3. The Metric 4-Link System
The pull bar, upper-third...
The pull bar, upper-third link allows the rear end to rotate under acceleration. The movement of the rear end rearward can be utilized to produce rear steer to the left only while under acceleration. That way, the car is correctly aligned at the rear during entry and through the middle of the turns. Rear steer to the left helps promote bite on flat and slick asphalt tracks.
- The metric 4-link is a widely used system that comes with some models of stock automobiles. It uses four links, as the name implies, that are not parallel to the centerline of the car. The top links are angled from a top view with the front pivots wider than the rear pivots. The lower links are angled from a top view with the front pivots narrower than the rear pivots.
With this system, the rear end stays located by virtue of the opposing angles of the upper and lower links. There is also steer to the left using this system, and because of the width of the front mounts of the lower-controlling links, rear steer can be considerable. Under most current rules, there is no adjustment for amounts of rear steer with these systems.
4. Leaf-Spring Systems - The leaf-spring rear suspension system locates the rear-end fore and aft, as well as laterally using the leafs. There can be a small amount of rear steer as the chassis rolls and squats, but it is both minimal and mostly fixed as far as adjustability. The advantage of this system is that it keeps the rear end squared up and the thrust under acceleration straight ahead, if that is what is needed.
Dirt Rear Steer
Some 3-link rear suspensions...
Some 3-link rear suspensions are built with the trailing arms angled from a top view with the front mounts located closer to the car's centerline than the rear mounts. This creates some amount of rear steer if the rear end moves laterally as the chassis rolls. This type of rear steer will tighten the car if the rear end moves to the right as the car rolls. The downside to this rear steer is that it will tighten the car in the middle of the turns, as well as off the corner.
There are four types of rear suspensions used in most dirt cars that are significant to study regarding rear steer. Characteristics of the metric 4-link, one of those four, are the same as discussed under the heading related to asphalt cars. Let's expand on the other three systems.
Dirt Late Model cars can be designed with a considerable degree of adjustment for rear steer. Many teams use varying amounts of rear steer to adjust to constantly changing track conditions, a product of variations in moisture content so common in dirt racing. Other teams may just stick with a fixed location for the mounts in the rear end and adjust handling with other means.
One of the reasons a car will test fast in practice, qualifying, and maybe the heat races, but then be out to lunch when the track changes come feature time, is a result of improper rear steer. Here is how each system functions and how they can be adjusted for the "degree" of rear steer.
The Standard 4-Bar System
The metric 4-link suspension...
The metric 4-link suspension has two links above the rear end and two links below the rear end. They are angled from a top view to prevent the rear end from moving side to side as the chassis rolls.
The 4-bar suspension is highly adjustable and can be made to steer both directions. The rule about never steering the rear end to the right on an asphalt car does not apply on a dirt car. There are times when we definitely want the rear to steer to the right.
Depending on the angles of the trailing arms or bars, each rear wheel can be made to move to the front or rear. The roll angles and vertical movement on a dirt car can be very pronounced. With so much movement, we can plan out our rear steer just about any way we need it.
The bars can be mounted on one side of the car so that only that wheel moves to create the rear steer. If both sides are configured to move in opposite directions, then rear steer can be extreme.
The leaf-spring suspension...
The leaf-spring suspension system usually has fixed mounts in the front and therefore no adjustment for amount of rear steer. The rear end stays mostly square in the car throughout the lap. The feedback we hear from the racers is that the leaf-spring rear suspension is good on tacky or wet tracks.
On a tacky track, the team would do well to limit rear steer on both sides of the car. These conditions call for a driving line that is more straight ahead. When the track goes slick, especially dry slick, rear steer is needed. In the past, drivers would set up the car for exit off the corners, throwing the car sideways by breaking the rear tires loose. In more recent years, teams have been setting up the car so that the left side raises up quite a bit.
The left-rear suspension is designed so that when that corner raises up, the arms are angled, pulling the left-rear wheel forward towards the driver. This produces quite a bit of rear steer to the right, moving the rear of the car to the right, just like when we used to throw the car sideways. The difference is that now we can maintain rear traction-having never broken loose-and the car is angled somewhat sideways, and pointed in the right direction to get off the corner.
How Much is Too Much?
There are limits to how far we go in steering the car this way. One disadvantage is pointed out by Masters. "High left-rear loading does not increase traction," he said.