Caster is challenging to understand because it's set up based on how the car will be moving at speed. If you could closely study the front tires while they are in motion, you'd notice the tire contact patch is different than when the vehicle is stopped. In motion, the tire contact patch moves rearward, which changes caster. Caster becomes more negative in degrees once we get going. This becomes even more apparent with radial tires, which make the car wander more from side to side. The solution is caster adjustment whenever tire type is changed from bias-ply to radial or vice versa. In fact, it's a good idea to have a front-end alignment any time you rotate or replace tires.
 Camber and caster adjustment...  Camber and caster adjustment changed forever for '67-'73 with an adjustable lower control arm that is adjusted using this eccentric at the fulcrum. Camber is adjusted at the eccentric; caster is adjusted at the strut rod. |  Camber and caster adjustment...  Camber and caster adjustment changed forever for '67-'73 with an adjustable lower control arm that is adjusted using this eccentric at the fulcrum. Camber is adjusted at the eccentric; caster is adjusted at the strut rod. |  Caster is spindle angle from...  Caster is spindle angle from top to bottom. Negative caster occurs when spindle angle is tipped forward at the top as shown. |
Mustangs like 0 degrees of caster, meaning both ball joints are 12 and 6 o'clock from one another. Ford calls for a minimum of 1 degree of negative caster and a maximum of 1-degree positive with no more than 1/2 degree of variation between the two on V-8 models. In short, caster must be within 1/2 degree from side to side for best results. Six-cylinder models need 0-degrees minimum caster and 2-degrees positive maximum due to the front-end weight difference.
 When the spindle is tipped...  When the spindle is tipped to the rear, this is positive caster. The '65-'66 Mustangs use shims on the upper control arm for caster adjustment. To shift caster to positive or negative, load shims at the front or rear attachment point. For '67-'73s, adjust the strut rod to move the lower control arm fore or aft to control caster. In either case, it's adjusting the spindle angle known as caster. |  Camber and caster can be summed...  Camber and caster can be summed up in this simple illustration. Camber and caster have to work together as a team for exceptional handling and control. Without proper adjustment, you're just wasting time and tire. |  Toe-in is adjusted by determining...  Toe-in is adjusted by determining the distance between the tire sidewalls or rims fore and aft. Marlon Mitchell of Marlo's Frame & Alignment, who has been doing front-end alignment for 25 years, checks toe-in fore and aft. He is seeking 3/32-inch to 1-1/32-inch toe-in. |
First-generation '65-'66 Mustangs caster adjustment takes place via shims at the upper control arms. Again, shims are added or subtracted to control caster. From '67-'73, the strut rods were adjusted to change caster. The same can be said for '74-'78 Mustang IIs, which also use a strut adjustment to change caster. The Mustang II's strut rods are attached to lower control arms from behind.