BMW motorcycle telescopic fork alignment procedure, Randy Glass, page 11

11 – Adjusting the X-Plane
fork diagram
      Before we begin with the X-Plane adjustment you need to understand the relationship of forces applied to the tubes and how they affect the alignment of the forks.  In this diagram, the cross piece represents the fork yoke and the two vertical tubes represent the fork tubes.  If a spreading force is applied between the tubes as shown at “A” then the tubes will move apart at that location.  The yoke is the fulcrum and the tubes will move together at location “B.”  This can also be reversed in this way- if a clamping force is applied at point “A pulling the legs together at this point, then the fork tubes will be spread apart at point “B.”  In all this, the forces applied are being used to align the yoke by applying subtle bending forces to the yoke through the force applied to the fork tubes.

      As you read in the last section, I needed to move the bottom of the tubes apart by .017 or so. I did that by using a clamp below the yoke as shown in the picture below this paragraph (these types of clamps can apply a spreading force as well as a clamping force).  The benefit of placing the clamp below the yoke is that there is more room to place the clamp down there, particularly with faired bikes.  One person can even apply the clamping pressure while measuring the amount of movement of the tubes with the gauge at the same time.  Place the clamp about six inches below the yoke.  Unless you get insane about it there is no real danger of bending the tubes themselves, but if you place the clamp too low much of the clamping energy will be absorbed by the flexing of the tubes instead of actually aligning the yoke.  Remember that we are not bending the tubes themselves here but aligning the softer yoke to hold the tubes parallel.

upper clamping

If you don’t have such a clamp as pictured here try a large “C” clamp.  You can try using two large blocks of wood (chunks of 4×4’s would work) with holes bored in them to slip over the fork legs so that there is a flat surface for the C clamp to push against as well as to protect the tubes. If not, try two lengths of PVC pipe slid over the tubes. It will be difficult to get the clamp to hold tight without slipping, but you want to protect those tubes!

I started my adjustment by clamping the tubes so that they moved about twice as far in the opposite direction.  Since they were about .017″ too close at the bottom, I clamped them to be about .040″ too far apart at the bottom, and I left them that way for a day.  I found that this was not enough pressure to get them to .000″ so I left them for another day, this time clamped about .150 further than .000″ which took a good amount of force from the clamp. After that I was getting very close, with the legs only out about .002″ after about 24 more hours of clamping pressure. I once again applied the clamp and waited another 24 hours. The last .001″ or .002″ or so took about two days to set.

lower clamping

You can also place the clamp above the yoke to do the same job. Here you see a clamp being used to move the upper end of the fork tubes towards each other.  This would have the effect of moving the lower ends of the fork tubes away from each other.

If the headlight ears were left on the tubes then using a clamp up here isn’t a good idea.

If your tubes are out by .015 or more then you might consider using clamps above and below the fork yoke at the same time- one on the outside of the tubes clamping inward and one pushing outward from between the tubes. Their pressure would have to be “balanced” but it might help speed the alignment process a bit.

Be patient.  It may actually take quite a few days for the yoke to take a “set” and hold the adjustment that you have forced upon it.  After you seem to have the forks properly adjusted, leave them un-clamped for a day or two and see if they are staying in the proper set.  After all this work you don’t want to bolt everything up to find that you are back where you started!  Worse yet would be to get them together, have them work poorly, and wonder if you actually “set” a correct adjustment or not.  I can say that it takes more force and more time to adjust the yoke in the tubes’ X-plane than in the Y-Plane.

      After completing the X adjustment, check the Y-plane once again.  It may be necessary to repeat the adjustments, one and then the other, a couple of times before everything is perfect.  Remember, patience is definitely a virtue here.  This is a process that takes time and rushing it will only be a waste of your energy and of your time and money.  It can easily take a week (or maybe even two) before you get all the adjustments right. After all, if you are taking the time and effort to do all this, you might as well get it as close as possible.

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