Braking Modulation/Feel At Low Speeds

Discussion in 'Prime Technical Discussion' started by kepani, Dec 9, 2019.

  1. kepani

    kepani Junior Member

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    After just over a month of ownership, I'm becoming quite educated about the car both from a specification standpoint as well as from a driver's standpoint.

    First off, I have so much praise about this car and whenever there is a chance to talk about this car with other Prius Prime and normal Prius owners, I get excited. When there is someone who is even partially interested in the car, I feel that is the green light to go ahead and "share"! :ROFLMAO:

    But with all the great, there's always a bad or...a nicer way of putting it...more to be desired. :cautious:

    I have gripe about the brakes on this car. I get an uneasy and not so confident feeling with the brakes - particularly at low speeds when eventually coming to a stop. The brakes are NUMB and/or have a DEAD BAND - they lack good modulation. I suppose it comes with the nature of the car since it has regenerative capability?

    Does anyone agree?

    Mind you, I'm used to the brakes on Hondas and Porsches, which have excellent pedal feel / modulation so I may be "spoiled".
     
  2. kepani

    kepani Junior Member

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    After some searches for "brake feel" and the like, there seems to be a lot of input on this very topic. ***Reading the archives!***
     
  3. bisco

    bisco cookie crumbler

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    ya, its tough for us to know what you're feeling, but in all likelyhood, it's just the normal prius brakes.

    two things, around 6-7 miles per hour, regen stops, and it's only friction braking. if the road is rough, that can cause pedal turbulance.
    the rotors can tend to rust, or if wet, the brakes can get get grabby when it's only friction
     
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  4. kepani

    kepani Junior Member

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    Thanks @bisco for the input. I did not know that regeneration stops at around 6-7 MPH. That explains why there is a feeling of transition when coming from moderate speeds to a complete stop.
     
  5. bisco

    bisco cookie crumbler

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    exactly
     
  6. VTBIGDOG

    VTBIGDOG Active Member

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    According to the display regeneration breaking continues to happen right up until the vehicle is stopped.
    Posted via the PriusChat mobile app.
     
  7. VTBIGDOG

    VTBIGDOG Active Member

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    Darn voice recognition...I meant braking.
    Posted via the PriusChat mobile app.
     
  8. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    The Prius has an amazing brake blender to let you operate both systems with the same pedal almost transparently. It isn't perfect though, and you've already felt why.

    In tougher traction situations (ice & potholes) the changeover from one system to the other can be more abrupt, and it can also surprise you with a sudden momentary lack of braking force. Once you see what's going on it's no big deal, you'll be used to it.

    Blending two brake systems to work (almost) transparently from one pedal is a pretty big technical achievement. I have some extra respect for Toyota now that I've driven it. Apparently a few EV/hybids out there didn't even bother trying.
     
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  9. bisco

    bisco cookie crumbler

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    that must be a new feature, but idk if the display is accurate
     
  10. kepani

    kepani Junior Member

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    I have yet to experience the lack of braking feel during low traction situations, but I'll be sure to watch for it!

    Much respect for the technology. Thanks @Leadfoot J. McCoalroller for the insight on this! (y)
     
  11. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    Mostly it just means stopping a bit short, then easing ahead to the appropriate stopline when you're in lousy conditions.

    W.........................X....Y..Z

    Where W is you in your car moving rightward, and Z is the stopped car ahead of you, Y is where you would naturally stop to queue at the light. Instead of stopping at Y, aim for X and then creep ahead to Y.

    It's a good way to handle stopping on ice in any car, but with the fancy brakes in a Prius the technique is a little more important.
     
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  12. kepani

    kepani Junior Member

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    I like how you "drew" it. I've been working on my braking technique in a similar manner to compensate for the blending cross-over. The trick seems to be to have deft feathering of the pedal to come to a neat stop.
     
  13. m8547

    m8547 Active Member

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    The Hybrid System Indicator display lies. The scale is not constant. For example if battery discharge current is limited (typically when it's cold and mostly discharged), when you floor it the bar will go all the way to the top, but you might accelerate only half as fast as normal (I've seen it limited to 30kW or less sometimes, on Hybrid Assistant).

