Braking question

Discussion in 'Gen 4 Prius Technical Discussion' started by Kimoy, Oct 14, 2020.

  1. Kimoy

    Kimoy Member

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    Idk if this is already discussed for Gen 4 but I couldn’t find anything on the search. Is the Gen 4 braking the same as the Gen 3? By that I mean that in Gen 3, friction brakes only kicks in when the car is down to 8mph or less or on sudden/hard braking, does this still apply to Gen 4?
     
  2. CooCooCaChoo

    CooCooCaChoo Active Member

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    Same, but now with radar, the car can brake in an emergency if it detects a collison.
     
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  3. ChapmanF

    ChapmanF Senior Member

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    That's not even true for Gen 3, or for any Prius generation.
     
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  4. vvillovv

    vvillovv Senior Member

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    What Chapman said.
    The description given in the OP is what I'd say is a misunderstanding of how regenerative braking works in the prius normally as apposed to some of the observations made of what regenerative braking can show at both extremes of use. ie: hypermiling or heavy traffic at freeway speeds......
    There is a lot more space inbetween the extremes where regenerative braking works without showing the driver obvious indications of what is happening.
     
    #4 vvillovv, Oct 14, 2020
    Last edited: Oct 14, 2020
  5. Kimoy

    Kimoy Member

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    What’s not true?
     
  6. Kimoy

    Kimoy Member

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    When you step on your brakes in Gen 3, the electric motor slows you down (regen) to 8mph then the friction brakes kicks in to bring you to stop. The question is, is it still true to Gen 4?
     
  7. ChapmanF

    ChapmanF Senior Member

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    No, that's not the case. Where are you getting your information?
     
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  8. Kimoy

    Kimoy Member

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    It was discussed in the Gen 3 sections years ago when I first got my 2010.
     
  9. ChapmanF

    ChapmanF Senior Member

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    It's been widely discussed in the Gen 3 sections. Plenty of threads get it right. Are there particular threads or posts you've been relying on to think it works the way you think it works?
     
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  10. Kimoy

    Kimoy Member

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    There was but I couldn’t recall which one it was. It’s been almost 3 years since I had my last Prius.
     
  11. ChapmanF

    ChapmanF Senior Member

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    Some places it has come up before, with the graphs from the Toyota manuals, are here in the Gen 3 section, and here and here in the Prime section.

    In that last one, Elektroingenieur kindly posted the Gen 4 graphs in the next post, #32. I typically include the graphs from the Gen 1 and Gen 2 manuals because I haven't personally downloaded the corresponding Gen 3 and Gen 4 ones; certain parameters get adjusted up or down over the generations, and they changed the style of the graph for Gen 4, but none of that ever changes the physics much.

    The physics pretty much dictate that very low speed has to be friction, and high speed and/or strong braking has to be significantly friction, and there is kind of a sweet spot in between those regions where regen is able to play its largest role. In any given stop your speed is changing, your pedal effort is changing, your battery SOC and temperature are changing, and the braking computer is recalculating how much of each form of braking to have in the mix, over and over again throughout the process.
     
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  12. Mendel Leisk

    Mendel Leisk Senior Member

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    Bottom line, it's pretty much always a blend of regular friction brakes and regen, sometimes more of one or the other. I'd think 4th gen and 3rd work very similarly.
     
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  13. Kimoy

    Kimoy Member

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    That’s why I said hard/sudden braking because if the driver is easy on their braking like slowly stepping on the brakes lightly not hard that way you don’t activate the Carson hydraulic brake system but rather slows down your car by it’s regen braking system. I guess it all depends on the driver then. Our cars uses regen braking as soon as you let off the gas pedals and if you just lightly tap on the brakes it will not activate it’s hydraulic/friction braking system but rather stays on regen but if you push that brakes hard then friction brakes kicks in together with regen (depends on speed).
     
  14. fuzzy1

    fuzzy1 Senior Member

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    There is still a huge amount of room between "easy" and "hard/sudden" braking, and the regeneration system simply doesn't have the capacity to serve much of that space.
     
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  15. ChapmanF

    ChapmanF Senior Member

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    That has bits of the right ideas, but to suggest that just "slowly stepping on the brakes lightly" necessarily means you "don't activate" the hydraulics is oversimplified enough to be misleading.

    That does get closer. Yes, the brake computer has to calculate how much regen and how much friction to use, every time, and how hard you brake and the vehicle speed are two of the inputs to that calculation. At highway speeds, even quite light braking would exceed the battery's limits if the computer tried to use all regen.

    Some people look at the ECO display on the dash and draw conclusions about what the brakes are doing, but there isn't any dash display that really shows exactly that. Old-time member hobbit got curious enough to build one once; one of the earlier threads linked above links to it.

    A typical stop from a cruising speed will start out with one mix of friction and regen, bend off in the direction of less friction and more regen as the speed comes down, and then bring the friction back in shortly before stopping.

    That's not quite as simple a picture as people sometimes want it to be, but making it sound any simpler than it is wouldn't really be fair to future readers who might come and read the thread.
     
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  16. Mendel Leisk

    Mendel Leisk Senior Member

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    If the brake disks have sat wet for a few days, gotten some surface rust, for the first block or two of driving you will audible confirmation that the friction brakes are being used.
     
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  17. kithmo

    kithmo Couch Potato

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    I suspect the rotors wouldn't last very long if not used much, it doesn't take much moisture in the air to start them rusting.
     
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  18. alanclarkeau

    alanclarkeau Senior Member

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    That was one of the things I was attracted to a PRIUS for - that the rotors are lasting for a very long time - in some cases, the useful life of the car.

    But then - they don't salt our roads, and I suspect the precipitation isn't the same here as in UK. We do get an extended wet period - for 2 or 3 whole days. And much of our rain is a fine day followed by an afternoon/evening thunderstorm.

    Other parts of the country might be different though.
     
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  19. CooCooCaChoo

    CooCooCaChoo Active Member

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    Yeah, unless the car has been sitting for a very long time, and I'm talking years here, its just surface rust that will come off with normal braking. Salted roads though are another matter.

    I'm actually surprised that they didn't throw on slotted and drilled rotors. Even with the usage that would be seen on a hybrid, I'd suspect that they would still outlast the car.
     
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  20. kithmo

    kithmo Couch Potato

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    I had a 2001 Ford Mondeo that had stainless steel rotors (I was told) which showed no signs of rust at all. I don't know why this didn't catch on, could it be something to do with the friction properties of stainless steel or just cost ?
     
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