Dirt Car Race Prep

Ten Critical Setup Areas You Need To Focus On

When developing a dirt setup, you have to consider where you will be racing, what the conditions will be like for the event, and what changes are possible with your car so that you can be good for each run you make.

Dirt is hard to set up for mainly because the conditions change so quickly. After hot laps, which will usually tell you nothing due to the track being so wet and sloppy, as the heats are run for the various classes, the track will dry out, the moisture will either soak into the dirt, or evaporate, and the amount of grip will change.

In working with the dynamics of chassis setup over the years, one very important thing I have learned is that as the lateral g-forces change, so does the handling balance of a race car. A particular set of springs combined with the front and rear roll centers, and more is balanced for a given side load.

On dirt, this side load amount changes considerably. After the track has been packed in and the surface is smooth, moist, but not wet, it will probably have the most grip it will have the entire event. So, a relatively high lateral force is generated and the balance must match that condition.

The key here is to be ready and prepared to make changes to your car, not if, but when the conditions change enough to warrant those changes. And, we must know what to change and how much, so that we keep up with the balance we need to be fast and consistent.

Now I have a short comment on consistency. It has long been a theory of mine that you don’t necessarily need to have the fastest car to win on dirt. Many times the winner is the one who makes the fewest mistakes. A car that is hard to drive causes more mistakes and loses more time during those missteps.

Today we see teams and car builders designing their moment centers more carefully and running setups where the left front tire is mostly in contact with the track surface. In other words, the car is setup with a more balanced dynamic.

What we get is not only speed, but consistency and the top teams know that is the way to Victory Lane. Here are 10 things you can do this season to be more consistent and give yourself a better chance at winning.

1. Front End Geometry

For any race car with a double A-arm front suspension, we always start with an analysis of the front end geometry. If the moment center (roll center) design on your car is not right, then the whole car will suffer, no matter what setup you have in it. This we have established as a fact.

The dirt car moment center design is different than that of an asphalt car. On dirt, the average g-force is much less than on asphalt because the track just does not provide as much grip. So, the MC needs to be located farther to the left in order for the car to work well.

Most dirt cars are not adjustable for upper or lower control arm angles, the parts we need to change in order to refine our MC location. So, we might need to make modifications to our chassis and/or spindles to improve our MC design.

This is not so hard to do. On a Dirt Late Model for example, we only need to know how far to move our inner mounting points and then drill new holes in the uppers. We have done this with many Dirt Late Models and it works well. The improvement is not only in the MC location, but in improved camber change.

2. Rear Geometry

The dirt car rear geometry layouts are varied and usually highly adjustable, not to mention complicated. Each car needs to be evaluated for where it is to be raced and then setup correctly. The trailing arm angles affect the rear steer and bite and the pull bar or lift arm can redistribute the rear loads upon acceleration and deceleration.

Many teams will tell you that there is a need for rearend steer to the right at times when the car is tight and you need to get it pointed in order to exit the corner. During tight and tacky conditions, a slight amount of rear steer to the left would probably improve lap times, but only if the car turns well. The use of rear steer to the left must only occur on acceleration and not at mid-turn. This is a possibility with certain designs.

It all comes down to trying to create better driver comfort and more consistency. If a jacked up setup causes us to make more mistakes than one that places all four tires on the track, speed being equal, then opt for the more consistent setup and you will gravitate to the front.

Time after time, I have seen where the top dirt teams have setups where the car runs more straight ahead and all four tires are on the ground. I have even seen a trend toward braking into the corners, driving straight ahead through the middle and then powering off going slightly sideways. I have seen this with not only Dirt Late Models, but Big Block NE Modifieds and I even saw Tony Stewart do this with his Sprint Car during a heat race.

3. Steering Geometry

Dirt car steering systems must be designed to work the same in both left- and right-turn attitudes. Mechanical effects such as Ackermann could be more beneficial on dirt than on asphalt, but again only to a small degree.

