Braking on rough surfaces

at any speed?

 

I have noticed this too - not at high speeds - probably below 30-40 kph.

 

It's happened to me, twice in 3 years.
Both times lower speed, rough surface...and slick surface.
The only other connection I make, is it seems to be quick braking, with these parameters, that trigger the traction control.

But it is scary, and one thing I don't like about my Prius.

 

"Old chestnut" is right ... this is one of the oldest, most-discussed features of the Prius, going back to Generation 1 ... just to link in some of that history, there's a post here that describes it better, and further links back to posts from 2008, and so on.

It seems to scare people who encounter it for the first time and don't know what's going on. It shouldn't scare people, since what it is doing is pretty much what you would want a car braking on two wheels to do when braking on four would be better. So the best advice has always been to just take the car out on a nice day and trigger the feeling on purpose and learn how it feels, and then worry about things that really are scary.

Just for keeping-things-straight's sake, the "anti-lock brake system" is what does this. There is also a "traction control system" in the car, but it does other things. (Even though since 2004 it's been the same computer in the car that combines both functions, they're still distinct concepts.)

-Chap

 

I don't know if I agree with this.
As both times this happened to me, both the anti-lock feature and the traction control light came into play.
And isn't it a moot point? Because if you are braking, and the anti-lock feature is activated, that is nearly always simultaneous with the traction control being activated.
I know there is a difference between "just" the anti-lock brakes, and the entire traction control system, but in these scenarios, at least for myself, I believe both the anti-lock brakes are kicking in, as well as some degree of the traction control system.

For me, it resulted in the momentary "slip" feeling...as well as the brake pedal going to the floor and a real loss of braking ability.
I actually had it checked out at my Toyota dealership service center, and they told me it was just how the traction control system was "correctly: working in that scenario.
Allowing wheels to spin...to maintain control.

Well...OK..but I think it's arguable which is safer. Maintaining control at the expense of the wheels NOT braking-allowing for more spin, OR skidding and maybe fishtailing...but the brakes holding and the wheels NOT turning.

 

As part of my training a truck driver in the army reserves, we spent a day each year on the skid pad. Vary scary going into a high-speed skid (and 360' spin) in a 15-ton truck or a double-decker bus. (To get a feel for what it is like, you are instructed to drive at high speed onto the skid pad, with the ABS disconnected: at some stage the instructor pulls on the hand-brake and you are completely helpless!)

Experiencing it a safe environment like a skid pad gives you the confidence that you could handle a skid problem in an emergency. It is something that I would highly recommend to any driver.

 

That sounds like you had an actual problem, something the dealership should have treated more seriously. The switch-from-regen-to-ABS-over-bumps is the really well known and widely experienced issue so often brought up here, but loss of pressure at the brake pedal is absolutely no part of that.

-Chap

 

it's the same feeling you get when you leap out of an airplane, even though you know you have a parachute.

 

I want to make sure I understand this correctly.

Is there any chance of losing total control of the car?

 

only if you take your foot off the brake. the natural reaction is to push harder, as it should be. it's the potential for heart attack that's the underlying problem.

 

Wonderful.

Just great.

I will add this to my list of: Just another Prius oddity....

It will go along great with my latest episodes of "surging and lurching."

 

They ran a full diagnostic. And it only happens under those very specific parameters. That is braking suddenly on a rough spot, and in my case also on a surrounding slick surface.
According to Toyota my brakes are fine. It's just the way the traction control works.

Their argument is that the vehicle is allowing individual wheel spin, to keep the car in a straight line and under control.
As opposed to the "old days" when your brakes would just lock or even with anti-lock...brake, and stop.

I don't know. To be honest, The Prius is the first vehicle I have had with traction control, so I have nothing to compare it to.
My 2010 Honda Fit...Previous Vehicle...had anti-lock brakes...but it was one model year before Honda made traction control standard across the line.

I've never had a vehicle that occasionally reacts as this Prius has demonstrated it can.
But the parameters have to be so specific, and knowing now what is happening, lets me mitigate the risk.

It is the reason however, I always tell people to gave GOOD tires on The Prius, and recommend specific winter Tires in winter driving conditions.
I think maintaining traction with The Prius is more important than with other vehicles.

 

Since it can be reproduced have you tried increasing the tire pressure?

 

God help us if we ever hit a patch of black ice. (But I suppose that's a whole different ball game)

So when this re-gen / anti-lock brake thing happens, the recommended advice is to keep your foot firmly on the brake? I just want to be absolutely clear. Thanks.

(Do any of you have a touchy brake pedal? When my Gen4 is cold, occasionally the brakes will be extremely sensitive. The slightest push of the pedal, and it comes to an abrupt stop. And then at other times, when I've been driving for awhile the brake pedal will sometimes feel very mushy, and it feels like the stopping power has diminished. Two more "oddities" to add to my list.)

 

If you hit black ice, braking is the last thing you want to do. If you are braking on black ice or deep water, you will have absolutely no control over the car.

 

Yes, you are correct. Thanks booke. I have no experience driving in snow or ice. I'm a life-long resident of sunny California, USA. If my car ever started slipping or sliding, I'm afraid my first reaction would be to slam on the brakes.

I did hydroplane two times in the past, and that was no fun. The car turned sideways with no control. (I was driving a work vehicle. A big Ford sedan.)

 

Yes, when my 3rd-generation is cold when backing slowly in "R". I think that's at least partially because even if you press the brake very gradually, the backward creep power shuts off simultaneous with the brakes grabbing. It's not as obnoxious when I back up in "N", so I use "N" instead of "R" whenever possible. That strategy obviously works only downhill, but backing is downhill more often than not anyway.

 

Yes, I've experienced the grabby brakes while in reverse, and backing out of the driveway (a slight down slope.)

So now I turn my car around, so that I can drive forward when exiting the driveway. The brake grab is still very sharp when going forward, and the car is cold. The first couple times, it tossed me forward in my seat!

I enjoy my Prius, there are a lot of high-tech nice things about it. Most of the time, it's fun and simple to drive. But then when I least expect it, some strange weird behavior will pop up... unlike any other car I've ever driven before. I don't think I will ever let anyone else drive it. There are too many oddball things to explain about it!

 

Sounds like what they're talking about is VSC (another distinct function in the Prius, different from ABS and TRAC). But in your earlier post you clearly mentioned the brake pedal going to the floor, which makes it sound like they were also BSing. To the best of my knowledge, there should be no feeling of pedal drop or loss of pedal pressure going with any of these functions being triggered. Unless you're talking about a reaction you had where you stood extra hard on the pedal and drove it further down.

I don't think I've triggered VSC yet (haven't had my Gen 3 in winter yet, and Gen 1 didn't have VSC), so I'll have to wait to find out how that feels first-hand.

The universal, all-Prius-generations, noticeable-switch-from-regen-to-ABS is usually noticed in light-to-moderate braking over a rough surface or bump, lasts for all of a split second, and is pretty easy for a new owner to trigger on purpose so as to learn what it feels like and get over worrying about it.

-Chap