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Question about Regenerative Braking/Brake Pads

Discussion in 'Gen 3 Prius Technical Discussion' started by nivekonbass, Nov 9, 2022.

  1. nivekonbass

    nivekonbass Member

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    Hi guys,

    Not sure if I'm in the right section of the forum. I had some questions about the regenerative braking system on these cars. Currently, I use it when slowing down from 35-0, or when I need to brake abruptly/going downhill.

    Ultimately the question is, at what speeds is the regenerative braking safe to use (can I use the "B" mode to get off an exit ramp from the highway)?

    Is there any sort of maintenance (besides normal brake pads/rotors/cleaning) that I should do to prolong the life of that system? Since the pads last so long, how frequently should I be cleaning/bleeding the brakes?

    I drive 100-175 miles a day, so when answering please keep that in mind. My brakes probably get real hot in some of the roads I have to drive through.
     
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  2. bisco

    bisco cookie crumbler

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    the car is constantly using regen braking when you lift your foot off the gas, or pess the brake.

    b mode is an entirely different beast, and not to be used indiscriminately.

    search here for threads and expert advice, as well as the o/m.

    the brakes should be inspected and slide pins lubed every 30k. that and fluid change interval are in the o/m
     
    #2 bisco, Nov 9, 2022
    Last edited: Nov 9, 2022
  3. Tombukt2

    Tombukt2 Senior Member

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    The regenerative braking is done inside your transmission with the electric motor when you step down further on the brake pedal you activate the hydraulic braking system which goes through a myriad of pumps and valves and other stuff to actually squeeze the pads against the rotors. Generally I change the pads even in these systems about every 3 years like I would on my regular Corollas and yaris. Just because after a few years the brake pad material starts to crack and just in general look like hell. You can usually look at your rotors and tell things are starting to get a little weird by then it's gone a little too far but you learn. If it gets like that then I change the rotors and the pads they're cheap enough now they're consumable items that should be changed that's why they're so inexpensive even the cross drilled slotted coated whatever pretty inexpensive when you consider.
     
  4. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    I think you have confused B mode with regenerative braking. B mode is a way to reduce regen.

    I doubt you ever want B mode in Illinois. If you have a downhill with more than a 600 foot vertical drop, you can fill the battery and have no regen braking. In that situation, switching to B mode at the top will add engine braking to help your friction brakes. So Colorado would be a poster child for B mode, but Illinois is not, 600 feet is the maximum elevation.

    There is no maximum speed for B mode, it does not change the gearing. One surprise in B is that the harder you brake the busier and louder the engine gets.

    Below 24 MPH in your Prius, and anytime below 61 MPH in a Prius plug-in, the engine may not rotate and then it may add to the regenerateve braking. At any speed that would help, B mode adds engine braking to lower the regenerateve braking.
     
    #4 JimboPalmer, Nov 9, 2022
    Last edited: Nov 9, 2022
  5. ChapmanF

    ChapmanF Senior Member

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    You use it more than you think you do. :) As bisco mentioned, the car is doing regenerative braking whenever it can. It only uses the brake pads to do the rest.

    The car always knows when it's safe to use regen braking. It handles that, so you don't have to.

    As bisco also alluded, the transmission "B" mode is something different. The car also knows exactly when that is safe, so you don't need to worry about shifting to B at any time. The car will only do what is safe to do. If you use B "indiscriminately", it won't hurt anything besides possibly your MPGs.

    The B mode does two things. First, it gives you a feeling similar to downshifting in a conventional car. All that is is some math inside the computer running the car. Normally, when you back your foot off the go pedal, the computer treats that as a request for x amount of slowing torque (this is also done to make the Prius feel like other cars). In B mode, when you back off the go pedal, it treats that as requesting y larger amount of slowing torque, and this makes the Prius feel like other cars in a lower gear.