The Ackermann must be developed in the design of the tie rod angles from a top view and not differences in steering arm length. That way the wheels always keep the same toe or toe gain in equal amounts while turning right or left.

There will always be times when a dirt car needs to be steered right. In fact, most of the time, even with the “straight ahead” style, the steering will be turned more right than left. So, the steering box or rack must be centered with the wheels pointed straight ahead and the caster split should be minimal or even reversed from what we normally see for asphalt racing.

4. Alignment

Alignment issues present just as serious a drawback for a dirt car as with an asphalt car. There really is no reason to misalign the rearend. In tests we have participated in, we have run the same lap times with the car “sideways,” due to excess rear steer to the right, as when running it straight ahead with no rear steer.

I believe that the rearend does not need to be any different in alignment than at 90 degrees to the centerline of the chassis and/or to the right side tire contact patches, and those patches need to be inline, even on dirt.

At the very least, a team needs to adjust the amount of rear steer in the dirt cars. There needs to be less rear steer for tight conditions and more for drier conditions. Most cars are built so that these changes are quick and easy.

So, study your system and learn how much rear steer is happening with each setting and make a decision as to which positions your trailing arms will need to be in for each track conditions in at least three different circumstances. Those would be 1) tight track with a lot of grip; 2) a transitioning track where grip is less, but it has not gone dry slick; and 3) dry slick to where there is very little grip and the lateral g’s are much less.

5. Setup Balance

Does a dirt car really need to be balanced? Of course it does. The balance, though, will need to be adjusted for the track conditions. If the track is tacky, then the balance needs to be more like what we would do for asphalt and that is to match the desires of the front and rear suspensions.

The term Balance means that both ends of the car have the same tendency to roll. The truth about setup dynamics is that at mid-turn, each end will want to roll to its own degree of angle. The relationship between those desired angles will determine the balance of the car. On dirt we can manipulate that balance relationship to adjust the car to different conditions.

The setup for slick tracks is with a controlled difference in balance in the front to rear relationship with the rear desiring to roll more so than the front. This provides more rear traction to give us more bite off the corners. If our MC design is correct, the car should still turn through the middle, but have better traction off the slick corners.

Just as with rear steer, we need to develop changes to the balance in order to be ready for changing track conditions. A stiffer right rear spring will cause a more balanced setup for tacky conditions. Then as the track goes more slick as it dries out, the RR spring rate can be reduced to produce more rear roll and along with that, more grip for bite off the corners.

6. Shocks

Research on shock influences on dirt has shown that there are a lot of gains to be had by concentrating on your shocks. This is evidenced by the influx of new designs of shocks into the dirt car market. The age old truth is that we need to perfect our setup first before working with the shocks.

Dirt cars show a lot of travel as they negotiate the four turns. This extreme degree of wheel travel means that shocks get to do more work than with other types of race cars. A shock does not have any influence if it is not moving.

Each corner of the car might need a different shock characteristic. The amount of difference is directly related to the installed motion ratio of the spring and the spring’s rate and amount of motion. A very soft spring would need more compression rate and less rebound rate, whereas a stiff spring would need a lot of rebound rate and much less compression rate.

Shocks affect the motion of the corners of the car and therefore the placement of wheel loads during transitional periods, and dirt cars are almost always in transition. That is the essence of basic shock technology related to handling influences.

Sometimes only one corner of the car needs to be adjusted in order to influence handling in the transitions. If I had a choice, I would make the RF shock the one to adjust for rebound and compression.

A stiffer RF shock in compression will tighten a car on entry. A softer RF shock in compression will help the car to turn in better. On the rebound side, a stiffer RF shock will loosen the car off and less rebound will tighten the car off.

For cars that are not allowed adjustable shocks, you can stock three or four different RF shocks to accomplish the same thing. Many shock companies will valve a non-adjustable shock the way you need them to be built. Label the shocks as “tight track,” “medium track,” and “slick track.”

