How would you make an airbrake for a car that is activated by the brake light signal ?
 

How would you make an airbrake for a car that is activated by the brake light signal ?
My first thought is using servos for large R/C planes, but how complex would that be ?

Arduino controller or similar. The third brake light can't distinguish a panic stop.

It would bleed off energy you paid money for most of the time.

Beware the level arm between your solution and the center of pressure (IOW put it at the rear like a shuttlecock).

Find the control signal for the abs system and use a mosfet to sample when it activates. Or a Throttle pot on the brake pedal set such that more pedal gives you more or less signal/voltage.

However there will be a lag time in milliseconds between activation and appropriate movement of any surface so use in a panic stop may require some janking about

Thanks guys. Great advice.
This is just a 'backburner' idea.
I think it would cost too much to just do as a hobby, but I have had the idea for quite a while now. ( years ! )
I wouldn't replace the brakes, but rather suppliment them with the airbraking.
I would just have an R/C servo that is trigger on and off by the brake light elecrical current.
In addition to braking, the flaps would also add downforce at just the right time that you need it most.

I wonder why something like this has never caught on.

Pedestrian safety laws perhaps ?

'caught on'
 

Currently, Bugatti and Koenigsegg are both using air-brakes.
We're talking 258-mph, and 310-mph cars though.
The retarding force, like any other aerodynamic force, is going to vary as the square of the velocity.
I know from observing passenger jet air-braking that, at rather high velocities, the effectiveness of spoilers and thrust-reversers kind of peters out, and they've got to introduce hydraulic braking.
I'm unsure what the breakover velocity would be on an automobile.
At Bonneville, I believe that they want drag chutes to precede friction brakes at around 175-mph, in order not to inadvertently burn the friction material off the pads and shoes.
The owner / driver of the Royal Purple streamliner told me that he shredded both parachutes on a run once, and it took eleven-miles to get the car to coast down to a safe braking speed. Floating Mountain must have looked pretty 'BIG'.

Pagani, et al, use dynamic spoilers for cornering. It doesn't require the discernment and reactrion time braking would need.
The 1955 Le Mans gave air brakes a bad name.

Quote:

https://www.wbur.org › onlyagame › 2018 › 10 › 19 › craig-breedlove-spirit-of-america-land-speed-record Craig Breedlove's Quest For The Land Speed Record | Only A Game - WBUR "He has no brakes. He's going 500 mph. He's running out of course, and he's got no way to stop." Images from Spirit of America's on-board camera during a run at Bonneville in 1964....

It was discussed and tested in this video series by the infamous Julian Edgar:
https://www.youtube.com/watch?v=oxn0RShug_I

Too long; didn't watch? While it does work, it's not really worth the effort at legal road speeds.

^^ I was going to make a similar comment.

In order for an airbrake to be effective at normal road car speeds, it would need to be massive.

Coroplast not fit for purpose.

Killing lift is one thing, but straight up drag is another entirely. I played with it with the beetle descent from '7000 on Mt. Rainier to ~'3000. It is pretty subtle to say the least at under 60 MPH.