Two Guys Rally

Aerodynamics: Drag [ August 10th, 2008 ] By:Mark Ozimek

Aerodynamics is quite an interesting subject, and also one of the more complex. Seeing as I’m still learning this stuff myself, this series will be an introductory lesson on aerodynamics, I’ll just cover the basic concepts that are a good framework to understanding a lot of other important things.

An important thing to keep in mind is that aerodynamics is more or less the study of how fluids move (aka: fluid mechanics), with the fluid in this case being air, and a car’s body pushing the air out of the way. As long as you think of it as air being pushed around, the rest of the concepts are pretty straight forward.

First up is drag. Drag is a force opposing motion. In the case of aerodynamic drag, it’s the force applied against the car as it moves through the air. There are a few variables that affect the aero drag. The faster you go (velocity, or V), the more drag there will be. Also, more total surface area and frontal area increases drag. The frontal area (Af) can be thought of how much area the car takes up when viewed from the front. Or if the car is moving sideways, the side area would be used, or some combination thereof. There is a coefficient of drag (referred to as Cd) that is a function of the body shape. The final important factor is fluid density. The density of air varies with altitude, temperature and humidity, as I have pointed out in the past.

There is an equation that puts all of these things together to find the aerodynamic drag that will be seen:

Drag = (Density / 2) * Cd * Af * V²

Just make sure the measurement system is the same for all and the answer will be a force. As an example, let’s look at how much drag there is on a VW Golf GTI from the late 80’s going 80mph. I have a book here, Theory of Ground Vehicles by J. Y. Wong that has a list of different cars and their Cd and Af. The GTI has a Cd of 0.35-0.36, and Af of 1.91 m². 80mph is 35.76 m/s. I’ll assume standard temperature and pressure, so the density of air is 1.292 kg/m³.

Drag = (1.292 kg/m³ / 2) * 0.35 * 1.91 m² * (35.76 m/s)² = 552.2 kg-m/s² = 552.2 Newtons

Now for some fun with math to see what this means. Let’s convert the force and speed into power.

552.2 Newtons is the same as 124.1 pounds of force. 80mph is 117.3 ft/s, multiply the two together to get 14,556.9 lb-ft/s. There are 550 lb-ft/s in a horsepower, so this hypothetical GTI needs 26.5 hp to overcome aerodynamic drag at 80 mph. If we increased the speed to 100 mph, that number changes to 51.7 hp! Note that this is power at the wheels, and is neglecting any incline, rolling resistance, drive train resistance, and so forth that increase the power requirements at the crank.

I’m going to cover lift and downforce in a later article, but while it may seem obvious, one major cause of drag is fins and spoilers that create downforce while the car is moving. The extra turbulence and changes in airflow usually turn up as an increase in the Cd. Why is this? Well, the fins are designed to push air up as their way to get downforce. When a car is going down the road pushing lots of air upwards, there will be similar amount of drag. Let’s look at F1 cars, since they make good examples. The car is basically covered in wings that make enough downforce to allow the car to drive upside down at speeds over something like 100mph. They have to make a tradeoff when setting up for every race to balance between downforce and drag, which effectively means they have to choose whether the car can corner faster, or have a higher top speed. Rally faces a similar dilemma, although in rally, there is a lot less space for wings, and there are many slow hairpin turns where wings don’t do a lot, so the emphasis on wing setup is diminished.

So hopefully now it is obvious why when driving down the highway, you hit a speed where the gas mileage suddenly drops off really fast: The power required to overcome aero drag increases with the cube of velocity! Stay tuned for the next part of aerodynamics, and feel free to suggest topics that you would like to hear from me on!

Aerodynamics: An Introductory Rant [ June 24th, 2008 ] By:Mark Ozimek

I’m going to take a brief break from the turbocharger! series to make a little segue into what I hope to by my next topic that I will take a close technical look at: Aerodynamics.

Really, we only care about two things here:

1. Downforce/Lift
2. Drag

Hopefully I’ll explain how these two things come about in a pretty simple fashion that explains it a little bit better than “Oh, the body pushes air out of the way.” However, before that, I have noticed some very disturbing trends among the modding community.

The most incomprehensible to me is the addition of only a rear wing to a front wheel drive car. This is really a double negative. The wing creates downforce in the rear of the car, behind the rear wheels. The wheels act as a fulcrum, and this downforce that is generated in the rear actually lifts the front of the car up at speed. Totally counter-productive, since it will decrease traction on the drive wheels, and increase the amount of understeer. Keep in mind that front wheel drive cars already tend to understeer a lot, a wing in the back will just make it worse.

The other negative of the rear wing is one you will have to deal with almost every time you try to generate downforce by pushing air up: Lots of drag. The engine has to put out a bit more power to overcome the extra drag, which increases with the square of velocity. So now we have a situation where when the wing is putting down the most downforce is when the front tires are applying the most force to the ground to propel the car forward through the air. If this downforce was applied up front, or if it was a rear wheel drive car, this would be a great thing. Since it’s not, the drag slows the car down, and increases understeer through turns even more!

The other are body kits that add ‘features’ to the body that don’t actually do anything. Some things are useful, like a front bumper that allows less air under the body of the car should reduce drag, however, many are just as counter-productive as a rear wing on a FWD car. A classic example is the intake vent on the hood and before the rear wheels on the some of the more recent generations of Mustangs. The protrusions do exactly that, protrude into the air stream. This adds extra turbulence to the airflow over the body, which is almost always a bad thing since turbulence usually increases the amount of drag on the car. It’s possible to use turbulence to your advantage, but that’s a complex topic to cover, and requires some pretty precise placement of fins to make the air go where you want it to, think F1 for an example here.

One thing that Charles said to me while we were discussing this article is that we have a pretty utilitarian view on what looks good on a car. If it improves performance somehow, we’re almost always for it. If it does nothing, is counter-productive, or just adds weight, we typically hate it and immediately reject the idea. The things that make the car ‘go fast’ also look/sound good to most people, since we associate that with performance race cars. Maybe what we should do when we get our car to modify into a rally car is mask the things that make it look fast. Sleeper race cars, ready GO!