Tag: 9 Lives Racing

The following article is made up of excepts from my wind tunnel report. You’ll get a more cohesive story, and a lot more data on many more aerodynamic parts, if you buy the report and read it end to end.

End plates on wings are necessary; they separate the low-pressure region under the wing from the high-pressure area on top of the wing. The suction side of the wing is what does most of the work, so by keeping the high pressure side from bleeding into the suction side, the wing makes more downforce.

The shape of the low-pressure region under the wing is different for every airfoil. However, for most wings designed for motorsports, you’ll find that the low-pressure region is at the front of the wing and often extends ahead of the wing. The low-pressure area extends about a chord’s distance below the wing as well.

Given that information, and after looking at the preceding image, you might conclude that a good endplate should be shaped to exactly cover the high and low pressure regions of the wing. And from that, you might surmise that a good endplate for a 10” chord motorsports wing should extend 10” below the wing, and should have a lot of surface area concentrated at the front. And that’s how I see it as well. However, some end plates have most of the area towards the rear of the wing, and I can’t say I understand that. But aerodynamics is full of weird contradictions, and perhaps some of those end plates work.

With the amount of companies selling improved end plates with different shapes and sizes, you’d assume there was something to be gained over a plain rectangular end plate. And because some of these fancy end plates cost a couple hundred dollars, and boast CFD-designed pedigrees, they must be doing something useful, right?

CFD and wing efficiency

Some of my wind tunnel data conflicts with published CFD (computational fluid dynamics) data. This isn’t surprising, as CFD is just a computer calculation, and not real-world data. Manufacturers typically test wings in free-stream CFD, meaning that the wing is suspended in mid-air, as only a computer simulation can do. This is the best way to calculate what happens when you change wing angle, add Gurney flaps, or change the shape and size of end plates. Free stream CFD is essential, because it eliminates everything in front of the wing. This is really the only way to compare one thing to another.

But when you put a wing on a car, everything in front of the wing affects its performance. Wind speed and direction, cars in front of you, and open windows can make a huge difference. Plus there’s the shape of your car, the angle of the windshield, aerodynamic devices on your car, like splitter, canards, hood vents, vortex generators, GPS antenna, wing stands, … you name it, every single thing that’s in front of your wing changes how it performs. You’ll never get the same amount of downforce from your wing as the free stream CFD data shows. Not even close.

You can research wings on Airfoil Tools or Bigfoil, or use tools like Javafoil and CFD, and you’ll find wings that have a 14:1 L/D ratio, or better. But when you put the wing on the car and adjust the angle of attack, you’ll be stoked when your wing has half of that.

For this reason, anything you do to improve the efficiency of the wing in free-stream CFD is meaningless until you put it on the car. For example, modifications to wing end plates can reduce drag, and this shows up in CFD as a gain in wing efficiency. But when you put the wing on the car, the drag of the wing is inconsequential to the total vehicle drag. Touring cars are essentially huge rounded bricks, and wings are tiny streamlined objects by comparison, and so you can understand that the drag from the wing is essentially nothing compared to the drag of the vehicle. 

In reality, the only thing that matters is the aerodynamic efficiency of the entire vehicle, and you typically get that by going after as much wing downforce as possible. Modifications to the end plate that reduce drag might increase free stream wing efficiency, but they do that by reducing wing downforce. And this makes the L/D ratio of the car worse, and the car goes slower. Ergo, it’s utterly worthless to optimize wing efficiency in free stream CFD by reducing drag. If you use CFD for anything, it should be for optimizing the wing for maximum downforce.

Testing end plates in a wind tunnel

Before I get to the testing data, let me tell you exactly how shit stupid I am. My aerodynamics sensei Kyle Forster had these things to say about end plates:

• Use a rectangular shape. Adding vents, cuts, and other tricks are more likely to reduce the performance of the wing than improve it.
• Optimizing the performance of the wing end plates is the least important part of the entire vehicle’s aero package.

You might think I would take Kyle at his word. He worked for the Mercedes Formula 1 team as an aerodynamics engineer during the manufacturer’s most dominant years. But I’m also a stubborn, pig-headed ass who believes in getting his own data. So I took three end plates (four if you count that I turned one backwards) to the A2 wind tunnel and spent my hard-earned money to see if he was right.

I tested four wings in the wind tunnel, but when I got around to testing the end plates, they were all swapped onto a 55″ (1397 mm) 9 Lives Racing wing. This is the benchmark motorsports wing for many good reasons, so I figured why not go with the industry standard.

I first tested the basic rectangular end plate, which is made from an aluminum street sign that I simply cut in half, rounded the corners, and called it done.

I then swapped those for a popular CFD-designed end plate. I’ve always found this design to fly in the face of reason – why is there a big cut out right where the low-pressure region is?

I then turned the end plate backwards and tested that. From my point of view, it seems like the end plate might perform better with more surface area facing forward and less at the rear. It wasn’t a great fit, though.

