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Traction Control Dangers and Engine Braking Savior

Discussion in 'Gen 2 Prius Technical Discussion' started by slvr_phoenix, Feb 9, 2007.

  1. slvr_phoenix

    slvr_phoenix Tinker Gnome

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    I'm lucky enough to live near a tourist-trap that during the winter has a large unused paved parking lot. It's handy for getting the feel of a car in winter conditions as the lot goes unplowed and is thus often filled with snow during winter. It's fun. :)

    But one thing that I've found that isn't fun about the Prius is the Traction Control. After driving in my test area for a while, I'm fairly certain that the TC actually causes a loss of traction during a rapid deceleration in slippery conditions.

    I've repeated the test several times now, and each time I get the TC to engage during a rapid stop on snow, the TC engages and suddely the stop goes from bad to worse. I'm guessing that the power drop we see to 'help' regain traction during acceleration does the inverse and limits the generator braking force if you lose traction in a deceleration. This forces the actual brakes to engage much sooner, causing the wheels to lose even more traction while the vehicle stutters from a sudden generator brake to wheel brake, throwing even more stability out the window because of the stutter. At least this is what it feels like to me. Not being an engineer or having a computer hooked up to the internals, it's hard to know for certain. But I definately feel a stutter followed by a much more likely loss of control whenever the TC kicks in during a brake-turned-skid.

    However, the Prius has one savior to overcome this awful situation for the skilled and quick-thinking driver: the Engine Brake.

    I've noticed that in a slippery situation if you start a brake force using the EB instead of the normal brake pedal, and you then apply the normal brake to supplement the EB, you get infinitely better braking. And that infinitely isn't much of an exaggeration. Because the TC limiter can't affect the braking force of the EB, you get a much more stable brake before the actual wheel brakes kick in when/if the TC engages. But even if TC doesn't engage, just judging by the feel and the response I get while braking it seems to me that the EB is also a more stable braking force than the generator is flat out. It just seems to drag the car with more control.

    So in nasty slippery road conditions remember that the Engine Brake is your friend. (And so far in my experience the Traction Control is more your enemy than helpful. Not just in braking, but also in accelerating, because it's almost always slower to react than I am by reflex, so I always get a double-power drop. The one I start when I feel the slip, and the one the TC forces on me even sometimes after I've already regained traction, because it's just darn slow some times.)

    Since I don't have Vehicle Stability Control I can't comment on if VSC fixes any of these problems, but my understanding is that VSC sits on top of TC and that in most cases you're still just using TC. So I wouldn't expect much better results unless you're turning while braking, and even then I'm not so sure. If it was All Wheel Drive it might be a different story.

    Now, these are all just my experiences. If anyone has a safe area to do their own testing, I'd love to hear more commentary on if these experiences are repeatable in other Prii. I say this because it seems some seem to behave differently than others in other areas, so it wouldn't surprise me if it is true for this as well. Complexity is the Devil's Playground. The more complex you make a system, the more places for variance there are, and the more likely something will break that will take down the system. But the gas milage is just oh so good. ;)
     
  2. Tom_06

    Tom_06 Active Member

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    Are you sure you're seeing traction control and not anti-lock braking? The word "stutter" makes me think anti-lock brakes which shares components, but is a different system.

    - Tom
     
  3. SomervillePrius

    SomervillePrius New Member

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    Also,

    How do you try to stop the car? This sound like a problem for people that are not used to ABS (and is made more appearent on the prius). On the priius the idea is to press as hard as you can on the break and it will try to stop as soon as it can, heck it even has a very advanced logic in there that will sense a fast break pedal depression and try to stop REALLY fast. I'm impressed with how well the car stops (as long as I press the brake pedal hard) even under slippery (parking lot) conditions.

    As soon as the Prius detects the wheels as locked it will disconnect the regenerative braking and engage the good old friction brakes. This feels a bit scary as there is a short gap where it feels like you're loosing braking power IT"S IMPORTANT TO NOT RELEASE PEDAL PRESSURE AT THIS POINT. The friciton brakes works with ABS like any other car.