    The other thing B does is what makes it kind of useful at some times. It advises the computer to do a little bit less regen than normal. It will go a bit sooner to spinning the engine to burn off momentum (which also sounds like other cars when downshifted) so that it does not put as much power back into the battery. When would you want this? At the top of a long downhill. You know the battery's going to end up at full charge before the bottom anyway, so you may as well charge it more slowly and treat it a little more gently. So, B then.

    None of this makes any essential difference to what the car does. It has all the same tricks in D mode, and B just suggests to balance them a little differently. In neither case will the car's computer do anything beyond the car's ability to handle.

    Brake bleeding isn't routine maintenance; it's only needed if bubbles are in the system, and they don't get there by themselves. So it is normally something you would do after opening the hydraulic system for some kind of repair. You normally don't do that even for routine things like changing pads. (You can open bleed screws to let old fluid out as you push pistons back, but normally you'll do that with care to let only fluid out, not air in.)

    Brake fluid replacement is sometimes done preventatively. Again, you do use the bleed screws for that, but with care to let only fluid out and not air in. Your incentive for being that careful is that a procedure does exist for fluid replacement without a scan tool, but no such procedure exists for bleeding air out. If air gets in, a capable scan tool is needed to kick off the car's computerized air-bleeding process. *

    The very most important thing to do regularly with Prius brakes is inspect them. Toyota calls for a visual inspection every 5k miles or so, roughly oil change or tire rotation frequency, and a more thorough inspection every 3 years or 30k miles. I split the difference and do a bit more than just "visual" inspection at every tire rotation, so I don't have to do special stuff at the 3 year points.

    Be holistic about the inspections. Don't get hung up on any one part. (There are a lot of posts here on PriusChat where you'd think the slide pins are the only brake parts the posters ever heard of. Ironic, because I hardly ever meet a slide pin that isn't just fine.) Check the lining thicknesses of course. Check for rust or weird wear patterns on the rotors. Check that all the rubber boots are intact, rubbery, not punctured or torn. Check that the slide pins move smoothly in and out with a "greased" feel; if they do, that's as obsessive as you need to be about those. Normally, they will feel a bit peanut-buttery at first (they move by very small amounts in normal braking, which lets the grease get that way), and you will feel the grease wake back up with a few in-n-outs.

    Check that the pads move freely back and forth in the little springy clips that hold their ends. Those clips are called the "fitting kit" and they have a dry, Teflon-like coating from the factory. It wears off. Later, they start to rust. If the pads aren't sliding in them easily, change the fitting kit. It's cheap, and if I ever do an inspection that shows any parts to be needed at all, most of the time it'll just be those. But attend to whatever you find that needs attention. That's what keeps brake problems away.

    Because I do these inspections as frequently as tire rotations, if I ever do find attention needed to some part I don't have on hand, there's normally no problem just reassembling and making a note to self to get the part and deal with it next time. It would be a rare problem that could go from undetectable at one inspection to urgent by the next one.

    * if the only place there's air is out at the wheel line ends, say because you had a caliper off, you might get away with using the "fluid replacement" to bleed that air out.
     
    #5 ChapmanF, Nov 9, 2022
    Last edited: Nov 9, 2022
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  6. Paladain55

    Paladain55 Active Member

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    Don't use b mode unless your life depends on it. Its just not great on the motor and the pcv system really can't handle it. For me if I see the cars battery is charged up and the engine is about to kick on to burn off the excess energy, I just go ahead and put the car in neutral and use the regular brakes.
     
  7. ChapmanF

    ChapmanF Senior Member

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    Probably a minority opinion.
     
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  8. Paladain55

    Paladain55 Active Member

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    I will say the premum recommendation is a brake bleed every two years. If you are having issues where the pedal goes deeper when the brakes are hot, the fluid is just full of water and the heat capacity of the fluid is not much.
     
  9. Mr. F

    Mr. F Active Member

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    When you say motor are you referring to one of the motor/generators or to the ICE? How does utilizing B-mode degrade either of those components or the PCV system?
     
