Because my Arrma Limitless is used both for drifting and speed racing, I want to control the downforce more precisely. However, the commercially available kits on the market are too expensive and far beyond my budget. So I decided to build this system myself - with just a few simple modifications, I can achieve active downforce adjustment without having to make major changes throughout the process.

In order to maximize the aerodynamic performance, I specifically optimized all the components exposed to airflow: using a gradual narrowing and smooth surface design to ensure that the airflow is stable and smooth, and does not get disturbed as it passes over various parts of the car body.

You can choose to keep the factory-standard rear wing, or you can let me know via private message the speed mark you want to engrave on the rear wing. If you choose to customize the text, it is recommended to use it with the LP stabilizer bracket, so that the text can be clearly displayed and the overall visual effect can be further improved.

Of course, you can also add a custom inscription to the tail fin directly in TinkerCAD using the included SCAD text generator or a custom font—but be aware that this might obscure some of the screw holes. If you're determined to do this, make sure to install the "911" font first.Oh, this isn't a typo of mine. Every time I see it, I can't help but feel both amused and annoyed~)，or choose other fonts according to personal preferences - you just need to specify them in a simple OpenSCAD file.

🛠️Required accessories
A high-speed standard servo motor with sufficient torque
A servo steering angle of at least 30 mm in length
An aircraft-style connecting rod about 55 mm long
Reserve an idle channel on the receiver/remote control
Complementary hardware

Update notes——I redesigned the servo motor mount, which not only improved the structural strength but also made it compatible with more models of servo motors. At the same time, I also specially introduced a low-profile servo motor mount. Compared with the traditional combination of servo motors and mounts, it can reduce the weight by up to 18 grams.

Installation Guide

First, disassemble the original factory wings and vertical tail wings on the rear assembly. You need to first unscrew the four screws on each side of the vertical tail wings - please note that two of them are located on the inside of the tail wings - after all the screws are removed, the vertical tail wings can be easily detached from the car body. Next, remove the two top screws that fix the original polycarbonate wings one by one.

In addition, you also need to remove the screws of the four small brackets used to fix the connection between the wing and the rear assembly - at this point, the rear assembly has been largely freed from constraints, and you can continue to disassemble the fixed bracket below

Next, it's time to install the active aerodynamic components that have been printed.

First, fix the servo motor you prefer to the "servo motor mount" - if you are using a regular large-sized servo motor - or the "low profile servo motor mount" - if you are equipped with a low profile servo motor - and secure it with 4 M3×12 screws.

Subsequently, remove the two "tail fin pivot axes", and use the screws dismantled from the original wing mounting brackets to fix them in two mounting holes near the inside of the car body. At this time, there is no need to fully tighten them. Just leave a little gap to allow the pivot axes to swing freely back and forth. For the specific installation effect, you can refer to the attached figure to have a clear understanding.

Next, pick up the tail wing and fix the "tail wing linkage mount" to the bottom of the tail wing with the two M3 screws left over from the old wing mount. When installing, be sure to pay attention to the direction, otherwise it may affect the airflow distribution and the smoothness of the tail wing rotation.

Finally, use two M4×8 screws to firmly connect the tail fin assembly to the tail fin pivot. After completing the above steps, reinstall the original vertical tail fin with the screws at the bottom back into the original rear diffuser.

It's a great success!

At this point, you just need to confirm that the wings can rotate flexibly on the predetermined axis. If everything is normal, you can fix the "upper stabilizer bracket" to the mounting hole at the top of the vertical tail with M3×16 screws - make sure that the conical edge of the bracket is facing the front of the car to optimize the airflow direction.

Next, connect the end of the selected linkage to the tail wing linkage mount, and the other end to a servo rudder angle with a length of at least 30 mm

At this point, your active aerodynamic system is officially in use! Enjoy the driving fun to the fullest~

Programming creativity

Basic programming

After setting a suitable travel endpoint for the wing channel, simply mix and control it with the throttle channel, and you can achieve the effect of the wing automatically lowering when accelerating and quickly rising when braking. If you are using a regular remote control or do not have too many demands for complex functions, this setting method can be simple and efficient.

Advanced Programming

On my RadioMaster MT12 remote control - in my opinion, the best surface remote control currently on the market - I have used a variety of clever programming methods to make the active downforce function truly deliver strong practical value.

Firstly, I preset multiple wing attitude modes for different driving scenarios: in regular driving, the wings are adjusted to a moderate angle, which not only provides sufficient downforce to help the vehicle smoothly take curves, but also does not hinder high-speed driving; at medium speed, the wings are adjusted to a more aggressive angle, providing a more solid sense of ground-hugging control; and in ultra-high-speed racing, the wings are almost parallel to the ground, maximizing airflow and helping the vehicle easily break the extreme speed limit of 100 miles per hour.

In addition to these preset modes, I have also set up two special priority controls: when the brake pedal is pressed more than 30%, the wings will instantly become almost vertical, transforming into an efficient air brake device; and when the throttle opening lasts more than 90% for more than 2 seconds, the wings will automatically return to a horizontal state, further enhancing the vehicle's stability and controllability.