Two brakes per rear rotor

[Musterion]

So in the event of a catastrophic brake fluid leak--- could the parking brake be used at the emergency brake and stop the car? In a regular car there'd also be downshift options but even with regen that might not be fast enough (like downhill).

I would imagine it's some kind of screw or cam driven by a motor and gearbox.

From inspection it is exactly what Doug_G describes. Once the cam locks an electrical power loss has no effect. Here is a sample spec sheet from one manufacturer. http://www.trw.com/sites/default/files/TRW_electricparkbrake_en.pdf
Interestingly, the same brakes can provide hill-hold functionality, perhaps to be enabled in the future by Tesla.

 

[Cal1]

Wow hard to imagine a electro mechanical device that can put out enough physical force to stop a rotating rotor disc. And how it would work without power is still a mystery. Wonder how much current this thing draws to be used as an actual brake. I spent a lot of years repairing test equipment in the Air Force but that was quite a while ago. Again the only thing I know of that would push a piston is a solenoid of some type. This article was good but still didn't give much detail except to say it's electromechanical.

 

[glhs272]

Wow hard to imagine a electro mechanical device that can put out enough physical force to stop a rotating rotor disc. And how it would work without power is still a mystery. Wonder how much current this thing draws to be used as an actual brake. I spent a lot of years repairing test equipment in the Air Force but that was quite a while ago. Again the only thing I know of that would push a piston is a solenoid of some type. This article was good but still didn't give much detail except to say it's electromechanical.

It is definitely not a solenoid. It is a electric motor connected to some type of gear box and or wormgear/powerscrew drive. It actually has quite a bit of stopping force. I accidentally put my S in park while still rolling a little bit and it came to an instant stop with just these brakes as soon as they clamped up to the rotor. The point of this design is so that the motor doesn't need to be powered to hold the brakes on. And I think there is a fair amount of mechanical advantage to put a lot of force on the pads. The only thing I could say about them is there is a small delay to actuate them before they bite. This would be annoying to me if they were going to use them in a hill hold application.

 

[Doug_G]

Wow hard to imagine a electro mechanical device that can put out enough physical force to stop a rotating rotor disc. And how it would work without power is still a mystery.

Nah, these are trivial problems for a mechanical engineer to solve. Just off the cuff, you could use a worm gear drive, as they provide huge reduction ratios and are pretty much impossible to reverse drive. Then make the whole worm gear into a cam, say having a ramp on one side of it. The ramp would push on the calipers as the worm gear rotated, producing an enormous mechanical advantage.

 

[Cal1]

It is definitely not a solenoid. It is a electric motor connected to some type of gear box and or wormgear/powerscrew drive. It actually has quite a bit of stopping force. I accidentally put my S in park while still rolling a little bit and it came to an instant stop with just these brakes as soon as they clamped up to the rotor. The point of this design is so that the motor doesn't need to be powered to hold the brakes on. And I think there is a fair amount of mechanical advantage to put a lot of force on the pads. The only thing I could say about them is there is a small delay to actuate them before they bite. This would be annoying to me if they were going to use them in a hill hold application.

This vehicle weighs in at about 4k lbs. Thats pretty impressive power.

 

[hoopty_yo]

In the event of a catastrophic brake fluid leak, you would still have half your brakes. The master cylinder and resevior are divided into two seperate circuits for this reason. RWDs are usually seperated into front/rear systems; FWD/AWD have a diagonal split -LF/RR on one circuit and RF/LR on the other.
This is because of a weight bias to the front. If you were to lose your front brakes on a FWD car, you don't have much chance of stopping with rear brakes only.

I have no idea how the MS is set up, but I would assume diagonal because it seems like a safer design. Volvos actually had split circuits to each front caliper (two inputs). Not sure if that is still employed.

As for the extra caliper, it should be noted that it is a parking brake, not an emergency brake. With regen and electric engagement it may actually stop the car, but for liability reasons, I would still call it a parking brake.

If you have a safe area, try stopping your ICE car with the parking brake. It doesn't give much hope in an emergency situation. That valuable time would be better spent downshifting and finding a ditch or something to scrub speed.

I'm just speculating, but I think the design of the MS parking brake would be more like an eccentric cam, kind of like how a deadbolt works. It would only need momentary motor function and no amount of load againt the piston will release it until the cam rotates back over the zero point.