There are a lot of racing organizations that have rules which allow cars to have a diffuser, but don’t allow a flat bottom. If you don’t understand how a diffuser works, you might think this is some kind of advantage. If you understand how a diffuser works, you might take a hard pass. So how does a diffuser work?
A diffuser helps air to expand both within the chamber, and in the wake of the car. As a result of this, the air in front of the diffuser drops in pressure and increases in velocity. The result is downforce. Contrary to popular belief, the diffuser itself isn’t where downforce is made; downforce is located where the greatest restriction is, in front of the diffuser.
So, if you have a car without a flat bottom, what’s in front of your diffuser? On the minority of cars, the manufacturer has done a good job making everything smooth, and it can almost replicate a full flat bottom. However, on most cars, and certainly my cars, there’s a shit ton of stuff in the way: transmission, differential, exhaust piping, suspension components, fuel tank, hoses, brackets, exposed frame members, etc. When air hits all of those pieces under the car, it creates local flow separations and drag. Accelerating air through that maze of parts doesn’t create downforce, it creates turbulence.
Clean airflow makes the most downforce, and thus turbulence is the enemy. Let me give you an example using wings, because I have solid data on that. My Miata has a DIY fastback that provides clean airflow to the rear wing, and it makes 390 lbs of downforce. If I use an OEM hardtop, which has more turbulence around the sides of the canopy, the downforce drops to 300 lbs at the same speed. If I then add AirTab vortex generators and thicken the boundary layer over the roof, the wing makes only 216 lbs. And finally, if I remove the top altogether so that the car is a convertible, the wing makes just 120 lbs of downforce. (These figures come from my testing at Watkins Glen report | data).
So while you can make downforce in turbulent conditions, clean airflow is obviously better, and the situation underneath the car is no different than on top. This is why proper race cars with diffusers have a flat bottom or tunnel under the car, so that they can get clean airflow.
But on a car that must to adhere to rules that don’t allow a flat bottom, the area in front of the diffuser is often a total mess. Moreover, because the diffuser has to begin before the rear axle, any downforce you create is only on the rear tires. Unless your diffuser has a better L/D ratio than your wing, why would you do it?
Maybe that’s a difficult question, let me explain first. On a proper racecar with a flat bottom and diffuser, the throat of the diffuser (where the downforce is located) is often ahead of the rear wheels, around the middle of the car. This means that the downforce is created equally over the front and rear tires. Some diffusers extend even further forward, making more front downforce than rear. Typically front downforce is harder to attain than rear downforce, so this is sort of the holy grail of underbody aero, the way I see it.
Conversely, if your car doesn’t have a flat bottom, and your diffuser begins at the rear axle, you are creating downforce over the rear tires only. A car with a rear wing already creates a lot of downforce over the rear tires, and it does so very efficiently. So unless your diffuser is more efficient than your wing, adding more wing is often a better way to make rear downforce (more wing angle, bigger Gurney flap, more planform area, greater coefficient of lift, etc..).
Nature abhors a vacuum
Let’s play the fantasy game where, without a flat floor, you somehow manage to create a low pressure area in front of the diffuser. Fact: the air everywhere around it is at higher pressure. Because nature abhors a vacuum, the air outside wants to invade the area inside, to balance the pressures as it were.
With great suction comes great responsibility, and so defending that low pressure area becomes your life’s work. There are many ways to seal off the area under the car, such as side skirts (barge boards), or by creating vortices on the sides of the car using canards, or under the car using strakes.
Having done all of that, you also have to attend to what’s happening with your wheels and tires. The most significant problem is that as your tires roll forward, they compress the air underneath them, like a supercharger. This tire squirt sends a high pressure jet of air out both sides of the tire, directly in front of your diffuser! You’ll recall that a diffuser only works because it creates low pressure, and so a jet of high pressure air is a significant problem. It’s less important, but air also intrudes through the spokes of your wheels, and lower caketin covers are required to block this air from going under the diffuser as well.
So as you can see, if you want to make downforce using a diffuser without a flat bottom, the odds are not in your favor. The air under the car is likely turbulent to begin with, and mother nature herself is actively working against you. You have to protect the area of suction as best you can, and if everything goes 100% to plan, you’re still only making downforce over just the rear wheels. When all is said and done, rear downforce is usually gained much more efficiently using a wing.
But I have CFD proof!
