Part One - Dirt Designs And Evolution
Rear steer in a circle-track car is an effect caused by suspension movement. Dirt racing generates more chassis movement than what we see on asphalt. Under the right conditions, RS can be beneficial and enhance performance. Under the wrong conditions, it can ruin your handling. Much of the difference depends on the track conditions, which we all know change quite a bit on dirt tracks. We don't 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 RS and what effect it has on the handling in our cars.
Here we see the extreme, lots...
Here we see the extreme, lots of rear steer and lifting the left front wheel off the track. A four-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 the changing conditions that occur on dirt surfaces. The attitude of the car on dry slick tracks can be quite radical.
The first and foremost fact related to RS is that it's caused by rear suspension movement. As the rear corners of the car move, along with the controlling arms that locate the rearend fore and aft, each side can move the wheel on that side forward or to the rear. Obviously, if both of the wheels didn't move or moved in the same direction by the same amount, we'd have zero rear steer. It's when one wheel moves more than the other that we have rear steer.
Rear steer can either tighten a car or make it very loose. Not only does RS affect the entry and middle-handling balance, it affects the handling under acceleration due to the thrust angle of the rearend being either right or left of the centerline of the car. In dirt racing, it may be advantageous to incorporate a larger amount of rear steer under certain conditions.
A rearend that is 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 rearend that is steered to the right of centerline causes the thrust angle to be pointed to the right of centerline and makes the car looser on entry and looser on exit under acceleration. Knowing these basics, we need to look at each type of racing to see how RS affects each type of car and how we might improve our performance using RS.
What Do The Experts Say
Is rear steer an important tuning tool? Opinions have changed for some. Five years ago we asked Joe Garrison of GRT Race Cars and Mark Richards of Rocket Chassis if RS was important in dirt racing and both said that having RS capability in a dirt car was critical. "Rear steer helps the driver get the rearend around on dry slick tracks without having to break the rear tires loose," Joe said. Mark said that "if the driver has to counter steer the car a lot, you need more rear steer."
Keith Masters of MasterSbilt Race Car Chassis told us that "the reason chassis builders build four-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 echoed what Joe Garrison said in that keeping the rear tires connected to the ground going in and through the middle helps to provide traction off the corners too. So a smooth entry provides a faster exit.
At that time, we also talked with Sanford Goddard of Warrior Race Cars and he told us that 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. He said, "Rear steer definitely helps get into the corners on dry slick tracks, 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, has been a driver as well as a car builder and echoed much of what the other builders had to say in an earlier interview. "We have always wanted rear steer," he said. "Rear steer is important in any design of dirt car." C.J. builds the swing arm-type of rear suspension that typically produces less rear steer than the four-bar cars.
Today, we see a different attitude among many dirt car teams and manufacturers. A trend that has taken hold is one where the car runs flatter, has less body roll, and where the rear bars are positioned for less rear steer. That's the evolution we alluded to in the title of this article.
With the rearend pointed to...
With the rearend 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. This never happens on dirt. Typically, dirt cars are hard to turn anyway and this would only make the situation worse.
With the rearend pointed to...
With the rearend pointed to the right of centerline, as typically happens with Dirt Late Model cars, the car will be freed up going in, through the middle, and possibly loose off the corners with the rearend wanting to run around to the right of the front. This is why you see dirt cars sideways. It's not that they are sliding the rear tires, it's the rear steer moving the rear to the right in relation to the front tires.
If you notice, the No. 0 car...
If you notice, the No. 0 car of Scott Bloomquist is driven and set up for a more straight ahead attitude as opposed to the No. 45 car. This has been the design for Scott for more than a few years. He discovered long ago the advantages of less rear steer and more balance in the setup.
I was at Volusia Speedway Park recently for the DIRTcar Nationals and observed the top cars in the Dirt Late Model divisions. All of the top cars kept the left front tire on the track. Most of the top cars exhibited much less body roll and hike and some looked more like asphalt cars, in that they drove more straight ahead and flat.
Rob, our editor, observed at East Bay Raceway Park the week before that a fast line around that track was up high and the cars drove through there with a very straight ahead attitude and were very fast. I spoke with Billy Moyer, often referred to as Mr. Smooth, and he has quite a bit of disdain for the trend of hiking the left front off the track. He told me that he would never do that again.
Types Of Rear Suspension On Dirt
There are four types of rear suspensions used in most dirt cars that are significant to study regarding rear steer. Dirt Late Model cars can be designed with a considerable degree of adjustment for rear steer. Many teams use varying amounts of RS to adjust to constantly changing track conditions, a product of variations in moisture content that is so common in dirt racing. Other teams may just stick with a fixed location for the mounts in the rearend and adjust handling with other means.
One of the reasons why a car will test fast in practice, qualifying, and maybe the heat races, but when the track changes be out to lunch come feature time, is a result of improper rear steer. Here's how each system functions and how they may or may not be adjusted for the "degree" of rear steer.
The Standard Four-Bar System
The four-bar suspension is highly adjustable and can be made to steer both directions. The rule about never steering the rearend to the right on an asphalt car does not apply on a dirt car. There are times when we may want the rear to steer to the right.
That being said, the trend over the past 5 years has been to set up the dirt cars with much less rear steer. With the introduction of meaningful technology on the front moment center and its overall role in front-end dynamics, many of the Dirt Late Model and Modified builders are designing better front ends that help the car to turn. This reduces the need to introduce rear steer to get the car to turn.
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 RS can be extreme in magnitude.
