. He/she may be in a blaming mood but not for long. Genuine sympathy is good policy, don’t be defensive. For a moment, wear the shoes of the spoke maker - apologize for the industry and material science.
(3) Do not act shocked, disappointed, sure, but stay calm. If you speak in a loud, agitated voice the rider will lose confidence in your wheels, and mechanical reliability in general.
(4) Examine the wheel and situation for any lessons. However, know that it was unlikely to be something you can see from the evidence.
Micro examination of spoke failure always shows the same thing. We know why and where spokes fail but cannot answer the “when.” The measure of science is not hindsight ability to explain but predictive power. By this standard, the science of spoke failure is as weak as most macroeconomics.
If breakage is wire material based, then consistency with adequate strength, is paramount. You bet. Wire consistency is an obsession of spoke makers and they have as little control as you once they accept the wire. There are too many variables over too many miles of wire. Variables include annealed state, surface finish, micro-hardness variability, occlusions and contaminants, crystal structure uniformity, and more.
Making strong wire (high tensile strength) is straight forward. Making it über consistent is very hard. Stainless wire for high end spokes is so extreme and unique in this parameter that it sees no other industrial use. No one would pay so much for consistency outside of medicine and there aren’t many medical uses for 14g wire.
Project calm confidence to those around you. Don’t let spoke breakage become a group sobbing session. Know the underlying science and know that odds favor spokes will sometimes break when you have done everything right.
Since spoke breakage strikes fairly randomly it is not an occasion to question everything. I have found it useful to squeeze parallel sets of spokes as hard as possible to gain insight about spoke integrity.
If no spoke breaks during the round of squeezing, you are nearly guaranteed none will break from fatigue in the next several hundred miles. With excess time available, you can also inspect the inside of each spoke elbow (what is visible) with a magnifying glass. Cracks that can't hide begin long before failure. I've caught infant cracks this way and felt like Sherlock Holmes.
So, choose your spokes carefully, don’t overreact to single events, and remember 98% of spoke breakage (while it occurs where expected) is not likely related to your building. It’s a spoke makers nightmare, not yours.
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