    The point is that if your wheels lock you will not use any regenerative braking until you let go of the brake pedal, it will simply use friction. If you release brake pedal pressure when you feel the ABS kicks in ("you loose braking power) THEN YOU LOOSE braking power! This is because you're telling it to brake less! If you keep as much pressure as you can on the pedal it will keep engaging and stops fine on slippery conditions (I can't stop faster)
     
  4. slvr_phoenix

    slvr_phoenix Tinker Gnome

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    <div class='quotetop'>QUOTE(Tom_06 @ Feb 9 2007, 11:34 AM) [snapback]387536[/snapback]</div>
    I'm 100% certain it's TC. And I wasn't happy using the word stutter either, but it's all I could think of to explain a single soft slam without getting too confusing. **shrug** It's not a repeated stutter, just one single discernable stutter in the braking. It's hard to notice when you're freaking out in an emergency situation, but do it enough times in a controlled environment and you can spot it.

    It was actually my inability to explain what the heck went wrong and almost caused an accident one day in an emergency situation that caused me to play around like this. I couldn't figure out what had caused such a loss in traction until I repeated the same situation over and over in a safe environment. Then I could feel it out. Now I know how to handle it better, but it's very counter-intuitive that the Traction Control actually causes a loss in traction. You'd think all that computerized shiny is working for you, not against you.

    And even now, I can't definitively prove it. That's just what it feels like to me. If I could disable TC somehow I could measure stopping distances to prove it, but I don't know how.
     
  5. Marlin

    Marlin New Member

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    <div class='quotetop'>QUOTE(Silver Phoenix @ Feb 9 2007, 01:55 PM) [snapback]387512[/snapback]</div>
    During your rapid stops in snow, it was undoubtably the anti-lock brakes (ABS), not the Traction Control (TC) that engaged. I beleive they both use the same dashboard indicator.

    And yes, when the ABS engages, it does take longer to stop then when the ABS does not engage. But, stopping with ABS engaged is a whole lot shorter than stopping with the wheels locked up, which would have been the case without ABS because the ABS does not engage until it detects that the wheels have locked up.

    And in any case, whether it was ABS or TC that you encountered, neither one caused you to lose traction, because neither would have engaged until you had already lost traction. Trying to stop faster than conditions allow is what caused the loss of traction.
     
  6. Marlin

    Marlin New Member

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    <div class='quotetop'>QUOTE(SomervillePrius @ Feb 9 2007, 02:48 PM) [snapback]387542[/snapback]</div>
    While I agree 100% with what you said, it reminded me that this behavior actually cost me a $1000 front axel repair two years ago on my 12-year old Ford Explorer.

    My Explorer was getting old and had a cronic problem with the front brakes. The ABS stopped working in the fall (again), and I chose not to fix the problem, because I was just wanting the car to hold out for another year or so before I was ready to replace it, and I wanted to avoid sinking even more money in to it.

    So, then comes winter and the snow, and one slippery day I was traveling about 10 MPH and started to slow down and turn into a parking lot. The brakes locked up immediately, the ABS didn't engage, but like I had been trained for 12 years, I pushed and held that brake pedal. I slid a good 50 feet and slammed the front wheel smack into a curb doing at least 5 MPH. Bent the wheel in about 20 degrees compared to the other side.

    Of course the moral of this story is not that you shouldn't keep the brake pedal pressed when ABS engages, but rather that you should get the ABS fixed when the error light comes on.
     
  7. SomervillePrius

    SomervillePrius New Member

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    <div class='quotetop'>QUOTE(Marlin @ Feb 9 2007, 01:21 PM) [snapback]387562[/snapback]</div>
    I've done the same thing three time, at the beginning of each winter, with my old car. For some stupid reason I bought a car with-out ABS last time even though I was trained on and had always driven ABS cars (I will never do this again). When something happens my instinct is to press the pedal hard and keep it there, my dads instinct is to pump or "feel" how much traction he has.

    We did a test in his Volvo and neither him nor I can stop a car as fast as ABS IN SLIPPERY or SNOWY conditions (far from), yet for him it's tough to unlearn his 'instinct'. On dry asphalt (with little or no gravel) slamming the brakes and keeping the wheels locked will stop your car a little faster then ABS, but for some reason every time I need to stop a car fast I never have perfect conditions.

