PFM???? Maybe it measures the distance from the front and rear tires, or front the front tires to the laser.
I did, google didn't know what I was asking for. I got all kinds of machinist links about laser guided things that had something to do with caster, but not for autos. The closest thing I found was a guy on some jeep forum that said to turn the wheel left 20 degrees and measure something, the right 20 degrees and measure and subtract one from the other and there is your caster. I didn't get what he was talking about. When it is up on the rack, they hook the laser thingies to the rims, and somehow it calculates it, but since it is on the same plane as the lasers and the mirrors/receivers, I can't figure out how it calculates it.
Caster is measured indirectly through camber. That is what you read about the method of rotating from 20 deg in to 20 deg out. Do this: Hold your left forearm straight up in front of you, wrist straight, bicep parallel to the ground, elbow bent at 90 degrees, palm facing to your right. Stick a pencil between your middle fingers, perpendicular to your hand (so your hand looks like a T). Pretend the outside of your forearm is the face of the wheel. Now, add some negative camber (move your hand in towards the center of your body). Add some negative caster by tilting your forearm slightly away from you. Now rotate your wrist, keeping your elbow stationary. As you "turn your wheel in and out", you will notice that the end of pencil will dive and rise as your rotate around. The difference of the angle from parallel to the ground (level) at the extremes is your caster. If you do this exercise, but keep your caster at zero, your pencil should create an arc, with the ends stopping at the same height (and thus same angle from level), and giving you a difference of zero, and hence zero caster.
So, to answer your question about everything being in the same plane... they aren't once you turn the wheel. You have to do a caster sweep (which is usually done on floating plates). The reason for turning the wheel in and out the same amount, or degrees, is so that you are taking symmetric measurements. In other words, if you rotated in 15 degrees, but rotated out 25 degrees, you can no longer simply take the difference to find your caster angle.
Enjoy! http://www.disco3.co.uk/gallery/albums/userpics/24543/steering-geometry-and-caster-measurement%5B1%5D.pdf
Alright! That makes sense. I figured the wheel would have to be turned to take into account a third dimension. I just don't remember ever seeing the alignment guy turning my wheels.
That was intresting.. I noticed there is no date to the information. The article mentions the Hunter C111 so I am assuming that the article, while still valid on all points, is outdated by equipment that has since become available. Newer equipment can do most if not all the calculations mentioned and can assist with displayed illustrations and bar graphs during adjustments.
