A 4-bar dirt Late Model rear...
A 4-bar dirt Late Model rear suspension is designed to have a large range of rear steer. The adjustability allows the racer the opportunity to make adjustments for changing conditions that occur on dirt surfaces. The attitude of the car on dry, slick tracks can be quite radical.
Rear steer in a circle track race car is a condition caused by suspension movement. Under the right conditions, rear steer can be beneficial and enhance performance. Under the wrong conditions, it can ruin your handling.
We do not necessarily need to know the exact amounts of rear steer our cars are subjected to, but we do need to have a solid understanding of what produces rear steer and what effect rear steer has on the handling in our cars.
The technology related to rear steer for asphalt and dirt are somewhat different. There are a few similarities, but many differences in how we evaluate and use rear steer for each group. Since we cannot lump them together, we will analyze them separately.
With the rear end pointed...
With the rear end pointed to the left of centerline, the steering will cause the rear of the car to want to run under the front end, causing a very tight condition, especially under acceleration.
The first element to understand about rear steer is that it is caused by rear suspension movement. As the rear corners of the car move, along with the controlling arms that locate the rear-end fore and aft, each side can move the wheel on that side forward or to the rear. Obviously, if both of the wheels did not move, or moved in the same direction by the same amount, we would have zero rear steer. When one wheel moves more than the other, we have a certain amount of rear steer.
Rear steer can either tighten a car or make it very loose. Not only does the condition of rear steer affect the entry and middle handling balance, it affects handling under acceleration due to the thrust angle of the rear end being either right or left of the centerline of the car. In dirt racing, it may be advantageous to incorporate a large amount of rear steer under certain conditions.
Tuning With Rear Steer
With the rear end pointed...
With the rear end pointed to the right of centerline, the car will be freed up going in, through the middle, and possibly loose off the corners, with the rear end wanting to run around to the right of the front.
To see if rear steer is an important tuning tool, we turned to some experts. Joe Garrison of GRT Race Cars and Mark Richards of Rocket Chassis both said having rear steer capability in a dirt car was critical. "Rear steer helps the driver get the rear end around on dry, slick tracks without having to break the rear tires loose," said Garrison. Richards added that "if the driver has to countersteer the car a lot, you need more rear steer."
Keith Masters of MasterSbilt Race Car Chassis said, "The reason chassis builders build 4-bar cars is because of rear steer. The entry is more important to design for than the exit as far as rear steer is concerned." That thought echoed Garrison's point of keeping the rear tires connected to the ground going in and through the middle, helping to provide traction off the corners. A smooth entry provides a faster exit.
The 3-link rear suspension...
The 3-link rear suspension system can produce rear steer in both directions. As the chassis moves down on the right side, the right-rear wheel will be moved back as the front mount approaches the height of the rear mounting point. As the front mounting point continues to move down, the right-rear wheel will be pulled forward. Normally, to produce a small amount of rear steer to the left on asphalt, we mount the front pivot point 1/3 of the total travel distance higher than the rear pivot.
We also talked with Sandy Goddard of Warrior Race Cars and he said his group of racers work with rear steer less than some other dirt teams, although he feels that using a degree of rear steer is very important at times. "Rear steer definitely helps get into the corners on dry, slick tracks," Goddard said, "but you can put too much rear steer into the rear suspension. A lot of guys take that to extremes."
C.J. Rayburn of Rayburn Race Cars, a driver as well as a car builder, echoed much of what the other buiders had to say. "We have always wanted rear steer," he said. "Rear steer is important in any design of dirt car." Rayburn builds the swing-arm type of rear suspension. Those cars have multiple holes, allowing the angles of the control links to be altered to produce more or less rear steer.
A rear end steered to the left of centerline will cause the thrust angle to be left of centerline and make the car tighter on entry and tighter on exit under acceleration. A rear end that is steered to the right of centerline causes the thrust angle to be pointed to the right of centerline and makes the car loose on entry and loose on exit under acceleration. Knowing these basics, we need to look at each type of racing to see how rear steer affects each type of car and how we might improve our performance using rear steer.
Asphalt Rear Steer
The front mounting block on...
The front mounting block on the 3-link suspension system is slotted vertically so you can adjust the angle of the trailing arm to fine tune the amount of rear steer. The car is very sensitive to changes in the trailing arm angle and the height of the front mount should be moved in small increments.
