Brake problems????

Looks like the FJ Cruiser people are saying the same thing about the brakes losing grip over rough patches in the road. If I could disable the ABS I absolutely would- this seems far more dangerous and I can't control it the way I can control brake lockup on my own.

Worst ABS brakes ever. - Page 3 - Toyota FJ Cruiser Forum


I don't think you accelerate in the sense your speed increases, but you very clearly stop decelerating so physically it is an acceleration you feel. I have been too scared to check my speedometer- had a biker or pedestrian jumped in front of my right turn I would have hit them.

 

I was not looking at the speedometer so I'm not sure if I was truly accelerating. It felt like it, but an apparent "surge" may describe it more accurately. It does grab your attention for sure, and make you very cautious with another car ahead.

 

This is not a brake problem, but an artifact of regenerative braking. Normal braking in the Prius is done only with the front wheels through electrical regeneration, where the wheels power MG2 and make electricity, which is stored in the HV battery for later use. If either of the front wheels lose traction, such as hitting a bump, the Prius stops regenerative braking and switches to friction braking. The short transition produces a reduction of braking force. This feels like acceleration, although it is not. Your body becomes accustomed to the braking force. When it lessens, your brain misinterprets the lower braking force as forward acceleration. The action is completely normal, and will not cause trouble if you keep braking.

Tom

 

I'm not so sure- When this happened to me I put the pedal to the floor immediately and nothing happened for a very long time (not more than a couple seconds but when you car stops braking entering an intersection any time is too long). Either way, my car went at least 5 car lengths further than I had wanted it to, and this was at 15 MPH preparing for a right turn. A driver with less skill could easily cause an accident- I could even see someone freaking out and just coming to a complete stop inappropriately.

I read through the FJ thread and people say if you release the brake pedal completely it will turn off the ABS and then you can apply the brakes again. Not good.

 

Search the forum. This is a well understood and often discussed issue.

Tom

 

I know the feeling of the brakes moving from regen to friction at low speeds. This is completely different and related to the surface of the road. The braking is completely released for much much longer.

 

In reality, if you think you are in danger of crashing because of this then you were going into the stop too hot anyway. Slow down, leave plenty of room between you and the car in front of you and it should be a non-issue.

 

This has nothing whatsoever to do with the low speed switch to friction braking. Where did you get that idea? This is the switch from regenerative braking to friction braking at speed, which occurs from loss of traction.

Tom

 

You could make the same argument for removing ABS or other safety features altogether- if you drive carefully you will never lock the brakes and lose control and need airbags etc.

I don't think I should have to plan on stopping 50 feet before the car in front of me just in case the car decides that braking should not happen like it normally does, nor should I come to a complete stop and crawl through turns in case the pavement is uneven.

If the ABS or even regular braking system would engage properly, than there would be plenty of room to stop. The issue I am describing adds an incredible amount of distance to come to a complete stop. Easily a full 3 car lengths if not more at 10-15 mph. I should be able to stop within 5 feet at that speed if a kid runs in front of me or the car ahead slams on its brakes. Unless the car has no traction whatsoever, the car should not suddenly deliver 10% of the braking power that was previously called for by the driver. This should certainly not happen when one tire loses traction over a pothole.

I did not think that the car would be designed to switch from regen to friction braking in milliseconds in one circumstance, but all of a sudden take a full second or longer to complete the switch in another circumstance. And I certainly didn't think it would take longer to complete the switch when traction was lost, so apparently I incorrectly assumed that the car switched over from regen to friction rather quickly whenever it had to. Searching through 10 pages of poorly titled brake threads did not help me understand either.

 

Then this isn't the normal traction induced switch to friction braking, but a serious problem with your individual Prius. What you describe is not normal and is certainly unsafe. You should have it checked out.

Tom

 

I have already made an appointment but from googling it seems that this is just the way the Toyota ABS operates on 2009 and newer vehicles. That's what my dealer said over email as well. They recommended slamming the pedal down harder when it happens, but that did not help me the times it had happened.

As an aside, what exactly and why is there a difference between the switch to friction braking at low speeds and the switch to friction braking from traction loss?

 

Back to the original post...yes I also have this problem (pkg IV). I'll wait awhile to see if brakes "break in" before I start complaining to the dealer (only 150 miles driven so far).

