Hatchback Aerodynamics part 1: Setting performance targets

A lot has been written about racecar aerodynamics, by now it must be clear to any reader that there are 5 external sources of force regulating the whole of a racecar’s behavior, the forces in the contact patches of the four tires and the aerodynamic forces that affect the car as it moves through the air. The body shape can be manipulated as a way to alter this force and vary the drag (force resisting the car’s motion through the air) and downforce (vertical force). As a rule of thumb, if you want to generate downforce, you will have to pay a price in drag. In many tracks it is preferable to have low drag levels, these are usually fast tracks where the car is operating at high speeds for most of the time or when the car is very underpowered. Drag will consume power at the cube of speed so the faster you are going the more horsepower your car is going to consume to move through the air, this can be seen in oval racing stock cars and land speed record cars amongst others. Other tracks, where speeds are lower and corners are tighter like autocross or time attack call for much more downforce as the increased vertical force on the tires will increase their cornering and braking ability, even if it means you have to pay a drag penalty. Downforce and drag are usually converted to the dimensionless coefficients Cd for the drag coefficient and Cl or -Cl for the lift or downforce coefficient, this is done in order to obtain a number that can be used to compare the force generated by one shape vs another of different dimensions. A lot has been published about sport prototype and single seater aerodynamics. Websites like Mulsanne’s Corner (www.mulsannescorner.com) and books from authors like Katz and McBeath mention these cars and their aerodynamics in great depth. The next cars to have more aerodynamic mention are GT cars and sedans having similar shapes. But not a lot has been written about the hatchback, this design has become more and more common in production compacts and the light bodies make a good basis for production based racecars. For this we will use examples from a project we have been doing on an Opel Corsa B. An economic hatchback produced by opel that is raced in Europe and Latin America. The class in which the car races commands for open windows and nets, this having an important effect on aerodynamics.

The first question is what are our targets? Where is the car going to race? What sort of power do I have to work with? Lap-time simulation software is of great help in this regard. Even though reliable tire models are very expensive to obtain, a generic model can be used and a lot can be learnt from varying the parameters of the car and studying the performance impact. The Corsa is on the lower end of power, producing 98whp at 6500RPM and it weighs 800kg as per the regulations. For all the simulations in this entry I will use the track at Imola, a renown twisty track with sweeping corners and hard braking, this is a nice example track. Most laptime simulation packages can create track maps from data or GPS.  There exist many methods for measuring downforce and drag both on the car with data acquisition or on Computation Fluid Dynamics (CFD) simulations. In this case we performed CFD simulations on a model obtained by generating a 3D scan of collector’s model of the car (Scanning a whole car is very expensive and after we scanned and scaled it, the model was found to be surprisingly similar to blueprints of the car). Creating a good 3D model from these blueprints is also possible. From the Baseline CFD with open windows and nets, we obtained a Cd: of 0.3225 and a Cl of .061 the positive cl implies the car shape is generating lift, which is not surprising as most stock car shapes generate lift.

The car with these numbers was put in the lap-time simulator and it lapped Imola in 2:23.13. The car in the lap-time simulator was then modified to the numbers that can be achieved with significant aero development of a Hatchback ie Cd: 0.39 and Cl:-1. In this instance the same car lapped Imola in 2:21.13. This marks a 2 second improvement in lap-time. The increased drag will mean a lower end of straight speed but this is more than made up for by the increase in corner speed.  

Laptime simulation software is very good for setting up targets for where you want to go with your car. In this case we used it to generate a surface, for many different downforce and drag settings and see how we can go about setting some design targets.

As you can see in the graph the advantages of increasing downforce and reducing drag generate an almost flat surface. This means the ratio between downforce and drag must be very important to lap-time. Looking at the graph you can see that you can get almost the same lap-time by reducing drag even if it sacrifices downforce than by increasing downforce even if it implies paying a price in drag. In our experience and testing, within the limits of the car and the class, lap-times will be greatly dependent of the lift over drag ratio. Thus, a downforce increase will only be worth it if the resultant drag price you have to pay rises at a lower rate. Within a given ratio some tracks will favor the low drag, low downforce setup and some tracks the high drag high downforce setup but setting out to optimize this ratio is a very good goal for aero development. In the case of this car, it will be run in tight twisty tracks at high altitudes so we will go for the high downforce end of the spectrum.

With these conclusions we will end this first entry in our blog. Keep posted for how you can change the aero of the car to increase this ratio and thus reduce lap-time. On the next entry we will discuss front of the car aerodynamics.

To end, the commercial side, contact us about CFD and lap-time simulation analysis on your car on your track. You will be surprised at what you gain and the costs and projects can be structured so they are attainable even to club level racers. Follow us on instagram @tizona_eng and Facebook @tizonaeng