Do you use parking brake?

Well, I couldn't do it in the Prime.

I think it does do harm, primarily by training a bad habit that you might be unable to lose during the winter when you really shouldn't use it at all because of freeze-up. It even says so in the Prime manual.

 

I've had that bad habit for nearly 40 years of cars, all rear drum brakes till now, never had the parking brake freeze. To be fair, our winters are mild.

 

I've personally had to help out people who have had their parking brakes frozen in the locked position several times here in Colorado.

 

Yeah and you guys also probably have to use an antifreeze mixture for your windshield washer fluid. Me, I'm just a completely spoiled California brat who has religiously applied his parking brake (there are hills around here) for over 50 years and has the mechanics unacceptably top off his windshield washer fluid with plain water! My previously all-stick cars felt much safer with parking brake engaged, especially after my GTV started rolling, engine off, in gear, on a very steep driveway! I did see that incredibly cold Colorado weather, -10°, in Snowmass, eons ago when our band played there the week of Christmas eve around 1973! I think we just need to be aware of circumstances and protect our cars. If we don't, we end up having to bug nice people like Mr. Jay to bail us out!

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When I got my Prime, one of the first things I did was siphon the washer-fluid provided and replace it with RainX washer fluid. The thought of putting water in there never occurred to me since it's intermittently below freezing here for about half of the year. The temp this morning when I drove in was 46°F, so not below freezing yet, but getting there.

I much prefer the cold to the heat. I consider 90F about the same level of misery as 20F. Unless I'm in the lake or pool, 50-60F is the comfort zone for me. The most uncomfortable I've ever been wasn't either of the times it was -41F where I was (one in Frasier Colorado, once in Steamboat, Colorado), it was the time I was in Washington DC, it was sunny, the temp was 99F, the humidity was in the mid 80s and I was outside walking around Arlington Cemetery. That was horrible!

 

I wonder what Toyota manuals for standard transmission cars say, use wheel chocks?

I've had my rear brakes freeze up 2 or 3 times. If they didn't break free within a foot of reverse movement on the gravel driveway, rocking once or twice freed them.

 

Even the Prime manual says that when not using the parking brake and when changing a tire.

For example:

"Park the vehicle and shift the shift position to P and block the wheel
under the vehicle without setting the parking brake. The parking brake
may freeze up, preventing it from being released. If the vehicle is
parked without setting the parking brake, make sure to block the
wheels."

 

Dead easy on wet grass, see my post #189 in this thread (hand-brake turns). Even on dry surfaces, most of weight is on front, particularly when slowing down, (hint: read up on Newton)

 

Sure, on slippery surfaces but not dry.

 

Second question first, the parking brake is a purely mechanical system where a pedal pulls on a cable that clamps down the rear brakes. So, it doesn't depend on any computer doing anything, and it doesn't depend on any of the hydraulic system of the car's service brakes.

Working back to the first question, well, you could consider using the parking brake in a pinch any time the car's regular service brakes (front and rear, hydraulic, power assisted, computer controlled) aren't doing what you want. The nightmare daydream example is some kind of total failure of the service brakes while careening down the highway. Something to think about, and maybe, Walter Mitty-like, practice stopping with the parking brake for, just to learn what it's like. (It's a lot like stopping a car with the wrong two wheels using a ratchety pedal that isn't built to serve the purpose very well.)

IRL, the times I use the parking brake with the car moving are more often when I'm doing a very close parallel park or trying to just sneak right up on something at creep speed. In those conditions, sometimes I find I can modulate the parking brake better (after double-pushing, of course) than the service brakes, which I find to be a little too touchy when I want a steady, really slow creep.

The best way to grok how the Prius brakes are plumbed is to stare really hard at the diagrams in the New Car Features manual. It isn't diagonal, and the first three Prius generations had three different approaches, and I just don't know what to say about Gen 4 because I haven't stared at those diagrams yet.