Finally I tested an end plate of my own design. It’s in some ways the opposite of the CFD end plate, having a lot of surface area forward and tapering towards the rear. There’s a very small relief cut on the upper back corner that’s supposed to reduce a vortex there (er… so I’ve read). But more significantly, this end plate has a very small wicker on the trailing edge.

Let’s see how the end plates performed:

End plate	cD	cL	Vehicle L/D
Rectangular	.467	-.382	.82
CFD	.475	-.386	.81
CFD backwards	.474	-.385	.81
Occam’s Racer	.480	-.398	.83

Wind tunnel data

So let’s unpack the coefficient data and translate that into more common figures, like pounds of downforce and horsepower consumed.

• Rectangular – Vehicle L/D ratio .82
  • Baseline to compare with other end plates
• CFD – Vehicle L/D ratio .81
  • +1.8 lbs total downforce
  • 4.1 lbs drag = 0.4:1 L/D ratio for end plate
  • +1.1 hp used from drag
• CFD backwards – Vehicle L/D ratio .81
  • +1.5 lbs total downforce
  • 3.5 lbs drag = 0.4:1 L/D ratio for end plate
  • +.9 hp used from drag
• Occam’s Racer – Vehicle L/D ratio .83
  • +8.8 lbs total downforce
  • 6.9 lbs drag = 1.3:1 L/D ratio for end plate
  • +1.8 hp used from drag

The first thing you’ll notice is that the CFD end plate increased downforce by almost 2 lbs at 100 mph. That’s more than the rectangular end plate, but not much. As a consequence of that additional downforce, there’s a bit more drag.

Turning the CFD end plate backwards resulted in less downforce, but also less drag. Overall, the CFD end plate performed the same forwards as backwards. Surprising.

Another surprise was that my Occam’s Racer end plates gained 8.8 lbs of downforce over the rectangular plate. Not surprising, this also resulted in more drag. However, these end plates resulted in the best vehicle L/D ratio.

Racing simulations

Those numbers are all very close, and you might be wondering how they affect the only thing that matters: lap times!

To find out, I put the coefficient of lift and drag values into OptimumLap and ran them around two race tracks, the autocross course from 2010 SCCA Solo Nationals, and Lime Rock Park. I typically use these two tracks because they are close in lap time, but are completely different with respect to speed. I’ve also included my local track Watkins Glen, because it’s very high speed and should spread the results out more. (My wind tunnel report shows lap time comparisons for every part that I test, as well as some useful combinations.)

End plate	cD	cL	Autocross	Lime Rock	WGI
Rectangular	.467	.382	61.79	61.03	134.74
CFD	.475	.386	61.79	61.05	134.81
CFD backwards	.474	.385	61.79	61.05	134.81
Occam’s Racer	.480	.398	61.78	61.04	134.81

On the autocross track, my end plates won by a whopping .01 seconds. At Lime Rock, the rectangular street signs won by the same insignificant margin. At Watkins Glen, the “No Skateboarding” sign went .07 seconds faster than either of the fancy end plates.

Discussion

The CFD-designed end plates were a disappointment, and put an exclamation point on my rant at the beginning of this post about using free stream CFD. Look, I’m not at all doubting that these end plates worked better in CFD and returned exactly what they calculated. But overall performance didn’t change facing either way, and shows how useless free stream CFD can be in the real world.

The custom end plates I designed are based on a hunch that I should put most of the area low and forward. More significantly, these end plates have a small Gurney flap on the outer edge, and it’s likely that the shape of the end plate was no better than the others, and it was simply the addition of the wicker that gave this end plate the most downforce. Most downforce doesn’t mean best, as it wasn’t terribly efficient and only returned a 1.3:1 L/D ratio above and beyond the rectangular plate. Using these end plates would make the car faster on a tight track, but slower on most race tracks.

In the end, there really is nothing wrong with a rectangular end plate. If you don’t have the means of CFD testing your entire vehicle and optimizing the end plates to your entire car, then a rectangular end plate is your best bet. I make mine from street signs that I buy from my local metal recycler for $1 per pound. And so both end plates are less than $2 and come with amusing graphics. It astonishes me that people will pay much more for something that performs worse.

But let’s face it, no matter what you do to the end plate, it’s not going to make it significantly worse, either. End plates with cuts and vents are a great place for personalization, they are a source of many silly conversations, and they throw competitors off the scent. In the end, I say do whatever you want, it won’t matter much anyway.

But if you want to hedge your bets and do the least work, just stick to a rectangular end plate. Learning this lesson cost me $300 plus a lot of time and effort, and it proves I should just have just believed Kyle Forster.

But it wasn’t a total waste of time and money if that keeps other people from making the same mistake. So if this article saves you money, please consider buying me a coffee, or if you want 50+ pages of the same kind of data, then buy my wind tunnel report. It’s only through contributions that I can afford to do wind tunnel testing and continue the lord’s work. Thanks!

Postscript

After posting this article, I got a bunch of great comments on the Professional Awesome Technical Forum on Facebook. This group has a wealth of knowledge that surpasses my own, so please see this post, scroll through the comments, and benefit from the global knowledge base.