anyway to know when old school friction brakes are used?

Discussion in 'Prime Fuel Economy & EV Range' started by The Big Sleaze, Jan 5, 2020.

  1. ChapmanF

    ChapmanF Senior Member

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    You would still be facing the fact that getting the data over the OBD port means you're getting discrete and laggy updates of values that can actually be changing pretty fast during real-world braking. And probably no guarantee that the value you got in the last reports of the braking power, accel rate, etc., were from the very same instant in time.

    I think hobbit's idea with the LEDs will still have the advantage there.
     
  2. Zed Ruhlen

    Zed Ruhlen Member

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    I think the real question is not "when are the friction brakes used" but rather "how much of the brake force is friction"? The distinction being that the friction brakes are ALWAYS used during braking, it just depends on how much.

    This is two-fold because first of all there is no regeneration on the rear of the car and you don't want to just brake the front, EVER. That's super dangerous and likely to cause the car to spin out.

    Secondarily the car is designed to stop, not generate power. That means the hydraulics need to be preloaded so they can properly provide braking as needed. The upshot is the moment you press the brakes the friction brakes are engaged. The amount of force they apply is then controlled by the braking system.

    If your goal is maximum regeneration and minimum brake wear then the proper method is to not use the brakes at all but rather allow the regen feature to gradually slow the vehicle down. Of course this isn't that effective. Also note that this is a 3300 plus pound car and the two electric motors only provide a maximum of 91 HP. I imagine that in braking they can only use a fraction of that power. So you need 300 HP of braking force to stop the car and you only have 91 HP in the motors and you can see the friction brakes need to produce a LOT of force.
     
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  3. ChapmanF

    ChapmanF Senior Member

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    As anyone who has ever felt the brief "whoopee!" when the car is surprised by a pothole or bump into going full hydraulic earlier than it had planned to, they are not really as preloaded as you might think. The amount also changed by generations. See, for example, the '04 to '03 comparison graphs in #9, showing the narrower 'stick' of the '04 hockey stick.
     
  4. Zed Ruhlen

    Zed Ruhlen Member

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    they are totally preloaded the second you touch the brakes. The "whoopee" effect is the proportion rapidly changing from regen to hydraulic. The amount of power used on the front brakes is dependent on a lot of factors but the amount on the rear brakes is ALWAYS hydraulic. That means that the brakes are by design ALWAYS hydraulic. It's far too dangerous to just use the front brakes. Cars would spin out of control on any slick surface if this were the case. This was a big problem with the first generation "X" cars by GM. Not enough rear braking would cause them to spin out.
     
  5. ChapmanF

    ChapmanF Senior Member

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    It's possible that the two of us use the word 'totally' to mean different things.
     
  6. Mendel Leisk

    Mendel Leisk 'Orrible Oracle

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    Totally...

    Sorry, sorry...
     
  7. FuelMiser

    FuelMiser Senior Member

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    This is actually a function of the ABS that senses a skid (wheels spinning at different speeds) and releases the brakes to get the wheels all spinning the same speed again.
     
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  8. ChapmanF

    ChapmanF Senior Member

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    [​IMG]

    It's not an extremely new topic on PriusChat, and the world is full of cars that have ABS yet do not have the whoopee. (Else everyone would be familiar with it and there would be far fewer new-owner freakout posts about it on PriusChat.)

    The key difference in this braking system is that before it can do ABS, it has to be using the brakes that are able to. (And during that transition, the early backout of the regen torque is a defensible choice, because the ECU can't be sure it isn't losing front-wheel traction, and traction under the wheels you steer with isn't good to lose.)
     
  9. Zed Ruhlen

    Zed Ruhlen Member

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    That's totally true :)
     
  10. Zed Ruhlen

    Zed Ruhlen Member

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    You want to lose front wheel traction first in a turn. That's understeer. Oversteer is extremely dangerous and was a key issue in "Unsafe at any speed." It's the same reason why if you only buy two new tires they always go on the rear wheels.
     
  11. ChapmanF

    ChapmanF Senior Member

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    During regen, there isn't anything to make the rears lose traction. Regen force applies only to the front wheels. Therefore when the car makes the 'whoopee' transition, it is not choosing between losing front traction and losing rear traction. It choosing between losing front traction or not losing traction. It prefers not losing traction.
     
  12. Zed Ruhlen

    Zed Ruhlen Member

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    Oh yeah, you're right about that.