Big Lies #6: aero doesn’t matter

This is post #6 in a series of articles about some of the biggest lies in high performance driving. In this post, I’m going to address one of my least favorite topics: aerodynamics.

Why I don’t talk about aero

If you are a long time reader of my blog, you know that my posts are mostly about safety, driving technique, sim racing, and data analysis. I sometimes talk about tires, rarely talk about brakes or suspension, and never talk about engine or aero. Why is that? Three reasons.

1. I value the driver first, the interaction of the driver with the machine second, and the machine itself last. However, I think most driving enthusiasts have the opposite priorities. How many people spend $2k improving their cars? Lots. How many spend $2K on coaching or sim racing? Not many. I’m generally offended by car enthusiasts for that reason.
2. This may be silly, but I don’t like the look of proper aero. I prefer the classic lines of the 1960s and such when aero was completely misunderstood.
3. Honestly, the most important reason for not talking about aero is that I don’t know much about it. It’s difficult to measure without the right equipment. To become knowledgeable, I’d have to do a lot of reading. I am similarly willfully ignorant of engines.

Aero matters at all speeds

Let’s ignore my biases for the time being and actually talk about improving the vehicle. Specifically, let’s talk about aerodynamics. Put your hand outside your car window at speed and you can get a profound appreciation for how the shape of your hand affects its disposition. Some shapes cause lots of drag. Others produce lift or downforce. As you drive around a track, the path the air takes around your vehicle similarly affects its handling and performance.

If you want to make your car faster, the best investment you can make is grip. Grip determines your speed out of a corner, your initial speed down the straight, and how late you can brake. A well sorted aero package makes you faster because it increases grip. The faster you go, the more important aero is, but even at autocross speeds it can be the difference between winning and losing. If you want to see some hard numbers, I suggest you check out my brother’s blog, Occam’s Racer, which is a site that focuses on Miata aerodynamics. He has done the right experiments with the right equipment. Aerodynamics isn’t just a few tenths of a second, but whole seconds. There are several ways to buy whole seconds of speed like dropping in a more powerful engine or upgrading tires. A basic aero package isn’t very expensive and is a one time cost.

Components

Aero modifications fall into 3 categories

1. Front
2. Rear
3. Underbody

One of the best things you can do to improve your aero is to install an air dam and splitter. Not only does this increase  downforce, it also reduces drag. So there’s really no reason not to have one.

The most common modification people make is to add a spoiler or wing. Both provide similar benefits. Most car shapes produce lift, and the faster the vehicle goes, the more lift is generated. Spoilers can negate this lift and wings can do one better and provide downforce. The combination of a splitter and wing is highly effective.

The underbody of a car is aerodynamically under-appreciated. If you’re going all-in, a flat bottom, side skirts, and diffuser should be part of your plan. Again, because they provide more grip. But stay away from vortex generators, which do nothing.

Results

Here are a few important results from Mario’s real world testing at Watkins Glen with thousands of dollars of sensors decorating his Miata.

• Adding a 4″ splitter to an airdam reduces drag slightly, (.01 Cd) and adds a lot of downforce (.38 Cl).
• If you race with an open top and a wing, it’s faster than an open top without a wing. However, an OEM hard top helps the wing generate 250% more downforce than an open top.
• My fastback measured 17% less drag than an OEM hardtop. When used with a wing, the fastback generated 130% more rear downforce. In other words, the 60″ wing behaved as a 78″ wing, but without any more drag.
• Don’t use vortex generators, particularly if you use a wing.
• A cheap dual-element wing created a lot of drag, but also created a decent amount of downforce. Using one is faster than not using one.
• The 9 Lives Racing single-element wing made less drag and more downforce than the cheap dual wing. By a lot.

Is aero worth the costs?

If your goal is to increase the speed at which you can lap a track, then aero is a worthwhile investment. It will make your car faster if installed properly. But aero isn’t free. It comes with costs:

• Money – Modifying your car costs money, and good aero products cost more than cheap ones. An aluminum eBay wing with 2″ uprights doesn’t have the same value as a fiberglass wing with swan neck supports.
• Weight – All aero adds weight to your car, and sometimes that weight is high up where you don’t want it.
• Drag – Aero may increase or decrease drag depending on the component, but if you’re maximizing downforce you will increase drag. The benefits of downforce almost always outweigh drag.
• Time – Aero adds an additional layer of tuning, which means addition time spent testing and tuning.
• Parts – Aero adds parts to your car and parts have a way of breaking.

If I was building up a new car, would I add aero to it? That depends on what I was using it for. For daily driving with the occasional track or coaching day, probably not. Driving around with a splitter would be a nuisance. However, if I was building a car for competition, then yes, of course. Aero is a relatively inexpensive way to make a car go faster.

Wings on FWD cars?

Do wings make sense on FWD cars? Oddly, it turns out that they make a lot of sense. Maybe even more sense than RWD cars. FWD cars should be set up to turn via lift throttle oversteer. Having an unstable rear end makes FWD cars easier to drive in slow corners. However, an unstable rear end in a high speed corner is nerve-wracking and dangerous. So slap a wing on there. At low speeds it won’t affect handling much, but at high speeds it will pin the rear of the car down. I’ve experienced this firsthand in my Yaris. To my shock, even a cheap eBay wing makes a difference in high speed corners. The next developments for the Yaris will be a splitter and a proper wing.