October 01, 2009
Steel wire makes great spokes. Take it from someone who's made a career of making and celebrating steel spokes. The wheel needs of a bicycle are beautifully met with steel wire, and one glance at the cycling world offers ample proof. Let's examine why this is true.
Why is steel wire so well suited to the bicycle wheel?
(1) High grade steel wire can support amazing tensile loads. Better spokes are good for over 600 lbs each, including the elbow and threads. The wire alone can hold 750 lbs per spoke, over 150,000 psi!
(2) Steel wire is very hard and can take lots of abuse, especially the motion and rubbing at the hub and where they cross.
(3) While wire can't support compression loads, they don't harm it. The wire just buckles harmlessly, ready for the return of tensile loading.
(4) Steel wire is very stiff. Working loads cause elongation of less than 1/3 of one percent of length. An elastic spoke would mean a rubbery feeling wheel. On a two wheeler, that means a wobbly ride.
(5) Steel has excellent fatigue properties. Within its elastic limits, the fatigue life is theoretically infinite. Just notice clocks whose very light springs and movements have been ticking for centuries.
(6) And, most importantly, steel wire is relatively cheap. Which is good, since more than 2 billion wire bicycle wheels are getting daily use on our planet. That's about 600,000 tons of steel!
(7) Unfortunately, however, thin steel wire is not torsionally rigid, so prone to windup when the nipples are turned. Thankfully, this windup is easily controlled.
What you may not know is that making cycling grade wire is seriously difficult. Standard grades lack consistency and purity. Variations in hardness, surface finish, and grain structure create weak spots. When spokes are pushed to their limits, small metallurgical problems become fractures.
The paucity of premium spoke makers is direct evidence of the challenges to making spoke-worthy wire. The elemental alloy must be extremely pure, practically pharmaceutical grade. No other wire customers, at least in the mass market, need such purity. So premium spoke wire must be drawn to order. It's not a typical industrial grade.
This pure metal is then drawn thinner and thinner to create the spokes we use. Each draw is a percentage reduction in diameter. The trick is knowing how large a percentage per draw, how fast to pull the wire through the die, what sort of lubricants to use, how often to change the dies, and what sort of annealing to perform after each draw. You see, steels, especially stainless, rapidly work harden. Annealing reduces the hardness induced by drawing. Without annealing (controlled heating and cooling) the wire becomes too brittle.
Even when you've perfected a strategy to make uber metal for spokes, there are plenty of sleepless nights and anxiety. The slightest missed step can produce thousands of unworthy spokes that broadcast hardship like a virus. Arguably, the metal standards applying to high grade spokes are more rigorous and less flaw tolerant, than the standards governing frame tubing or other metals in a bicycle. Spokes are in their own class for quality issues.
So now you'll understand if you catch me gazing absently at a pile of new spokes. I haven't lost my mind, I'm just adoring what I see. Can't really get enough face time with spokes. And, by the way, as long as you have a spoke cutter/threader, there are few safer places to put your money.
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