 

Maybe this is an anomaly on your car or the GenIIIs because I have never experienced anything so drastic in the 94k miles I've driven my GenII. Like I stated before I have had similar issues with GM trucks but stopping distance was much less than 3 car lengths. If it is that big of an issue for you and not an exaggeration then I would schedule a dealership test drive so you can reproduce the effects and show them how serious the issue is.

 

as you slow down from 10 to 9 to 8 mphs, you have regen and maybe friction braking, then at 7 mphs regen stops and its all friction

 

At around 7 mph the speed is too low for effective regeneration, so the braking system switches over to pure friction braking. This is a planned transition - the control system does it as smoothly as possible. As regeneration ends, friction braking smoothly picks up the slack.

At higher speeds there is no planned transition. The transition only occurs if there is a loss of traction. Since you are going faster, and traction is lost, the transition is not as smooth. Braking drops to near zero for a split second, which to an unsuspecting driver feels like acceleration. After the transition you have normal ABS brakes.

Tom

 

This explanation from above is what has happened to me:

"1. When I go over a pothole as I am applying the brakes (stop sign just ahead), the car accelerates slightly, which is quite scary when others are waiting at the stop sign."

For me, it sure felt like it accelerated, even if it didn't. But bottom line, I ended up 3 or 4 feet further in distance than I would have expected/planned on...nothing in front of me this time. For me it wasn't a pot hole, but rather a rough patch of road coming up to a stop sign. If this ever happens with a car or person in front of one of us, they could get hit. I'm wondering if this is a defect, as a car can't react this way without resulting in increased accidents.

 

If it is then it affects quite a few cars brands/models out there...

 

At the risk of once again being called a "Prius Apologist", I can say with certainty that this is not a defect. It is simply the way the braking system works. There are a large number of trade-offs in the design of a braking system, many of which affect the ultimate stopping distance. Choice of tires is a big factor, yet Toyota doesn't use the best stopping tires. Why? It's a trade-off. The best stopping tires would be more expensive, have poor tread life, and hurt mileage. Why don't we consider this a braking defect? It's because most drivers understand this trade-off.

Regenerative braking is also a trade-off. It recovers energy, avoids fade, and saves wear on the friction brakes. On the negative side it increases cost and complexity, and only brakes with the front tires. Front tire braking makes the Prius more susceptible to loss of traction during braking. The loss of traction can lead to a small increase of braking distance. Why is this an issue when many other braking factors are not? It's because it is new and unique. Toyota has done a good job making the regenerative brakes work in a transparent fashion. The differences are subtle and not particularly dangerous when proper driving techniques are used. In the end it's up to the individual driver to understand the characteristics and quirks of a car. Some stop faster, some corner better, others accelerate like rockets. You have to know your car, and drive within its limits.

The next argument will be that yes, my point is true, but this is a different and dangerous issue because it unexpectedly increases stopping distance. The answer to that is that it doesn't increase stopping distance any more than other random variables such as sand and dirt on the road, hot temperatures, and a host of other things. If you measure your stopping distance with a micrometer you are asking for trouble.

Tom

 

It sounds like there are two issues here:

1. (and probably the reason the thread started) In my three plus years of Prius driving, I've noticed that my breaking technique has changed dramatically. We just took delivery of a Tacoma. I tried driving it. It felt like I had to press the break pedal halfway to the floor to just start breaking. My wife, who will be driving the truck, thought the brakes were normal. This leads me to believe that there is a learning curve for the Prius. Remember that when a Prius "creeps" (brakes off no accelerator pressed), it isn't the engine pulling the car. It is the motor simulating how a standard automatic transmission behaves. Similarly, the "brakes" are almost never the cause-and-effect behavior of a standard car or truck. The pedel pressure is sensed and then the hybrid braking system does what it thinks you want. This difference is subtle in many cases, but some, like backing down a hill, are more dramatic cases of the computer overthinking a bit. It won't take long before all this becomes second nature. In fact, the behavior that amazed me when I first got my Prius was when I stopped on a steep hill (Seattle has lots of them) and then just touched the gas to start. The car moved as though it were on level ground. That's when I began to undertand what "drive by wire" really means.

2. Dirt and rust. This was discussed in the thread. I never notice it as a problem, but it will cause uneven braking at times.