In my younger days I was an avid bicyclist, and a bicycle was my main mode of transport until my senior year of college. One thing I learned early on is that a bicycle is a pretty high maintenance machine. I worked as a bicycle mechanic during and for a while after my college years. Wheels are one of the most vulnerable parts on a bike, and absorb most of the shock from the road and all of its imperfections, such as curbs, potholes, and tree roots. Wheels on racing bikes are particularly vulnerable, due to their thin, high pressure tires and rims constructed for light weight and low rolling resistance, rather than durability. The wheels on my bikes were frequently getting warped due to hard use, so I taught myself how to repair them. Warped rims are more than a minor inconvenience, they compromise braking ability with cantilever or caliper brakes.
(there are several sizes available, make sure you buy one that fits your spokes)
Professional Wheel Truing Tools
Wheel Truing Stand
(Nice to have, but if you have one of these, you probably don't need to read this writeup)
Wheel Dish Gauge
(nice to have, but not absolutely necessary}
The process of straightening a warped wheel is called truing, and involves manipulating the tension on the spokes to produce the desired result. There is a little bit of an art to it, so you might want to practice on an old wheel first. Before starting to true your wheel, look it over. Remove the wheel from the bike and remove the tire and rim strip. Make sure the axle bearings are properly adjusted, and not loose. Next, check for missing or broken spokes. If the wheel has any serious flat spots due to hard landings or accident, it may not be salvageable. Flat spots more than 1/4 inch deep are probably not repairable, but under emergency conditions, you may be able to reduce the flat spot by loosening the spokes around the affected area and pounding the rim out. If the wheel has a serious flat spot, there may also be a bulge on the braking surface on the rim. Again no guarantees, you may be able to reduce the bulge by carefully squeezing the rim with a pair of channellock pliers or vise-grips.
Truing the wheel
After evaluating the wheel's condition, put the wheel back in the forks, or the truing stand if you have one. Replace any missing or broken spokes, and tighten them up with the proper spoke wrench to the same spproximate tension as the other spokes. This in itself will probably greatly improve the trueness of the wheel. Spin the wheel slowly, and note where the rim wobbles to one side or the other, and also note high and low spots on the rim, using the brake pads or your truing stand's gauges as a guide. Find a spot where the wheel is warped. If the wheel is warped to the right, loosen the spokes on the right side of the rim in that area, while tightening spokes on the left side. Work a little bit at a time until you get a feel for how much the rim moves for a given amount of tightening of the spokes. To deal with high spots on the wheel, tighten the spokes in the area around the high spots, and loosen the ones 180 degrees to the high spots. Keep spinning the wheel to monitor your progress. If you encounter spokes in an area that you want to loosen that are already loose, leave those spokes alone and tighten the opposite spokes. This will help correct the warp, but it is a sign of a damaged rim.
Wheel Dish is the degree that the rim is offset from the centerline of the hub that is necessary to center the rim between the forks at the brake pads. Imagine the wheel as a plane (in geometric terms). The plane made by the wheel must be in line with the plane made by the triangle made by the bicycle's frame triangle, or else bad things happen to the handling of the bike. On the front wheel this is fairly easy, if the rim is straight but off-center, loosen all the spokes on one side of the rim and tighten the spokes on the other side until the wheel is centered. Start with half a turn, and go from there. On the back wheel on a multispeed derrailier equipped bike, things can get a little more complex due to freewheel clusters. The best way to think about this is to think of the wheel's centerline as the midpoint between the outside of the axle cones. The rear wheel on a derrailier equipped bike will be flatter on the side facing the freewheel cluster.
Finishing the job
Once the wheel is true and centered, plink the spokes. If the spokes feel loose, tighten all the spokes on the wheel equally, a bit at a time. It may be necessary to touch up some of the adjustments. A tight wheel will be stronger, handle better, and stay true longer than a loose wheel. Before remounting the rim strip and tire, check each spoke nipple
for protruding spokes, which will quickly puncture the inner tube, and file them flush. Reinstall the rim strip, and remount and inflate the tire, and you are done. After the wheel is reinstalled, adjust your brakes to take advantage of the wheel's newfound trueness and increase your braking power. If you know that there are no spoke heads getting ready to protrude on a particular wheel, you can probably get away with touching up the wheel from time to time without removing the wheel, though I recommend deflating the tire first.