Wheels give a bicycle its effortless motion. Carrying the entire load, spinning at hundreds of rpm, they encounter everything we ride over.
This feat of strength comes from enormous tension in each spoke. Spokes of a bike may carry a total tension load of over 5,000kg, but comprise less than 1/3 of one percent of vehicle weight (bike plus rider).
In structural engineering, these are staggeringly impressive numbers. So, yes, it’s all about the spokes.
We’re spoiled
Ever since the bicycle became widely commercialized it’s used high tensile steel wire for spokes—with nearly the same capacity as today. That early mechanical breakthrough has been driving the bicycle forward. Hard to imagine reaching today’s high level of development and popularity without this early role of steel wire.
In fact, it’s hard to imagine our world without steel wire. Like ball bearings, it’s everywhere and critical—aircraft control, bridge and building reinforcement, power transmission, and on and on.
Beyond steel
No other material rivaled steel spoke wire until the 1990’s:
1990—Spinergy Rev-X, wide carbon fiber band spokes.
1993—Fiberflight UD carbon spokes with steel ends.
1994—Lightweight carbon spokes in fully bonded wheels.
1997—Spinergy PBO (Zylon) aramid fiber woven spokes.
1999—Mavic Ksyrium aluminum spokes.
Some of these efforts had good success, but none challenged steel wire in the global bicycle industry.
Today
Lately, a host of non-metal (mostly carbon) spokes are making headway at the very highest end:
Lightbicycle—Thrust-4 UD carbon spokes.
Newmen—Vonoa thermoplastic UD carbon (used by DT, Enve).
Arris—MTO thermoplastic UD carbon spokes (Specialized Roval).
Alpina—moulded UD carbon.
Berd—braided Dyneema.
Sapim—RC-1 flexible PBO spoke.
Gulo—braided composite spoke.
Serious research and proprietary processes precede these products, more than we can discuss today. And there are certainly more examples on the way.
Carbon spokes are especially tempting if they can be integrated with the hub and rim, eliminating joints and fasteners, like Lightweight, Syncros, Partington, Corima, and Mad Fiber. Such wheels are impractical for big markets but the design beauty is hard to deny. Composites love integration.
Pro’s of non-metal challengers
• Most have much higher tensile strength. Granted, tensile failure is rare with bicycle spokes. Bonded/cured on end pieces are likely where failure would occur.
• Fatigue strength should be excellent.
• Shock absorption in some cases (Berd, Spinergy PBO) is superior to steel. UD carbon is certainly worse. Interestingly, the woven braid of Berd creates a natural spring defined by braid angle for an otherwise very inelastic material (Dyneema).
• Wild looks is a huge advantage in an advertising driven, consumption market.
Con’s
• Non-steel spokes are 5–10X more expensive, adding $200–$1,500 to wheelset cost.
• High modulus materials like carbon fiber are known for a relatively catastrophic failure mode. Sudden, explosive, cascading failure is bad news but these materials are very strong. Importantly, early publicized failures with Fiberflight and Mavic R-Sys are well in the past.
• Composite spokes generally need special purpose hubs to fit them, limiting choice and adding weight.
• Wheelbuilding cost is higher as some systems take twice as long to assemble.
• Extra high stiffness is not without issues. Riding causes brief reductions in tension. With more inelastic materials, we see more moments of zero tension. These can, if not addressed, affect nipple loosening and even spoke ejection. Wheel design seeks a balance between spoke and rim choice. Historically, light rims are paired with light, more elastic spokes. Heavier rims with heavier, stiffer spokes. Today we see very light rims with super stiff spokes, a combo we used to avoid. There is much still to learn.
• Uncertainty is higher as we're early in this phase. How will resin systems and adhesives fare with age, toxic exposure, UV light, temperature extremes, humidity, rough handling, etc.?
Practical takeaways
• Many of these have already won big races and accolades by riders and mechanics. Many have been developed by gifted and motivated engineeers. There are certainly lots of possible upsides.
• Builders must master tension management because composite materials are stiffer and their inelasticity means small nipple adjustments may make large tension changes.
• Damage should be monitored as fracture propagation is very different in composites than in metals. Stainless steel care is casual by comparison. Pay attention to grime (sweat, energy drinks, brake dust, sewage, petrochemical debris, etc.).
• Be careful of storage and transportation. Carbon spokes deserve extra caution compared to stainless.
• Stainless wire has relatively uniform properties. Each composite spoke is unique—nothing can be inferred about performance from appearance.
Design thoughts
• Aerodynamics is driving much component development. With frames and rims, carbon more easily affords exotic shapes. What about 1mm x 3mm and 4mm aero profiles—shapes abandoned by steel spokes? Why no data about aero benefits? Features are not necessarily benefits.
• Is there such a thing as "too light?" Gyroscopic and momentum forces of wheels, proportional to mass, are important to stability and handling. At Mad Fiber we sold hundreds of 1kg wheelsets which some found too light for gnarly, high speed descents. Of course, wheels that save grams in spokes can spend them with rims but remember that weight loss for its own sake is not an automatic plus.
• Much of the bicycle’s beauty is owed to elegant simplicity. 150yrs has been spent reducing mass, simplifying load paths, and avoiding complexity. Stainless wire is an elegant contributor. Globally, the recycled content of stainless averages 44% and 95% is being recycled. It is spared plating and painting, and working with it—for lab, cuisine, aeronautical, marine, architectural, and medical applications—is special.
• My bias is clear! I love high function delivered by modest ideas. Bicycles are brimming with this sort of cleverness.
If your ride is headed to carbon spokes, best of luck! Share how it works out, be a positive part of change, and congrats for volunteering as a test pilot!
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