    I love that the prius senses when you press the pedal hard/fast and will apply maximum brake power right away (faster then your foot is traveling). I have had this kick in a couple of times and while I haven't needed it I"ve been surprised how hard the Prius can stop (in asphalt).
     
  8. jayman

    jayman Senior Member

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    The TC or Traction Control only works during acceleration, when you have your foot on the gas pedal. When your foot is on the brake pedal, the ABS is involved.

    Overall I prefer a car with the modern safety features like ABS, brake assist, VSC, TC, etc. But I expect these features to be well-engineered, not poorly designed.

    A good example is how GM implemented ABS in their pickup trucks in the mid to late 1990's. The system was so aggressive that it could actually make a stop longer than without. I knew a few owners in Utah who actually pulled the ABS fuse just to disable it.

    I've posted about how - in my opinion - poorly implemented the Prius traction control system is. Contrary to popular belief, it does NOT pulse the brake of a spinning wheel. Rather, it cuts power until wheel spin stops. It appears for the majority of Prius owners the system works ok, but for some of us, Trac has left us helpless at an icy intersection with one wheel on dry pavement and the other wheel on ice.

    One situation where ABS can dramatically increase stopping distance is on a gravel road. If you can lock the brakes, you "dig in" and usually stop quicker. Or if you run studded tires in winter like I do, very slight wheel spin during acceleration and very slight wheel lockup during braking helps the studs "grab"

    The whole point to ABS isn't to decrease the stopping distance - since it usually cannot do so - but to keep *control* during a panic stop. Once the front wheels lock, you lose steering ability.
     
  9. narf

    narf Active Member

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    The problem isn't the ABS or traction control, it's the crappy tires that come stock on the prius. I've been driving through the worst winter Denver has had in the last 30 years and have been out and about every day. with good snow tires I have not gotten stuck, even in places where other people in SUVs and other 4WD cars have. I haven't lost control, and the traction control hasn't shut the car down.

    It you have ZERO traction, the car will kill power to the drive wheels, but don't fool yourself into thinking you could somehow drive out if it if only the car would spin it's tires. Zero traction is still zero traction.
     
  10. Marlin

    Marlin New Member

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    <div class='quotetop'>QUOTE(jayman @ Feb 9 2007, 03:57 PM) [snapback]387584[/snapback]</div>
    Yup. When my Explorer slammed into the curb (story above), it was not because the car didn't stop, but rather because the car didn't turn. My original goal had not been to stop my Explorer, but rather to slow down and turn into a parking lot entrance. It was even a gently curved entrance, rather than a right angle. The problem was that once the wheels locked up, my Explorer didn't turn and took a nice straight line into the curb.
     
  11. hobbit

    hobbit Senior Member

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    I pointed out near the end of my rundown on B-mode that the "falls
    on its face" behavior of letting up the pedal is favored by some
    autocrossers, and is definitely useful in the slippery stuff.
    .
    _H*
     
  12. jayman

    jayman Senior Member

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    Hobbit thanks for the writeup. I'll try B around the city and on the gravel road at the hobby farm to see if there is any difference.
     
  13. ncc74656m

    ncc74656m New Member

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    Oh boy, I hate ABS...

    Not that I hate the concept or even application of ABS. I hate the fact that my Corolla's salesman lied to me and told me it had ABS (could've just been wrong, but saying that he lied makes me feel better), and I almost skidded right into an intersection one night when it was rainy and leaves were all over.

    I distinctly remembered being told I had ABS, and so I squeezed the life out of those brakes. If I had known, I would've released and pumped.

    On a funnier note (because I stopped in time), you should've seen the look on the guy's face in the next lane. He saw my sheepish "oops" grin and just kinda nodded and smiled.

    Anyway, now that i know I don't have them, I haven't had a problem. Of course, I will be making sure I get them on the next car (still hoping for the Prius).
     
  14. statultra

    statultra uber-Senior Member

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    the ABS on the toyota prius is very sensitive, maybe too sensitive, my 01 prius almost got into a accident one day when there was a patch of ice near a 4 way intersection, the light was red, and i was trying to stop, but ABS kept kicking in making the brake pedal shake ( it was braking every second almost and on the ice it was braking every half second ) , to the point where i stepped really hard on the brakes and not much happened. I then used the handbrakes since i was down to about 15 mph.

