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Number of Complaints Growing on 3rd Generation Prius Brake Performance

Discussion in 'Gen 3 Prius Technical Discussion' started by Marine Ray, May 9, 2017.

  1. Marine Ray

    Marine Ray Senior Member

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    I don't recall having this experience. How about you all? "As the driver applies brake pressure, the regenerative brakes kick in and slow the car. When the vehicle hits a bump or pothole the regenerative brakes switch to friction braking. The friction braking slows the vehicle at a lower rate than the regenerative brakes which in turn makes the driver feels as though they are monetarily accelerating. The driver must then apply the brakes more firmly to return to the expected level of deceleration."

    Number of Complaints Growing on 3rd Generation Prius Brake Performance
     
  2. jerrymildred

    jerrymildred Senior Member

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    I hadn't heard of that before. BTW, that article is from Dec., 2009 so if it did happen, it was in some of the very first Gen 3s.

    I have heard (but not experienced) that on a bumpy road under hard braking that the VSC or anti lock braking can kick in and make it feel pretty weird.
     
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  3. xliderider

    xliderider Senior Member

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    There are numerous threads on the subject. Some claim that the surge forward/loss of braking almost caused them to hit something in front of them, or to enter a road with cross traffic.
     
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  4. fuzzy1

    fuzzy1 Senior Member

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    Old news, 2009-2010 era. Lots of threads about it. The recall happened long ago, reducing but not eliminating the disconcerting feeling. But it doesn't happen during hard braking, because such is already beyond the regen-to-friction transition, so isn't a real hazard.
     
  5. ChapmanF

    ChapmanF Senior Member

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    It's not just a Gen 3 thing, my 2001 (which I bought in 2008) did the same thing. I learned about it right here on PriusChat the week I bought the car. I saw a post suggesting I learn what the brakes feel like when mildly braking over rough road, so I went and did that, learned what it felt like and why, and so it never bugged me.

    It can be useful to think about how you'd want it to work differently. Under mild braking, the friction brakes aren't engaged (or only lightly so), and braking is being done electrically in the transaxle. That only uses the front wheels, so the necessary traction between tires and road is roughly twice what it would be when braking equally hard using all four tires to do it.

    The rough road, picked up through wheel speed sensors, leads the ABS computer to think traction may be failing. It switches as quickly as possible to four-wheel braking, to cut the necessary traction in half and establish four-wheel-independent control. But that takes a short moment to ramp up the fluid pressure. Meanwhile, it has already backed off the front wheel brake force because it already detected signs of possible traction loss, so that was priority 1. So for that split second it takes for the friction brakes to catch up to the earlier braking force, you feel that brief decrease in deceleration.

    You won't experience it when making an urgent stop, because at a certain speed of stomping the pedal, it classifies you as making an urgent stop, and goes straight to friction with four-wheel ABS.

    -Chap
     
  6. K-Dub

    K-Dub Junior Member

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    Hit any pothole or bump under braking in a car with anti-lock brakes and you will get this effect.
     
  7. bisco

    bisco cookie crumbler

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    o/p's link is 8 years old.(n)
     
  8. Former Member 68813

    Former Member 68813 Senior Member

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    Folks, it's never a good idea to brake on bumpy road. I never do it and have no braking issues whatsoever.
     
  9. fuzzy1

    fuzzy1 Senior Member

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    The regen transition makes it worse than in an equivalent non-hybrid.
    Sometimes, we must, because that is all we have between here and the things we mustn't hit.
     
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  10. sleekitwan

    sleekitwan New Member

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    This is useful, I have a phenomenon like this sometimes happens. The problem with our roads in the vicinity is they are urban and ruined. Terribly bumpy, as we are the other side of spending cuts and pandemic slow-downs. There isn’t a single time braking, my Prius won’t ride a bump, locally anyway.

    I thought perhaps there was a little more to it. Not acceleration, but the sudden withdrawal of regen braking, must mean there is an MG been rotated fast, that now needs to be brought under control…now I understand why that ‘emergency clutch’ exists I think? The MG in this state, has inertia and a rotational speed greater than the (slowing) vehicle’s speed warrants ie there’s a mismatch. I suspect, the car IS being made to accelerate slightly, then the system senses this - it’s watching for it - and suddenly there’s a ‘bump’ or shudder as that little emergency clutch pops, perhaps.

    This theory works, anyway?

    The only warning about that article is the wording. ‘…regenerative brakes kick in…’ does sound like one of those people who are motoring journalists because their dad knew somebody who could wangle the job. !

    In other words, it fits with that one I saw, a video review, where the otherwise confident young woman carried it off, right up until she said ‘Obviously the regenerative brakes also will also need maintenance, so that’s a small extra to consider’. If this were true, manufacturers could presumably just fit ALL cars with regenerative brakes, and they’d be hybrids. I didn’t hear anyone else make their ignorance of the regen braking being a principle, not a component, quite as explicitly as that one quoted, but an awful lot of the journos around 2012 sounded like they too, thought ‘regen brakes’ were an actual component, as opposed to regen braking.

    Thank you all here though for the filling-out of what’s happening, when my Prius gen 3 2010 wants to ‘carry on’ sometimes when I brake. It seems worse, or more likely to occur, when I have cruise control on.
     
  11. ChapmanF

    ChapmanF Senior Member

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    You're overcomplicating it. The system has been producing braking force through regeneration in the transaxle. It decides to reapportion that force to the actual brakes at the four wheels, which up to this point have not been much in the game. It takes them a split second to catch up to the braking force that regen was supplying a moment ago. During that split second, the car is not 'decelerating' as strongly.

    I put 'decelerating' in quotes because some physicists don't even use the word; they'll just speak in terms of acceleration, using a number that can be positive or negative. Define the positive direction as forward, and braking is acceleration with a − sign. And during that split second of force reapportionment, there is less of that, meaning you feel acceleration change toward the + direction for that split second.

    And yes, for a long time there was a pavement irregularity on my way out of the neighborhood here where I could enjoy the effect every morning without fail.