[Note: this is #4 in a series of 20 wheel building tips to be published during 2009.]
Wheel builders often try to "pre stress" their wheels as a last step in the building process. All builders, especially novice, are nervous, wanting the new wheel to be as good and stable as possible. Like the rituals that preceded hunting in earlier times, we want to stack the cards in our favor. After all, hunting was important, uncertain, required skill and luck, and was potentially dangerous. Sounds like bike riding. Consequently most wheel building methods involve some sort of stressing routine.
Two popular lines of thought underlie wheel stressing. First, spokes that wind up during tightening and truing should be unwound so the new wheel makes no noise if this unwinding occurs in initial riding. Unwinding spokes are also liable to make a wheel untrue and we want it to stay straight from new. In Tip No. 3, about spoke twisting, I recommend building in a way that leaves no residual windup. If you master such a technique, then there's no reason to pre-stress for spoke windup.
The second idea involves stretching the component parts. If some outside stress can provoke stretch or yielding in the hub, spokes, or rim that might occur later in the wheel's life, then it will be more stable and require fewer touchup's. This makes sense but is not too important as the changes within a wheel during use mostly involve periodic reductions in tension, not increases. So yielding in the future, except during a traumatic event, is not much of a threat.
Give your wheels kind attention and they'll return the favor.
The Science of Stress Relieving
Well then, what is a solid reason to "pre-stress" a wheel? Here we refer to material science and the well established practice of stress relieving. What a new wheel can really use is not, technically speaking, pre-stressing. It's stress relieving. Any assembled structure involves new forces within its component parts. These forces involve static loads but they also involve trapped stress. Sometimes this stress is beneficial but generally it will make the structure less stable and encourage fatigue failure.
A good example (thanks, Jobst) is the grappling hook used by an industrial crane or derrick. Such a hook is expected to carry huge loads without deforming or wearing out. When it's formed, a straight section of metal is bent into a "U," the shape of the eventual hook. However, if the U is over bent a bit and then pulled open to the desired geometry, pent up stress in the metal is relieved. Such a hook is more rugged than one simply bent to shape. The over bent, then opened hook is stress relieved.
How to Stress Relieve
With a bicycle wire wheel, stress relieving can be done in two ways. First, as spokes are inserted into a given hub, you'll notice that they often don't exit the hub flange at exactly the correct angle. Tension of a finished wheel is enough to pull the spoke straight but it would have residual stress. It would secretly want to return to its pre tension shape. Each time tension was reduced, the spoke would tend to flex back to its original shape. This flexing would accelerate fatigue. The answer is to forcefully bend each spoke to the new shape PRIOR to tension. Even a bit of over bend is good. Then, with cycles of tension change during riding, the spokes will not be changing shape so much. That's stress relieving.
A second stress relieving opportunity exists after the wheel is at full tension. If you grab side-by-side parallel sets of spokes, one in each hand, and give a forceful squeeze, their tensions will be momentarily increased without damaging the wheel. Increasing spoke tension and then lowering it back has the wonderful effect of reducing trapped stress. Such stress relieved spokes have enhanced fatigue resistance. Every handbuilt wheel deserves this simple touch.
Grabbing parallel spoke pairs.
After a few such squeezes or with aero spoks, you'll begin wearing gloves.
All too many pre stressing or stress relieving strategies are simply too rough. Pushing a wheel sideways with enough force to flex it is dangerous. It's easy to damage the structure since this is its weakest dimension. I've seen builders step on their wheels or drop them from heights. In skilled hands, such techniques might have benefit but they take unnecessary time and often cause damage that requires additional repair. Some of these procedures are born more of superstition than science. But who can blame builders of the past? So much of a wheel's secret is hidden from view and its seemingly magical balance of forces and strength in use is hard to explain. If there's a superstitious part of your mind, I recommend you perform something gentle to the wheel, like a chant or burning some incense. If there are spirits who watch over our work, they'll get the idea.