Editor's Note: The material for this story was provided by Steve Smith, whose book Building the Mustang Ministock addresses the issues of the MacPherson strut suspension. Information about the complete work is available at the end of the story.
The law of supply and demand can play a critical role in a racer's life. Mini- Stock racers are finding this rule to be a hard reality. The supply of once popular models for race cars has diminished. Racers are looking for different options. One of the options gaining popularity is the '79-'93 Ford Mustang.
The MacPherson strut system used in the 1979 and up Mustang is different. The coil spring is located on top of the lower control arm instead of being part of the strut unit.
Popularly known in the Ford world as the "Fox-bodied Mustang," these cars, on the surface, appear to be likely candidates for the Saturday night Mini-Stock wars. However, there is one drawback that has caused some to shy away from this option: The car is built on a suspension platform that is different. It uses a MacPherson strut front suspension and incorporates a splayed four-link rear that can be susceptible to binding.
With a little knowledge of the general principles and application of acceptable theory, these suspension hurdles are quickly overcome. Steve Smith Autosports, with the help of Gary Ebeling and Esslinger Engineering, saw the challenge of creating a competitive Mini-Stock on this platform. Their finished product was competitive right off the trailer. They've also graciously agreed to share some of the tips used in building this car, an '84 Mustang.
Camber/caster plates replace the stock upper mounting hardware on the top of the strut tower.
MacPherson Struts
The main problem with a MacPherson strut suspension is its tendency to gain positive camber at the outside front wheel as the wheel moves in compression and is steered. In building this car, the first chore was to reduce the positive camber gain and to set enough static negative camber to overcome the tendency.
A true MacPherson strut combines the shock absorber, spring, and upper control arm into one unit. The bottom of the strut is bolted onto a steering knuckle or spindle. The top of the unit bolts into a strut tower, built into the car's inner fender.
An '89 Mustang control arm (left) was used on this car. It is an inch longer than the stock arm (right) and will help get more right-front negative camber.
This design is not found in the Ford Mustang. The coil spring is not part of the unit, but instead placed between a lower control arm and a bolt-in frame crossmember. The spindle is attached to the lower control arm with a ball joint. The strut itself bolts to the top of the spindle.
In a strut suspension, the upper mounting point of the strut moves outward as the chassis rolls to the outside during cornering. At the same time, the inner mounting point on the lower control arm is pulled inward, creating positive camber, meaning a need for static negative camber. One way to create negative camber is by moving the inner mounting point of the lower control arm higher on the chassis. But this is not legal in most Mini Stock classes. A more practical approach is to use a longer lower control arm at the right front.
Slotted holes in the front suspension crossmember were elongated. This piece was set ahead 3/16 inch to build in additional positive caster.
The Right-Front Corner
Positive camber at the right-front corner of the car will make the car push because the tread face of the tire is losing contact and reducing grip potential. The right-front tire develops its maximum grip during cornering when there is 1.0 to 1.5 degrees of negative camber. In order to achieve this, there must be higher amounts of static negative camber to start. With Mustang strut cars, the range is 6.0 to 8.0 degrees negative.
To set large degrees of negative camber, you need to use some caster/camber plates at the top of the strut tower. A longer right-front lower control arm will be beneficial. In the case of the '84 Mustang, a newer model ('89) control arm was used, which was about an inch longer. The longer control arm moves the lower mounting point of the strut outward 1 inch, which adds negative camber. The longer control arm also creates a wider swing arc, which reduces camber loss in bump travel. With the large amount of static negative camber, the car will push on corner entry.