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Prius Brakes

  1. Feb 8, 2010 #1
    Whatever the outcome of the Prius brake recall, it sounds like educating the public on antilock brakes will be part of the answer.

    Wozniak is quoted as indicating the car would coast for a bit on an icy road, rather than stop immediately, and that this was "scary".

    It's "scary" if you assume without the antilock brakes the car would have stopped immediately; not so scary if you realize your brakes would have locked up with loss of steering.

    Wozniak may need some one-on-one education on the fact that it's friction between the road and the car tire that stops the vehicle.
     
  2. jcsd
  3. Feb 8, 2010 #2

    mgb_phys

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    Perhaps the solution is to have the car stereo play a sound file of a hollywood stunt style skid with all wheels locked up and screaming tires?
    For extra realism you could have smoke generators in the wheels
     
  4. Feb 8, 2010 #3

    FlexGunship

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    Hah, yeah. Antilock brakes only preserve the rotation distance between gripped wheels and non-gripped. The end result is a pulsing effect on non-gripping wheels. Furthermore, the repeated application of the static coefficient of friction instead of the constant kinetic coefficient of friction help to reduce stopping distance.

    However, the fact remains, if you have not locked up ALL of your ABS wheels (i.e. ABS engaged on four wheels, and not just three), then you are NOT stopping as fast as you can.

    Pro Tip: Find a strip of ice, align the left or right wheels with it, being sure to keep the other two wheels on pavement. As you are moving apply the brakes lightly. You can get ABS to engage ALMOST without even slowing down. This is because your slipping wheels are pulsing to match the rotational movement of your gripping wheels allowing for the repeated application of the static coefficient of friction. Press the pedal harder and you will STILL stop faster even though you can already feel the ABS pulsing.

    TRUE!
     
  5. Feb 11, 2010 #4

    Moonbear

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    The problem is that MOST of us who are drivers learned to drive without ABS. When you start skidding with regular brakes, you don't lock up your steering, you take your foot off the brake or pump it (depending on what you're feeling and how fast you REALLY need to stop). WITH ABS, you're supposed to keep your foot firmly planted on the brake pedal or it defeats the function of ABS. After many many many years of driving one way, it's very hard to unlearn and start driving another way.

    Personally, I HATE having ABS brakes on my car. It's the first car I've ever had them on, and I can't tell what the heck the car is doing when they start activating when braking on icy roads. They may work fine if I need to slam to a stop on hard, dry pavement to avoid a collision, but I've never needed to do that anyway because I pay attention to what the idiots around me are doing before I get that close to needing to avoid them. I'm really not convinced they work the way they should on ice or gravel and I think they increase my stopping distance in those conditions considerably from what I've been able to do in other cars without ABS (in fact, the other day, the only way I managed to finally stop in a downhill slide in slush was to cut the wheel hard to the right and completely defeat the ABS by not letting the wheel roll in the direction of the slide...after sliding 20ish feet, stopped in about 2 feet cutting the wheel). I would love an option to turn off the ABS when snow driving or on gravel roads just to see if my hunch is right.
     
  6. Feb 11, 2010 #5

    mgb_phys

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    My first car with ABS and I love them, going down a very steep hill to a stop light. I pressed the brakes and heard this banging noise - didn't even know there was ice.

    The only time I skidded without ABS I came round a corner hit the brakes and never knew a thing until I was in the ditch.
     
  7. Feb 12, 2010 #6

    Borek

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    That's assuming you are a driver good enough to do it. Many people just press the pedal harder and watch in horror what is happening.
     
  8. Feb 12, 2010 #7
    Your correct moonbear ABS does not work properly on ice, snow or gravel. When the wheels lock up on these surfaces the build up a barrier of snow/gravel infront of the tire which helps slow the car down, with ABS everytime the wheels lock and the barrier starts forming the brake pressure gets released and you roll straight over the barrier.
     
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