Ahh, firmware (Subaru this time)

"9. Substantially similar U.S. vehicles: 2015MY Subaru Legacy/Outback and 2015MY Subaru WRX are substantially similar to their counterparts for sale in the United States. Subaru determined that this is not a safety issue since there is no comparable US safety regulation to UN No.

13-H. However, Subaru will initiate a service program to remedy affected vehicles in the US market"

I think that is what is known as a "silent recall". They'll fix your car if you know to ask them.

What I'm wondering is why they feel the need to activate the parking break twice?

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Rick C

Perhaps to compensate for expansion and/or contraction of various bits of the mechanism after a (potentially very hot) brake cools down.

On aircraft, it's common to avoid setting parking brakes if the brakes are hot, or if a significant temperature changes is expected. Although the mechanism there is usually rather different (most aircraft parking brakes usually just trap pressurized hydraulic fluid at the brake end of the system, and expansion/contraction of that trapped fluid can do interesting things). So it's not strictly comparable.

I've never needed to put my parking brake on twice. Where's the difference? I think the expansion/contraction would be small compared to the amount of stretch in the cable pulling the brake in a manual brake. The electronic brake would be hydraulic I expect so no stretch. But the actuator could easily be designed with some compliance to compensate for a few thousandths of an inch as the parts cool down.

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Rick C

If the "electronic" brake is mechanically hydraulic, I'd expect similar temperature-related issues as seen on aircraft. If there's a large drop in the temperature of the trapped hydraulic fluid, a significant reduction in pressure is possible.

Why would that happen in a car?

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Rick C

If the brakes are hot, presumably due to heavy use, just before the parking brake is set, they might significantly heat the hydraulic fluid near the brake.

There are a few nasty things that can happen with a hydraulic parking brake:

(1) The caliper and fluid are hot. You set the brake, the fluid cools, and the car rolls away ;-) (1b) You set the brake at noon. In the evening, the fluid cools, and the car rolls away ;-)

(2) The caliper is OK, but the brake disc is really hot. You set the brake, and the hydraulic fluid boils as the caliper overheats (this is aircraft panic-stop scenario).

Hydraulics can be entertaining. In my Antares, running the charger generates enough heat in the engine bay to heat the hydraulic cylinders that open the engine bay doors, and the doors open an inch. Unintended but provides better cooling during charging ;-)

See ya, Dave

Am 25.05.2016 um 05:52 schrieb rickman:

Probably in that you've never had a hydraulic or electro-mechanic parking brake.

I guess that expectation is formed a bit prematurely. AFAIK the currently available automatic / "electronic" parking brakes are actually electro-mechanical, i.e. there's a servo motor that opens/closes the brake.

This is often accompanied by a hydraulic pressure store that holds the normal brake closed while the vehicle is stopped, so you can take your foot off the pedal --- even if the engine is stopped, too. The car will then only (start the engine and) begin to move again once the pedals are pressed to indicate "go" (clutch for manual gear shifts, gas for automatic).

In this combined system, pressure in the "auto hold" system is monitored, and if it drops far enough, either the engine is restarted, or the electric brake is activated automatically. If you park (key leaves the car), the parking brake takes over right away

See the other replies on why a hydraulic system may move on its own, particularly without an active compressor feeding it.

Note that disk brake pads only move about 10 mils on actuation, anyway. I.e. one mil may already be all the difference it takes for the car to get moving.

This is a bit overstated. The actuator simply needs some compliance. Whatever is pressing on the brake needs to have a spring to take up the slack when the fluid volume changes.

See above.

How do you think the manual parking brake works? Fluid is not the only thing that changes size with temperature.

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Rick C

Which is very, very easy to deal with in a parking brake.

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Rick C

But (A), they didn't do that, and (B) they proceeded to screw up the software. At least it's not in something safety-related, so it's not an inexcusable screwup -- but give them time.

--
Tim Wescott 
Control systems, embedded software and circuit design 
I'm looking for work!  See my website if you're interested 
http://www.wescottdesign.com

If I am not mistaken, the idea that they are applying the brake twice to deal with thermal issues is speculation. So you don't really know what they did or didn't do.

I don't think it was a screw up in the software really, but maybe I missed that. I thought it was a systems design issue where they didn't catch that applying the brake at the same time the ignition was turned on would cause a problem.

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Rick C

Nope.

Sure, except a spring-loaded accumulator can cause brake response issues...

As you already posted, the Bowden cable is elastic (though because the housing compresses; the tension cable doesn't stretch much).

From wasting far too much time debugging brake and hydraulic issues... Best Regards, Dave

"Brake response issues"? This is the emergency brake. It doesn't need to "respond" in ms times.

What housing? The ones I've seen are just a cable. It might not stretch "much", but we seem to be talking about very small amounts.

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Rick C

Why I think it's "software" rather than "systems" (even though there's significant overlap in most companies):

1: Software engineers are the systems engineers of last resort. This shouldn't be -- but it is. 2: The service bulletin says it's a software problem. This is about 1% of the necessary proof -- but still, it's there. 3: Only the left rear wheel is affected. If it were a system problem, you'd expect the entire emergency brake system to be affected. 4: The effect is to damage hardware. Excluding software in bombs and emergency fail-safe systems, software should never, ever, damage hardware. That points to a bug severity known as "crazy way bad" in the industry. Even if this were a job that was assigned to the system's people, and that assignment was enforced by goons with baseball bats, the software people should still make it not happen (see item 1, above).

So -- it's software. It is, almost certainly, a dumb-ass race condition, and very possibly one that is very similar to the one that Toyota was using to kill people with their cruise control software a few years ago. Perhaps Toyota and Subaru buy their software from the same super-very- good consulting company, and the critical code was written by the same engineer.

--
Tim Wescott 
Control systems, embedded software and circuit design 
I'm looking for work!  See my website if you're interested 
http://www.wescottdesign.com

parking brake, since dual circuit brake systems was mandated eons ago cars haven't had emergency brakes

if the parking brake uses the same hydraulic system as the regular brakes you can't just add a spring some where it be like having air in the system

-Lasse

They share a lot of technology now there's joint ownership (BRZ/GT). It could be the same in-house team.

If the primary brake system is shared with parking, you can't introduce compliance without causing a spongy primary brake response.

A Bowden cable has a center tension cable, and (typically) a wound outer compression housing (which looks suspiciously like a spring). The outer housing compresses much more than the inner cable stretches. A 6 foot light-weight Bowden cable can easily compress 1/2 inch.

Try it on your bicycle: Wedge the brake caliper so it cannot move (using something really hard). Squeeze the brake handle hard and watch what happens.

I've had to rework a number of applications where the mechanical engineer forgot to allow for the compression, and so much travel was wasted compressing the cable that the actuator ran out of travel before the desired results were obtained. (Some of you know a few of these: think Tost). Sometimes the fix is as simple as a stiffer housing.

In the case of the parking brake, the springy Bowden cable is a feature and not a bug.

Hope that explains OK! Best Regards, Dave

I think you are mistaken. The parking brake and emergency brake are one and the same, always have been on typical passenger cars.

It is easy. The *actuator* has a spring involved, but not when it is inactive. Then the actuator has a hard stop.

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Rick C