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 Heavy-Duty Rear Lower Control Arms utilize specially designed 3-piece urethane bushings at the chassis end. These bushings have a hard center section to prevent fore and aft deflection, and softer outer sections to allow the angular motion necessary to prevent bind. This design, with softer outer urethane sections, prevents torque-box damage due to binding of the lower control arms. At the axle end, we use a high quality Teflon-lined spherical bearing to precisely locate the rear axle. Unlike the other bushing types listed above, a spherical bearing allows freedom of motion for both pivoting and angularity. As with our 3-piece urethane bushings, this freedom of motion improves handling over other styles of bushings, and prevents damage to chassis components. We place the 3-piece urethane bushing at the chassis end, rather than at the axle end, for two reasons. First, this reduces the amount of noise, vibration, and harshness (NVH) that is transmitted into the chassis. Second, because the spring is mounted to the control arm and the chassis, it is best to keep the control arm and chassis from getting too far out of line with each other. If the spherical bearing is placed at the chassis end of the control arm and the urethane at the axle end, the control arm will tend to stay in alignment with the axle. This would cause the spring to arc sideways with body roll, and change the effective wheel rate.