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. Now, in more recent years, teams have been setting up the car so that the left side rises up quite a bit.
The left rear suspension is designed so that when that corner rises up, the arms are angled so that the LR wheel is pulled forward, toward the driver. This produces quite a bit of rear steer to the right and the rear of the car moves 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.
The high angle of the bars...
The high angle of the bars on the left side cause the LR wheel to move forward as the left side of the chassis rolls us through the corners. This can be extreme, whereas the LR tire moves well forward in the wheelwell. We are seeing less and less of this attitude in the top divisions of dirt racing.
If the bars on a four-bar...
If the bars on a four-bar car are set all the way to the top of the mounts on the right side, then as the car rolls down on the right side, the right rear wheel will move back. This, in conjunction with the left rear wheel moving forward will cause the rearend to steer to the right quite a bit. This can help the driver get the car turned to prepare to exit the corner and keep the rear tires in contact with the track surface under extremely dry slick conditions.
If the bars on a four-bar...
If the bars on a four-bar car are set in the correct holes, the movement of the top and bottom mounts at the rearend will compensate for fore and aft movement resulting in zero rear steer. This is best for tight, wet tracks where we don't need rear steer and where we need the thrust to be pointed straight ahead.
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 Keith Masters. He says, "high left rear loading does not increase traction." As the left side of the car travels up, the front of both of the trailing arms are angled upwards so that the LR tire tries to drive up under the chassis loading it considerably. This is a trend that now sees much less use.
We can have too much weight end up on the LR tire and lose traction and/or cause the car to push off the corners because all of the forward thrust is concentrated in the LR tire. In racing, we have the maximum amount of traction from a pair of tires on the same axle if they are equally loaded. Excess loading of either the LR or RR tires decreases traction in most cases.
The Z-link System
The Z-link rear suspension, or swing arm as it's sometimes called, is another system used on dirt cars. Compared to the four-bar cars, it has more limited adjustment for rear steer and historically has worked well on the tighter and more highly banked racetracks because the direction the rearend is pointed is 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. Mark Richards told us that today, the Z-link suspension can have nearly as much RS as the four-link, without the excessive loading of the LR tire.
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 that the car feels is much less, usually around 50-55 percent, than the actual installed spring rate.
A Z-link suspension system...
A Z-link suspension system uses a link extending from under the rearend forward to the chassis and one from above the rearend rearward to a mount on the chassis there. Most designs use very few mounting holes that would enable the team to adjust for high amounts of rear steer in those cars. Note that the spring is mounted directly onto the front link.
A 200-pound spring in a Z-link car feels more like a four-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 four-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 wheel 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 car. The front of the car felt the actual 400 pound rate while the rear "felt" like 200-pound springs. The track in this case was banked and had a lot of grip. So the car was set up and driven more like an asphalt car and it was fast. Those conditions rarely exist on dirt.
Along with the chassis adjusting...
Along with the chassis adjusting holes, the birdcages have numerous holes so you can make changes to the link angles to regulate the rear steer. We used this combination of positions to design our 2008 Dirt Late Model for minimal rear steer. The car was very easy to drive and had plenty of forward bite.
The Metric four-link System
The metric four-link is a widely used system that comes with some models of stock automobiles. It has become the standard for Street Stock-type dirt cars. It uses four links, as the name implies, that aren't 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.
The leaf-spring rear suspension system locates the rearend fore and aft as well as laterally using the leaves. There can be a small amount of RS as the chassis rolls and squats, but it's both minimal and mostly fixed as far as adjustability. The advantage of this system is that it keeps the rearend squared up and the thrust under acceleration straight ahead, if that's what is needed.
The height of the front eye of the leaf spring in relation to the axle centerline determines the amount of rear steer. Spacing the leaf different heights from the axle tube can be a way to change the height relationship of the front eye, which will change the amount of rear steer for this system.
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 rearend 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.
Tuning On Dirt With Rear Steer
We should learn to read the conditions of the track and tune the amount of rear steer. We need much less, maybe none, for tacky and wet conditions and then we can add some RS as the track gets slicker. On extreme dry slick conditions, many teams use a lot of RS to the right. This is accomplished by causing the RR wheel to move back and the LR wheel to move forward as the car rolls and hikes up on the left side. Soft springs, a high center of gravity, a left chassis-mounted track bar running at a high angle, and easy up shocks on the left side all promote the body roll that produces rear steer to the right.
The metric four-link suspension...
The metric four-link suspension has two links above the rearend and two links below the rearend. They are angled from a top view to prevent the rearend from moving side to side as the chassis rolls. With this system, the rearend stays located by virtue of the opposing angles of the upper and lower links. The roll center in this system is very high compared to other systems. To get the rear to roll, teams use a much softer right rear spring than the left rear. Using a stiff right rear spring will cause the car to be very loose.
What we need to think about is, do we need a lot of rear steer? In my observations, and this has been fortified by comments I've received from some of the top drivers in Dirt Late Models, a flatter and more straight ahead running car will do very well in 85 to 90 percent of the conditions teams encounter as they travel the circuit. There are times when changes must be made in small amounts for drier conditions and times when the tracks are so dry and slick the teams must go to extremes.
For most events and for most of the race day, minimal rear steer will produce a more consistent lap. Some tracks will yield only so much speed in the turns no matter what you do, but at the end of the day, most will agree that the driver who makes the least mistakes usually has the best chance of winning. So, consistency counts for a lot, just ask Mr. Moyer. 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.