Spoke length has no effect on tension. That is, dish has nothing to do with spoke length BEFORE lacing the wheel. In other words, using longer or shorter spokes does not determine dish at all. Good spoke length is important, but it does not by itself dish the wheel.
However, if you consider spoke length to be "effective" spoke length as distance from the hub spoke hole to the rim, the right spokes will be shorter than the left.
Try a simple experiment. Take somebodies front wheel, and cut a spoke. Cut another one in another place. Now, replace one spoke with one 2-3mm shorter, and the other with one 2-3mm longer. True the wheel. The long spoke and the short spoke have the same tension as the cut ones. One may have poor thread engagement and may be barely holding on, and the other may have the spoke sticking up in out of the top of the nipple and may even poke the inner tube, but the tension is the same as all the others.
By using different spokes on left versus right side you get better thread engagement at the nipple. View a wheel from the top as a two dimensional triangle. There is a longer leg from the left side and a shorter leg from the right. So, the left side gets a longer spoke.
On multi-speed rear wheel, and also on front disc hubs, the two flanges are not centered to the middle of the hub. If the left and right side spoke tension were even, the rim would be centered between the flanges. On non-disc front wheels the left and right flange are symmetrical and centered to the middle of the hub center. The tension is the same of left and right side, and the rim is centered over the hub center, between the flanges, and between the hub locknuts. It is most critical that the rim be centered between the hub locknuts, not flanges.
However, on a rear wheel, the right flange is closer to the hub center by about 12-18mm as compared to the left flange. You want the rim centered over the hub locknuts, not between the flanges. You want the rim closer to the right flange than the left.
It is common on rear wheels to have as much as 30% less tension on the left side than the right. This is why pulling up a wheel tight is so important. If the flanges are moved together by design, you can get this figure lower, but there is always some difference.
Last edited by Calvin Jones; 07-31-02 at 03:17 PM.