Bicycle Wheel

The bicycle wheel is a paradox of strength for its light weight. The spokes seem to push from the hub and axle to support the wheel. The paradox is that the spokes are pulling, and it is the equal pulling of all the spokes that is transferred to the circular rim. It is that tension distributed around the circle that gives the wheel its rigidity and strength through its resistance to deformity. It’s as strong as solid metal center, but only the weight of the spokes.
But there are also forces from the side the wheel on turns when the rider leans the bicycle right or left. The hub of the wheel is about 4 inches across and for every spoke originating on the left side of the hub extending to the middle of the rim there is an opposing next spoke from the right side of the hub to the middle of the rim. The opposing tensions of this spoke positioning scheme prevents sideways deformity of the rim circle.
The front and back wheel are substantially different. Looking down at the front wheel, the spokes angling from the hub to the rim on each side have a shape similar to two dinner plates on edge, face to face. Looking down onto the back wheel the spokes on the left angle in from the hub, but on the right side a substantial portion of the hub is taken up by the rear gear cluster or cassette, and attachment of the spokes has to be much nearer the center of the hub. The right side spokes are therefore more vertical and are also tighter to achieve a balance with corresponding spokes coming from the left side of the hub. Its shape is more like one dinner plate on end. This is called “dish” of the back wheel.
When a spoke breaks, that short segment of the rim is pulled sideways opposite to the broken spoke producing a rim deformity that often rubs the brake pad, increasing the pedaling difficulty. As a replacement spoke is tightened, it pulls the rim back to a true center of the wheel, or into the plane of the rim circle. Fine adjustments are made to one side by tightening the spoke opposite to the deformity of the rim (about 90 degree turn), then loosening the spokes in front and behind the replacement (a 45 degree turn each). The loosening is important so that the tension in that segment is stable and similar to all of the other spokes. Otherwise, with tightening spokes only, the rim would be deformed to a slightly elliptical shape giving the rider a sensation of a “hop” or bump every time the wheel goes around.
How do you adjust the spoke tension? At the rim, the threaded end of each spoke is engaged by a nipple protruding from the inner rim. This nipple has a square shape that can be turned with a spoke wrench. It is opposite of what you expect. If you look from the hub toward the nipple you must turn the wrench to the left to tighten. Progress of the adjustments can be followed by spinning the tire on the bike and watching for a wobble toward one of the brake pads when the wheel spins. In a shop, the wheel is placed on a truing stand that can be set to make small wobble side to side or “hop” out of round more easily observed and corrected.
In the setting of spoke replacement, it is easy. Remove the old spoke and put the new one in its place at the hub and at the empty nipple. There are two points of passing interest. First, each hole at the hub is counter-sunk. Each spoke goes through the hole such that that the head is on the side opposite from the counter-sink. This puts the sharp bend of the spoke, just beyond its head, in the counter-sink, eliminating the sharp edge of the hole that would have cut into the spoke. Second, on most wheels, each spoke passes over the head of the next spoke and crosses two more spokes. This is the “three crossing” pattern which is the most popular compromise among strength, “give” or comfort, and turning stability. (0 crossing is strong but stiff and less stable in turns and four crossing is less strong, but more flexible and more stable in turn)
That’s the magic of the light weight strength of bicycle wheels, front and back..