05 July 2010

Your street car is not a race car (so don't pretend it is)

This is the first post in a (probable) series of posts on street cars and race cars; specifically, when street cars are modified in ways to make them appear like race cars and/or (attempt to) perform like race cars.

So what exactly are my qualifications to judge these sorts of modifications? Well, weak, admittedly. As a naval flight officer, I had a number of courses in aerodynamics and nearly 1,000 flight hours over the course of about 4 1/2 years, so I have a pretty good understanding of Bernoulli's principle, of lift and drag, and thrust and weight. I consider myself a fairly serious fan of racing (NASCAR specifically), have been watching races since I can remember, and generally think I am well-versed on the technical aspects of racing. On the other hand, I have never owned or driven a race car, nor I have I ever owned a street car that was ever modified in any way to resemble or perform like a race car. So take that as you may.

In this first post I am going to talk about aerodynamic changes to street cars, and more specifically, spoilers and wings. But before I begin, I need to define the difference between a spoiler and a wing. I also recognize that there is some controversy as to exact definitions, but I am going to go forward with what I think is the prevailing opinion. Also, in reference to spoilers, I'll be writing specifically about rear spoilers. I recognize that front "air dams" are often considered spoilers, but perhaps we'll save that one for another day.

First, the spoiler, and be advised I am copying and pasting some of this directly from Wikipedia because it says exactly what I intend to say:
A spoiler is a device whose intended design function is to 'spoil' unfavorable air movement across a body of a vehicle in motion; it functions by disrupting or diffusing the airflow passing over and around a moving vehicle. This diffusion is accomplished by increasing amounts of turbulence flowing over the shape, "spoiling" the laminar flow and providing a cushion for the laminar boundary layer.
The goal of the spoiler in racing is to improve handling and fuel efficiency by decreasing the lift that is generated by the car moving forward.

Second, the wing:
A wing is an aerodynamic device intended to increase drag and/or generate down force on an automobile.
While an airplane wing produces lift upwards, an automotive wing does just the opposite by providing lift downwards. This increases downforce on the rear of the vehicle but also increases drag. By increasing rear downforce, the tires grip the road better, providing for better handling and higher speeds.

Now I fully recognize that many passenger cars come with spoilers and/or wings pre-installed, although these are usually very innocuous, and are solely designed for cosmetic purposes. So I'm not talking about these devices. I am speaking specifically to obviously after-market spoilers and wings.

Here are my issues, generally, regarding spoilers and wings:
  • The desirable aerodynamic effects that you would intend from spoilers and wings do not really present themselves until you reach typical highway speeds, and even then, the effect is not considerable.
  • Surely, many people do drive faster than highway speeds, but I suspect that driving considerably faster (like, 100+ mph) isn't an every-mile occurrence, and even then, I would argue the aerodynamic effects still aren't that pronounced.
  • But...you actually do race your street vehicle? I guess that's possible and undoubtedly happens, but I'm not sure why you'd want to do that on a regular occasion.
  • The wing, which is intended to provide downforce on the rear wheels, is often installed on cars with front wheel drive. In most (although not necessarily all) cases, this is counter-intuitive, and tends to increase understeer. While understeer is designed into most passenger cars, it exists for stability and control.
  • Lastly, and I understand this is just my opinion, but large wings on street cars just look dumb. :-)
Is it possible that some after-market spoilers and wings are added with ignorance to the above information? I guess it's possible, but if you're going to spend money, I'd hope you'd know what you're doing it for. Is it also possible that some after-market spoilers and wings are added with full knowledge of the above information, but still did so simply for decorative or "status" purposes? Again, no doubt it's possible.

In summary: generally speaking, your street car doesn't travel fast enough to have the desired aerodynamic effects that are associated with after-market spoilers and wings.

What are your thoughts?
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