MM offers two different types of rear lower control arms: Heavy-Duty and Extreme-Duty.
The difference between the two is the bushing configuration. The Heavy-Duty arms are suitable for cars with moderate horsepower levels that are not drag raced. We recommend the Extreme-Duty control arms for any car that is drag raced, and any car with the higher horsepower levels that come with superchargers, larger displacement engines, etc.
The MM Extreme-Duty Rear Lower Control Arms should be used for any form of drag racing. Repeated standing-start launches will eventually damage urethane bushings. Once again, MM's engineering expertise led to a unique design. These are the only control arms available that have spherical bearings at both ends, and yet do not require a coil-over conversion kit. MM's Engineering Team solved the problem of keeping the control arm upright, stable, and aligned with the chassis, when the springs are left in the stock location. We did this by designing a urethane "bumper" that is located around the spherical bearing, between the chassis and the end of the control arm. The bumpers are at only one end of the control arm, the chassis end. The bumpers keep the control arms aligned with the chassis, but are not so stiff as to cause an increase in suspension bind. The large Teflon-lined spherical bearings at both ends of the MM Extreme-Duty control arms completely eliminate the deflection allowed by urethane bushings during hard launches. This reduces axle windup, and allows the car to react more quickly. Unlike more commonly used bushing materials such as hard urethane, Delrin, or steel, spherical bearings allow proper articulation of the rear suspension. This eliminates torque box damage caused by suspension bind.
While many people have expressed concern about the potential for increased NVH when a control arm has spherical bearings at each end, we have found that the Extreme-Duty control arms are still suitable for street use. There is only a slight increase in noise and vibration over a stock control arm. The increase in noise is usually only noticeable in a car that has the stock, quiet mufflers. An increase in road vibration can be felt by rear seat passengers, but not in the front seats.
Muscle Mustangs and Fast Fords magazine tested the MM Extreme-Duty Rear Lower Control Arms in the January 2003 issue, as part of their test of the MM Street & Strip Box. They found the car's 60-foot times to be remarkably consistent, varying only .02 seconds over the course of 10 runs.