Effect of changing front or rear height on caster

[Genthar]

Wanted to put this somewhere where it might be easier to re-locate. Hope you don't mind me quoting your info, DJ.


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Changing rear shocks and sway bar bushings will not change the alignment. Adding one or more leaves to the rear springs might change it slightly because it might change the "ride height" of the rear end. An approximate way to figure the change is measure the height from the ground to a point on the frame at about the rear axle position. Do it before and after changing the springs. Now compute the "change" in the height due to changing the springs, where a "decrease" in the height is "positive". The net change in caster is

0.45 degrees x (change in rear height)

For example, suppose the rear drops 1/4". The net change in caster would be

0.45 degrees x (0.25) = 0.11 degree
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DJ, is that equation only for the rear or does it work for the front as well?

Caster is the lean of the steering axis toward the rear, i.e. when viewed from the side. So, if you lower the rear end, the vehicle leans more toward the rear, and so does the steering axis, and so caster increases. If you raise the front end, the same thing happens.
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[bloodypulp]

after changing your weight distribution, tires or suspension components you also need to re-adjust your camber and toe.

here's an excerpt from Progress Auto Suspension:

A high performance chassis is set up to provide maximum levels of grip; road holding ability. That means generating forces applied to the tires to keep them in good contact with the road surface. All the chassis dynamics are transferred to the road by the tire contact patch; literally where the rubber meets the road. Keeping that patch as big as possible means that more force can be transmitted to the road without a loss of traction.


There are three main criteria for keeping that patch maximized:
TOE
CASTER
CAMBER

TOE:
Toe is easily explained... Stand up. Now look down at your feet, and imagine you are looking down at your front wheels! If the toes of both feet are pointing straight ahead, your TOE setting is zero. If you move your feet so that the toes are pointing towards each other, you now have a condition called toe-in. If you move your feet so that the toes now point away from each other, you have created a toe-out condition. The vehicle will roll most easily with a toe setting of zero, but the car can become "darty"; requiring constant minute steering adjustments to keep it pointed straight. So some small amount of toe-in is normally built in to your alignment settings. Toe-in takes the tire contact patch and places it at a slight angle relative to the forward motion of the vehicle. Too much toe scrapes off speed, as the tire wants to roll in a slightly different direction compared to the vehicle. Since it can't, the tire has to slip just a little as the car moves forward.

CASTER:
Caster is a bit harder to understand. Now that you are standing with your feet planted firmly and your toes pointed ever so slightly in, you realize that you can rock forward and backward to a limited amount. Think of the angle created by your lower leg; from the ankle to the knee as the caster angle. Since your knees don't bend backward, the analogy isn't perfect, but if they could, a backward angle would be negative caster. Now crouch down a bit, and you see the angle of your leg has changed. The knee is now forward of the ankle, and you have a positive caster angle. Most cars have a slight negative caster built in to the factory setting, however caster is a very dynamic setting. It changes as the suspension moves. You want your inside front suspension to dip a bit on corner entry, and the outside to raise up slightly. Caster angle helps to achieve this. Drop one knee and see how you are naturally now leaning into that direction. Static (standing still) caster can also help keep the car pointed straight, since in order to move right or left, one side of the suspension has to "lift" the weight of the car. Since doing so would require an energy input of some kind, (turning the steering wheel), the absence of such input will allow the car track straight naturally. CASTER moves the tire contact patch forward or aft of an imaginary line drawn through the centers of the ball joints in the upper and lower control arms. In a McPherson strut suspension, the strut replaces the upper control arm and ball joint assembly, and so you would use the upper mounting point of the strut as one anchor of that imaginary line.

CAMBER:
CAMBER is everyone's favorite! It is VERY dynamic, meaning it changes quite a bit as the suspension moves up and down, and the wheels turn left or right. Still standing in a "normal" position, you realize that you can sway a bit side to side. Again this analogy isn't real good because your knees can't bend to the left or right, but you can simulate that flexibility by moving your feet wider apart. Now your knees are inside your ankles, and you have a negative camber angle. Bring your feet together, and your ankles are slightly inside the knee, creating a positive camber angle. Try and adjust all three settings by moving your feet around, and you see how hard it is to keep track of all these changes! A professional alignment job is necessary to make sure these specs are right. With your feet set very wide apart, and slightly toed-in, you can feel how much more pressure is being applied to the insides of your feet. Your tires experience the same kinds of loads! Too much negative camber and you will wear out the inside of the tire, while the outside of the tire looks brand new. That´s because the outside of the tire is actually being lifted out of good contact with the road. Camber can actually change the shape and size of the contact patch, and THAT´S why it is so important!

[DJ]

Originally posted by bloodypulp
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CASTER:
Caster is a bit harder to understand. Now that you are standing with your feet planted firmly and your toes pointed ever so slightly in, you realize that you can rock forward and backward to a limited amount. Think of the angle created by your lower leg; from the ankle to the knee as the caster angle. Since your knees don't bend backward, the analogy isn't perfect, but if they could, a backward angle would be negative caster. Now crouch down a bit, and you see the angle of your leg has changed. The knee is now forward of the ankle, and you have a positive caster angle. Most cars have a slight negative caster built in to the factory setting, however caster is a very dynamic setting ...

You have it backwards. Positive caster describes the lean of the steering axis toward the REAR, not toward the front. I know of NO vehicle that is aligned to factory specs having NEGATIVE caster, because negative caster is unstable to the point of making the vehicle undriveable.

I suggest you go read DJ's Alignment Setting Recommendations