7. Brake Bias

Turn entry on dirt is important and dictates how well we are able to negotiate the middle of the turn. So, we need to evaluate our turn entry characteristics related to the amount of brake bias. We may want to try to solve turn entry problems with the brake bias on dirt.

As we said earlier, I see many top drivers using braking into the corners on dirt to slow the car rather than throwing the car sideways to slow the car down. This technique is used for Dirt Late Models, Modifieds, and Sprint Cars under certain conditions.

Brake bias influence can easily be determined for any race car by entering the corner with medium to heavy braking first and then entering with light braking to see if there is a difference in the cars attitude. If there is, try to adjust the brake bias to improve the entry conditions under heavy braking to what it is under light braking.

Brake bias can also be used between the front wheels. More bias toward the left front will allow the right front tire to maintain lateral grip and help turn the car more so that if it were also trying to slow the car and using up its available grip.

8. Bite off of the Corners

We never seem to have enough bite off of the corners on dirt. The exit portion of the track provides little traction and most corners are usually more flat. We almost always need to develop more rear traction upon acceleration. To give the car more rear traction, we need to understand a little about the dynamics at work on the car when we are accelerating.

We should work to develop ways to create more rear traction on acceleration only so we don’t ruin our mid-turn handling. There are several ways to do that without changing the handling at other points around the racetrack. One way is to reduce the “shock” of sudden application of throttle and torque to the rear wheels.

We can use lift arms and pull bars with various stiffness of shocks and springs. More and slower movement is needed for slick conditions and much less movement for the tackier conditions. Another way to gain bite involves the use of a spring loaded pushrod that allows a certain amount of forward right rear wheel movement to steer the rearend more to the left.

Adjusting the amount of loading that falls on the rear tires as the loads transfer on acceleration helps increase forward bite. If we overload one of the rear tires, then we don’t have as much grip as if we evenly load these tires.

On the other hand, for very dry slick conditions, it has been shown that putting more loading on one tire will cause it to push through the sand and grip more than if we evenly distribute the loads. That is why loading the left rear works on dry slick tracks.

Much of what you read here has been observed during my three years on Tour to tracks around the country

9. The Anti’s

Antidive and antisquat are mechanical influences that can help our transitional phases of entry and exit. We can regulate the amount of both depending on the need. A small amount of antidive on our dirt cars can help prevent sudden nose dive on entry by utilizing mechanical resistance to the downward motion of the suspension.

Antisquat results from the pull bar trying to straighten out, or become more horizontal as the car accelerates and the rearend desires to rotate. The more pull bar angle you have, the more antisquat there is. The lift arm also creates antisquat and can actually lift the rear of the car on acceleration. Lateral movement of the front and/or rear of the lift arm can alter the loading among the two rear tires upon acceleration.

10. Aero Package

Racers discovered the need for better aero designs some years ago. Just look at the way Dirt Late Model bodies have evolved. The front ends are wedges that scoop the oncoming air up and over the car. The wheelwells are shaped to route air out and away from the front tires creating low pressure under the hood and more downforce.

The degree that you need to get involved with aero for your car depends a lot on what you run and where. Aero influence varies with the speed of the vehicle. There is an algebraic increase in both drag and downforce associated with increases in speed through air. That is why a car with twice the horsepower does not go twice as fast.

Try to understand how aero downforce is created and then configure your car so that you take advantage of every area where you could produce more downforce. On dirt, we need the most loading on our front tires that we can get and aero generated load is not weight we have to accelerate.

Conclusion

The ideas we have presented and the methods we preach have a basis. Much of what you read here has been observed during my three years on Tour to tracks around the country. No one has to buy into any of what is presented here, but you do need to think out your particular setup and how you attack your racetrack.

To be more successful, it helps when you get your car setup correctly for the basics of geometry, alignment, and balance. Develop a proper approach to the setups for the tracks that you intend to race at and be prepared and willing to make changes to your setup when the track conditions change. That ideology will never change.