The asphalt racing surface provides a lot of traction, even on those flat "slick" tracks. Because there is very little slip of the tires on asphalt, the range of useable rear steer is very small. We never need our suspension to steer the rear end to the right of centerline on asphalt. It has been a practice for teams to align the rear end-and/or have it steer slightly to the right-to fix a tight mid-corner condition, but this goes into the category of crutches, and should not be necessary if the car is set up properly otherwise.
Asphalt stock cars have four predominant rear suspension systems and all of them produce some amount of rear steer. They are:
1. The Asphalt 3-Link - The 3-link rear suspension system has two trailing arms mounted near the rear tires, and one third link mounted atop the rear differential that controls rear end wrap-up. The trailing arms can be mounted parallel to the centerline of the car, or angled with the front mounts closer to centerline.
As the pull bar extends under...
As the pull bar extends under acceleration, the rear end rotates back, causing the rear wheels to move to the rear. The bottom pivot point is the back end of the trailing arms. If we mount the right-side arm higher than the left-side arm...
Rear steer in this system is caused by chassis movement, which can produce several secondary effects. Usually, the right-rear corner of the chassis moves more than the left-rear. On most flat- to medium-banked tracks, the left-rear moves very little. This has been confirmed by studying data from onboard computer systems during testing that show shock travel amounts in the turns. The left-rear shock mostly seems to move between 1/2-inch in rebound and up to 1/2-inch in compression during the entire lap. The right-rear shock shows from 3 to 4 inches or more of travel, depending on the spring rates used.
On most asphalt 3-link cars, the right-rear trailing arm mostly controls rear steer due to body roll. We usually need to position the angle of the trailing arm so that the front mount is higher than the rear mount by roughly one third of the distance that the front mount will move down during cornering. The variation of height for the right-rear trailing arm is very small. Changes in the height of the front of the trailing arm as small as a 1/4-inch can be felt by the driver.
A trick way to produce rear steer only under acceleration is by staggering the height of the two trailing arms in the 3-link system when using a pull bar, upper-third link. If we mount the left-side trailing arm lower than the right-side trailing arm, then as the rear end rotates under acceleration due to the pull bar extending, the left-rear wheel will move rearward more so than the right-rear wheel, causing rear steer to the left to a small degree. This promotes forward bite without causing the car to be tight on entry or in the middle of the turns.
...the radius on the left...
...the radius on the left side will be longer from the lower mount to the axle, causing the left-rear wheel to move farther rearward than the right-rear wheel. This causes a slight amount of rear steer to the left, the exact amount depending on the difference in height of the mounting points and the amount of third-link travel.
Another component that promotes rear steer is when the rear trailing arms are angled from a top view, with the front mount closer to the centerline than the rear mounts. With this design, lateral movement of the rear end causes rear steer. If the Panhard bar is mounted on the right side of the chassis and level to the ground, the rear end will be pulled to the right, and will steer to the left when the chassis moves during cornering. This is caused by the rear end swinging around the instant center, created from projecting lines through the arms to the front until the lines meet.
2. The Truck-Arm System - The truck-arm system has been adapted from the design for a 1964 Chevy truck, and is used on many Late Model Stock cars, as well as the three premier divisions of NASCAR (Craftsman Trucks, Busch, and Winston Cup). These systems only steer to the left and have a limited amount of steer. The roll of the chassis and the movement of the Panhard bar are the two components that influence the amount of steer in these systems.
As far as the geometry related to rear steer is concerned, this is an ideal system. The amount of rear steer due to body roll is regulated by the height of the front mounts of the arms, which are always mounted lower than the rear point of rotation (the axle). Rear steer amounts-due to the Panhard bar angle-are regulated by the angle. A downside to using the truck arms is not related to steering characteristics, but due to a narrow spring base when the springs are mounted directly on top of the truck arms, creating a narrow spring base in the rear.
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.
If the bars on a 4-bar car...
If the bars on a 4-bar car are set all the way to the top of the mounts, the rear end will steer to the right quite a bit as the car rolls up on the left side...
As the left side of the car travels up, the front of both of the trailing arms are angled upwards so that the left-rear tire tries to drive up under the chassis, loading the left-rear tire considerably. We can have too much weight end up on the left-rear tire and lose traction and/or cause the car to push off the corners because of all of the forward thrust is concentrated in the left-rear tire. In racing, we have the maximum amount of traction from a pair of equally loaded tires on the same axle. Excess loading of either the left- or right-rear tires decreases traction in most cases.
