Could use a variable pressure switch, that could open new possibilities.

 

Most of the brake pressure switches I have messed with that trigger brake lights have very low pressure setpoints so that the lights will turn on with the slightest pressure from the pedal. It would be nice to have the switch only activate at higher pressures. I am not sure what would be considered moderate brake pressure on a miata. Keith, I know you did a good bit of brake system work a while ago, do you have any idea what a good setpoint or range would be for moderate brake pressure? I will have to go take a look at the Napa book. I have sent them questions before online. They might be able to suggest some options if I can give them an actuation pressure range.

As mentioned earlier it would be nice to have something that could read a proportional signal for proportional control or just a higher setpoint so that your wing would only move to the high drag position when brakes are applied with moderate to high pressure. This should eliminate concerns with the wing actuating during light braking.

I was wondering if anyone is familiar with newer traction/stability control OEM setups that might have more useful switches/sensors that provide brake pressure info to the ECU.

 

A 1991 has an approximate 17:1 pedal/fluid pressure ratio. Since "moderate" is such a general term and the actual pedal pressure for a given braking amount will vary depending on what you've got for calipers, pads and rotor sizes, I'll let you measure your "moderate" pedal pressure and do the multiplication.

 

I work 8 years on AC-130 Hydraulics and there are plenty of different switches. Just like on the boost pump activates @ 20 psi and deactivates @30 psi just to give an idea.

 

Switch: McMaster-Carr

However, I would rather have a brake pressure transducer hooked up to MegaSquirt and/or Arduino. The wing would be driven by a MOSFET, just because I hate relays.

Transducer: XT Racing 1/8" Pressure Sensor | MotoSport

 

Oh thats a great idea using brake pressure, how hot do the actuators get just from the continuous use?

 

I didn't check them, I was checking something else after the long test session and forgot. They certainly have lots of cooling airflow!

 

Not a brake pressure switch, but a pressure transducer should be used. You can get one for $150, maybe less if you search around.

What other data should be used as inputs to rotate the wing?

VSS and/or deltaVSS?
Accel?
Other?
Is a time component needed?

Further, I don't see why a position sensor can not be added, allowing for variable control of the wing element, either multi stage or continuously variable.

 

Ben, check this little guy out: XT Racing 1/8" Pressure Sensor | MotoSport

VSS, deltaVSS, Accel, any sort of gyro input along the vertical axis, and definitely time. For safety, I would include some logic that holds the wing up until all inputs are "down" for a second or so. The reason why active aero is banned in most racing classes is because of the danger in transient conditions. If you are pushing the car with active aero, failure of the aero to do what you expect means an off or worse. Once the actuator is told to go down, it needs to complete a full rotation to get back up (1.2 seconds). A "down-delay" would prevent a lot of problems.

The Miata headlight actuator works off position sensors and has its own little logic system and cams in it. It is not designed for intermediate positions. It's a one-and-done deal for up or down, so variable position would require something else. The point of this exercise is to use the cheap and available headlight actuators to do this, so I'm happy living in the constraints of pure up/down.

 

That looks like it will work perfect for the purpose.

Same train of thought I had.

How do they respond when de-energized in mid-position?
The same controller could operate both 2-position and variable position actuators. Just a minor software difference. A position reference sensor can be incorporated for fault detection with either type of system.

It may also be worth looking at other cars with flippy headlights and low cost actuators to see if there's possibly a better option out there. S13 240SX comes to mind as a possibility, but that's because I think there's a pair of them around here somewhere.

 

 

The Concentric 5:1 linear actuators that scenturion mentioned would be a good option for full control, I think. They've got position sensors in them and the Ardunio controls are already worked out. Search eBay for linear actuators and you'll find all sorts of interesting options. I never did that before his suggestion because the SPAL unit basically fell into my lap.

If you pull the power from a Miata headlight motor mid-cycle, it will complete the cycle when power is restored. And they only spin one direction, so you'd have to go all the way down and up again.

 

Thoughts on using a wiper motor instead of a headlight motor? It would allow for variable position, variable speed, and bi-directional control.

 

The headlight actuator is basically a wiper motor with built in positional controls. You could reverse the drive polarity of the headlight actuator and apply PWM with an H-Bridge to accomplish all of that open-loop. Using a wiper motor would definitely require closed loop feedback through an encoder and home switch, or if you're feeling confident of your controls, a potentiometer.

 

How about this guy for us cheap bastards? $36

Pressure Transducer or Sender 1000 PSI for Oil Fuel Air | eBay

The same seller has a 1600psi sensor. I asked the seller for a datasheet(attached). These sensors can withstand 200% full scale pressure so 1600psi sensor should be good to go for our aplication and even the 1000psi could work.
The 0-4.5V input would be easy to use on Arduino.

 

Nice find. For $36, it's worth a shot to see if it gives good data.

 

Wiper motors like to just spin and spin in one direction and they only have logic to return to a single position, not two (or variable). Without complicated controls, the headlight motor is much better.

My only concern with this being hooked to the brake light is triple digit trail braking at the limit where you are counting on a certain level of grip that might not be there with the wing in air brake mode.

Edit: I typed this about 2 hours before my phone sent it. Psi switch on brakes would be better to trigger.

 

Indeed, that reliance on a complicated system is why active aero is banned across most racing classes. Still, it's fun to play with!

I wonder if anyone has ever applied this to a diffuser, where you would have more aggressive channels on a multi-element diffuser that are opened at low speed...You could probably make the system quite stealthy and invisible for TT classes...

 

The only work you need to the defuser is a motorcycle engine with a giant fan on it to suck the life out of the ground and give you 400 mph downforce at 20mph. We're messing with banned mods, might as well go nuts at it.

 

Sucker..