    Traction control doesnt bother me, its a good thing in some conditions ( in snow and ice, it actually helped me rather than held me back like other drivers ) up here in NJ well, last year snow was kinda on the bad side and the prius proved to be a good car, minus the ABS.

    btw the ABS on the prius is very sensitive, i believe this is due to the interconnection of the ABS and the hybrid management system,
     
  15. Skeptic357

    Skeptic357 New Member

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    <div class='quotetop'>QUOTE(narf @ Feb 9 2007, 03:10 PM) [snapback]387594[/snapback]</div>
    That's not entirely true. I've seen my wife get through short patches of ice by furiously spinning the tire until friction generates enough heat to melt through to pavement. The downside is that there is a distinct odor of burning rubber--I wonder how many miles of wear the tire suffers in such an event.

    I'm not suggesting that the above is a good idea, but it sure is frustrating to sit in my prius with the accelerator floored, unable to move because one tire is sitting on a patch of ice. Couldn't the car at least pretend to do something? My dad always said that having too much power available makes for bad drivers who try to power through every situation--maybe Toyota agrees, and traction control is their way of chiding, "Stop driving like an idiot, it's slippery out there."
     
  16. jayman

    jayman Senior Member

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    <div class='quotetop'>QUOTE(hobbit @ Feb 9 2007, 03:22 PM) [snapback]387609[/snapback]</div>
    Hobbit

    I did try it out in city driving and it feels just like having a stick shift again, actually felt pretty good in city driving. No difference that I could tell on ice, with the studded tires I don't have a problem now.

    I can tell though the city fuel economy would really suck if I drove in B all the time, so I won't use it anymore.

    <div class='quotetop'>QUOTE(ncc74656m @ Feb 9 2007, 03:53 PM) [snapback]387633[/snapback]</div>
    They appear to lie for a living. Good training if they're seeking a future in politics!

    <div class='quotetop'>QUOTE(Skeptic357 @ Feb 12 2007, 12:47 AM) [snapback]388737[/snapback]</div>
    Or if you have studded tires like I do for winter driving, very *slight* wheelspin really helps as the studs can claw down into the ice and get you moving.
     
  17. slvr_phoenix

    slvr_phoenix Tinker Gnome

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    For the record, only my first car didn't have ABS (also was my only manual transmission), so I've been driving with ABS all my life. And I'm not talking about a full on hard brake stop. I'm talking about a normal slow braking process on a slippery road.

    <div class='quotetop'>QUOTE(Marlin @ Feb 9 2007, 12:07 PM) [snapback]387554[/snapback]</div>
    I have a feeling that we're arguing technical semantics. The TC is responsible for killing your power the instant there's a skid. That's what kills the generator brake. The ABS is responsible for pulsing your brake discs/pads so they don't lock up the wheel. It's the TC killing the generator that causes a bad situation to become worse. ABS in general I have no problems with. It's handy to have. Don't get your technical terms confused.

    <div class='quotetop'>QUOTE(Marlin @ Feb 9 2007, 12:07 PM) [snapback]387554[/snapback]</div>
    You'll have to define 'locked up', as a minor temporary loss of traction during deceleration is not 'locked up' IMHO, yet that's what causes the situation. It's funny actually. When I'm accelerating the TC doesn't engage fast enough. I always beat it, causing the dreaded double-drop. In a deceleration however the TC drops your power before you can even flinch.

    <div class='quotetop'>QUOTE(Marlin @ Feb 9 2007, 12:07 PM) [snapback]387554[/snapback]</div>
    Sorry, but you're extremely wrong. That's exactly what I'm trying to point out so people are aware of it. You're happily driving along on a snowy road. You're braking normally (even extremely safely) with the generator braking conveniently providing a more secure braking mechanism than the real brakes. Then oops a minor slip. Because gosh, that never happens on snow or anything. Most cars wouldn't have a problem. They'll at most anti-lock while you're braking and keep your traction the same or better.

    But the Prius instantly drops out of generator-braking and kicks in the real brakes the milisecond a single wheel (of all four) slips. The resulting jerk caused by switching from one brake system to another at the drop of a hat when the other three wheels could all have perfectly reasonable traction turns a minor slip for one wheel into an actual skid for all four. It's this switch that causes a loss of traction. (If you ever fully lost it to begin with, which is debatable as the other wheels could have still had it.) And letting off the brake (even accelerating) doesn't regain you any traction at all either because the TC has grounded you. And if it happens while steering/turning you can't even seem to turn into the skid for that matter, because again, the car seems to think that it's a better driver than you are.

