Our Dirt Late Model project...
Our Dirt Late Model project car goes to Victory Lane. All of our hard work has paid off with Bobby Clark's very first Late Model win.
The Circle Track Dirt Late Model race car project, otherwise known as Project DLM, started in March 2008 in the garage adjacent to our offices in Tampa, Florida. We joined forces with Bobby Clark, a driver and team owner who had limited experience with driving Dirt Late Model cars. We wanted to build our project with a team that could grow with it. From the very start, we realized that Bobby was just the guy we needed.
He proved that on July 11 at North Florida Speedway in Lake City, Florida, when he won the feature event. Watching the video (online at www.circletrack.com), I saw a more confident and comfortable driver now that he had a few races under his belt, and especially since he had tuned the car to suit his liking, using much of what we write about in Circle Track. Through numerous restarts, he held his ground and used patience and determination to bring in his very first Late Model win.
You can tell when someone is open to learning his craft. Bobby dreamed of driving and winning in a Dirt Late Model from early on in his life. He was so eager to learn that we had to pull back on the reins a bit at times, walking him through the processes at a slower pace so that the details would sink in-and boy did they ever.
I have to say I learned a bunch along with Bobby. Some of the things I wanted on the car just did not work. This is a story about how we tuned our car for performance and Bobby's preferences. We have more work to do for next season, but with the win to boost us, we are well on the way.
Early Development of the Chassis
The rear links we observed...
The rear links we observed were similar to the original Rayburn, but had some strange angles to them. I thought this design would have produced serious rear steer to the right at mid-turn and the bumpsteer might cause an unstable feel for the driver as he went through bumps and holes in the track. We initially made wholesale changes to the mounting points and link angles to reduce the rear steer and bumpsteer. Bob Bolles
We started out with a 2002 Rayburn chassis that, as you might know, is a Z-link rear suspension in which the bottom link goes forward from the birdcage to the chassis and the upper link goes to the rear. The link ends are adjustable for height to provide more or less rear steer. In our original configuration, we mounted the coilovers to the lower links, making the suspension a swing arm-type, with a motion ratio for the spring rate to wheel rate.
Lift Arm to PullBar The lift arm design that was in the car at that time was replaced by a dual spring third link with double shocks to control rearend wrap-up and rebound. This is a unit that we experimented with at Eldora in 1998 and was used in the winning car for the Dream and the World 100 that year. That track was very high banked at that time, 19 degrees in the upper groove, and had a good amount of bite.
Frontend Geometry Our overall design goal was to build a car that was correct for front suspension geometry and that would be adjustable within a certain range in order to provide a more balanced setup. The new front geometry design we established did help make the car turn well through the middle and the balance we established made the car both fast and consistent.
In the end, we reintroduced...
In the end, we reintroduced rear steer, although with the swing arm design and our right-side mount of the J-bar, we would still not see a great amount of rear steer. The birdcage we used had plenty of adjustment for arm angle in conjunction with the mounting points on the chassis. Bob Bolles
What we did to this chassis might and might not work with your car, even if it's a Rayburn car, due to the fact that we couldn't determine if the spindles or control arms were the same as came on the original car. Bobby bought the car used in 2006. So we aren't saying this chassis is as CJ would have released it new. We modified it to work just like many of you might do.
Reduced Rear Steer Our goals for setup are to provide a more balanced car on tight tracks where the level of traction is high, and also one that will provide more bite on the drier surface tracks. Most tracks we ran were the latter-dry and slick, like you probably encounter.
Along with the spring layout and roll center locations, we proceeded to design less rear steer into this car and a little different weight distribution than what many racers are used to. That would change. We positioned the links off the birdcages to be more level, so when the chassis moved, the ends of the rear axle tube would remain in the same position fore and aft.
This view is staged to simulate...
This view is staged to simulate the position of the rearend in relation to the chassis at ride height. Note the angles of the links on this left-side view. We ended up with about what you are looking at here. This increases the amount of rear steer to help the car around the slick turns. We will still have adjustments to where we can introduce more rear steer if we want to. Bob Bolles
Initially, we had to move some weight to the front to get a better front-to-rear percent, to around 52-53 percent rear, so we moved a 30-pound piece of lead from the back of the car to a point at mid-chassis and at around shoulder height, next to the driver. This piece came from the rear bumper and should not have been located there for reasons of negative polar moment tendencies. Keeping your moveable weight between the axles is a design goal that has proven itself over and over again.
If you have a car that won't turn, putting weight or ballast far to the rear of the car will help the initial turn-in and middle by making the rear loose, but will make the car very loose off because there will be no bite once the rear tires have lost contact with the track surface.
