The total coefficient of drag on my racing Miata (hard top, airdam, splitter, wing) was measured precisely at Cd .48. Where does that drag come from, and how can you reduce it?
The following table shows estimates of where that drag comes from. You’ll notice a splitter reduces drag, but every other feature adds to it. This data is largely from Katz, but I’ve supplemented this with some Miata values and racing parts (airdam, splitter, 9LR wing) and included a value for the windows being open (we are racing, after all).
|Rear wheels + suspension||.023|
|Front wheels + suspension||.025|
|Underbody and chassis||.085|
|Total drag coefficient||.480|
There are lots of ways to reduce drag. One way is to use a fastback, which reduces the drag in three places: bodywork, rear surfaces, and open windows (my fastback is narrower and considerably less air goes in the windows).
My fastback reduced drag by .07 to a Cd of .41. What does a 15% reduction in drag do for lap times? I’ll run a quick simulation in OptimumLap (2375 lbs, 125 hp, 1.15 grip).
Right away you can see that drag reduction at a course like Pineview (or autocross) is nearly worthless. At a medium-speed track like New York Safety Track, it amounts to less than half a second. But at a high speed track like Watkins Glen, drag reduction is worth 1.2 seconds! So the value of drag reduction depends entirely on the venue.
For endurance racing, drag reduction is almost always worthwhile, but you need to make large gains. A 15% reduction in drag works out to only 2% less fuel usage at Watkins Glen. But if you’re right on the cusp of a 1:55 stint time, that’s huge.
Every once in a while someone talks to me about reducing the drag from their wing, or choosing one single-element wing over another because it’s more efficient. If you look at the table above, my 9 Lives Racing wing only adds .03 drag. Even if you could reduce your wing drag by 20% (doubtful), that would be a .1 second per lap at WGI and .03 seconds at NYST. Seriously, there are bigger fish to fry!
When setting a course for drag reduction, start at the bottom of the table, where the biggest items are, and work your way up.
Wing Angle, Efficiency, and Downforce
If you have a wing, your next question might be, is it better to run less angle and have a more efficient wing, or more angle for downforce? Assuming you can set the car up to handle equally in each situation, which would be fastest?
Let’s run another simulation. I’ll use my data for a 9 Lives Racing wing and use three values: 0-degrees AOA representing the most efficient setting, 10 degrees with a 1/2″ Gurney flap representing the most downforce and drag, and 5 degrees no Gurney to split the difference. I’ll use the same car and the same three race tracks.
|Zero angle of attack, L/D 14:1, Cd .461||2:21.22||1:39.90||1:15.66|
|5 degree, L/D 13:1, Cd .48||2:20.55||1:39.40||1:15.38|
|10 degree, 1/2″ Gurney, L/D 8.5:1, Cd .52||2:20.76||1:39.33||1:15.27|
What’s interesting to note here is that the most efficient setting (zero degrees angle of attack and no gurney flap) is the slowest at all tracks. The setting with the most downforce (10 deg AOA, 1/2″ Gurney) is fastest at Pineview and NYST, but not at WGI.
The best wing angle for your car is obviously track dependent, but if I was going to set it and forget it, I’d run the wing at 5 degrees and add a 1/4″ Gurney flap. I’m not going to run that simulation for you, I have to keep some speed secrets for myself.