For most of the test I used a 60″ 9 Lives Racing wing, but I wanted to see what would happen without it. The coefficient of drag went down by .03 for all configurations, but total downforce took a nosedive. Significantly, this was the first time in the test that the car generated lift instead of downforce.
While the wing and splitter were off the car, we figured we’d test the other tops as well. First we threw the Chop Top back on and did another run. Then we attached the rear section, making it into a fastback again. All of these runs are captured in the following table. We couldn’t test the OEM hard top without the VGs (they being stuck tenaciously to the roof), so I made an educated guess on the Total Cl and Cd values. Theses are in italics in the table below.
| Configuration | Front Cl | Rear Cl | Total Cl | Cd | L/D % | HP @ 100mph |
| Hard top, VGs, no splitter, with wing | 0.18 | -0.64 | -0.47 | 0.53 | 0.88 | 58.75 |
| Hard top, VGs, no splitter, no wing | 0.22 | 0.38 | 0.59 | 0.49 | -1.20 | 54.53 |
| Chop top, no splitter, no wing | 0.11 | 0.20 | 0.31 | 0.48 | -0.64 | 52.93 |
| Fastback, no splitter, no wing | 0.16 | 0.18 | 0.35 | 0.38 | -0.80 | 42.00 |
| OEM hard top, no splitter, no wing | .50 | .45 |
What’s surprising here is that without a wing, the chop top has the best L/D ratio. This is largely because it creates the least lift. Remember that negative lift values are what we’re looking for (downforce), and the chop top’s .31 Cl has the least lift. The fastback creates more lift than the chop top, but it does so with less drag, and in the end, this is makes a faster car.
Unfortunately we didn’t get data for a bare OEM hardtop (without VGs, wing, or splitter), so we don’t know if it’s the shape of the OEM roof, or the VGs that create so much lift. But the total Cl value of 0.59 is quite a bit worse than either the chop top or fastback. Based on the data we obtained doing the wing tests, a bare OEM hard top should have a CD of about .45. It’s hard to imagine the vortex generators adding more than 10% lift, and that would put the total lift around .50.
Let’s see what happens in OptimumLap when we remove the wing.
| Wing or no | Watkins Glen | Waterford Hills | 2010 SCCA Nationals |
| VGs, no splitter, 9LR wing | 2:25.65 | 1:21.12 | 1:03.89 |
| Hard top, VGs, no splitter, no wing | 2:29.14 | 1:23.14 | 1:04.91 |
Yikes, that sucks! The wing is worth 3.5 seconds at WGI and even 2 seconds at a short track like Wateford? Amazing.
Now let’s just compare the different tops without wings.
| Tops without wings | Watkins Glen | Waterford Hills | 2010 SCCA Nationals |
| Hard top, VGs, no splitter, no wing | 2:29.14 | 1:23.14 | 1:04.91 |
| Chop top, no splitter, no wing | 2:27.74 | 1:22.85 | 1:04.63 |
| Fastback, no splitter, no wing | 2:26.51 | 1:22.42 | 1:04.63 |
| OEM hard top, no splitter, no wing | 2:28.17 | 1:22.87 | 1:04.80 |
Here we can see that the fastback is still the fastest configuration, but it’s not a huge difference unless you’re at a high-speed track like WGI. Without a wing, I’d be happy to use a Chop Top at most tracks for the light weight and convenience of mucking with stuff in the cockpit. And it beats the OEM hardtop without factoring in the 38 pound weight loss.
One thing that’s conclusive here is that if the rules allow it, use a wing. This probably even applies to classes like NASA ST6/TT6 that carry a substantial penalty for running a wing.
Occam's Racer 