To vertically true the rim, or to get the, "bounce," out, adjust the caliper close to one of the rim's edges. Spin the wheel and look for high and low spots. This will look like the wheel is hopping. To get rid of the bounce, tighten the spokes over the high spots and loosen the spokes over the low spots. Remember to start with only a quarter turn at a time and turn all the spokes evenly or you'll end up taking the wheel out of a lateral true.