Active rear wing test
 

So, I've been reading a lot of Can-Am books and followed a McLaren MP4-12C around the track a few months ago - so I had to try this. Not legal in almost every race class, although it is legit in Targa Newfoundland Open class ;) Still, there's better aero discussion here than just about anywhere else, so I'll post this here. Really it's just fun to play with.

The initial plan was to use a SPAL linear actuator with a cool three-position controller. I'd have a low drag setting (triggered by a momentary button on the steering wheel), normal (the default) and high drag (triggered by the brake lights). Unfortunately, the speed of the SPAL part was just too slow, 1/2" per second. I could speed that up with linkages, but it was less than ideal.

Warren at Exomotive had been playing with headlight motors and thought they'd make good actuators for an active wing. They're strong, fast, repeatable and cheap. They also have internal logic that's easy to control. 12V on one wire makes them go up, 12V on another makes them go down. The motors spin at 50 rpm, and with the standard levers that means about 6.5" per second of stroke. You can change this by changing the length of the levers.

The goal here was to build a proof of concept without cutting up the car. I took a prototype COT mount and attached the motors to the sides in a convenient place, then triggered them with a SPDT relay attached to the brake lights. There's an override switch in the cockpit that turns off the active wing. The wing takes 0.6 seconds to change position, although it's effectively faster on retraction because of the way I have the various linkages clocked.

https://www.miataturbo.net/attachmen...ine=1379000568

Test video. The fastest test was at about 75 mph, which is the best I can do without attracting undue attention. No sign of any deformation to the wing mounts. I'll be at the track on Saturday and will give it a try with a Traqmate measuring deceleration as well as lap times. I'm not as familiar with this weekend's track as my home one, so there may some noise in the lap time data due to driver inconsistency.


A timer circuit has been suggested to delay the return after the brakes have come off, to avoid a flappy wing if I get on and off the brakes a few times in rapid succession. Moving the motors would also be preferred, although I'm less concerned with drag than many other Miata drivers for some reason ;) That would require longer pushrods, which may get into flex problems.

Interesting!

So it simply goes from "normal" mode to "brake" mode at the moment.


Could you in theory make it infinitely adjustable based on speed? Or would the simple nature of the headlight motors be too cumbersome for that?

The motors don't have any sort of feedback for positioning, they simply cycle between up and down. You'd have to simply base position on a timer, and if they get slow under heavy load you might find yourself drifting away from your intended position.

The SPAL actuator does have a position sensor, so it can be set to repeatable positions and would be a better choice for that sort of control. The speed of movement would be less of a concern in that case as well.

Besides there are times when you will want all the high-speed downforce you can get.

Id rather see a pressure sensor in the brake line so it only goes up during intense braking, rather than a delay timer for release. All the little brushes of the brake pedal during around town driving (as seen towards the end of the clip) would seem annoying.

Well, obviously the video was to show the movement. I have an override switch that locks it into the flat position by simply interrupting the ground wire to the relay.

I'd rather you shut the hell up about "around town driving" on a race car showcasing a prototype active aero rear wing.








:party:

Great concept, you may want to introduce some g loading logic so the wing does not change settings in mid corner, the timer will work in the track but not sure that will be all you need at the targa

Agreed.


On the concept though...

I think its very cool and great. I feel like this may require some logic on when the spoiler brake would activate.

Would it make trail braking more difficult/unpredictable? especially in high speed corners, where the spoiler has an effect on aero balance?

If the wing is changing settings mid-corner, it's because I stabbed the brakes. Might be a good time for a little stabilizing aero, actually.

Will it make trail braking harder? Dunno. That's exactly why I built it instead of imagining it, although I don't tend to trail brake much. Here's a peek at what the rear wing on a McLaren MP4-12C does under braking and cornering (starting at 35 seconds or so). I think the brake light flickers on that car are the stability control, not the driver. The wing seems to deploy at the same times regardless of cornering load, you're basically seeing the same spots I'd be braking on that track.

The brake light flickers are probably from the torque vectoring system that uses the brakes to help the car rotate around a corner. McLaren is a big fan of brake steer, which the actual MP4-12 used in the 1997 F1 season (there was a second brake pedal and a switch for the driver to select which rear wheel it acted upon). There's really no other way to get a car to change direction like it did around the 37s mark in your video. Cool stuff.

Yeah, "stability control" is a bit of an over-simplification :) I did find it odd that it flashes the brake lights in that situation, though. Might be a federal requirement.

I realize you're joking, but please remember the PUBLIC roads this race car was specifically built to drive on. He is indeed putting around town for small portions of it.

So suck it.

What's annoying about it? It's not like it's going to bring the car to a screeching halt every time it moves.

Man, you MiataTurbo kids really like to argue about anything. Back to my old lurkur status. :inout:

Hey Keith, if youre looking for something with a bit more adjustability, I would recommend a Concentric 5:1 linear actuator. I imagine the operation is similar to the Spal, but the speed is about 2 in/s. It cant be backdriven (rated at 34lb dynamic load, 450lb static), and has potentiometer feedback for position. You can drive it with any standard DC motor controller, or get an arduino or fancier controller for more advanced functioning. I have a lot of experience with them and theyve proven to be very reliable in our application.

Pololu - Concentric LACT4P-12V-5 Linear Actuator with Feedback: 4" Stroke, 12V, 1.7"/s

If youre interested I may have a spare one sitting around you can use for testing.

Looking at the specs and the pictures, I think the SPAL is actually the 20:1 version of that actuator with different branding. So I already have a controller for it. Thanks for the link, I didn't know about that option.

The 34lb limit when moving is a bit of a concern, but that would be double if they were paired. That would make the linkages easier as well. They would be well suited to constant fine adjustment, full airbrake might not be effective.

I'd be interested in laying my hands upon one for some testing if you'd be willing to loan it. Or two :) What I REALLY need is some sort of strain gauge to figure out just how much load is on this system.

How have you addressed possible "chatter" of the wing when in the down (brakes off) position?

It looks like the rod from the actuator to the wing would be almost horizontal in the down position (judging by the rub marks on the upright). I'd think that would make it hard for the motor/rod-ends to keep the wing "locked" in position at that angle without some sort of serious stopper/bumper on the upright, and an "over-center" sort of motion of the arm (if that makes sense). I'd hate to see that nice wing chew a hole in itself bouncing off the tops of the metal uprights.

I'm hoping from the first picture that you've replaced the brittle white plastic doohickeys that attach the headlight bucket to the arm with something more substantial?

LOL WUT?

Picture is of the wing in the down position. There is no torque load on the actuator, and the rod sees only a compression force.