Data acquisition not only...
Data acquisition not only tells us exactly how our drivers use the throttle, but also how the traction control devices are working. In this example, we see a graph from a dirt Late Model car at a test at Eldora with driver Kevin Weaver. We can see that as the throttle is applied (purple line), the pull-bar (red line) begins to extend. Kevin initially applies about 30 percent throttle and then gradually increases throttle all of the way down the straightaway to 100 percent near the end. The engine rpm (black line) never runs up, meaning the rear tires never break loose. The pull-bar has controlled the engine torque and the driver used throttle control. This was a very fast lap.
The shape of the track for both dirt and asphalt can influence the available traction in several different ways. As we apply power, we need to know a little about how the track is banked, how the banking angle is changing coming off the corners, and how the radius of the turn might be changing. A highly-banked racetrack is very forgiving when it comes to needing bite off the corners. There is so much downforce due to the banking and associated lateral forces, that many times the tires are loaded to the extent that the available amount of horsepower cannot break the tires loose under normal conditions with a balanced setup. The tracks we often worry about getting off the corners are the ones that are flatter and with less surface grip.
The severity of change in banking angle of the racing surface in the portion of the track where we are initially accelerating can cause changes to the pitch angle of the chassis that works to unload one or more tires, reducing traction. A track that goes from high banking to low banking fairly quickly can cause the left rear tire to unload quickly, making the car loose.
There are two ways this can happen. One is when the outside edge of the track drops in elevation and the right front tire follows the drop-off. This lifts weight off the left-rear tire, causing loss of traction in that tire.
The other problem occurs when the inside edge of the track rises up to match the elevation of the outside edge of the track. As the left-front tire rises up, the left-front and right-rear pair of tires become more loaded, momentarily causing loss of loading in the opposing pair of tires. The loss of crossweight percentage (right front to left rear) makes the car lose traction in the rear.
A track that has a decreasing radius in the latter portion of one of the turns can cause a car to develop a loose condition at that point. Usually, older tracks that were originally dirt and then paved retain a straight front stretch and a rounded-out back "straightaway". This "D" shape causes Turns 1 and 4 to be a smaller radius than Turns 2 and 3 for that reason. So, it is difficult to accelerate from Turn 4 because of the decreasing radius.
Remember, we said traction increases for a set of opposing tires when we increase the angle of attack (simply put, this is when we turn the steering wheel more). If the car is neutral in and through the middle of the turns, then as we approach the tightest portion of the turn past midway, where the radius is less, we need to turn the steering wheel more and that produces more front traction than rear traction. The balance we enjoyed through the middle of the turn is now upset and the car becomes loose just when we are getting back in the throttle. This causes loss of rear traction. We will study ways to compensate for this later.
The surface largely determines the amount of traction available under power and we will look at dirt and asphalt tracks separately. On dirt tracks, the amount of moisture dictates the amount of grip the track gives us. Bumps, grooves, banking angles, and the overall radius all help determine how much grip is available for traction off the corners. The setup related to shocks, springs, and rear geometry help determine how much traction will be available for a certain set of conditions.
On asphalt tracks, and even some "dirt" tracks that have been oiled to the point of almost being asphalt, the surface is more consistent. Other than holes or bumps and rises in the surface, we can expect the grip to be the same over the course of the event. Flatter banking and older asphalt dictates the need for more traction control efforts.
Now that we have some kind of understanding of just what affects traction in the rear tires, we need to examine how we can use that information to enhance the tractive properties of the rear set of tires. Next month, we offer some suspension tuning suggestions for over-coming the problems some teams have getting enough bite off the corners.