    In the Prius, an incredibly minor slip of a single wheel while braking turns your car into a useless cage where all you can do is grab on tight and hope for the best. No driving skill can save you because the car prevents you from being able to use any of your skills. Where as other cars won't switch brakes like that on you to catapult you into a skid at the slightest hint of lost traction.

    As much as I love my Prius, I'm also beginning to hate it. I drive well. I have good instincts and reflexes. I could make even a '98 Mercury Sable for all its weight look like a rally master. I've grown up driving in snow, ice, and gravel. But in a Prius I'm at the mercy of some very badly designed 'safety' features that turn emergency situations into nightmares.

    The last thing that the Prius should do is drop the generator-braking in a skid. Besides the chunk when it switches causing a minor skid to suddenly become a major one, the generator-braking is also a much more stabilizing braking force than the real brakes. What you want if anything is for the overall braking force to remain identical, which it doesn't do. But if you're going to change the quantity of braking force, at least do so by kicking in more on the transmission, not grabbing the tires harder. If it has to change anything it should not only kick in generator-braking during a skid, but should even automate engine-braking during a skid. Instead you get is the worst option of all, a sudden switch over to pure use of the least effective braking mechanism in the whole car out of all three braking forces that it has available.

    In other words, it's an extremely bad design. TC shouldn't drop the generator-braking if ABS kicks in. The only way to mitigate it is to use the engine-braking in conditions when you could expect a loss of traction, and to put some actual tires on the car. The stock ones just plain suck.
     
  18. slvr_phoenix

    slvr_phoenix Tinker Gnome

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    <div class='quotetop'>QUOTE(narf @ Feb 9 2007, 01:10 PM) [snapback]387594[/snapback]</div>
    No flirking schiznit! I'm very much growing to hate the stock tires. Because I want to avoid these TC problems with the Prius I'm even contemplating wider wheels as well as new tires. Unfortunately at this exact moment I just bought a new car, so I don't exactly have money for new tires let alone new wheels. The Prius is the first 'normal' driving car I've ever owned where I've even considered seperate summer and winter tires instead of all-weathers. I abhor the Prius TC and it's awful that the stock tires ask the TC to come out and play so often. The only worse tires possible would be those infernal low-energy tires. So far I'm not convinced that the Prius tires aren't low-energy tires for the traction they get...

    <div class='quotetop'>QUOTE(narf @ Feb 9 2007, 01:10 PM) [snapback]387594[/snapback]</div>
    Zero traction, sure. However zero is exceedingly rare. Zero is statisticly theoretically possible, but highly improbable. So zero power is not a good solution to almost-zero traction. Chances are if you can spin the tires slow enough, you can get out. (And then there's rocking.) But if you can't spin the tires at all...

    The TC as implemented is just awful. The drop of power is such a horrible solution. It should at most limit the RPMs so that if you've already dropped the power by beating it to the punch then it effectively does nothing. Protect the system from damage without trying to kill the driver. And the TC should most definately not kill generator-braking when ABS kicks in. The absolute last thing that you want if you lose traction on a single wheel is a change in braking force. The only time I could remotely consider dropping the generator-braking would be if all four wheels had sustained zero traction for more than 500 miliseconds. (And even then I'm still not for it.)

    I can understand why from a technical perspective they do this, to protect sudden changes in traction from causing damage to the car. However there are solutions to this that would be safer for the driver and keep the parts all within spec. They'd just cost a tiny bit more.
     
  19. jayman

    jayman Senior Member

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    <div class='quotetop'>QUOTE(Silver Phoenix @ Feb 13 2007, 01:57 PM) [snapback]389553[/snapback]</div>
    I agree. I'm sure most Prius drivers have felt the "lurch" of losing regen braking when a wheel has very briefly slipped. It really gets your attention!
     
  20. richard schumacher

    richard schumacher shortbus driver

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    SP, sorry if I missed a description of it above, but exactly what are you doing to make this behavior appear?