Brakes - Scared the SH!T Out of Me

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by disc0din0, Jul 9, 2009.

  1. jburns

    jburns Senior Senior Member

    Joined:
    Oct 13, 2005
    829
    111
    0
    Location:
    Archdale, NC
    Vehicle:
    2010 Prius
    Model:
    III
    This is not correct. When a tire loses contact with the road while braking it does not spin faster. It stops spinning. When the ABS engages it releases the brake discs on that wheel so that when it regains contact with the road it does not skid. Upon re-contact of the tire with the road ABS allows the the input of the driver from the brake pedal to resume control until the wheel loses traction again. The cycle then repeats over and over many times a second. This is the pulsing you feel in the pedal.

    The first action of an ABS system when it engages is to release the brakes on one or more wheels. ABS does not have the ability to apply the brakes. It only has the ability to release the brake on individual wheels even while the driver pushes hard on the brake pedal.
     
    1 person likes this.
  2. cwerdna

    cwerdna Senior Member

    Joined:
    Sep 4, 2005
    12,544
    2,117
    1
    Location:
    SF Bay Area, CA
    Vehicle:
    2006 Prius
    Your 30% figure is a total exaggeration. From Log In - Consumer Reports (you need a subscription to get see this page), there are 321 vehicles w/ratings for all sorts of figures including braking from 60 mph to 0. The shortest was the Porsche Boxster at 112 ft. The next best were the Porsche 911 and Dodge Viper at 113 ft. The worst was the Dodge RAM 2500 at 176 feet.

    The (2nd gen) Touring Prius got 133 ft and the (2ng gen) base Prius got 143 feet.

    143 * .7 = 100.1. No car did this is under 112 feet, let alone 100. Feel free to ask me about what other German cars got in this test, similarly priced or not.
     
  3. accordingly

    accordingly Member

    Joined:
    May 5, 2009
    268
    63
    0
    Location:
    Madison, WI
    Vehicle:
    2010 Prius
    Model:
    III
    I don't agree the braking force is always consistent. Perform a search and you will find pages and pages of threads of people expressing concern that the braking force noticeably slips during the transition from regen to traditional hydraulic braking and people assuring them this is simply how the braking system functions. This is even one of them! :D

    Of course I don't expect the brakes to stop the car in the same distance it would on ice as it would on dry pavement, loose gravel or a manhole cover, but I do expect consistent braking force to be applied equal to my brake pedal pressure regardless of the surface or what mode of braking is currently being engaged.
     
  4. wave_slider

    wave_slider New Member

    Joined:
    Jun 5, 2009
    201
    26
    0
    Location:
    the aloha state
    Vehicle:
    2010 Prius
    Model:
    III
    I have a hunch that cwerdna may be on to something. I have suddenly hit the brakes on a Corolla in the parking lot and had the ABS kick in due to the brake assist feature. Scared the sh!t out of me. : )
     
  5. Sho-Bud

    Sho-Bud Member

    Joined:
    Jan 5, 2006
    190
    0
    0
    Location:
    Amsterdam, Netherlands
    Vehicle:
    2006 Prius
    IMO you cannot compare this: the truck is probably heavier and has different tires
     
  6. qbee42

    qbee42 My other car is a boat

    Joined:
    Mar 2, 2006
    18,058
    3,052
    7
    Location:
    Northern Michigan
    Vehicle:
    2006 Prius
    Obviously the braking force as applied to the ground is not consistent, or there wouldn't be any issue. The question at hand is the braking force applied to the wheels. During regenerative braking all of the braking force is applied to the front wheels through a normal differential. When one of the two front wheels looses traction, all braking force is lost. Given the way humans perceive acceleration, this loss of braking action is often described as "shooting forward" or "jumped ahead" or "suddenly accelerated". In fact the car does none of these. What it does do is stop slowing down. The reduction of braking makes it feel like it is accelerating. As soon as the traction control system senses the loss of traction the friction brakes kick in. The transition is very quick, but it can seem like an eternity when you are heading for something solid.

