A four-link Dirt Late Model...
A four-link Dirt Late Model rear suspension is designed to have a large range of rear steer in either direction. This full range of adjustability allows the racer the opportunity to make adjustments for the changing conditions that occur on dirt surfaces. The attitude of the car on dry slick tracks can be quite radical and we’ll study why this might work.
The parallel four-link rear suspension is common to Dirt Late Model, Dirt Modified stock cars, and Sprint Cars. The original purpose for using the four-link suspension was to have a suspension system that produced very little, even zero, rear steer as the chassis moved vertically. Racers, being true to their nature decided to experiment with various angles in the four-link and found that zero rear steer was not the most efficient under all circumstances.
Today, we have various schools of thought on where to position the links on a four-link system and many more theories on why. Let’s examine what happens with the various changes and look at the big picture to try to understand what is really happening to our cars. Even though the current trend among top Dirt Late Model racers is to minimize the steering characteristics of the rear suspension, there are times when even these teams must get radical.
Basic Four-Link Designs
Example 1: If the right side...
Example 1: If the right side of our chassis travels up or down in the turns, we’ll see near zero rear steer by positioning the links as shown (reverse the vertical arrows and the horizontal arrows will be reversed too). The same would be true for the left side. The vertical movement of the chassis will cause the top and bottom of the birdcage to move in different horizontal directions which will result in minimal fore and aft movement of the rearend.
We’ll be analyzing two basic designs of the four-link rear suspension, the “standard” four-link, that we’ll call a four-link, where both links are forward of the rearend axle tube, and the “Z” link where the top link is rearward and the bottom link is forward. Both of these designs can be positioned to produce near zero rear steer. The four-link design can be made to produce somewhat more rear steer than the Z-link.
One of the big differences between these systems is the effect of weight jacking and forced loading of the left rear tire with the four-link system. As we introduce more rear steer which moves the LR wheel forward, the angle of the bars is such that the wheel is forced down and in trying to lift the chassis, load is transferred onto the LR tire. This loading is desired by some in order to produce more bite under dry and slick conditions.
Example 2: Just as in the...
Example 2: Just as in the Z-link example, movement of the chassis with these settings will produce minimal rear steer in the four-link system.
If you look at the way the car is situated when the rear is severely steered to the right, the LR tire is pointed more to the middle of the front tires and is driving from the middle of the car. Because of the added loading of this tire, most of the rear weight is on this tire. The RR tire is helping to locate the rear of the car, but does little to drive it off the corners.
Both of the suspension types are usually attached to a birdcage that isn’t locked to the rear axle tube and where the rearend is free to rotate. A separate structure is attached to the rearend to control rotational movement of the rearend upon acceleration and braking. This could be a “third” link or pull bar, similar to that used on a three-link suspension, a lift arm that runs forward and is attached well in front of the rearend or a combination of several systems.
Here we see a car that is...
Here we see a car that is raised up on the left side, and there appears to be a lot of rear steer as the left rear tire is positioned well forward in the wheel well. This would indicate that the links are positioned for maximum rear steer on this four-link car. The front wheels are not turned because the rear is steering this car around the radius of the turn.
If the link brackets were mounted solid to the rear axle tube, then as the car rolled in the turns, there would be a significant amount of binding going on because the birdcages would be moving different amounts and possibly in different directions. The suspension would be trying to twist the rearend as each axle tube would be rotated separately.
If we change the angles of the links so that one side of the car produces more fore/aft movement at the birdcage, we cause that end of the rearend to move in a direction that will cause the rear of the car to steer away from straight ahead. This is called rear steer and most of what is used for dirt racing is steering to the right.
Under some conditions, rear steer is less desirable, especially on hard and tight dirt tracks that act more like asphalt than dirt. Rear steer on dirt tracks that are slick is not only acceptable, but downright necessary under certain conditions.