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Intermittent pulling when low speed braking over bumps

Discussion in 'Gen 3 Prius Technical Discussion' started by bromli, Apr 30, 2023.

  1. bromli

    bromli New Member

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    Vehicle:
    2012 Prius
    Model:
    Four
    Got a 2012 Prius that is exhibiting weird behavior. Intermittently, when braking from say 35mph down to zero, and usually after driving over an imperfection over the road, she will pull real hard left or real hard right. Usually its pulling to the left but it pulls both ways. It doesn't happen all the time, and there is no other problems with braking like roughness in the pedal. In fact, I can decelerate hard or softly on smooth highways and the like with no issue. The car only has 62k miles on it and the friction braking system doesn't seem defective.

    The only thing that I can think of that is related is that the alignment seems to be slightly off (drifts to the left at highway speeds) and that it seemed to start after my mechanic rotated and balanced the tires, but I think that is just pure coincidence. Is this some weird quirk of the regen braking and the power steering interacting together? I have driven cars with funky alignment and they haven't caused a braking issue like this.
     
  2. ASRDogman

    ASRDogman Senior Member

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    Vehicle:
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    Pulling one way or the other can be caused by tires.
    Swap the front wheels left to right and right to left and see if it still pulls
    more, or pulls less.
    Mine drifts slightly right. Whoever did the alignment didn't take in effect that
    the road is not level, there is usually .5 degree slant to allow the water from rain
    to drain.
    The crappy tires I got last time made it worse. I swopped the front tires and it drove
    straight.

    You should check all the bushings for control arms. I would bet that they are worn.
    The ones on my van where shot. I had to turn completely one way in a circle once so
    that it would drive and brake straight....

     
  3. ChapmanF

    ChapmanF Senior Member

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    Vehicle:
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    IV
    This sounds to me like a combination of things.

    There is a classic Prius normal behavior that many people notice (and are often startled by, the first time), when the car is being gently braked and then encounters some road imperfection. The normal feeling is that the car let up on the brakes for an instant, or slightly leapt ahead.

    What happens is that the gentle braking is being handled by regen, using only the front wheels, and the road imperfection makes the car think "whoops! sketchy traction, I'll switch from regen to friction brakes, they can grip with four wheels, not just two". That's not the same as applying ABS, but it's a needed step so the car is prepared to do that if it later decides there's still sketchy traction and just switching to four-wheel friction brakes hasn't helped enough.

    The car lets off the regen braking right away and it takes an instant for the friction brakes to reach the same braking rate, so that's why you normally feel a sort of mini-lunge ahead right while it's in that transition.

    It does not normally feel like a pull to either side, just a mini-lunge straight ahead. But I am thinking if that normal behavior got combined with some issue where your friction brakes are pulling to the side, that could feel as you've described.
     
    mr_guy_mann likes this.
  4. bromli

    bromli New Member

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    Location:
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    Vehicle:
    2012 Prius
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    Four
    Chapman, I think you nailed it. I do feel that "lurch" that you're describing, as if the car is suddenly going full beans on the brakes just for quick moment. Sometimes the trac light flashes during this, but usually its on gravel or wet roads (which makes sense). I think I will chalk it up to slightly misaligned tires and the quirks of a hybrid car. Definitely a bit more going on than the beater saturn that I had before this! Next time I get the oil changed I'll have the tires rotated again and see what happens.