So what's the ideal caster angle? Opinion is split, and every suspension and alignment expert has his own opinion, but general consensus puts it between 1 to 6 degrees. Chassis Engineering recommends 3 to 5 degrees for a beam axle, while Brent VanDervort of Fatman Fabrications told us they build 1 1/2 degrees into IFS systems with a manual steering rack, and 3 to 4 degrees with a power rack, for optimum steering response with each system. Gary Heidt at Heidt's Hot Rod Shop said, "We build 1 degree of caster into our independent front suspensions, but the builder can go to as much as 3 degrees if they so wish." Art Morrison of Art Morrison Enterprises, on the other hand, told us, "It really depends on the application and what the car is going to be used for, and the kind of speeds it will be driven at, but typically we'll build a minimum of 4 degrees into the frontend on one of our chassis."
The greater the caster angle, the higher the stability of the vehicle, but at the expense of ease of steering. Picture the way an old Top Fueler's front wheels "fall over" when turned hard. Stability is paramount over steering with anything up to 15 degrees of caster in this instance, but that's not what you want on a street-driven car. Stay within the 1- and 6-degree parameters-though from the advice above it would seem the angle is dependent on whether you're using a beam axle or IFS-and all should be well, providing you have built in correct camber.
CamberCamber is the measurement, in degrees from vertical, of the inclination of the wheels when viewed from in front of the car. It is known as negative camber if the top of the wheels lean toward the center of the car, while it's positive camber if they lean outward. Excessive camber will cause tire wear, but a little negative camber is conducive to good road-holding. Art Morrison said, "We like our cars to handle, so our Tri-Five frames, for instance, will typically have 8/10-degree of negative camber on the front end, with 1/16-inch toe-in."
With beam or tube axles, the camber is built into the axle and not easily altered if incorrect. It should be very close to vertical, with a half-degree of positive camber. With an independent suspension, such as with twin A-arms, if it's taken from another vehicle, such as our fictitious Camaro clip from earlier, the camber should remain unchanged from the donor vehicle factory specs, unless excessive weight is put upon it and the A-arms are almost at their full extent of travel (not an ideal situation in which to use them anyway). If you have an aftermarket IFS, the camber (and caster to a certain amount) is usually adjustable by moving the inner mounts of the upper A-arms to suit.
Most manufacturers of Mustang II-based IFS systems build in some form of adjustment for just this reason, be it slots, shims, eccentric shafts, or threaded rod ends. Back to Brent VanDervort at Fatman Fabrications: "We prefer the slotted or shimmed adjustment technique, as it preserves the axial alignment of the upper control arm shaft and bushings. An eccentric shaft puts the bushing out of alignment and can cause wear and binding. Threaded rod ends cause additional difficulty since disassembly is required to make any adjustment." Incidentally, '74-78 Mustang II camber should be 1/4-degree negative.
Toe-InToe-in is the angle at which the wheels point when viewed from above, but this is going to be virtually impossible to see unless you drive a fenderless rod or your tires are hanging out of the wheelwells. If they point slightly toward each other, it is referred to as toe-in, with toe-out obviously when they point slightly away from each other. On rear-wheel-drive cars, the forward-most edges of the tires will attempt to point outward under acceleration, so they are set with a minimal amount of toe-in as a counteractive measure.