Have a rough season last year? Take advantage of the off-season to refine your chassis setup. Here are the top 10 most important areas of concern facing teams in dirt track racing.
Preparing your dirt car for the upcoming season should involve some processes and knowledge of technology that have been developed over the past 10 years. The approach we suggest has resulted in a lot of success and has made racing a lot more enjoyable for a lot of dirt teams.
Let's review the most important aspects of chassis setup we've presented over the last few years. Racing knowledge isn't usually shared among race teams, and for good reason. Information shared can be used against us. That's why we at Circle Track keep our eyes and ears open as we talk to racers across the country. Here are the top 10 topics for setting up a dirt car.
1. Front End Geometry For any race car with a double A-arm front suspension, we always start with an analysis of front-end geometry. If the moment center (roll center) design on your car is not right, the whole car will suffer, no matter what setup you have in it. This we have established as fact.
The primary complaint coming from dirt drivers is that their car does not turn well. The number-one reason for that is usually because the front geometry is incorrectly designed. Just improving that aspect of the car's overall design can make a huge difference in performance.
The mono-ball joint is adjustable for height so you can change the height of the center of
The dirt-car moment center design is different from that of an asphalt car. On dirt, the average g-force is much less than on asphalt because the track 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.
Trust me when I tell you that most top touring dirt-late-model teams have learned the importance of correct MC location. If not, they are being beaten by those who have. The MC can start out left of centerline and move, after dive and roll, to around centerline or somewhat left of centerline. If you don't know where your MC is located, then you don't know the most important thing about your car.
2. Rear Geometry The dirt car rear-geometry layouts are varied and usually highly adjustable. Each car needs to be evaluated for where it is to be raced and then set correctly. The trailing-arm angles affect the rear steer and bite, and the pull bar or lift arm can redistribute load upon acceleration and deceleration.
Many teams will tell you there is a need for rear-end 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. But 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.
Pushrods and pull bars that move substantially and lift arms that are allowed to move more freely are all different ways to create more bite off the corners in dry and slick conditions. We've seen conditions where no matter what we tried, there were no improvements in lap times. The track would just give so much traction, period, and no more.
The angle of the control arms determines the location of the front moment center. On this
It then 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.
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, 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.
This modified has a "four link" left rear suspension linkage and a Z-link swing arm on the
If you were to run different length steering arms to create Ackermann, turning one way might produce positive Ackermann and turning the other way would produce Reverse-Ackermann. This is unacceptable for any race car. So, other means (such as is done with stock passenger cars) must be employed to create Ackermann.
4. Alignment Alignment issues present just as serious a drawback for a dirt car as an asphalt car. There really is no reason to misalign the rear end. In tests we've participated in, we have run the same lap times with the car "sideways," due to excess rear steer, and when running it straight ahead with no rear steer.
We believe the rear end doesn't 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.
It is difficult to maintain a correct driveshaft alignment on a dirt car. We should nonetheless try to start out with something that's close to ideal. The commonly accepted rule is, the angles between the driveshaft and both the pinion shaft and transmission-output shaft need to be equal and in opposite directions. This creates the lowest loss of power and the fewest vibration problems, which translates to longer U-joint/seal life.
The driveshaft doesn't care which view these angles are resulting from, only that they are equal and opposite. If we have a top view differential in alignment between the engine (crankshaft and transmission output shaft) and the pinion shaft and they are parallel, the angle created by that misalignment might be sufficient to provide needed angular differentials to load and lubricate the U-joint bearings.
A 1 1/2-inch misalignment with a 44-inch driveshaft results in nearly a 2-degree angle at both the tranny and pinion shafts. The engine should always be aligned perpendicular to the rear end and/or parallel to the centerline of the car.
5. Setup Balance Does a dirt car really need to be balanced? Of course it does. The balance, though, must be adjusted for the track conditions. If the track is tacky, the balance should be more like what we would do for asphalt, which 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 determines the balance of the car. On dirt, we can manipulate that balance relationship to adjust the car to different conditions.
Greatly unbalanced setups exhibit certain characteristics, such as an unusually high degree of wear and temperature on one tire versus the other tires, and an unwillingness to turn the corner. The unbalanced car will have less bite off the corners, and the handling will not be consistent.
