SPOKES FOR DIF­FER­ENT FOLKS

WHY DON’T ADV BIKES AND MOTOCROSSERS USE CAST WHEELS?

Cycle World - - News - By Mark Lin­de­mann

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!

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