When we designed our MOD1 coilover kit, increased caster was part of our design criteria. To start with we need to understand what the customer goes through to understand how caster comes into play in their car, what happens, and how to remedy the situation. This normally starts with who did the alignment and what settings they aligned the car to. The response is generally answered by Uhh.. the local shop? Originally, the Mustang suspension was designed to have little to no caster. Caster is how much the upper ball joint is set back from the lower ball joint. Caster is responsible for stability on the freeway among other suspension/alignment changes while steering your Mustang. Back in the sixties the thought was to use very little caster and this would make the front tires slide before the rear and also have a very light steering wheel feel. Things have changed quite a bit since then. Now days more caster to a point is a good thing. It creates stability at speed on the freeway and inspires confidence in the driver.
First we need to make sure that the customer has taken the car to an alignment shop that does not think that making the box green on the alignment machine means that the alignment is good. Those specifications for the alignment were created in the sixties to a bias ply 14” tire. An alignment is a guestimate of how much the tire flexes from the bead of the wheel. Those old tires did a ton of flexing so the alignment shop had to over compensate to get the tire to go the right direction. Now days with modern tires if you used those alignment spec’d the car would feel horrible. Our tires do not flex like those old bias ply tires, not to mention that we are mainly using wheels in the 17” range now. One of these specs in the alignment is the Caster setting; this like we said earlier was how much the upper ball joint is set behind the lower ball joint.
Since the original specs were about 0 degrees, plus or minus a touch. The car would wonder and feel unstable at speed. Through years of development we, among multiple other groups have employed increased caster settings. Historically manual steering cars would use up to 3 degrees before the steering got too heavy and power steering cars used about as much as they could get. This usually ended up around 4 degrees before the tire was pulled into the front fender to achieve this setting. When we designed the MOD 1 coil over system we saw the challenges of trying to get this caster out of the 1st generation Mustangs suspension design. We here at MMI decided to offset the upper ball joint back to get the base caster setting increased out of the gate. This keeps the 1964.5, 1965 and 1966 Mustangs from over shimming the upper A arms and the 1967, 1968, 1969, 1970, 1971, 1972 and 1973 Mustangs from over pulling the lower strut rods and miss aligning the sway bar and wheel location in the wheel well. By offsetting the upper control arms we got our base caster settings up to about positive 4 degrees.
Another added bonus of increasing caster is that when to employ more caster the camber is affected when you turn the steering wheel. Camber is when the tire leans into or out of the wheel well and how much. Negative camber is when the top of the tire is leaned in and positive camber is when the top of the tire is leaning away from the car. Many companies including MMi do the Shelby drop to the inner mount of the upper control arm. By doing this we are increasing the amount of negative camber as the car goes through roll. Many companies go beyond the classic 1” drop. In our humble opinion more is not always better. We want the tire as flat as possible while driving and increasing the caster settings will increase the outside tires camber and decrease the inside tires camber while turning. This will keep the tire flat to the ground when side loading is applied to the tire, thus pulling the tire under the bead of the wheel.
Over the years we have learned all sorts of tricks to get these old cars working better and better. We can go on for hours just about alignments. This is one of the reasons why our MOD1 suspension system feels so good. Proper alignments coupled with giving the customer the proper tools to get the desired feel is what it comes down to. It’s not just bolting parts on, you also should have an idea of how to tune it. We hope this has helped you to get an idea that there is more to it than just building a tubular arm.