    Tom
     
    1 person likes this.
  7. jayman

    jayman Senior Member

    Joined:
    Oct 21, 2004
    13,439
    630
    0
    Location:
    Winnipeg Manitoba
    Vehicle:
    2004 Prius
    The curb weight of my FJ Cruiser and the Ford pickup truck are within 250 lbs, the FJ Cruiser being the *heavier* vehicle. Similar tires

    I should mention that with an FJ Cruiser, if you shift into 4L the ABS is automatically disabled. On that same washboardy gravel road, if I then go up to a reasonable and safe speed, such as 50 km/h, and stand on the brakes, it will stop about a car length shorter

    I will reiterate that for normal driving, especially on highways and public roads, I very much prefer ABS and VSC. There are only a very few situations where you don't want ABS
     
  8. accordingly

    accordingly Member

    Joined:
    May 5, 2009
    268
    63
    0
    Location:
    Madison, WI
    Vehicle:
    2010 Prius
    Model:
    III
    When something "stop slowing down" that is defined as acceleration. The question is the magnitude of acceleration caused by the transition period, which translates into real distance the car travels forward. It can be felt regardless of loss of traction or not, and I think the concern many have raised with the length of time it takes and the added stopping distance due to a minor loss of traction is valid. Simply slowing down and anticipating it is not always an option.
     
  9. HTMLSpinnr

    HTMLSpinnr Super Moderator
    Staff Member

    Joined:
    Dec 8, 2003
    5,339
    913
    251
    Location:
    Surprise, AZ (Phoenix)
    Vehicle:
    2018 Tesla Model 3
    Model:
    N/A
    I think our expectation is that braking force to the wheel should be fairly consistent (+/- ABS modulation) despite the friction between the wheel and the ground up until the loss of traction, and then immediately after the event. If regen is to let go, back it up w/ an equal amount of hydraulic braking. In the scenarios we're mentioning, the wheels have only lost traction for a fraction of a second, however the duration of the loss of braking force can be several seconds.

    What we're observing is that regeneration is a high portion of our total braking force, and when it lets go, we have to compensate pretty highly w/ additional pedal pressure for exclusive friction braking. If time permitted, I suppose we could release and reapply (which I will try the next time I can reproduce this.

    One place in my area where this is easily reproducable is heading southbound on Litchfield Rd (near Luke AFB), making a left turn to Northern Ave. There's a concrete "gutter" which the asphalt doesn't mate up with cleanly (1/4 - 1/2" or better). If you're crusing w/ light/moderate braking through that turn and hit this "gutter", the regen will let go.
     
  10. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

    Joined:
    Apr 13, 2004
    15,140
    610
    0
    Location:
    South Puget Sound, WA
    Vehicle:
    2013 Nissan LEAF
    Model:
    Persona
    if you want effective brakes other than ABS, you need to get a MacLaren. it has vents that run thru the entire car that are louvered. with 600 HP and a carbon fiber weight of just over 2000 lbs, the coefficient of friction simply aint there under normal circumstances.

    on turns and braking, the louvers adjust the airflow running thru the car to exert more downward pressure to the tires which essentially creates artificially controlled weight which greatly aides in stopping a 200+ mph car.

    now that braking system is effective but there is a minor issue on price (estimated cost for braking system alone is over $100,000)

    now, expecting a equal and consistent braking force on an inconsistent road surface is a bit presumptuous. ABS, TRAC and VSC all play a part in helping the driver to maintain control. the only real thing that needs work is driver training
     
  11. yardman 49

    yardman 49 Active Member

    Joined:
    Mar 26, 2009
    606
    76
    0
    Location:
    Northern Kentucky
    Vehicle:
    2009 Prius
    Drew:

    I have a G2 and I really like the brakes on it.

    And if you think that the brakes are really that bad, why would you give the car to your mom?
     
  12. jayman

    jayman Senior Member

    Joined:
    Oct 21, 2004
    13,439
    630
    0
    Location:
    Winnipeg Manitoba
    Vehicle:
    2004 Prius
    Life insurance policy??
     
  13. qbee42

    qbee42 My other car is a boat

    Joined:
    Mar 2, 2006
    18,058
    3,052
    7
    Location:
    Northern Michigan
    Vehicle:
    2006 Prius

    Any change in velocity is acceleration, where velocity is a vector of speed and direction. By definition a = dv/dt where where a is acceleration, v is velocity, and t is time. So yes, your statement is both correct and obvious. Indeed the core issue is the increase in stopping distance related to loss of traction and transition to friction braking. This brings us full circle back to my original point that the increase in stopping distance is marginal. The issue is mostly one of perception, i.e., having the sh*t scared out of you by the sensation of acceleration. The small difference in stopping distance is well within the normal variation caused by road surfaces, tires, and weather.