Dirt Track Aero
One theory related to having the attitude of the car sideways involves the use of the aerodynamic aspects of the car. If you look at a modern dirt Late Model, the sides are made up of big, flat panels similar to the sides of a Sprint Car wing. We can see the effect of the Sprint Car wing in the turns-as the cars actually roll left due to the pressure differential developed on the flat wing sides-as the cars go sideways at a high speed. On a Late Model, this air pressure difference may help keep the car on the track by virtue of having the car go through the air at an angle, causing both a high pressure on the leading (right) side and a lower pressure on the trailing (left) side. Masters added, "Any time we can press air against the sides of the car, we can help the car to turn." His company has experimented with dirt track aero in the past.
...and down on the right side....
...and down on the right side. This helps the driver get the car turned to prepare to exit the corner, and keeps the rear tires in contact with the track surface under extremely dry, slick conditions.
The reason rear steer aero might be important comes on dry, slick surfaces, where the tires do not grip well. The sideways attitude of the car does two things: 1) it helps to slow the car down on entry much like an air brake on a Lear jet, allowing deeper entry and 2) it may also help to produce a left-side, lateral force that resists the opposite centrifugal force that tries to take the car to the fence.
The Z-link System
The Z-link rear suspension, or swing arm as it is also known, is another system used on dirt cars. Compared to the 4-bar cars, it has more limited adjustment for rear steer and historically has worked well on the tighter and more highly banked race tracks because the rear end is pointed more straight ahead. Some manufacturers have added multiple mounting points on the front and rear chassis mounts. This helps make the rear steer characteristics more adjustable for the changing conditions. Richards added that today, the Z-link or swing-arm suspension can have nearly as much rear steer as the 4-link, without the excessive loading of the left-rear tire.
Spring Motion Ratio
If the bars on a 4-bar car...
If the bars on a 4-bar car are set in the correct holes, the movement of the top and bottom mounts at the rear end will compensate fore and aft, resulting in zero rear steer. This is best for tight, wet tracks where we cannot use any rear steer and we need for the thrust to be pointed straight ahead.
Most of the Z-link systems utilize a spring mounting system that attaches the coilover spring to the front link. This produces a motion ratio that causes the spring to move less than the chassis per degree of roll and/or inch of squat. Therefore, the rate the car feels is much less, usually around 50 percent, than the actual installed spring rate. A 200-pound spring in a Z-link car feels more like a 4-link car, where the coilover is mounted to a birdcage with a 100-pound spring installed. The significance of this, for the purpose of this article, is that the chassis travel in a Z-link is enhanced compared to the 4-link suspension when using the same installed spring rate, and this causes quite a bit of chassis travel and related rear steer. So, teams need to take this rate difference into account.
There are reports from the past of a team winning a race using four 400-pound springs on a Z-link type of car. The front of the car felt the actual 400-pound rate while the rears "felt" like 200 pound springs. The track in this case was banked and had a lot of grip. The car was set up and driven more like an asphalt car and it was fast. Those conditions rarely exist on dirt.
Tuning With Rear Steer
A Z-link suspension system...
A Z-link suspension system uses a link extending from the rear end forward to the chassis, and one from the rear end rearward to a mount on the chassis. Most designs use very few mounting holes that would enable the team to adjust for the amount of rear steer in those cars. Note the spring is mounted directly on the front link.
We should learn to read the conditions of the dirt tracks and tune the amount of rear steer-less for tacky and wet conditions, with more rear steer as the track gets slicker. On extremely dry, slick conditions, use lots of rear steer to the right. This is accomplished by causing the right-rear wheel to move back and the left-rear wheel to move forward as the car rolls. Soft springs, a left chassis-mounted track bar, and easy-up shocks on the left side all promote the body roll that produces rear steer to the right.
On asphalt, do not make large changes to components that influence rear steer. Make small adjustments if you feel a need. When you find the correct amount of rear steer, stay there and tune the handling with the other components. When racing on dirt, watch the conditions and be prepared to make changes accordingly, not just to the setup, but also related to rear steer. That way, the car will stay as fast and balanced as it can be throughout all of the changing conditions.