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How do you brake in an emergency?

Discussion in 'Gen 3 Prius Main Forum' started by Higgins909, Dec 4, 2018.

  1. ChapmanF

    ChapmanF Senior Member

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    If you look for any driveshafts or motor-generators connected to the rear wheels, you won't find them. Regen is only capable of acting on the front wheels. The hydraulic, friction braking system acts on all four.

    Now, if you dig, there's this graph that gets reposted from time to time showing that the car does a more complicated dance of balancing regen and hydraulic brake force than most people guess:

    [​IMG]

    The car can be constantly adjusting the balance during the stop, depending on vehicle speed and battery conditions. Those particular graphs are from the Gen 1 manual, and show that some amount of hydraulic pressure is always present when braking in that generation, even under the lightest braking. The Gen 1 hydraulic system design made that unavoidable.

    Starting with Gen 2, they were able to eliminate the little sliver of hydraulic running down the lightest-braking edge of the graph:

    g2ecb.png

    Still, the car can be caught at a time when all or even most of the braking force is from regen at the front wheels, and be spooked by a wheel-speed signal and think "yikes! I'd be better off balancing the force over all four wheels!" and the regen-to-friction whoopee! moment follows.

    Even Gen 1 had the whoopee! moments ... its little sliver of hydraulic force during all braking was still not the same as balanced braking force to all four wheels. Later generations may have slightly more pronounced whoopee!s, as their friction brakes might have to clamp in all the way from zero pressure at the moment the car decides to switch.
     
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  2. Mendel Leisk

    Mendel Leisk Senior Member

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    I'm skeptical that the rear brakes would be disengaged, or near-disengaged, when the front brakes are mostly using regen.

    Searched a bit, without much result so far.

    The rear brakes have a dedicated hydraulic pump, somewhat unique.
     
  3. ChapmanF

    ChapmanF Senior Member

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    In what car are you talking about?

    I have never known of any Prius generation or model with more than the single hydraulic pump that fills the accumulator.
     
  4. Mendel Leisk

    Mendel Leisk Senior Member

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    Sorry, mis-worded, I have a limitted understanding.

    What I was thinking of: when bleeding the brakes, the fronts are bled conventionally, foot on the pedal, and a quick open/shut of the bleed bolt. The rears though, foot on the pedal, you can open the bleed bolt and leave it open as long as you want, something keeps pumping.
     
  5. ChapmanF

    ChapmanF Senior Member

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    Yeah, that's just using the pressure built up in the accumulator, and that's how the car works the rear brakes, when it wants to operate them. Just opens a valve and the pressurized fluid flows back.

    That's also how the car works the front brakes in normal operation. But you don't use the same procedure to bleed the fronts, because the fronts are also on a fail-safe hydraulic circuit straight from the master cylinder that is supposed to be there to stop you even if all the electronic controls fall over. So you bleed the fronts using the fail-safe circuit, because it's most critically important to have all the air out of that.

    (In normal operation, the system can sort of mask the existence of smallish air bubbles in the line: there's plenty of pressurized fluid in the accumulator, the valve just stays open a bit longer and enough fluid whooshes into the line to squeeze the bubble, and you don't notice much difference as the driver. But if you ever find yourself in fail-safe mode, it's just like an older car then: there's a limited distance from your foot to the floor, and you can't afford to use it squishing bubbles.)

    If you're skeptical about when the car does or doesn't add friction when braking by regen, you don't have to take Toyota's word for it; you could make like hobbit and add a brake-pressure display. :)

    [​IMG]
     
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  6. Mendel Leisk

    Mendel Leisk Senior Member

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    Lemmee guess: front brakes on the left... :cautious:
     
  7. Higgins909

    Higgins909 Member

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    Here's a question. Does the abs light not flash or anything when abs is activated? I know there is ESP, but that only seems to go off when I slide or do a burnout. Sometimes when I park it will flash and beep. (I think it beeps but it's only happened 2-3 times. I pull in to the spot and press harder on the brake and use the foot parking brake and then P button) I found some wet spots and was testing my braking capability and got a few pulses of abs, but no lights.
     
  8. ASRDogman

    ASRDogman Senior Member

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    The light only comes on when there is a problem or doing the check at start up.
     
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  9. ChapmanF

    ChapmanF Senior Member

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    I'm pretty sure the ABS light only lights to indicate a problem with the ABS.

    That's in distinction from the skid light, which does light when the skid control is doing stuff.

    I'm not sure he said. He mentioned he only hooked up the FL and RR, so he had one from the front and one from the rear, and they were the easiest wires to get to (in a 2004).

    If you follow the link, he describes a bunch of observations he made, including of "several intriguing dances that happen between front and rear at times".