Building an RC Car with Extreme Downforce!

What would happen you modify a 100mph RC car with a massive fan to increase downforce? Well... we did just that!

You should check out the full video here, or read on for more depth about this fun Project Air experiment!

Starting with the Stock Car
The Arrma Limitless is fast but it has its limits in cornering. Being very powerful, but a little on the hefty side at over 6kg, this car tends to understeer, especially at high speeds. With fresh tires, I first measured its cornering speed and 0-60 acceleration, establishing baseline performance figures for comparison after modifications.

The Ground Effect Plan
I wanted to see if adding a fan could improve downforce enough to:
1. Tackle corners faster.
2. Beat its stock 0-60 time, potentially matching an F1 car’s acceleration.
The goal was ambitious, especially on a real-world road full of bumps that could compromise the ground effect.

Understanding Ground Effect
To create downforce in F1 cars, engineers manipulate air pressure. By shaping the car’s underside to accelerate airflow, they create a "venturi effect," reducing pressure beneath the car, pulling it down toward the road.

A Bit of Racing History
In 1978, F1 team Brabham took this concept further by introducing the BT46B, a fan car that actively sucked air from underneath to maximize downforce. While effective, this design had a major drawback—any fan failure instantly eliminated downforce, leading to potential crashes. Banned shortly after its debut, the BT46B remains legendary in racing circles.

Building the Fan-Powered Arrma Limitless
First, I stripped down the Arrma Limitless chassis and installed a flat aluminium ground-effect floor. This would serve as the “roof” of a sealed, low-pressure area under the car. Adding L brackets allowed me to attach a skirt to maintain this low-pressure seal, though I anticipated a few revisions.

The Fan
Emma designed a custom fan using 3D-printed parts and a powerful RC plane motor. After testing several prototypes to see how well the fan blades held up at high speeds, we were ready to mount it on the car and see if we could create enough of a pressure drop for genuine downforce.

Emma tweaked the fan design to add more blades, ultimately achieving 6.4kg (14.16lbs)—equal to the car’s weight! But as we pushed further, a 10-bladed fan caused a motor burnout, so we settled on an 8-blade design.

The First Skirt Attempt
For the skirt material, I initially tried foam board, hoping it would stay intact at high speeds. Testing showed the foam could achieve around 1.8kg (4.04lbs) of downforce.

Using foam was a temporary fix; I needed something that could handle uneven surfaces without shredding. Inspired by the 1970s Chaparral 2J, I tried building a sliding skirt to maintain ground contact, but this proved too complex. Instead, I locked the car’s suspension, added wooden strips, and sealed them with tape to create a consistent 3mm gap from the road. Was wood a good idea for a car, though?

Real-World Road Testing
The big question: would this ground effect work on the bumpy road corner.
The initial runs were underwhelming; the car was bouncing, and the wooden skirt wasn’t sealing well on the uneven surface, leading to a total loss of downforce and grip. A switch back to the foam skirt allowed for better pressure consistency, although the fan stuttered and struggled to keep up on the rough terrain.

0-60 Test
Testing on a smoother road improved performance drastically. With fresh tires and maximum fan power, we achieved a 0-60mph time of 2.28 seconds, rivaling real F1 acceleration! The fan was clearly adding downforce and grip on this smoother surface, allowing the car to use every bit of its speed potential.

What We Learned
Despite challenges with uneven surfaces, the fan setup created impressive downforce on smooth roads, making this one of the fastest fan-assisted RC cars around. We learned that while ground effect is powerful, it’s also sensitive to road conditions, as evidenced by the RC car’s difficulty on bumpy terrain—a phenomenon akin to F1 cars' "porpoising."

This project mirrored the journey of real ground effect cars, from skirt experiments to pushing limits with fan power. It was a wild ride, complete with some spectacular accidents along the way.

If you enjoyed this, check out other extreme Project Air RC builds.