Alignment: 3 variables, 4 corners
1st variable: toe in or out at each wheel, most important to tire wear of the variables. Tires that are scrubbing wear. Too much toe out and the insides wear; too much toe in and the outsides wear. The tires will chamfer or bevel for the first couple beads because the tire is basically rolling in under the strain of the steering input. 928s like to be straight ahead or slightly toe in with factory camber settings (about 1/6° is stock+. Adjusting the tie rod at the end of the rack changes toe.
No need to lift the car. Use a straight edge with the laser level that only contacts the wheel rim. Use a shortened bungee cord to hold it in place so it’s hands free. Adjusting the eccentric at the front lower control arm attachment does rear wheel toe. You need to loosen the outboard nut on the long eccentric bolt and then rotate the head of the bolt to adjust and then retighten. These can be messed up and require disassembly and cleaning to do a good adjustment on older cars.
Next variable camber: changing camber changes toe so you need to do these in order. Get camber close first before dialing in toe (or caster). Camber is the tilting away from vertical of the face of the wheel. Negative camber is when the top of the wheel is canted in toward the center of the car and it’s what we want. The factory setting is about .5° for the fronts and about .66° rear. Using trig and a 16" wheel as an example this means that if you put a level on the wheel so it contacted the bottom center of the rim the top center of the rim should be .139 from the level. The rear should be .186 from the level. This is an easy way to measure camber.
A plumb bob is probably the MOST accurate tool for setting camber if it’s on a fixture that can be set on the wheel properly. A 39" high mast plumb bob is very accurat (Shocki"s method). At 39", 1/16" is 1/10°. I put a flat piece of plywood up against the rim about 6" x 18". I attached the mast to the outer edge so that the string on the plumb bob sort of swing across the plane of the front of the plywood. The string on my rig was a stranded piece of wire about .005 thick so I could see tenths of sixteenths or HUNDRETHS of a degree. A piece of paper on the plywood with marks so that I could see how far the wire moved from vertical as the plywood leaned against the wheel. 5 x 1/16" equaled .5°.
I got a digital level that’s accurate to 1/10°. I also made some sliding clamps that allow me to use it on different wheel sizes. With this level bungee corded to the wheel I can set camber directly to 1/10°, which is accurate enough for me. I can see the display when I am lying on the ground next to the car making the measurements so it’s very convenient. Camber is adjusted on the front wheels, with a long handled 8mm allen wrench, by rotating the eccentric washer under the clamp nut on the ball joint attachment stud closest to the wheel. The one furthest away from the wheel is the caster adjuster. Both nuts need to be loosened in order to make the adjustment. Rotate the eccentric washer to adjust camber monitoring the measurement with your level, plumb bob, and digital meter.
If the measurement will not move in the desired direction move the caster eccentric. These can be misadjusted and will limit the travel of the caster adjuster. The rear wheel camber adjuster is the rear inboard attachment of the lower control arm. There’s a nut to the rear of the pivot and the front is the head of the eccentric bolt. Loosen the nut and adjust the camber with the adjuster. Check during adjustment and after final tightening. They move as you tighten them. Camber does not wear tires quickly, even a lot of it. Race settings can be in the 3° range and it doesn't wear tires unevenly. Camber tire wear is even across the entire tire not just at the edges. The camber you set is far less critical than making real sure you have it the same side to side. Unbalanced camber feels like a dragging brake but the feeling gets stronger proportional to speed.
Last variable, Caster: Caster like in a bicycle is the tilting of the steering axis. This is done in our cars and many others by having the upper ball joint farther back than the lower with respect to vertical. The inboard eccentric on the lower ball joint pivots the ball joint carrier on the camber adjuster making the ball joint move fore and aft to adjust. Most cars need to have their ball joints adjusted all the way forward to get anywhere near a proper measurement. For most this means that you can look at the ball joint carrier move and make it go as far forward as the eccentric will make it go near the eccentric. This drives the ball joint by the wheel as far back as possible. Most cars will steer well and not wear tires using this technique. Cars that have had damage, or have differing measurements from the centers of the wheels side to side may not do well using this method.
Caster is a derivative measurement. It cannot be measured directly. It’s important to have it be balanced side to side and it’s not important what the actual value is especially with power steering. Caster is measured and set using turn plates and camber measurements at a set steering angle from straight ahead. Most ways of measuring caster require turning the wheel from 10°, 15° or 20° each side of straight ahead and then measuring the camber at that point. The difference between the camber measurements can then be mathematically converted to camber. Suffice to say without showing the derivation that at 14.3° of turn left then right caster is 2.0 times the difference between the two measured cambers at the left and right positions. So if the camber with the wheel turned right is 2.0° negative and the left is .5° negative the difference would be 1.5° and the caster would be 3.0° simple enough to describe. The wheel must be steered straight ahead before turning left and right to make this work accurately. Caster like in a bike is a centering force, the more caster the stronger the centering force, and is necessary for good stable, non divergent, steering feel. Bad caster is especially noticeable when backing and turning tightly.
Height at each corner is critical to a good alignment. Some will say that height has to be correct to align a car properly but that isn't true. The car is somewhat tolerant to height changes as long as they have been stabilized before final setting of alignment. It’s less the height value and more the settling. A car with somewhat wacky corner balancing can handle well, most drivers can't tell. The reason the front needs to be settled is that the car toes out as a result of and increase in negative camber as it settles. That means that the tires will wear bevels on the inside edges rather quickly after a bad alignment and it won't feel hooked up to a sensitive driver as the tires are not evenly loaded and so not generating their max grip.
It’s possible for tire wear to be more pronounced with large camber settings. All things are a compromise, large camber, tire pressure and intended driving purpose are all interrelated. If you put large negative camber on a car, don't change the toe to compensate, go beyond what the tire likes to run at, continue on with high tire pressures, etc. But it’s also for tire wear to be normal with large camber settings. Higher than stock negative cambers can give more even tire wear if the car is setup and used for very aggressive cornering.
I have about $360 in my alignment tools including the digital level and the turn plates and I think more accurate then any machine,. And it’s certainly more accurate than a fancy machine and someone who doesn't know how to use it which is the biggest problem out there. These methods require flat ground, not level but that’s nice.
93 GTS 5-spd