This article will discuss the use of the WAG-4 Wheel Dishing Gauge and the WAG-3 Wheel Dishing Gauge. For procedures on wheel truing see Wheel Truing. The dishing gauge procedures between the two tools are similar in concept.
Park Tool wheel dishing gauges will check the rim centering over the hub locknuts of most bicycle wheels. The gauge compares the position of the rim and hub locknut from one side of the wheel to the opposite side. To use a dishing gauge accurately, the wheel should be laterally (side to side) true.
WAG-4 Wheel Dishing Gauge
The WAG-4 comes with two sliding blocks on the feet. These blocks allow the tool to check off the wheel rim even when a tire is still mounted.
It can be useful to check dish while the wheel is still in the bike. Begin by noting dropout thickness. Equal thickness dropouts can be treaded simply as spacers. Measure from the outer edge of each dropout.
Dishing Procedure
To use the WAG-4, slide the two blocks so they rest on the rim and the sliding gauge is centered to axle. Make note which side of wheel is being checked. This example will assume the right side is being checked first and will act as a reference for the left side. Lower the sliding gauge until the end rests on the face of the locknut. Do not rest gauge on end of axle. The wheel will reference the locknut face when mounting in the frame.
Turn wheel over to check opposite side. Place WAG on rim and note results. There are three possible results.
Situation A: Both feet of WAG rest on the rim and the indicator point lightly contacts left locknut face.
The rim is centered to locknuts, no correction of centering is required.
Situation B: Both feet of the WAG rest on the rim, and there is a gap between the indicator point and locknut face.
This situation shows rim is off center toward left side. Rim should be moved to right. Rim is off-centered to in bike only half the distance from indicator pointer to locknut. For example, if indicator point is 3mm from locknut, wheel is off centered to mid plane of the bike by 1.5mm.
Generally, it is better to view the error (if any) at the rim rather than at the hub. If after checking the wheel you find Situation B described above, it is best to reset the indicator using this side as a reference, and recheck the opposite side. You will find there is now a gap between rim and WAG foot when the indicator rests on locknut face. This method makes it more obvious that the rim should be moved toward the foot. If you see the error or gap at the hub, the rim is actually pulled toward the opposite side to close the gap.
Situation C: Three point contact of both feet and indicator point at locknut is impossible. If indicator rests on locknut face, only one foot will contact rim, with other foot higher than rim, as seen in the left image below. If both feet are on rim, indicator is below level of locknut face, as seen in the right image below.
A gap between tool and rim indicates an error in centering.
This situation indicates the rim is off center toward right side. The rim should be moved to left.
Corrections to Dish
Spokes in a bicycle wheel are held by the hub flange. Spokes from the left flange pull the rim toward the left, while spokes from the right flange pull the rim toward the right. By adjusting spoke tension the rim can be moved to either the left or the right. To correct dish or centering, turn same flange-side spoke nipples evenly. For example, tighten left side spokes evenly to move rim left. Tighten right side spokes evenly to move rim right. Alternatively, loosen left side spokes to move rim right. Loosen right side spokes to move rim left. After turning spoke nipples, check rim for adequate lateral true, and check centering again. When making corrections to dish, remember that it will also make changes to over tension. If you correct by tightening one side, it will add to overall tension. If you correct by loosening, the entire wheel will become looser.
WAG-3 Wheel Dishing Gauge
The WAG-3 operates on a similar principle as the WAG-4 above. It is necessary to remove the tire and quick release or axle nuts to use the WAG-3. The WAG-3 uses a threaded pointer. Hold the pointer just to the side of the axle, and adjust the end of the pointer so it is flush with the axle locknut face. Turn the wheel over and compare the setting to the other side of the wheel. The same situations and resolutions apply as above.