But wait, you say, I’ve seen CFD that proves that a diffuser works without a flat bottom!
In the Verus Engineering blog they did a neat CFD study called Is a Flat Underbody Necessary for a Rear Diffuser to Function? The data shows that, compared to a car with a dirty bottom, a diffuser without a flat bottom reduced drag by 26.2 lbs and made 10.2 lbs of downforce. Anytime you can reduce drag and gain downforce, you take it, so this looks like a clear win.
But you could also read their data in a completely different way. Note that the flat bottom alone (without a diffuser) made 23% more downforce than the flat bottom with a diffuser, and only gained 3.5% drag in doing so. So based on this CFD data, one could conclude that this diffuser reduced the effectiveness of a flat floor.
Now that’s a pretty strange conclusion, because I would imagine that any diffuser would help a flat bottom work better. It makes me wonder if the CFD model is too simplistic. Let me not be too critical, because Verus and everyone else publishing CFD is doing us all a favor showing us this data. Computers are simply tools, and with a refinement of those tools, we’ll get better and better data. Let’s just keep moving ahead.
Next, I’ll take a look at Kyle Forsters videos. He has two CFD videos examining flat floors and diffusers, using a NC Miata:
- Cut bumper vs Diffuser – Kyle’s first video is just a cut bumper vs diffuser. He didn’t test a flat floor, and the muffler got in the way a bit.
- Flat Floor vs Diffuser – In the second video, he uses a flat floor and gets different results.
I watched the videos, and made the following notes. (The downforce and drag values are at 180 kph, or about 112 mph.)
- A cut bumper added 6 kg downforce and reduced drag by 1 kg.
- A diffuser with the muffler in the way made 4 kg downforce and 4kg more drag. This is not as good as the cut bumper.
- Improving the diffuser by removing the muffler added 11 kg downforce, but drag remained the same. Based on this, the cut bumper is still better than the diffuser without a flat bottom.
- A flat floor with a cut bumper made 49 kg of downforce and reduced drag by 15 kg.
- A flat floor with a diffuser made 50 kg of downforce and reduced drag by 18 kg.
Gleaning this data was a bit difficult, because the comparative data is split across two videos. There are inconsistencies as well; In one video he says the diffuser is 3x more effective at creating downforce than a cut bumper (both with flat floors), and in another video the cut bumper data is virtually the same as the diffuser. So, just like with the previous CFD, there are inconsistencies with the tool, or the operator, or the person interpreting it (mea culpa).
I’ve taken Kyle Forster’s course on aerodynamics, and there’s a lot of CFD on underbody aero, but not much without a flat bottom. However, I’ve had some private consultations with Kyle, and in one of those he showed me a hush-hush diffuser from overseas that was designed to work without a flat bottom. He wouldn’t go into details on the design, and when I asked him how many points that was worth, he shut me down quickly, saying that was private information. But know this, it is possible for a car manufacturer and surely an F1 aerodynamicist to make a diffuser work without a flat bottom. The question is, can you or I modify our cars to do that?
My DIY diffuser
I wanted to know if a diffuser without a flat floor was worth it, so I built one and tested it in the A2 wind tunnel. It’s actually quite neat looking and follows basic aerodynamic principles of vertical and lateral diffusion. With it mounted on the car I was like… damn, that’s cool looking!
Obviously the results in this section apply only to this diffuser on this car, and your results will certainly differ. If you are building your own diffuser, you might take some of my build details and do them differently. (Or better yet, do something else with your time.)
The diffuser is as wide as I could make it between the wheels and extends to the rear axle. It has about 3″ of ground clearance at the front, and the leading edge angles up slightly. My thinking here was to make the area of least restriction (where the downforce is located) within the diffuser itself. I thought this was pretty clever, but this might have been a mistake.
I added strakes that diverge from the centerline. These are supposed to spin a vortex off the trailing edge, which should help defeat some tire squirt, by sealing off the center compartment. Thus, the outer chambers of the diffuser are mostly sacrificial in nature, allowing the center tunnel to do most of the work.
The strakes are 3″ off the ground, because at least two of the rulebooks I was looking at state that the minimum ground clearance was 3″. Perhaps the diffuser would have worked better if the strakes were closer to the ground, but rules be rules.