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.
This car is equipped with sliding clamps mounted on square tubing. Adjustments to the link
6. Shocks Research on shock influences on dirt have shown 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 in 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.
This dirt car has the battery mounted high and just to the left of the center of the car j
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. If one corner of the car is shocked stiffer, as that corner desires to move in compression, more load will be retained by the tire on that corner as well as the opposite diagonal corner of the car during the compression cycle only.
If the same stiffly shocked corner is put in rebound, some load will be lost by that corner, as well as its diagonal corner, during the rebound cycle only. That's the essence of basic shock technology related to handling influences. A coilover shock mounted on a swing arm will move less than the chassis and therefore would need to be rated higher than if it were mounted to the axle or birdcage.
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 brake bias. We may want to try to solve turn entry problems with the brake bias on dirt.
Brake-bias influence can easily be determined for any race car by entering the corner with medium to heavy braking first, then entering with light braking to see if there is a difference in the car's 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.
With the pull bar third link, you can usually mount the bar in several locations left to r
Once you've made the turn entry better, check to see if your brake adjuster is centered. If it's too far to one side, changes to the brake master-cylinder sizes and/or pad compounds might be necessary in order to solve the bias problem and still maintain a centered bias adjuster. Off-centered adjusters can be inconsistent.
8. Bite Off the Corners When on dirt, we could always use more bite off the corners. 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 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 rear end more to the left. As the car accelerates, the right-rear wheel moves forward, creating a slight amount of rear steer to the left. This points the forward thrust to the left of centerline and propels the car more efficiently, much like a sprint car.
9. The "Anti's" Anti-dive and anti-squat are mechanical influences that can help our transitional phases of entry and exit. We can regulate the amount of both depending on the need.
Chet Fillip shows me a remote adjustable shock at PRI last December. This design makes the
A small amount of anti-dive on our dirt cars can help prevent sudden nosedive on entry by utilizing mechanical resistance to the downward motion of the suspension. We do this by using the rotational forces created through braking.
This pull bar has a long spring that probably will not coil bind as the car accelerates. T
As the brakes are applied, the caliper grabs the rotor and the motion of the wheel/rotor tries to rotate the spindle. This force is resisted by the ball joints. The upper BJ is being forced in a forward direction, and the lower BJ is being forced in a rearward direction.
If we arrange our arm-shaft angle, from a side view, correctly, then as the car dives the upper BJ would need to move to the rear, and the bottom BJ would need to move to the front. Since the anti-dive forces are in the opposite directions, there is a serious resistant force to dive that helps prevent the front suspension from moving quickly.
The amount of resistance is directly related to the degree of side-view shaft angle we put in our control arms. The left-side suspension should be designed with about half the angle of the right side in a conventional design. This has an overall benefit on dirt by reducing the sudden camber change that can cause the front end to lose grip and push.
The upper control-arm attachment on a dirt late model offers the opportunity to easily rem
Anti-squat results from the pull bar trying to straighten out or become more horizontal as the car accelerates and the rear end desires to rotate. The more pull-bar angle you have, the more anti-squat there is. The lift arm also creates anti-squat and can actually lift the rear of the car on acceleration. Lateral movement of the front end and/or rear of the lift arm can alter the loads among the two rear tires upon acceleration.
10. Aero Package Some years ago, racers discovered the need for better aero designs. Just look at the dirt late-model cars and how they 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 downforce.
The degree to which 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's why a car with twice the horsepower does not go twice as fast.
Try to understand how aero downforce is created, 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 we can get on our front tires, and aero generated load is not weight we have to accelerate.
Dirt cars have been modified over the years to a more aerodynamic wedge shape where the on
Conclusion As every dirt racer knows, dirt is a tough medium to set up for. Have your car set up correctly for the basics of alignment and geometry. Develop a proper approach for the tracks where you intend to race. Be prepared and willing to make changes to your setup when track conditions change. With all forms of motor racing, remember that success comes in stages.
Improving your finishes is where you start on the road to winning. Don't think that just the work done to improve the setup will quickly lead you to Victory Lane. Learning how to win is a process; when the moment is right, you'll find yourself waving that checkered flag on the Victory Lap.