SPOKES FOR DIFFERENT FOLKS
WHY DON’T ADV BIKES AND MOTOCROSSERS USE CAST WHEELS?
Why do some bikes have wire spoke wheels while others have cast wheels? We cast light on the subject and discuss the merits of each design.
WE ARE A NATION DIVIDED.
Forget about that red-state/blue-state thing, or which bathroom to use, or burgers or burritos, boxers or briefs. We’re talking about the state of the motorcycling union. Wheels, specifically. Wire or cast?
Incendiary rhetoric aside, here’s the real question. Why do the best dirt bikes in the world still use wire wheels, while the best roadbikes embrace cast construction?
Journey with us back in time some 6,500 years, when the first wheels appear, those proto-harleys lumbering on solid, Fred Flintstone ox-cart discs. Spoked wheels (think Ben-hur chariot) go back about four millennia. These are compression-spoke construction— the load path pushes down from the hub and tries to compress the spoke(s) below. The first wire tension-spoke wheels show up around 1800. They’re still radial construction; the spokes radiate straight out of the hub. The real breakthrough comes with the tangentspoked tensioned wheel, courtesy of James Starley in 1870. These are the wheels we see on bicycles and motorcycles today. Why is this important?
All motorcycle wheels need to demonstrate strength in three planes: radial, axial, and torsional. Radial strength keeps the wheel round and resists rim deformation when hitting a bump. Lateral strength means side loading—less important on a bike because we lean when we turn. Torsional strength—the genius of the tangent-spoked wheel—lets us transfer drive and braking torque from the hub to the rim. The number of times a spoke crosses other spokes on the same side (most motorcycle rims are laced in a three- or fourcross pattern) increases torsional strength.
What about cast wheels? They’ve been rolling longer than you might think. Remember the 1927 Böhmerland? How about Morris Mags (1973)? By 1976 any teenage squid could get factory mags on his Yamaha RD400C. Honda’s Comstars showed up a year later, and BMW’S iconic snowflakes made their debut in 1978. Today, cast wheels are everywhere.
A semantic digression: The vast majority of our current one-piece motorcycle wheels are cast. A few are forged, and the Comstars were composite metal construction. In our discussion we’re going to use “cast” for all contemporary one-piece metal designs.
Cast wheels offer plenty of advantages. They facilitate tubeless tires, generally hold their true better than a laced wheel, and require little maintenance. They can transfer more torque and carry more load. Depending on the design, the overall assembly can be lighter. God knows they’re easier to clean.
Yet for dirt bikes, wire wheels remain superior for two principal reasons. First, most use 6000- or 7000-series extruded aluminum rims. This is an excellent, durable material. Hit it hard—say, on a sharp-edged Baja rock— and it tends to deform (dent), where a cast wheel may crack and fail.
Second has to do with construction. Kenny Buchanan, of Buchanan’s Spoke and Rim, took us to school. Most off-road wheels use 36 or 40 spokes, and each spoke can have up to 1.500 pounds (!) of tension. These pull equally on the entire rim’s circumference. Introduce a sharp load in one spot (our nemesis the rock) and the wheel tries to take an oval form. But because the entire rim is in tension, this deflection transfers the load maybe 270 degrees around the balance of the wheel—and that means 27 spokes at 1,500 pounds each are all trying to pull the wheel round again. Now hit that same rock with the same bike at the same speed, but rolling on a cast wheel, and directly over one of the large cast spokes. The force is localized over a small area. There’s no give. Plus, the alloy used for your cast wheel is less malleable to begin with. The wider the rim (big adventure bikes), the more exacerbated the problem.
Sure, cruiser riders love wire wheels for their traditional looks. And it’s possible to straighten or replace a dented rim. But the real reason to respect the tangent-spoked, tensioned wire wheel is its elegant, Cartesian answer to a problem that’s 6,500 years old. Roost on, James Starley!