The Metric four-link suspension design is a bit different than most other rear suspensions. This configuration is common in General Motors cars built in and around the 1980's as well as some Ford cars, such as the Thunderbird, from the same time period. Because these cars are being used in great numbers in the Stock classes of racing here in the U.S., we need to have a better understanding of the design and how it relates to the setups.
First of all, the design is intended to solve several problems. It locates the rearend fore and aft, it reduces rear steer when the chassis rolls, it prevents axle wrap-up when braking or accelerating, and it provides a relatively stable roll center location.
The stock Metric four-link...
The stock Metric four-link rear suspension is designed to control lateral movement of the rearend and prevent axle wrap-up when braking and accelerating. The reverse angles of the upper links and lower links create a bind effect that hold the rearend steady. The result of the design is a very high rear roll center.
Conversion to Race Car
When we convert a Metric four-link suspension to a racing application, we need to be aware of several important factors. First, we don't want to restrict the movement of the arms too much. Adding neoprene bushings to all of the connection points will unnecessarily restrict movement and cause the chassis to be stiff in roll. We can add the stiffer bushings to most of the points, but leave several, one at the top and one at the bottom, as a rubber bushing to absorb the torque created in roll.
We also need to establish a ride height that will both be appropriate for our type of racing and also help to establish the best roll center height. The RC height will move with vertical movement of the rear of the car. As the car is lowered, the rear RC will generally move down. So, it might be the best for asphalt racing to run as low as possible or as much as the rules allow to create a lower center of gravity.
The rear connection for the...
The rear connection for the lower links at the rearend are very close to the axle tube. This prevents raising the link at this point. If changes to the link angle are to be made, they must be done at the front chassis mount.
We also might want to change the locations of the chassis and/or rearend mounts for the links. We can lower the rear RC by making changes to the mounting points. Check with the rules to see if it's even mentioned. Subtle changes often go unnoticed, especially when they're hard to see without jacking the car up and crawling underneath. Remember, we're still using stock components, we're just making small relocations of those points.
The Metric four-link Roll Center
The metric roll center is quite high compared to other rear suspension systems, such as the three-link with Panhard bar, the leaf spring system, and the Watts link. This is because of the construction using angled, from a top view, arms that converge to points called instant centers. There is one IC for the upper links and one for the lower links. The upper IC is to the rear of the rearend and the IC for the lower links is well in front of the rearend. Both of these ICs act like hard points of resistance to lateral movement of the chassis. It's the average height of the instant centers that determine the roll center height.
Aftermarket spring holders...
Aftermarket spring holders can be welded to the axletube to position the stock springs inline with the upper spring bucket. This is usually done after the rearend has been mounted with the correct pinion angle and link angles.
If we can raise the upper front link mounts, we can lower the rear RC by lowering the upper IC. In the same manner, we can lower the rear mount of the upper link and lower the upper IC.
For the lower links, lowering the chassis mounts at the front of the link lowers the rear RC. Raising the rear of the lower links also lowers the rear RC, although in reality this is very hard to do because the links are already mounted close to the rearend axle housing.
Why Lower The Rear RC?
A high rear roll center causes a short moment arm in the rear suspension and will stiffen the rear suspension similar to installing very stiff springs. The rear won't desire much roll and there will be excess load transfer to the outer wheel and tire. This will loosen the car, and that's undesirable, especially on dirt. So, we either lower the rear RC or soften the springs, especially the right rear spring, or preferably do both.