Adjustable Four-Link Arms
A trigulated four-link rear suspension is very effective at controlling the pinion angle of the rearend during articulation since both the upper and lower sections are captured. For street cars at stock ride height the factory setting are generally fine.
The design of the four-link controls the pinion angle through suspension travel, which means that it is dependent on the vehicle's ride height. At stock or slightly lower ride height, non-adjustable arms will suffice, however, the lower the ride height the more the pinion angle is altered, which will alter the traction under acceleration. Adjustable control arms can compensate for this by allowing the length to be altered to bring pinion angle back into check. Racers also use adjustable arms to dial-in exactly the angle they want in the driveline when the car squats to accelerate off the starting line.
For handling, extremely lowered cars that also have a lateral location device installed may benefit from replacing the bushings at one end of the arms with a Heim joint since they will allow full articulation of the suspension without bushing bind.
Panhard Bar & Watt's Link
Both Panhard bars and Watt's links are lateral location devices used on solid-axle cars. While leaf springs and four-link rear suspensions work well for straight-line acceleration and mild street use, they lack positive location for the axle under high lateral loads experienced when cornering.
Panhard bars are simple devices that are literally nothing more than a straight rod with a bushing at either end that connects one side of the axlehousing to the opposite side of the chassis. The bar prevents sideways movement of the axle while still allowing articulation. Though they work well, Panhard bars inherently induce some suspension bind and slightly differing characteristics in left versus right curves since they cause the suspension to travel in an arc when moving up or down. To minimize the arc, Panhard bars are always designed to be as long as possible. This can result in an arc that can be as little as .25 inch over 3 or 4 inches of travel.
Watt's links have the same principle goal as a Panhard bar, but the locating bar is split by a central bellcrank between the chassis and axle mounts. The crank is mounted to either the rearend housing or an independent structure, and rotates during suspension articulation and allows the rear suspension to move without bind. This is the ideal lateral location device for solid-axle performance irrespective of spring choice, but packaging can be a challenge and impractical in some cases.
A car's handling, steering, braking, and ride quality are all dependent upon the bushings being tight and within spec. All suspension and steering systems are interconnected through bushings in some way, and slop at any point in the chain will result in decreased vehicle dynamics. As a matter of fact, just replacing control arm, sway bar, and body bushings (if your car has them) can make a dramatic improvement in how a car drives with no other new parts. If you're on a tight budget, start here.
As far as material, we're advocates of using increased durometer polyurethane bushings at most points since they offer less deflection while keeping NVH levels acceptably low. The less deflection in the bushings, the more precisely the components can do their jobs; that's why you'll see Heim joints and solid bushings in race cars. We wouldn't recommend the full solid bushing route on a street car since you'll feel every pebble, but you can safely opt for solid body bushings and near-solid Delrin bushings without creating excessive NVH.
Torque arms are another traction device used to control the motion and wrap of the rearend in a solid-axle car. Every vehicle has an imaginary point where the suspension links would converge if they were extended forward, known as the instant center. Dialing in the location is key to both straight-line acceleration and braking. Whereas the instant center can move around on three- or four-link rear suspension, torque arms have the advantage of a fixed instant center location and a very long swing arm length.
The length of the torque arm matters, because it directly affects the antisquat of a vehicle. A shorter torque arm increases the antisquat, which provides more bite during acceleration, but for a road race suspension it's possible to have too much antisquat, which can lead to rear brake hop under threshold braking. In general, a torque arm suspension allows a significant amount of antisquat to be designed into the suspension while still maintaining good roll steer characteristics.
Though generally paired with a coilover conversion, a torque arm actually works very well for leaf-spring suspensions as well. Under-links on leaf springs only help significantly with acceleration, while overriding links help with braking. A torque arm does both by not allowing anywhere near the amount of axlewrap possible from short three- or four-link designs. Additionally, since the arm is centrally mounted to the gear case of the rearend, there is very little pinion-angle change or driveshaft slip during suspension articulation.
Subframe Connectors & Chassis Braces
A unibody chassis is a great way to engineer a vehicle, and it also allows easier integration of impact-absorbing crumple zones for safety, however, in the early days of such technology, the elimination of a full frame without properly reinforced substructures created very flexible chassis. That loss of rigidity results in twist in the midzone, compromising performance and control on all fronts.
Luckily there is a brutally simple solution that works in all cases: subframe connectors. Subframe connectors tie the front and rear of the unibody together with a weld-in or bolt-in set of steel rails that effectively become frame members that reduce chassis twist (see “Weld-In Subframe Connectors," p.70). Subframe connectors can be purchased for many applications, or easily created with standard 2x3 rectangular stock and a little cutting. When practical it's recommended to keep the connectors as straight as possible and cut the floorpans to accommodate them. This makes them a true structural element of the chassis.
Engine compartment braces function on the same principle of eliminating deflection in an area with little reinforcement. Different platforms have differing needs for braces. For example, shock tower–equipped cars like '60s Fords benefit from tower-to-cowl braces that prevent both tower sag and movement from hard driving. Subframe-equipped GM products benefit from down bars that connect to the upper part of the firewall and extend forward to the end of the factory subframe.