Spokes are angled so that...
Spokes are angled so that half brace the wheel under acceleration while the ones angled the opposite way support it during braking.
Despite technology that's nearly a century and a half old, the wire-spoke wheel still has a lot going for it. Wire wheels are light, and when properly configured and maintained, strong and stiff, although not so stiff that they don't flex slightly when loaded, a feature that was particularly desirable in the days before front and rear suspension made the scene-and still isn't a bad thing. The traditional look of a wire wheel is also undeniably attractive, which goes a long way toward keeping them popular, especially with the cruiser/retro/vintage crowd.
On the downside, wire wheels do require slightly more maintenance than their cast or machined cousins. But unless you've let things get completely out of hand, it's relatively easy to perform and needs doing only occasionally.
The Theory
Wire wheels are composed of three main components-the hub, rim and spokes. The hub is basically an aluminum-alloy tube that connects the wheel to the bike. It contains the wheel bearings, and normally has two flanges on either side that are drilled to accept the spokes. Besides providing the motorcycle-to-wheel connection, the hub also acts as a convenient spot to hang things like sprockets, brakes and sometimes the speedometer drive. Outside of the occasional bearing check, most hubs need just about zero maintenance.
The rim is little more than a band of metal formed into a circle. Constructed of either aluminum alloy or steel, its job is to hold the tire in place (and give us something difficult to polish). Rims can be dented if struck hard enough, and I've seen more than one develop cracks between the spoke holes, especially after coming off a harsh encounter with a New York City pothole. Like hubs though, rims are relatively trouble-free and will perform well enough even with a few dings in them. After years of hard use, my trail bike's rims have acquired the general shape of a stop sign and they still work fine, although I'll admit I rarely run the bike at 75 mph down the interstate.
Cracks usually radiate outward...
Cracks usually radiate outward from the spoke nipple seats.
The spokes are the third piece of the wheel pie, and in many ways the most important. Spokes are thin, steel rods that are threaded at one end. The opposite end has a domed head that fits snugly in the hole drilled through the hub flange (though some designs are reversed and locate the spoke nipple there.) This end is normally turned 90 degrees or close to it; however, in some applications, a straight spoke may be used for increased strength. The threaded end passes through a hole drilled in the center of the rim, or occasionally through the outer edges (the latter design allows the use of a tubeless tire). An elongated steel nut called a nipple fastens the spoke to the rim and allows spoke tension to be adjusted.
Because spokes need to cope with driving forces from one direction and braking forces from the other, they are arranged in a tangential pattern-that is, they run at an angle from the centerline of the hub in a crossway configuration to their location in the rim. This allows one half of the spokes to brace the wheel under acceleration, while the other half braces braking forces. The spokes also cross from one side of the hub to the opposite side of the rim, which results in a triangulated structure that prevents the hub and rim from twisting in relation to each other, and provides a measure of lateral strength.
That being said, a lack of lateral strength is still the wire-wheel's weak point. But because a motorcycle banks through turns and sideways thrust loadings are relatively low (forces tend to be felt as downward pressure through the center of the wheel), lateral strength isn't that big an issue unless the wheels are poorly designed or have become weakened due to loose or missing spokes.
Of course if you're a sidecar enthusiast, the situation is reversed. Because sidecars turn like cars, side loadings on the wheels are enormous, especially at the front, and breaking some spokes is fairly common, which is why most chairs use either solid or cast wheels or have someone like Buchanan's lace them up a set with spokes as big around as a No. 2 pencil.