Spoiler Roundup

Over the past several months, I’ve tested eight different spoilers on my car. I could test more, but I’ve reached a point where I would rather install something permanently and move on to other things, leaving this spoiler project behind. Hence this Spoiler Roundup. Let's meet the competitors:

Straight Gurney flap

Sinusoidal Gurney flap

Commercial lip strip

10° board

20° board

20° board with slot

30° board

Hellcat. I want this one to win only because I enjoy saying "Hellcat."

Which one should I use, if any? That depends on a few things, including my goals, my design limitations, and how much I want to permanently alter the car.
Goals: I want to reduce lift and improve stability as much as possible without increasing drag or, if possible, even decreasing drag.
Limitations: I don’t want to extend the factory spoiler backward at all. Upward is fine, but I use a hatchback tent when I roadtrip that won’t fit if I elongate the spoiler.
Alterations: I don’t care if I have to drill into the stock spoiler or fabricate my own from plastic. I also don't care if I permanently alter or even destroy the stock spoiler because I have an extra spoiler in storage. If I ever want to go back to stock it’s as easy as swapping that one on.
I’ll make a decision based on the data I’ve collected for each spoiler, mainly pressure measurements on the back window and trunk. Here are the pressure differences from standard for each spoiler, laid out in one chart:

I’ve left out the spoiler + fins tests since they weren’t much different from the spoilers alone.
An important step in analyzing experimental results like this is the “gut check”: do the data look reasonable? For instance, if I graphed the pressure changes like this and found that the huge Hellcat spoiler made a smaller difference than the short Gurney flaps, that would stick out as something unexpected and require further testing. Everything looks good here; bigger spoilers or steeper angles generally increase pressure on the window and decrease pressure on the base more than smaller spoilers, as expected. Check.
Next I’ll estimate the changes in drag and lift using the method I outlined in my last post:

Positive change on each axis shows reduction in drag or lift.

Are these reasonable? Well, they seem to be, just comparing them to the pressure changes. I haven’t had a chance yet to install the ride height sensor I purchased a couple months ago, and unfortunately I don’t have an easy way of measuring drag changes on this car. But I did perform some coastdown testing recently and verified that the expected change in drag from adding the Hellcat spoiler (+15% or so) correlates with the change in magnitude of acceleration (+13-14%) I measured in that test. That suggests that this method of estimating drag and lift changes is good enough for comparison since the calculation was fairly close to reality for one of these spoilers. Bigger spoilers or steeper angles generally decrease lift and increase drag more than the smaller ones, as expected. Check.
Decision Time
Now the question is: out of the spoilers I’ve tested, is there an optimum? That is, does one of the spoilers fit my goals and design parameters better than the others?
Yes—and it turns out, it’s the spoiler I’ve had installed on my car the last two years.
The Hellcat and steeper board reduce lift a lot but also increase drag a lot—so those are out. The small Gurney flaps and shallower board don’t increase drag as much but also have a smaller effect on lift. And the middle boards moderately decrease lift with a small increase in drag. But the lip strip—which is similar in height to the Gurney flaps but has a nice curve rather than just sticking straight up abruptly—decreases lift as much as the flaps and may also decrease drag slightly. It does that without extending the spoiler at all and with minimal increase in height, so my tent will still fit just fine. And it’s a ridiculously easy installation since the strip is just taped onto the existing spoiler. We have a winner!
Pay attention to the process I used to get here. This is how you should design and test to optimize changes to your car. Don’t build something and hope it works, or assume that measurements from another car transfer to yours, or guess at what you think the air will do over your car. Test a lot of designs, analyze the results, compare them to your goals, and then make a permanent modification (or no modification if nothing you tested did what you wanted). This is the process that OEMs use, and it results in interesting designs such as this example:

Those are small dams inside the slots of the Ioniq 5’s rear spoiler. How do you think Hyundai engineers figured out that those do something beneficial (whatever that may be)? Do you think they guessed and went right to production? Or did they test a bunch of designs, analyze the results, compare them to their design goals, and select the one that worked best?
This time around, I carefully removed the cheap double-sided tape that came on the spoiler and added stronger Gorilla VHB tape. Hopefully this will stand up better and last longer than the 2 years I got out of the first installation.

On the car and ready to go:

Go try this on your own car, with whatever modification you're interested in investigating. You will be amazed at what you can learn through testing.