Wheel True, Round and Dish
Wheels should be true, round, and centered. Rear wheels and disc brake wheels have different length spokes on each side and need to be dished.
Round. Vertical true is roundness. A wheel should be very round--within 1-2 mm tolerance. Roundness is less important if you don't have rim brakes.
Lateral True. Lateral true is being parallel to the plane of the wheel. While rotating the wheel, watch the gap between the rim and your brake pads. If the gap varies, the wheel is out of lateral true. A wheel should be within 1-2 mm tolerance in lateral true.
Centered. Wheels should be centered. That is, the plane drawn though the center of the rim should be on the plane of the bike--a plane drawn though the center of the bike. Presuming the bike frame is properly aligned, if the plane of the wheel intersects the center of the hub (center between the locknuts), the wheel is centered.
Centering is easier on the non-disc brake front wheels than on rear wheels. The front hub flanges (where the spoke holes are) are the same distance from the hub's locknuts, and the front spokes are the same length. The front hub spokes form an isosceles triangle (a triangle having 2 equal sides). Usually a simple truing of the front wheel results in a centered wheel.
Dish. A rear wheel is harder to center. The rear hub flanges are not the same distance from the hub's locknuts. The drive side flange is further from the locknut, because the cogset must go between the flange and the locknut. Therefore, the drive side spokes are usually shorter than the non-drive side. On a properly centered rear wheel, the plane of the wheel will intersect the center of the hub (center between the locknuts)--but it will not intersect the center of the flanges--it will be much closer to the drive side flange. Look at your rear wheel. Notice it looks kind of like a dish. That's why we call centering the rear wheel dishing. A centered rear wheel is "dished" and the tool to measure dish is the dishing tool.
Front hubs designed for disc brakes have the same issues as rear wheels. The spokes do not form an isosceles triangle, and the wheel must be dished.
Spoke Tension. Spoke tension should be as uniform as practical. On the front wheel, all spokes should have a similar tension. On the rear wheel, the drive side spokes will have much more tension than the non-drive side, but the tension on each side should be as close as practical. (See the section on wheel tensioning for spoke tension details.) A poorly tensioned wheel is much more likely to come out of true or fail.
In summary, a well trued wheel will be:
1. round
2. laterally true
3. centered or dished
4. equally tensioned.
Tools needed:
spoke wrench that fits
truing stand
dishing tool
tension meter (optional)
Standard spoke nipples come in a number of exterior sizes, with just 2 sizes fitting almost all bikes. Park's black spoke wrench (0.127") fits nipples on nicer wheels. Park's red spoke wrench (0.136") fits cheaper wheels, and almost all wheels made in the Far East. Many high end wheel makers use spokes that require special spoke wrenches only useful for their products. It is extremely important to use the right spoke wrench, or you will strip your nipples.
Procedure to true a wheel:
1. Remove the wheel from the bike. To check round you must also remove the tire and tube.
2. Make sure the hub is correctly adjusted. A wheel with hub play cannot be properly trued.
3. Place the wheel in a properly tuned truing stand. A tuned truing stand can measure round, lateral true, and center/dish. You can make do with a substitute--even eye-balling it. A simple substitute to a truing stand is the brake pads. Turn the bike upside down and rotate the wheels while looking down at the brake pad gap. This lets you measure lateral true, but nothing else.
4. Survey the field of issues: Rotate the wheel while moving the truing stand's calipers close to the side of the rim. Notice lateral true issues. Move the stand's calipers to where they can measure round, to notice problem areas. Pull on all the spokes to notice their relative tension. Examine the rim, spokes, and nipples for damage. Evaluate whether your wheel is reparable.
5. Laterally true the wheel.
Looking down at the inside of the rim, notice that half the spokes go to the left flange and half go to the right flange. You will pull the rim to the right by tightening the spokes that go to the right flange or by loosening the spokes that go to the left flange. Conversely, you will pull the rim to the left by tightening the spokes that go to the left flange or loosening the spokes that go to the right flange.
Remember righty tighty lefty loosey. While looking down at the inside of the rim, to tighten a spoke, twist the nipple to the right (clockwise). To loosen a spoke, twist the nipple to the left (counterclockwise). The rim will adjust in the direction of the spoke you tighten and away from the spoke you loosen.
Make very small adjustments--1/8 rotations. Go slow.
So, your wheel is in the truing stand, and you have moved the stand's calipers so that the right caliper barely touches the rim in one place. Tighten the nearest spoke on the left side 1/8 turn. Continue by small adjustments, tightening the opposite side spokes or loosening the spokes on the side of the bulge until the caliper no longer touches the rim. Move the caliper in just a tiny bit until it again barely touches the rim, and repeat the process. Go slow with small adjustments.
From time to time, squeeze the spokes. This may cause the nipples to move, effecting the true--better to happen in the truing stand than on a ride.
Shoot for near equal spoke tension. On the front wheel, all spokes should be close. On the rear wheel, the drive side spokes will be tighter than the non-drive side, but each side should be close. You can tell a little by squeezing and plucking the spokes, but a tension meter is necessary to do it right. Park sells the cheapest reliable tension meter, the Park TM-1.
6. Round.
The wheel must be close to laterally true and nearly centered before it can be rounded. Move the truing stand calipers so they are directly below the edge of the rim, looking for a hop or dip. Fix a hop by tightening the nearby spokes. Fix a dip by loosening nearby spokes. Much greater adjustments are necessary to impact the roundness of a rim. Twist the nipple one rotation at a time. It is very hard to round a rim when there is already much tension on the spokes. You may have to considerably reduce the spoke tension on all the spokes before rounding. Simply loosen all the nipples, a half rotation at a time, taking many revolutions of the wheel. Laterally true and dish the wheel before adjusting for round. Once round is achieved, you will have to laterally true and dish the wheel again, and bring the overall spoke tension up to an adequate level.
7. Centering/Dish
On a properly tuned truing stand, when the rim is equal distanced between the calipers, the wheel is in dish. However, I only know one professional bike mechanic who regularly tunes his truing stand, and sometimes he screws up. So, even after your truing stand tells you the wheel is dished, measure it with a dishing tool.
The tire and tube must be removed. The wheel must be true to measure dish.
Hold the dishing tool with the arms squarely on the rim, and apply the measuring device to the locknut. Hold the measurement, and do the same thing to the other side of the rim. Adjust as necessary. Back in the truing stand, bring the rim to the right by tightening all the spokes on the right side. There is zero tolerance on dish--get it exact.
8. If you stripped any nipples, replace them.
9. Replace the wheel on the bike. Readjust the brakes and rear derailleur. Make sure the pads hit the rim properly. Check your shifting.