The battery location in this...
The battery location in this car is excellent. It's mounted high and on the right side, in front of the fuel cell. Our driver weighs around 260 pounds, so right-side weight is not a problem. We actually need to create more right-side weight to reduce left-side percent. Note also the lead bolted to the chassis just in front of the battery. Bobby ended up with 55.1 rear percent-more than I had wanted, but we'll experiment with that next year. Bob Bolles
We mounted the J-bar to the right side of the chassis so that we could be more consistent. This is something we had done in the past on tracks with more bite. This drastically changes the attitude of the car in the turns and coming off the turns. We did a dirt test a few years ago and made this change in the process of making lots of changes. It did not help or hurt the lap times, but it did make the car much more manageable to drive. We maintained this configuration through the final race we won, but might reconsider moving the bar to the left-side chassis mount in future races.
Spring Rates We started out with more conventional spring rates up front using a 450 LF and a 500 RF. In the rears, we used a 425 LR (equal to a 212 on the birdcage) and a 450 RR (equal to 234). Remember, we're using the swing arms on both sides, and we calculated the motion ratio squared for the left side was 50 percent and 52 percent for the right. So multiply the installed spring rate by the percentage to get the rate the car "feels," or equal to a spring mounted on the birdcage.
In looking over Bobby's notes, I noticed that he changed the rear spring rates to 450 LR and a 400 RR. He noted that the car was "a little tight in the middle and off." The softer RR spring rate was now making the car tight. We would later return to a stiffer RR spring.
The lift arm system was reinstalled...
The lift arm system was reinstalled after we disengaged it at the beginning of the project. In the end, we used that plus the original Rayburn pullbar to control axle wrap-up. We also mounted a 30-pound chunk of lead to the chassis rail just in front of the oil filter to get weight up in the car. Later on, after the installation of the Roush-Yates motor, we had to install a 30-plus-quart oil tank beside the driver and this increased the rear percent of weight distribution. Bob Bolles
As the season went along, Bobby made a few changes to various components to try to find a feel he liked and one that would provide more bite off the corners. A few crashes here and there caused some of the frontend settings to be altered. When we checked them, he reset them.
Caster and Camber In the final races, the caster split was set to 1.5 degrees LF, and 3.5 degrees RF. The cambers were set to 2.5 degrees LF and 4.5 degrees RF.
The team did take tire temperatures, but from my estimation, they cooled much too quickly from the time the car was making laps until they could get the temperature gauge on the tires. So the readings were very low. Nonetheless, adjustments were made based on those readings. We might check tire wear in the future to get a better idea of proper cambers.
One of the changes Bobby made...
One of the changes Bobby made was to move the right rear spring off the swing arm and onto the front of the birdcage. When he went from the 450-pound spring that netted 234 pounds of "wheel rate" to the 225-pound spring on the birdcage, he actually maintained the same spring split. Then he reduced the left rear spring rate to 375 pounds, increasing the spring split to 38 pounds, right rear being more. Bob Bolles
Also, if we get into more rear steer, which we did, the caster split might need to be closed up a bit to provide a more comfortable steer to the right as the rear comes around. With the caster split we have, it has to feel awkward to steer right against the split.
Lift Arm Reinstalled In the initial runs with the car, Bobby noticed that the pullbar was bottoming out. When it did, the car would snap loose. I'm not sure if he used the adjuster to tighten it to provide more resistance or not, but the decision was made to go back to the lift arm setup along with the original Rayburn single-spring pullbar. He then moved the pullbar 3.5 inches to the left of center for more forward bite. This placement puts more load on the left rear tire when the car is accelerating.
With the final changes made,...
With the final changes made, the car looked much better and felt like one Bobby could control well and that had better bite off the corners. Note how the attitude is more straight ahead with a slight drift from limited rear steer, and we are still able to keep the left front down on the track for better steering through the middle. Tanya Clark
The lift arm is much longer than any pullbar and provides lots of control for both acceleration, which we controlled with the pullbar, and for deceleration. We will be tuning this device in the future to improve our entry and exit performance.
Rear Steer Is Back Bobby also reset the link angles to provide more rear steer. Because we were still running the J-bar on the right-side chassis, the rear attitude in the turns was not what it would have been if we had mounted the bar to the left-side chassis. The left-side mounts cause the rear to raise up quite a bit more and enhance the effects of rear steer.
