Indianapolis Motor Speed (IMS) is a 2.5 miles (4 km) race track, it has four corners. All of them with same radius and banking, this last one is 9°. This circuit is not an oval track, it is a sort of rectangular one. However, each corner from this track has some different detail. The first corner has stands on both sides, while the second corner has stands only at one side. Hence, the effects due to wind on the vehicle behaviour is quite different.

Throttle and speed

Figure 1 illustrates some data about the four wheel speeds and the throttle signal. First point is that left and right rotation speed are different due to the stagger. Hence, the left rear tire is rotating at lower speed that the right rear. This is a dimensional variation of the rear tires that creates a differential effect.

In the case of the left and rear front tires, the speed different is the same. However, the reason is the different traveled path that each them run. The front wheels are also different, the right one is smaller than the left wheel. Even though the centrifuge effect is the same on the front tires, the right one is exposed to compression due to the higher wheel load. The speed sensor on the wheels measures the rotation speed. Hence, when calculating speed, it is common to multiply it by a fixed radius. However, this is not correct. The best approach is to multiply it by the loaded radius, which can be done by a math channel. This is based on the applied load and on the centrifugal effect (Read more).

Figure 3 illustrates two plots of RPM and speed. It is possible to notice that, the RPM signal is the same for all speed signals. However, these exhibit variations due to the different wheel radius. Hence, the ratio between RPM and speed is the same during an outing because of the loaded radius.

Push-rod loads

The push-rod loads (Figure 4) are identical left to right. As can be seen, the right rear push-rod is exposed to a higher load when compared to the left rear one. For the front suspensions, the situation is the opposite, the right front push-rod is exposed to a lower load when compared to the left front one. This is approach called, wedge. With the car on a scale, it is possible to pre-set the push-rods in order to induce the length of the left front push-rod. Consequently, the right rear push-rod is loaded. Hence, it is left the job of the right front push-rod in order to pre-load the left front push-rod, thus the right rear one too. In this way, it is possible to change the understeer-oversteer balance.

Tire	Left	right
Front	+ 15 kg	– 16 kg
Rear	– 16 kg	+ 15 kg

For instance, considering a situation which a car that exhibits the configuration observed in Table 1. When the car enters in the corner, the pre-load in the inner wheel (left front – LF), thus the right front wheel will be less saturated. However, the right rear wheel will suffer a bit more. If the car is understeering, this is an approach to reduce it. In the oval racing, race teams created a math channel called, wedge. This is given by the sum of the loads on the left front and the right rear push-rods divided by 4.

wedge = (LF + RR)/4

At the corner entrance, the right front push-rod is loaded, the left front on is a bit relieved, but the diagonal does not vary significantly. At the mid corner the is flat and on the corner exit, LF push-rod is loaded while RR one is relieved (Figure 4). Since the speed is very high, this parameter is not just a matter of load, but also the variation of these loads. Hence, it is possible to create a math channel that performs the wedge derivative. This information is very helpful to tune dampers. The wedge is not only example of the “asymmetries” of the oval racing. Actually, the driver seat can be off-set with respect to the center line. Another example is the wing adjustment, which can be different for each flap. Even the wedge can be different in some circuits, but in this case it depends on the track shape. The car ride heights, when the car is in the pit lane, is not flat, instead it is tilted. Hence, is like in the corner, thus it loses some downforce on straights. In the corners, the car produces its maximum downforce, because it becomes flat. Hence, whenever spring and anti-roll bars are changed, it is necessary to change wedge, stagger and weight jackpot. This last one is a device, a sort of cylinder on only one spring, which the driver can adjust the wheel loads inside the car.

Longitudinal and lateral accelerations

The longitudinal and lateral accelerations are plotted on time-distance graph as the one seen in Figure 5. As can be seen, the lateral acceleration is not zero on the straights. Actually, it is around zero, because the adjustments made on those cars require a constant steering correction. The perfect car on speedways is the one that it is able to race without steering inputs. The reason is that on corners, that are ran at 3 to 5 G, any steering correction is definitely not desired. Hence, the caster, camber, toe and king pin inclination (KPI) are adjusted in order to provide a wheel alignment that steers the car by itself.

Ride heights

The ride heights on ovals have a characteristic variation, this occurs in corners, where the car is compressed on its right side. The is bottoming on the corner, not on the straights. In this part of the track the car is a bit higher.

Steering

Figure 7 illustrates a plot of some parameters as the lateral acceleration, the steering, the kinematic steering and the understeer. As can be seen, the car exhibits some counter-steer on the straights. In addition, it is easy to spot the four corners and the lateral G developed during the lap.

References

• This article was based in the lecture notes written by the author during the Applied Vehicle Dynamics lectures attended in Dallara Academy;
• Knox, Bob. A Practical Guide to Race Car Data Analysis. Rev. 1. ISBN: 978-1456587918;
• Segers. J. Analisys Tequiniques for Racecar Data Acquisition, 1° Edição. Warrendale, PA. SAE International. 2008.