Another detail that didn’t work in its favor is that I made a quick-release diffuser that attached to the car via the trailer hitch and two zip ties. On the upside, I was able to remove the diffuser in one minute. On the downside, there’s a longitudinal cross member behind the muffler that blocks air moving rearward above the diffuser. See the image below.
Is this significant? I don’t know. I was mostly concerned with air going under the diffuser, and reckoned anything that snuck between the muffler and top of the diffuser wouldn’t be important. If I had to guess, this probably added some drag, but might have also kicked up some air like a spoiler or big Gurney flap. I’ll have to retest this on my Miata in the future, with topside designed so that air can travel cleanly on top and below.
So how did my DIY diffuser work? If you buy my wind tunnel report you’ll get the full story, but the abridged version is the diffuser made about 15 lbs of rear downforce at a cost of about 3.5 lbs of drag (at 100 mph). That’s not a lot of downforce, but a favorable lift/drag ratio. Except that isn’t the whole truth.
The front of the car lost 14.5 lbs of downforce (when you push down on one end of the car, it goes up on the other), and so the net gain was only .5 lbs of downforce for 3.5 lbs of drag. The data shows that the diffuser would make the car slower. Yuck.
The loss of front downforce is normal, but for whatever reason, the ratio is really bad. I tried 16 different rear aero options in the wind tunnel (5 wings, 3 Gurney flaps, 4 end plates, 2 spoilers) and they all added rear downforce at the expense of front downforce. But all of them did so much more favorably than the diffuser. (In fact one spoiler added both front and rear downforce, but that’s a story for another day.) I don’t know why the diffuser lost an equal amount in front, it’s downright puzzling.
I’m an amateur aerodynamicist, but I’m well studied, and I have good DIY fabrication skills. I generally feel like I know what I’m doing, but I made a diffuser that made my car slower. But this was my first stab at a diffuser, and I can only improve on this. I also didn’t throw all the tricks at it, and I’m sure that side skirts could have helped. But note that the A2 wind tunnel doesn’t have a rolling road, and so the negative effects of tire squirt aren’t quantified here.
But even if I could have made the diffuser work better, I’m still only making rear downforce, and losing front downforce in the trade. In my mind, the entire game of budget touring car aerodynamics is getting more front downforce, that’s the real limiting factor in an aero package. From all of this I can conclude one thing, which is that a diffuser without a flat bottom is fuggin useless to me; it’s much easier for me to gain rear downforce by adding more wing angle, a larger Gurney flap, or using a larger wing.
Rules and reasons
Given this, why do the people who write racing rules allow a diffuser, but ban flat bottoms? I can think of three reasons:
- Cost control – Rule makers are trying to reduce the cost of a full-scale aerodynamics war. Enh, I guess that’s a concern, but compared to engine mods and tires, aero is the cheapest way to get performance, and a flat bottom is dead simple. I mean, it’s flat.
- Give the people what they want – Rule makers don’t want to restrict people from bolting on parts that look cool. This one I sort of understand. If someone likes the look of a diffuser, they should be allowed to use one. Even if it makes the car slower. But what if someone likes the look of a flat bottom? I mean, give the people what they want, right?
- Copy/paste – Rule makers are lazy or don’t know any better, so they copy and paste someone else’s rules. We’ve all been there.
Given those reasons, it’s understandable that some racing rules allow a diffuser, but not a flat bottom. Which rule sets are we talking about? I’m sure there are many more, but here’s a quick look:
- Grid Life – Street Modified
- NASA Time Trials – TT4
- Ontario Time Attack – All classes
- SCCA Time Trials Nationals – Max classes
But… not all cars are created equal. Some manufacturers have done a great job fairing and flattening the underside, and a diffuser will work well on those cars. Whereas on many cars, a diffuser without a flat bottom is going to be totally useless. For parity across different cars, I feel that any rules that allow diffusers should allow a flat bottom. Full stop.
Next stop… flat bottom
Despite failing miserably in the wind tunnel, I think my diffuser design is pretty good, and so obviously the next thing to test is how the diffuser works with a flat bottom. But this time I’ll test it on a proper racecar, not my Veloster. In the Spring, I’ll trailer Falconet down to A2 and test every combination of underbody aero including flat bottom, diffuser, side skirt, etc. And maybe I’ll acquire some diffusers manufactured by various companies and test those. Finally, I’m really curious about side skirts with no other aero, that should be an interesting one to throw into the mix.
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Occam's Racer 