For half a century, wings and spoilers have helped race cars stick to the track. Lately, though, another piece of equipment has joined the mix: underbody rear diffusers. Originally seen only on pro cars, these diffusers quickly made their way to the amateur ranks. Stroll through the grid at a club race, time trial or track day, and you’ll see these devices peeking out from beneath back bumpers.
Is it a case of monkey see, monkey do, or do these diffusers actually help a car stick to the road? To find out, we asked a real aero engineer, Steve Stafford. He cut his motorsports teeth in autocross, but since then he has done aero for Formula 1, Indy, NASCAR and Grand-Am efforts.
What Do Diffusers Do?
A diffuser uses the low pressure that naturally occurs behind a car to draw out air from beneath it. The result: downforce that hugs the tires to the track.
The walls of the diffuser contain the suction and increase the airflow through the unit’s entrance, or throat—normally its forward-most and lowest point. This increased flow at the throat reduces pressure by way of a venturi effect.
The length and angle of the diffuser dictates how far forward the increased downforce can act on the car. The longer and shallower the diffuser, the farther the downforce will reach.
A steeper angle on the diffuser translates linearly into more downforce, provided the diffuser doesn’t stall. A diffuser stalls because the air can only expand up to a certain rate. Stalling limits the downforce that can be created.
In a diffuser that is too steep or too short, the air will not be able to follow the surface smoothly; it will break away from the surface to find the path of least resistance. This places a theoretical maximum angle of 9 degrees, but height from the ground has much greater influence than any other factor.
How Can You Maximize Your Diffuser?
Rules may limit the size and location of a diffuser—if they don’t ban them altogether—so maximizing its performance can require a great deal of development for just those last few percent. There are a few ways to accomplish this, but they each come with caveats.
Photography Credit: David S. Wallens
- Incorporating the Exhaust: Allowing the exhaust pipes to dump into the diffuser helps accelerate air flow, providing more downforce.
The diffuser loses a significant portion of downforce as the car transitions from open throttle to closed throttle. This can shift the balance to the front when the driver least wants that to happen.
- Adding Strakes: Adding strakes or fences inside the diffuser can delay the stall of the airflow or allow more angle to be used.
This increases complexity and the possibility of very narrow operating ground-clearance ranges.
- Adding Wickers: Wickers can make the outside faces of the diffuser more efficient by reducing the inflow from the sides and keeping the low pressure contained in the unit. They can be used at the vertical trailing edges as well.
Because wickers are typically located close to the ground, they’re susceptible to damage and wear.
- Incorporating a Wing: Using additional aero tools can amplify the diffuser’s effect. A wing or other device that creates very low pressure near the diffuser exit can reduce stall tendencies or allow more diffuser angle to be run.
Now there are more parts in the puzzle to build and tune.
How Do You Build a Diffuser?
Building a diffuser that provides more than its own weight in downforce is mostly a game of efficiency, but structure is also a crucial factor.
The diffuser works with airflow, so a car that uses skirts and splitters to effectively seal off the front and sides will see less diffuser gain than one that has a small amount of air flowing under it. Remember, like a wing, a diffuser needs airflow to work.
The diffuser’s throat should be as far forward as possible and at the lowest part of the vehicle. This increases downforce without significantly affecting car balance. Plus, this location is unlikely to become sealed off unless the entire car bottoms out.
The throat also needs to have a relatively smooth flow leading into it. That may mean trying to have a flat area ahead of the diffuser to reduce any disturbances in the flow to the diffuser.
A diffuser can generate a large amount of downforce, and the effective suction on the part can be several hundred pounds. First, ensure that the material doesn’t flex; diffusers are most commonly made of carbon fiber, but fiberglass, wood and sheet metal can work, too—they’re just a little heavier. Second, make certain that the device is securely attached to the car. Spreading out the attachment loads to several locations will help to reduce the deflection of the diffuser. Plus, if a mounting point fails, there will be several backups.
Photography Credit: David S. Wallens
If the car exhibits more than 2 inches of suspension travel, consider running sliding skirts along the sides—if they’re legal for your class—to maximize the suction. Since the distance to the ground is such a major factor in the diffuser’s effectiveness, limiting the travel or using sliding walls that attempt to close off the sides will help. Sliding skirts can be created by adding a second wall panel pivot near the front of the diffuser and limiting it before it opens a gap at the back edge.
Since most cars don’t have unobstructed rear underbodies due to their suspensions and exhausts, sometimes a squared-off diffuser profile shape can be adjusted to maximize the benefits. This may mean that the diffuser becomes shallower but wider in those areas. This isn’t ideal, but it is a way to continue the diffuser as far forward as possible.
Parting Words
Since the diffuser is making downforce, there is likely to be an increase in drag. Generally this effect is small at normal speeds, but it can be substantial as a car approaches terminal speed. The key, of course, is designing an effective diffuser and then properly testing and tuning it for maximum efficiency.