    We completely agree on the physics of this issue. Our differences come from defining what is acceptable in relation to stopping distance variation. I maintain that the variation in the Prius due to regenerative braking and loss of traction is well within the normal variation found in day to day driving. You obviously don't agree. You probably follow a lot closer than I when driving, and drive in a more aggressive fashion. It's a personal issue. You think the Prius brakes are dangerous, I think aggressive driving is dangerous. Our priorities are different.

    Tom
     
  14. jayman

    jayman Senior Member

    Joined:
    Oct 21, 2004
    13,439
    630
    0
    Location:
    Winnipeg Manitoba
    Vehicle:
    2004 Prius
    How about these numnut punks who have seen "Fast and Furious" one time too many?

    Last winter, there was a punk about a block ahead of me who, while on ice/snowy roads, was having fun pulling the handbrake while driving, making his used up rusted out s***box ricer slide sideways. Loads of fun

    Then he came to a corner and tried the same thing. The car spun like a top, and went nice person-first into a huge snowbank. So the scrawny little bugger gets out and starts madly waving his arms for me to stop

    So I stop. I get out. The scrawny little punk suddenly seems afraid of me. He sheepishly asks me if I can help him out. I glare at him, get back in my FJ, and say

    "Tell your story walking"

    and drive off
     
    1 person likes this.
  15. qbee42

    qbee42 My other car is a boat

    Joined:
    Mar 2, 2006
    18,058
    3,052
    7
    Location:
    Northern Michigan
    Vehicle:
    2006 Prius
    I think we have all tried that, at least those of us lucky enough to live where it snows. The difference is that you try it in an empty lot with lots of room, not on a street. :doh:

    Tom
     
  16. Sandy

    Sandy Hippi Chick

    Joined:
    Apr 28, 2009
    860
    53
    0
    Location:
    Ocala,Fl
    Vehicle:
    Other Non-Hybrid
    Model:
    N/A
    Why use Sh!t to get your point across....
     
  17. djasonw

    djasonw Active Member

    Joined:
    Jul 13, 2004
    939
    107
    0
    Location:
    Coconut Creek, FL
    Vehicle:
    2004 Prius
    Model:
    N/A
    The ABS activating on bumps is not just a Prius issue, most cars will respond this way but in varying degrees. I've had my car for nearly six years and I don't think I've ever had a short stop that scared the sh*t out of me. Exercise defensive driving skills, and you'll never have an issue where you soil your pants.
     
  18. RobertMBecker

    RobertMBecker New Member

    Joined:
    Jul 6, 2009
    13
    3
    0
    Location:
    New York
    Vehicle:
    2010 Prius
    Model:
    V
    While it is "interesting" to hear the explanation about why ABS reacts the way it does, the bottom line for people driving in cities is that bumps, potholes, and manhole covers will occasionally result in your car braking a couple of feet later which can be the difference between stopping when you want to/are supposed to and hitting a car in front of you or worse, a pedestrian crossing the street.

    It's really a safety issue and not a matter of getting used to it. It's not about driving too fast or braking too hard, etc. For those of us experiencing this issue, it's scary, frustrating and annoying not to have the issue validated or addressed appropriately.
     
  19. qbee42

    qbee42 My other car is a boat

    Joined:
    Mar 2, 2006
    18,058
    3,052
    7
    Location:
    Northern Michigan
    Vehicle:
    2006 Prius
    You assume the car would stop significantly faster without ABS. Once traction is lost, it's going to take longer to stop with or without ABS.

    This is a lot like the discussions we have when people post "VSC NEARLY KILLED ME!!!". The truth is that they lost control and VSC helped them recover, but instead of being thankful they blame VSC for causing the initial loss of control. The safety system kicks in so fast that there is no way for a human to perceive the difference between the cause and the effect.

    Tom
     
  20. jim256

    jim256 Member

    Joined:
    Jun 18, 2009
    317
    69
    0
    Location:
    Eastern NC
    Vehicle:
    2010 Prius
    Model:
    IV
    Agreed--if your ABS is pulsating the brake on that wheel, traction is already lost and it sensed it well before drivers might have. You still are stopping with the other wheels/tires.
     
Loading...