[Tim Brewer]

I'm not sure what rack you have, but there should be an adjustment on yours. It will be on top, a large nut with a hex nut that you adjust with an allen wrench. Loosen the large nut, and with someone sitting at the wheel, have them slightly move the wheel back and forth while you tighten the allen nut until the slack dissapeers, then lock it down with the large nut. Just don't overtighten the allen nut.
Tim