Traction Control Dangers and Engine Braking Savior

[slvr_phoenix]

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.

[Tom_06]

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

[SomervillePrius]

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)

[slvr_phoenix]

<div class='quotetop'>QUOTE(Tom_06 @ Feb 9 2007, 11:34 AM) [snapback]387536[/snapback]</div>

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.
[/b]

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.

[Marlin]

<div class='quotetop'>QUOTE(Silver Phoenix @ Feb 9 2007, 01:55 PM) [snapback]387512[/snapback]</div>

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.[/b]

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.

[Marlin]

<div class='quotetop'>QUOTE(SomervillePrius @ Feb 9 2007, 02:48 PM) [snapback]387542[/snapback]</div>

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. [/b]

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.

[SomervillePrius]

<div class='quotetop'>QUOTE(Marlin @ Feb 9 2007, 01:21 PM) [snapback]387562[/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 when 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.
[/b]

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).

[jayman]

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.

[narf]

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.

[Marlin]

<div class='quotetop'>QUOTE(jayman @ Feb 9 2007, 03:57 PM) [snapback]387584[/snapback]</div>

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.
[/b]

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.