Feature This is a new section where Cliff attempts to shed some light on technical terms and jargon, plus highlight some of the more interesting aspects of the Sprintcar itself.
This feature - Sprintcar Aerodynamics.
Sprintcars are well known for their staggering cornering speed. Much of this is due to the wings in use on the cars. In this article I will attempt explain just how they aid the cars and how changing their setting changes the cars characteristics.
(Please note that I am no aerodynamics expert, the information given here is what I have compiled while researching the subject. I can really only touch the surface of the subject here - and there may be irregularities contained within this information)
Firstly, lets look at aerodynamics.
How do 'wings' work?
This is not an easy question to answer as surprisingly, even in this day and age, there is widespread debate over how lift (or 'downforce') is actually generated. There are two theories, one is the 'Skipping stone' theory and the other is known as the 'Flow turning' theory. Lets take a look at both these theories. We will focus on downforce as it applies to race cars more than lift.
The 'Skipping stone' theory.
Many believe that downforce is simply generated by large volumes of air colliding with the surface of a wing and bouncing off it, forcing the wing downward (the image below illustrates this). This theory takes into account none of the air flow that travels under the wing. A simple enough theory. Many of you would have held something out of your car window when traveling at speed before and seen how the wind affects it when you angle it one way or the other. The theory seems to hold true in that context.
The 'Flow turning' theory. This theory is more widely recognised as being correct. It works on this basic principle "downforce is a force generated by turning a flow". I will not go into this in any greater detail as it is too complicated to try to explain. Suffice to say that the more the flow of the 'fluid' (in this case air) is turned, the more force will be produced. This can be demonstrated by placing teaspoon under a running tap and observing how it lurches around in the flow of the water. The faster the water is flowing the greater the spoon bounces around. The Flash animation below does a fairly good job of explaining the theory.
So how does all this affect the car? This is where it gets a little more complicated. A race car with wings has two centres of gravity. One is the normal CG (Centre of Gravity) and the other is an aerodynamic centre of gravity (ACG). The key to the aerodynamics and Sprintcars is how the two COGs interact with each other.
Sprintcars are different in that they do not rely on their wings for just downforce purposes. The top wing in particular works as a type of 'air brake' when cornering. In fact, downforce is greatly reduced during cornering as the large outer side board of the wing obstructs much of the air flow over the critical areas of the wing. This is a good thing as the air that is lost from under the wing is put to good use on the outer surfaces of the side boards (sometimes called the 'tip plates') and works to 'lever' the car into a good attitude (the 'leaning over' that a sprintcar does in the turns). You may have experienced standing at the pit wall when a Sprintcar drives past at speed - the wind that it produces is pretty amazing. If you never have, I would suggest you do! I have never really got used to the speed of these cars up close.....
This image shows how a Sprintcar can lean into the turns due to the top wing's 'Tip Plates'.
OK, so how do you set the wings up on the race car? Wing angles, a rough guide....
If the track is drivey, the wing is more critical. Speeds will be higher and using the wing will be a trade off of downforce versus drag (the 'weight' that the wings pulls along behind itself when it moves through the air). It comes down to personal preference for many drivers, but as a general rule, you should start the wing off at an 'angle of attack' anywhere between 12 to 20 degrees. Any greater with the angle and the drag created by the wing can slow the car down on the straights. On short tracks the angle can be higher as speed on the straights is not as fast as it would be on say a half-mile track. Western Springs is a quarter-mile in length.
For slick tracks, many drivers stack the wings as high as they can get them. As the track is bound to be slower in these conditions, straight-line speed can be sacrificed for grip.
The higher you angle a wing, the less 'tip' you get from it (this is when the can leans over into the turns), so again this is something that needs to be sorted out mainly by the driver as he is the best judge of how the car is handling.
Other Adjustments....
The wing on a sprintcar can be shifted from side to side (up to four inches on some cars) to alter the leverage that the wing has over the car. The wing is normally shifted to the right for slick tracks, as this increases the effect that the wing has over the right rear wheel. Again, the effects of this can be neglegible at times. Especially on shorter tracks.
Cockpit levers can be used to move the wing forward or back while the car is on track. They wings are normally shifted into position with a ram cylinder that uses power steering fluid (the system is run off the power steering pump). Moving the wing forward normally lessens the angle of attack, and moving it back makes the angle greater. The wing is often moved back by the driver when the track dries off. You may have seen this while watching from the stands before.
I hope this article has helped you understand a little bit more about how the wings on a sprintcar work. Unfortunately, there is so much information to take in that I cannot possibly cover everything in this article. I hope I have given you enough to work with!
If you have any comments on this feature article, or questions you would like answered, please e-mail Cliff.
