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.
Note the upper spring bucket...
Note the upper spring bucket with adjustable jack screw. This is a very easy way to adjust the weight distribution in your Stock class race car. Just be sure your rules allow these types of adjusters. Most teams use internal adjusters that don't show.
Most Metric cars I've evaluated have rear RC's up around 14.5 inches off the ground. If the car's center of gravity was say 16.5 inches (around average for an asphalt car that has been lowered), there would only be a 2-inch moment arm to try to roll the car. Compare that to an asphalt Late Model car with a rear Panhard or J-bar and a 10-inch rear RC height and a 15.5 inch CG height.
The moment arm is 5.5 inches for the Late Model and is much easier to set up. If we can lower the Metric RC to 13 inches, we would increase the moment arm length in the rear to 4.5 inches and that's a huge improvement in compliance. In short, you'll get more rear grip by lowering the rear RC.
Setup Changes For The Metric four-link
Even if we can lower the rear RC to around 13 inches, that's still a high rear RC. More has to be done to help the rear roll to match the front. So, in most cases, we need to run a softer spring in the right rear. A spring split of 250-lb/in spring in the left rear and 200-lb/in spring in the right rear is common.
If you find that the left rear is the coolest tire on the car, then you have a setup balance problem where the rear is too stiff. Lowering the rear RC and softening the RR spring will help solve the problem. There's a happy medium between running just the right soft right rear and going too soft.
Pay attention to the left front tire temperatures versus the left rear. They should be matched up within a couple of degrees of average temperature across the tread. When the LF is too cold, the RR spring is too soft. If the LR tire is too cool, then the RR spring is too stiff.
The Metric four-link rear suspension is here to stay for a while. So, we must make due and set up these cars the best we can. Understanding the relationship between the rear roll center on these cars and the setup will help us find a more balanced setup. The car will turn better and we will have more bite off the corners.
If you need to know the height of your Metric four-link rear roll center, then send an email and I will send you a free calculator to use. Send your request to firstname.lastname@example.org.
A top view sketch of the layout...
A top view sketch of the layout of the Metric four-link shows the reverse angles of the upper (inner links) and the lower (outer links) links and how the extension of a line through the pivot points will intersect at a point we call the Instant Center. These points act like hard points and restrict the lateral movement of the sprung mass or chassis similar to a Panhard bar in a three-link suspension.
We need to design our suspension...
We need to design our suspension so that the rear roll center is low enough to create a longer moment arm. This promotes roll in the rear and helps us balance the setup so the car is dynamically balanced. We need to force the two ends to work to achieve the same roll angles in order to have a balanced setup.
A side view of the Metric...
A side view of the Metric four-link shows the approximate locations of the two instant centers, one for the upper links and one for the lower links. It's the average height of the two IC's that determines the rear roll center height for this system.