All of the (first three) gens do have a dual-chambered master cylinder, like most conventional cars. The two chambers are plumbed one front and one rear, not diagonally, but then the weirdness piles on, because both circuits disappear into the ABS actuator and its mess of little valves. (To be fair to the Prius, most of this weirdness is just any-car-with-ABS weirdness, not specifically Prius weirdness.)

Then there's the electric pump and the accumulator of pressurized fluid to give you brake force assistance.

In Gen 1, the pressurized fluid works a lot like the giant vacuum diaphragm in other cars. It enters a 'boost' chamber built into the master cylinder, behind the piston, so its own pressure pushes the piston forward in the same direction your foot is. A reaction-disk and valve at the front of the piston regulates that pressure to be proportional to your own effort, so you can modulate the brakes and just feel that your strength is somehow amplified. But the Gen 1 ABS can also kick in, and admit the pressurized fluid from the accumulator directly into the brake circuits. Gen 1 only has independent control of: left front, right front, and the rears as a pair.

Between the front and rear circuits in Gen 1 is an old-school Proportioning&Bypass Valve ensuring the right distribution of brake force for the car's weight distribution (but without disabling the rears if front pressure should be lost for some reason). This is a purely mechanical/hydraulic valve with a spring and doesn't need any electrical or computer control.

If the Gen 1 computer decides to go failsafe, valves switch in the ABS actuator, so the front brakes are operated directly by fluid from your front master cylinder chamber, and the rears are unused. If there is pressure left in the accumulator, you still have the boosted master cylinder assisting you. That booster also being a purely mechanical, hydraulic design, it has no dependency on the computer or actuator, and will keep assisting you until you've used up the stored pressure, which is usually good for around 30 brake applications, if the pump has just run. (Compare that to conventional cars with vacuum boost, where you might get maybe half a dozen pedal applications before the vacuum is used up.) Once the boost is used up, you can still stop, but the pedal feels a lot harder and you have to really step on it.

Gen 2, for some reason, did away with that hydraulically boosted master cylinder design. There is still a master cylinder and it's still hydraulic with two chambers, but essentially, when not in failsafe, the fluid you push out of the master cylinder goes nowhere except a stroke simulator and some pressure sensors in the actuator. You're basically telling the computer how hard you want to brake, and it uses the ABS valves to send pressurized fluid from the accumulator directly into the wheel circuits to do that. Gen 2 has independent control of all four corners, instead of ganging the rears as Gen 1 did. It also does away with Gen 1's old-school P&B valve. The computer is controlling all four wheel pressures directly anyway, so it can balance the front/rear braking force (what the P&B valve used to do) as just another part of its job, which Toyota calls EBD for Electronic Brake-force Distribution. (Unlike the simple P&B valve, which is just matched to some predicted weight distribution for the car, the EBD can proportion the force according to how the car really is handling in the moment.)

If Gen 2 goes failsafe, the valves flip, and once again you're applying the front brakes directly and the rears not at all. It has an extra valve that takes the stroke simulator out of the circuit in that case; that valve was weirdly missing in Gen 1. In Gen 2, you don't have a purely hydraulic boost effect helping you out in failsafe. But you do have a new, giant box of capacitors back by the aux battery, to sustain electric power to the brakes and make them less likely to go failsafe in the first place.

Gen 3 keeps the all-four-corners-independent, EBD proportioning control, but brings back the hydraulic boost in the master cylinder, and does away with the box of capacitors. It also closes the stroke simulator out of the circuit when in failsafe, like Gen 2.

Gen 4 can be described in this space _________________________________ by somebody who is game to download its New Car Features manual and study the same diagrams there.

Summing up, you can see that pretty much all of the effort to make sure you can stop safely, under the widest range of conditions, has been put into engineering the car's regular, service brakes. They are also real-time monitored all up one side and down the other and the computer can let you know at the first sign of any of a hundred-odd possible problems that might be developing with them so you can proactively keep them from deteriorating anywhere near the point of any unexpected failure to stop the car. That way, even if you practice using the parking brake to stop with, because why not?, you'll generally use it to park, and do your emergency stopping with the brakes best designed to handle emergency stopping.