  10. Paladain55

    Paladain55 Active Member

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    ICE. Its not really hurting the motor but it puts more blowby pressure into the crankcase which causes windage and heats up the oil which allows the pcv to suck more oil vapor out of the motor and burn it and clog up the rings faster. Oil burning leads to all sorts of problems. I guess we only really notice as toyotas last long enough to notice the oil burning whereas other cars might fail before that.
     
  11. ChapmanF

    ChapmanF Senior Member

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    The Prius brake system has a quirk, or feature (however you want to look at it), that gets in the way of making those judgments by pedal feel that work in other cars.

    In the Prius, the electronically-controlled braking (ECB) system is responsible for the fluid pressure in the brake lines. It picks the target pressure and works the actuator valves to keep it there. If there's an unbled bubble in the line, or if wet fluid turns to vapor, ECB just sends more fluid down to maintain the pressure, and the pedal feels the same. Other cars aren't like that.

    This is how you can have some bubbles in a Prius brake system and feel complacent about the brakes working well, until some day when a problem puts the brakes in fail-safe mode and then they don't work so well.

    It turns out ECB is so good at masking those pedal feels that in the repair manual, in the troubleshooting steps for C1214, they have a whole procedure to follow to check for warped rotors. In a normal car, you'll feel a warped rotor as a pulsation in the pedal. But in a Prius with ECB active, you don't. The computer just makes the pressure stay constant.

    So the repair manual makes you disable ECB first, and then you can feel the pedal pulsation if a rotor is warped. They can't just have you use Invalid Mode, because that self-cancels if you drive the car. So they make you go create a stroke sensor error, forcing the fail-safe mode. Then you can feel things in the pedal, like in other cars.
     
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  12. Paladain55

    Paladain55 Active Member

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    I agree. i actually monitor brake pressure so i can do my best to not use the friction brakes besides like under 7mph when the system swaps from mixed to friction brakes only.
    If you have ever towed a boat and didn't use engine braking and sunk your brake pedal to the floor and noticed the smoke coming off the brakes is what i'm talking about. Like a prius goes down a massive hill into a corner and somehow overcharged the battery the engine will go ahead and kick on and start engine braking and using more mechanical pedal. In my head i will just go ahead and go into neutral before it does that at like 80% soc or whatever and just brake normally unless there's too much heat then just use b or regular so you don't die
     
  13. CR94

    CR94 Senior Member

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    That's confused on several counts. There's no "blowby pressure" in the crankcase until blowby gas volume flow rate exceeds the PCV valve flow rate, and even then is small, because any increased pressure is free to back out the PCV fresh air intake. Blowby volume is at its maximum at max torque output, not when pressure in the cylinders is low, as it would be coasting in "B". Windage loss is a function of engine speed, not pressure.
    Oil that exits via the PCV valve ordinarily is in the form of mist (fine droplets), not vapor as is commonly believed.
     
  14. Paladain55

    Paladain55 Active Member

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    that's possible but max suck would be at highest rpm with the throttle closed. The pistons will suck and the crankcase will blow. Blowby pressure does make it in to the crankcase i'm not focusing on the pressure though. Oil mist/vapor whatever it is will get sucked into the motor and burnt between the two forces. like every chevrolet motor that has the exact same problems as our priuses. Idk, if you don't have engine issues just use B mode then. cars aren't designed to last forever anyways. Windage just means the flow of air, not windage loss though. You always hope the pcv system can keep up but a lot of the time on older cars it doesn't depending on your definition of keeping up.
     
  15. ASRDogman

    ASRDogman Senior Member

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    Remember, when 2 pistons are going up, 2 are going down..... it equalizes.
     
  16. Paladain55

    Paladain55 Active Member

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    True. I like theory but sometimes we don't know. Maybe its just the pistons sucking the valve seals? Too many different things going on.