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Panic stopping...doesn't

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by TheLandYacht, Jan 2, 2021.

  1. TheLandYacht

    TheLandYacht Member

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    So...I found out that panic-stopping in the 2010 Prius isn't the most reliable in the world.

    Going about 30mph in a neighborhood I was unfamiliar with, I come across one of those "sudden dips" like there are over some "low water stream crossings", where it goes from flat to a quick-downhill-to-creekbed suddenly.

    I hit the brakes and the car...ALMOST stopped. Got down to about 5-10mph & it felt like the brakes just let go.

    Brakes feel fine otherwise, both before & after the event. No brake squeal, no grinding/etc. My foot didn't "go all the way to the floor".
     
  2. rjparker

    rjparker Tu Humilde Sirviente

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    Probably dropped out of regen and the hydraulic brakes were not pushed enough. If you have to stop, step on it and let the traction control and abs take care of any excess.
     
  3. Higgins909

    Higgins909 Member

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    I have never had to actually use the "panic brake assist". It's only been a pain in my buns. The last one I can remember is I was going to a stop and a car pulled out in front of me kinda, but at speed and I had to brake a bit harder and my head almost hit the damn windshield. I only braked a little bit harder, but it was fast enough for the system to activate.

    Several times it been a yellow light and I need to go straight to mild braking, but that causes it to spazz out and lock up the wheels and come to a grinding halt. I usually let up right away after realizing whats happened and that disables the system for the while. Hasn't saved me once in the 100+ times it's gone off.

    It's possible all 4 wheels locked up at the slower speed and slid? I think I lied, the system did save me once, but the whole time it felt like I was breaking super hard but sliding and didn't hear the ABS go off. I went from about 40 to 10-20 in about 30ft, it was downhill too.
     
  4. vvillovv

    vvillovv Senior Member

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    What happened the second time you tried to stop before getting stuck in the middle of the creek?

    was that in thelandyacht?
     
  5. TheLandYacht

    TheLandYacht Member

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    I'll admit, I've grown out of the habit of "mashing" the brake pedal as was my wont in the previous vehicle...which needed quite a bit more coaxing to stop, as you can imagine.

    As a side-note...driving the Prius has rapidly re-trained my driving reflexes considerably. I find myself accelerating & braking much more...gently...even in my full-size pickup.

    No, and it was a dry-creek anyway. When I took my foot off (at 5-10mph) & reapplied...the car came to a stop.

    And no, it wasn't in The Land Yacht...which is a reference to my previous Business Nom de guerre.

    You might notice a tiny bit of difference between that and your average Prius.
     
    #5 TheLandYacht, Jan 2, 2021
    Last edited: Jan 2, 2021
  6. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    If you are on the brakes when you hit a significant pavement defect or transition from one surface to another, or even the sharp threshold of a grade like you describe, that makes a bit of a 'thump' in the suspension. You feel this as you drive and think nothing of it.

    But to the braking computer, that event 'feels' very different. It can feel like a momentary loss of traction, which is sometimes enough for the computer to react, and it nearly always decides to cancel all regen braking and switch in the hydraulic braking system- which takes a brief fraction of a second.

    While brief, it feels like a lifetime when you have just gone over "a bump" and your foot is still on the pedal, and you aren't slowing down as much as you'd hoped or calculated.

    You get used to it. You learn to use more brakes before the bump, and less while on it. You train yourself to remember that when you feel the brakes drop out from under you, just hold the pedal- they'll be back stronger very quickly. It only feels wrong for a moment.
     
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  7. rjparker

    rjparker Tu Humilde Sirviente

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    So you have had 100+ wrecks? Abs in its traditional mechanical sense is different than a computer controlled brake by wire system. As long as you have traction it should be seamless and not rapidly pulsing. Even the old abs would only pulse on slippery pavement. There are recalibration techniques if you think your system is faulty.
     
  8. ChapmanF

    ChapmanF Senior Member

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    This. But one thing puzzles me in this particular thread. The original post says it was a panic stop ("... flat to a quick-downhill-to-creekbed suddenly. I hit the brakes...").

    The car behaves differently when it thinks you're making an urgent stop: it goes straight to the hydraulic brakes anyway, so that occasional regen-to-friction dropout feeling isn't a possibility in that case.

    I would have to guess that the original poster went on the brake pedal kinda quickly, just not quickly enough to meet the car's definition of urgently.

