Adjusting the X-plane assures that the forks are parallel from top to bottom- that is, if you measure the space between the fork tubes, their distance apart is the same under the yoke as it is at the bottom ends as well as all along their lengths, as diagramed in this photo:
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Clymer recommends using a caliper to do this job, but it is an inaccurate method at best, good only for showing that a fork was badly damaged in accident or so out of alignment that the forks would most likely be approaching a nonfunctional state or working so poorly that the measurement would be unnecessary. To most accurately check the X-plane alignment you need the special tool that should have accompanied this document. Before using it, please cleanse your hands of oil and clean the tool as well so that the possibility of it slipping from your hands is minimized. Also, place a folded towel or old bedspread on the floor under the bike so that if the tool is dropped it may be saved from a hard impact with the floor. The tool is seen in position in this next image:
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Apply the tool to the fork tubes so as you move the aluminum guide towards proper contact with the fork leg, the operating shaft of the dial indicator is being compressed into the gauge (it works right handed or left handed). When the guide rod comes into contact with the fork leg, the indicator’s shaft should be touching the center (widest part) of the fork leg. If not, loosen the 1/4″x20 nut and then the bolt and adjust the angle of the gauge to the tool for proper contact. Loosening the little silver locking knob visible at the 10 o’clock position in the picture allows the face to be turned so that the needle will align with “0” which will make recording of the amount of error a bit easier. Move the tool to the top of the tube and set it to zero.
As you can see, the tool can be easily operated with one hand (although please have a hand under the tool for security!). My thumb is gently holding the tool with just enough pressure to keep it in full contact with the fork leg and my fingers are holding the guide rod gently against the opposite tube. The tool can then be slowly slid down the tubes. It really is not important to know what the measurement is along the entire fork, but it is important to know that you are applying only the minimum amount of pressure to make the measurement and to know the difference in the measurement between the top and bottom of the legs below the fork yoke (lower triple tree).
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As you can see in the digital “multiple exposure” to the left, I am sliding the tool downward along the fork tubes. It will take a bit of practice to do this because you can actually press the tool inward hard enough just using your thumb to deflect the fork tube five thousandths of an inch or more depending on how close you are to the end of the tube at the time. You can demonstrate this by holding the tool at the bottom of the tube and lightly pressing against the damper rod protruding from the fork tube. Even a light pressure down here will deflect the tube and cause the needle to move.
Notice how I am holding the gauge? That is wrong. I held it that because I was snapping the photos with the other. If using the tool as shown in the photo, wrap your fingers around the tool where it meets the tube on the left of the photo (the right fork tube) and lightly hold the tool against the fork by using your right hand behind the tool.
If you are getting a measurement that is grossly far from parallel it is not important to be totally accurate, but when making the final measurements after adjustments have been made and the fork is close to .000″ out, the smallest of sideways pressure will make the reading inaccurate. In the X-Plane you are going to see that there is a lot of flex in the fork tube itself. Up near the yoke it is very easy to get an accurate measurement with this tool, but down towards the end of the fork tubes it is very easy to apply enough lateral force to deflect the tubes .003 and more without realizing that you are doing it. Practice with the tool for some time so that when you approach “parallelicity” you will have a good feel for the tool. It takes practice.
A method I came up with was a combination of factors. With the tool located near the end of the tube, protect the tool from dropping with your free hand, and then loosen your grip on the tool. While loosening your grip, move your hand (the one holding the tool on the tube) slightly, almost imperceptibly, “wiggle” your supporting hand left and right. It is sort of a “slow vibration” of your hand to relieve any lateral tension in your wrist.
There is no mistaking that this will take practice. The tool can be easy to use when measuring deviations of .010 and more, but when you get much below that the deviation of the tube that can be caused by manual pressure against the tube can be sufficient to make accurate measurement very difficult. How easily are the tubes deflected in the X-plane? The pressure of the spring in the measuring tool itself can cause a slight amount of deflection in the tubes!
Try this to quantify how well you are using the parallelness tool: Make marks with a small marker on the inside of the tubes every six inches. Zero out the tool while holding it at the top of the tubes, right under the yoke. Now, move the tool down to the first 6″ mark. Check the error. Let’s say that it is .002″. Now move to the second 6″ mark. The error should be double the first, or in this case it should read .004. Now move to the third 6″ mark, or 12 inches below the starting point. The error should now read .006″, etc. There are other factors at work here, but this should give you a good idea as to whether or not you are working accurately and not deflecting the tubes with hand pressure.
If you find that the tool does not slide down the fork tube smoothly, try a light coat of WD-40 or a little fork oil on the fork tube to make it slick. Repeat this measuring process from top to bottom a few times until the readings you receive are consistent.
BMW specs call for .1mm or .004″ as maximum, but Duane Ausherman found that .004″-.006″ could cause a severe wobble with his /5’s. If you have the time and patience, you should attempt to get your tubes within about .001″ or less out of parallel from top to bottom.
When I first measured mine as shown in the multiple exposure image they were out of parallel about .017″ (about four times worse than are called for in the BMW specs) when measured from just under the fairing to the bottom of the legs (being closer at the bottom than at the top). At this point it became clear to me that there was quite a bit of room for improvement.