    For braking, it doesn't necessarily show you how much of the braking is regen. If you press the pedal fast you might get friction braking right away, even if you are only braking gently. If you slowly ease onto the brakes you're more likely to get regen.

    And if you hit a bump while braking, it will quickly switch to friction braking. There might or might not be a momentary interruption in braking force, the traction control light might or might not flash, and the ABS might or might not kick in. The display that shows the amount of braking won't change.

    There is probably always a little bit of regenerative braking, but in some cases it is so small that it is negligible. But it is enough that the energy monitor will probably always show power going into the battery while braking.
     
  14. CharlesH

    CharlesH CA HOV Decal #5 on former PiP

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    My understanding is that Tesla is one EV that doesn't do brake blending. You are expected to do "one pedal" driving with the throttle, and if you let off the throttle, regeneration slows you down. But the brake pedal is always the friction brakes.
     
  15. fuzzy1

    fuzzy1 Senior Member

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    Yes, this is very important on ice in any car, and something that catches many winter neophytes by surprise. Absent perfect winter road grooming, the degree of tire-induced ice glazing normally increases the closer one gets to the stop line. I.e. the closer one gets, the slipperier the snow-ice combination becomes, making for less possible traction.

    Thus, it is always essential to leave some expected buffer. Don't try for a perfectly steady constant-force medium or mild braking all the way up to an arm's reach of the bumper ahead. And especially don't start mild with a gradually increasing brake force all the way to the stop, as some of my car pool partners did.

    Do the harder braking early, so less braking is needed at the end. So if traction becomes unexpectedly inadequate, there is still room for alternate maneuvers.
     
  16. ChapmanF

    ChapmanF Senior Member

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    If I remember right, this was also the solution to a homework in my high school physics class, just having to do with optimizing the energy lost in coming to a stop (versus not having to stop completely, if the light changes as you're approaching).

    High school physics for me was well before hybrids and regen braking ....
     
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  17. fuzzy1

    fuzzy1 Senior Member

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    I very much agree here. The sooner some speed is scrubbed, the less total speed will need to be scrubbed at all, thus saving kinetic energy and improving trip efficiency.

    Of course, with most of us lacking visible countdown timers indicating when the light will turn green again (I do see these in some foreign locations), we can't do this with any perfection. But even with guesswork it still produces energy savings.
     
  18. PiPLosAngeles

    PiPLosAngeles Active Member

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    I find this maneuver almost impossible in the Prime, although never an issue in the PiP ... or the Camry, the Corolla, the Civic, the Accord, the Jetta, or Passat of years past. The brake booster in the Prime is really "jerky" for lack of a better term. It is impossible to make fine braking controls with it. One of my little driving hobbies used to be stopping in such a way that you can never feel when the car actually stops. That takes fine control of the brakes, but that maneuver is impossible with the Prime. Just try letting off the brake in a Prime just barely enough that you're creeping so slowly you can't tell if you're moving or not for sure. Impossible. When moving at low speed, the difference between wheels locked and nearly free-rolling has to be less than 2-3mm of pedal movement.
     
  19. m8547

    m8547 Active Member

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    For me it's usually perfect smooth, indistinguishable from any other car. But sometimes (10-20% of the time) it gets stuck in a jerky mode. When it does that it also feels like the pedal motit is almost sticky. It seems more likely after pressing the brake hard.
     
  20. PiPLosAngeles

    PiPLosAngeles Active Member

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    Mine is that way 100% of the time since I got the car. I think it's the hill assist. It causes the calipers to remain engaged even though some pressure has been released from the pedal. So if you slowly release the pedal you aren't slowly disengaging the pads, instead the caliper continues to grip until the pressure passes some threshold and then they release instantly to the position 'normal' brakes would have eased into.

    If I had an unlimited budget and my own laboratory I would put a pressure gauge at the calipers and map pedal position vs. pressure applied at the brakes.
     
    #20 PiPLosAngeles, Dec 15, 2019
    Last edited: Dec 15, 2019
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