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.