Let it not be said that we at Motorcyclist are unresponsive. Recently (er, make that about three and a half years ago), after running a short bit on how to make sure your bike's wheels are in a line, we received several suggestions for a better method. Our original thought was the time-tested string method -- wrapping the string around the front wheel and checking alignment at the back. We tried the alternative method, and it works, further proof that we have smart readers.
In fact, the second string method is nearly the same as our initial suggestion but backward, using the rear wheel as the primary reference and checking for misalignment against the front wheel. Here, we'll walk you through it.
Obtain an eight-foot or longer length of thin string or fishing line. (We prefer brightly colored string because the fishing line can be awfully hard to see in a dark garage.) Find the midpoint of the string and pass it through the rear wheel (1). The idea is that you will wrap the string back across the trailing edge of the rear tire. Choose a location on the tire that will allow the string to run to the front wheel, parallel to the ground, without hitting the bellypan or stands (2). Pass the string around the backside of the tire and pull it forward toward the front wheel (3). If you want, tape the string to the tread where the two free ends to keep it from shifting (4).
Now bring the free ends of the string forward to either side of the front tire. Turn the front wheel so it's pointing approximately straight ahead. (It's not important to have it exactly right just yet.)
Get yourself down on the ground ahead of the front wheel so that you can see all the way to the back wheel. Grab the string ends and pull them taut. Start with the free end of one string several inches from the side of the front tire and draw it toward the tire, looking at where the string touches the sidewalls of the rear tire. Your job is to place the string so that it just touches the front side of the rear tire's sidewall (5). Hold it there. Repeat this step on the other side of the tire, being careful to hold the other string in the position you just established.
When the string is taut against the rear-tire sidewalls and pulled straight forward, it will form a reference for the rear wheel's alignment, in effect projecting the angle of the tire toward the front.
Now, with the string still carefully positioned, see how it looks against the front tire. Have a helper nudge the front tire left or right to get it parallel to both sides of the string (6). That done, compare the distance of the string from one side of the wheel to the other. Properly aligned, the string will be both parallel to the front wheel and equidistant on both sides (7). If the string is closer to the right side of the front tire -- as you are viewing it, looking back -- then the rear wheel is cocked leading-edge right (8). Obviously, if the string is closer to the left side (looking back), the rear tire is leading-edge left.
Make your adjustments at the rear axle and check your work. Once you have the front and rear tires tracking the same line, just be careful to make subsequent chain adjustments equally on both sides.