We probably will try a left-side J-bar mount in the near future to see how it works with the new link angles. Ultimately the LR forward (lower) link was 22 degrees up, the LR top mount was 20 degrees down, the RR forward link was 15 degrees up, and the RR top link was 9 degrees down. This provided some rear steer, and did help on the dry and slick tracks.
Final Weights For the final weights, Bobby added some rear weight and mostly ran a high range of crossweight, or left-rear weight. In talking with many dirt racers, most run the lower range of left rear, or around 65-75 pounds or so. The high range is upwards of 240 to 260 pounds of left rear. Here are our final weights and percentages. The rear percentage was 55.1, left-side percentage was 53.4, and the left-rear weight was 249 pounds.
Bobby being a big guy would have a hard time reducing the left-side percentage, but we may try doing that for the dry/slick tracks to get more load on the right-side tires. We might also change to a lower left rear weight and see how that does. If we move the J-bar to the left side of the chassis, we might need to put more load on the right rear tire for added bite.
Spring Rate Changes
We installed a new Roush-Yates...
We installed a new Roush-Yates Dirt Late Model motor into our car. The engine uses NASCAR Cup technology but is designed for dirt racing. Bobby quickly adapted to the additional horsepower in the new motor, winning in only his second race with it. Tanya Clark
For the final spring rates, Bobby made a change to the right-side mounting of the spring. He moved the spring from the swing arm to the birdcage and changed the rate to 225 pound spring. Note that the "felt" rate of the 450-pound spring on the swing arm was 234, or very close to the one now installed. So the net change in spring rate did not change.
Bobby did change the left rear spring to a 375-pounder, or the equivalent as if it were a 187-pound spring mounted to the birdcage. This increased the spring split to a stiffer RR spring. I have found that over the past five years or so, many top Late Model teams are going to stiffer RR springs for some conditions and having a lot of success.
Other Changes Some of the other changes that were made include trying a different brand of tires, installing a new motor, using a new trailer, and wearing a new driver suit. Here are the details.
Tire Changes Mid-season, Bobby changed from Hoosier tires to American Racers. He says that the side walls were softer on the AR tires and he felt that he was able to get more side and forward bite as a result. We don't really know if it was a compound difference or side wall construction. We do know it made the car faster.
He also increased the rear stagger from 3.0 inches to 4.75 inches, and that made a big difference in how the car turned through the middle. He also started working with the grooving and siping adding more sipes to increase the heat in the tires.
The team gives due credit...
The team gives due credit to Roush-Yates by running its logo on the hood of the car. Getting that first trophy was a big step in the introduction of these types of powerplants into the Dirt Late Model racing arena. Tanya Clark
One big change we made and detailed in the December issue of Circle Track is the installation of a brand-new Roush-Yates Dirt Late Model motor to replace the 520hp motor we were running. Using NASCAR Sprint Cup technology and components, this new motor was built by Yates' veteran employee Jeremy Anderson.
In our first outing at Volusia Speedway Park, the car had a handling problem that was later sorted out, but we did easily lay down a pair of black steaks down the straights.
One can never totally use 822 hp on many of the dirt tracks we have traditionally raced with this car. But you try to get as close to full throttle as you can. Throttle modulation is an art that must be mastered, and watching the winning race video, I'm convinced that Bobby is well on his way to getting it right. I never saw him spin the tires or loose ground. And the motor pulled strong all of the way through the range of rpm we were running.
Other Changes Bobby told us that a couple of other things made a difference. We began using our Circle Track enclosed trailer and that was a big help over his old open trailer. It allowed him to bring a bigger assortment of tools, provide a retreat from the action, and overall made him feel like he was moving on up in the racing world. Don't discount the power of confidence new and better equipment can provide.
Leaf Racewear also set the team up with new crew shirts and a new driver suit for Bobby that both looked great, but also fit better and provided a greater measure of protection. Having the right safety equipment has a positive affect on your mental attitude. You lose that lingering question in the back of your mind: "Am I really going to be alright if this thing bursts into flames?"
Bobby and crew chief Pete...
Bobby and crew chief Pete Epple share a funny moment. Can you tell this is a Circle Track project car? That Firestorm Graphix wrap job sure does look good. Tanya Clark
In closing, we want to thank Bobby and his girl, Tanya, crew chief Pete Epple, Anthony Griffith, and all of the others who helped this effort. We are learning this sport, just like many of you. This is just a start and we're going to get into more detailed experimentation for the 2010 season.
We'll have a new Mastersbilt chassis to play with, our new Roush-Yates motor, and more. Stay tuned as we attack new and uncharted setup parameters. Whether we improve or go backwards, we will provide you with the details. It's all good stuff.CT