    There are two ways the car recognizes "urgent". It has a pedal stroke sensor mounted up at the pivot of the brake pedal, and if it sees the pedal being pressed quickly enough, that's urgent.

    The other rule, I've noticed, is that if cruise control is on, any use of the brake cancels it (no surprise there) and is also always treated as urgent and goes straight to hydraulics, no matter how gentle (that was the surprise). Since figuring that out, I've formed the habit of always pulling the cruise stalk to cancel before any ordinary, non-urgent braking.
     
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  9. Higgins909

    Higgins909 Member

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    I haven't thought about seeing if it could be recalibrated. I kind of wish it could be disabled. The manual only talks about it a tiny bit under "brake assist". I usually call it "panic brake assist" because any time it goes off, the car must think I need to stop fast. I haven't been in 100+ wrecks, but It has gone off 100+ times. My 04 Accord had a brake pedal that had like 2-3" of free play before engaging the brakes. The Prius uses this for regen and has less of it. If I go off the throttle too fast and straight to full regen, it will possibly detect it as a panic stop and activate the "panic brake assist". In my Accord I went straight to to needed braking amount and it was always the same. Not on the Prius. It will cause the "panic brake assist" to go off. I have to brake slowly to my needed brake amount or it will go off. I think I just registered my car for the 4th time (2021 will be 4yr ownership) and am still not used to it. It doesn't help the braking amount is random.
     
  10. TheLandYacht

    TheLandYacht Member

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    This. Didn't "slam" the brakes (all the way to the floor). Apparently not enough for the system to recognize the urgency.
     
  11. rjparker

    rjparker Tu Humilde Sirviente

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    I can tell you this: My younger brother used to build 1/4 and 1/2 mile oval track cars for use on local venues back in the day. His go to daily driver is a conventional Camry but he has pickups and motorcycles as well. Earlier this year he borrowed my v for the first time and was blown away by the braking g force it was capable of compared to his old race cars, Camry, trucks or motorcycles. I explained regen creates braking far in excess of engine braking or hydraulics alone.

    I does take a bit to get used to it or you may be decelerating fast with g forces. I don't believe there is a "panic brake assist" other than four wheel disks commanded along with the normal and most frequently used regen. Sure there is abs and traction control for slipping and sliding situations, most of which we never experience.
     
  12. ChapmanF

    ChapmanF Senior Member

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    As far as I know, "urgency" is not detected from how far or hard you press the pedal (down to the floor or not), but how quickly you go on it, judged by the speed of the stroke angle sensor up at the pedal pivot. You could push the pedal just moderately far down, but if you did it quickly enough, it would count as urgent, and go directly to hydraulics.

    Brake Assist is a separate function, and it is triggered based on the master cylinder fluid pressure sensor, not the pedal stroke angle sensor, so it is also separate from the simple judgment of urgency that skips regen and goes to hydraulics, based on the angle sensor. If you also stomp hard enough to trigger Brake Assist, it increases the hydraulic assist gain for you, as if your leg suddenly got stronger.

    ba.png

    'Random' probably isn't the right word really; I'm sure it's quite thoroughly algorithmic. But you're not used to it. :) The best medicine is probably just to cultivate a smoother braking style ... I'm not sure if I've ever triggered Brake Assist in mine.
     
    #12 ChapmanF, Jan 2, 2021
    Last edited: Jan 2, 2021
  13. fuzzy1

    fuzzy1 Senior Member

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    No, regen braking is quite limited to what it can put into the battery or into engine braking. The battery can't take more than 27 kW, the engine braking seems somewhat similar. So regen is limited to just light to moderate braking.

    Hard and panic braking are essentially all hydraulic friction braking, which on a fully loaded car at highway speed can exceed 500 kW.

    If you brother found stronger braking forces on your Prius than on his race cars, then it is a testament to your tire choice and to the many brake control features on your Prius: ABS, EBD (Electronic Brake-force Distribution), BA (Brake Assist), and SST (Smart Stop Technology).

    The few times I've been in panic braking, either intentionally or by unintentionally triggering the Brake Assist, it felt as it the car was trying to dig up or plow the asphalt.
     
  14. vvillovv

    vvillovv Senior Member

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    The Karaoke Kid loves the video Thanks :love:
    Hope the damned prius brakes stop acting up on ya....:whistle:
     
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