-Chap

 

I am not worthy of that response, my two brain cells just shorted out. But really, thanks for trying to enlighten me.

I will try to grok what I can.

 

Have you guys all read, "Stranger in a Strange Land", where the Martian-raised human uses the term, "grok" to signify analysis/understanding??!! I love that Heinlein novel!!

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But others have said that it is becoming more common for the parking brake to be electrically actuated, and this is how it is done, for example, in the Volt.

 

True enough. In that post I was replying to PriusNeckBeard, who was asking about the parking brake in a Prius.

I might have been one of the people kvetching about the electric motor approach ... my mom has a Volt that works that way, and my first reaction was "Wth!?" (which, oddly enough, even though I have typed "Wth!?", will be displayed on PriusChat as "Wth!?", just as if I had actually typed "Wth!?", which I didn't).

By now I'm well past my first reaction, and it's still the same. It's hard for me to imagine you can beat a pedal or lever and a cable for simplicity.

-Chap

 

First off, every one should be aware that their car has an auxiliary braking system in their engine and transmission. In the event that the main brakes fail, your first reaction should be to downshift.

Next, the advancement of the brake technology is the reason the parking brakes are no longer emergency brakes. Go back to the '60s, and the driver didn't have the luxury of power brakes. The brakes needed more surface area, and were larger, to apply the same amount of braking force as the brakes on modern cars do. The parking brake was an emergency brake back then, since the amount of brake force on the rear wheels the driver can apply with it was around the same as they did through the service brakes.

Go ahead and use the parking brake in an emergency, but also use downshifting, increase regen braking, or whatever auxiliary braking the car has available.

 

That's kind of like having it hard to imagine an electric motor being less simple than an ICE.
The electronic brake is just an small motor at its heart, and running wires to the switch in the cabin and to the power supply is simpler than running a metal cable that needs to take as direct of a path as possible between the the switch and brakes in order to avoid hang ups. Getting rid of the cable is likely where the weight savings come from.

Going from no software is a complication, but adding software makes the brakes safer by allowing auto application in the event that a car that shouldn't be moving starts to. Plus it allows the parking brake to be integrated with the ABS system and apply more force than the driver in the event they need to be used in an emergency.

 

The electric parking brake releases on its own if the car is put in gear and starts to move. No leaving them dragging.

 

In my mind, the emergency brake should be the ultimate failback to stop the car when all the more sophisticated and efficient braking systems fail. Those other systems, such as ABS, already have "safe mode" failure states which make the need for mechanical backup highly unlikely, but what if there is a total collapse of the electrical system? Even standbys like downshifting don't apply with a powertrain design like in a Prius. A pedal pulling a cable that clamps down on the brake pads is about as failsafe as one can get. If you are worried about someone driving off with the emergency brake engaged, then put sensors on them to yell at the driver if they attempt to drive off with the brake engaged. With all the paranoia over safety, like a gazillion expensive air bags, and drooling liability attorneys itching to sue for millions if a person bumps their knee, I don't understand how manufacturers got this one (electric parking brakes) past their legal team and regulators.

 

If there is a total collapse of the electrical system, you might not be able to steer well enough to even give the parking brakes the distance they would need to stop the car. The unlikeliness of total failure of the service brakes has lead to parking brakes no longer being effective emergency brakes.For the the power loss scenario, electronic brakes can be designed to clamp down on power loss. A deadman switch can be even more failsafe than a cable.

Electronic parking brakes (EPB) started in Europe where the large majority drive a manual transmission. EPBs make those more convenient and safer.

 

The steering will work just fine, just without assist. It has a direct mechanical connection and plenty of ratio to control until you get to very slow speeds and very high amounts of turning.