Braking problem

As far as the behavior of the ABS and regen, my 2004 behaved in a similar fashion. I've driven work trucks, primarily early 90's Chevy's, that had far worse behavior

I no longer have my 2004 Prius, but thought last night I'd go through my 2007 FJ manual, to see if they mention this. What do you know, they DO mention it. There is actually an entire page of yellow highlighted text, with the header a Triangle Of Doom and "CAUTION"

2007 FJ Cruiser owner manual, page 212

Do not overestimate the anti-lock brake system .... Anti-lock brake system is not designed to shorten the stopping distance

Here is the interesting part

"Compared with vehicles without an anti-lock brake system, your vehicle may require a longer stopping distance in the following cases:

-Driving on rough, gravel, or snow covered roads
-Driving with tire chains installed
-Driving over the steps <sic> such as the joints on the road
-Driving on roads where the road surface is pitted or has other differences in surface height"

Another bit of yellow highlighted text got me thinking about tire pressures

"Install all 4 tires of specified size at appropriate pressure: the anti-lock brake system detects vehicle speeds using the speed sensors for respective wheels turning speeds. The use of tires other than specified may fail to detect the accurate turning speed resulting in a longer stopping distance"

With the FJ Cruiser, when I'm in 2H, the traction control, VSC, and ABS are all engaged. When I shift into 4H, the traction control is detuned along with ABS, and VSC is disengaged

When I shift into 4L, the ABS, VSC, and traction control are all disengaged. As an option, I can manually engage "Active Traction" and/or the rear differential lock

Driving on a washboardy gravel road, in 4L, my FJ stops *much* quicker than if I'm in 2H with all the electronic features engaged

 

Tom, having appreciated many of your posts, maybe you can confirm a technical explanation I read on another site, saying (I'll simplify here) that the Prius brake pedal will always generate hydraulic pressure, but that an intelligent valve will block or allow that pressure to be released to the wheels depending on the capacity of regenerative power to slow the car as needed. As more pressure is applied in a hard stop, it will release the pressure to the wheels, and in the event of a panic stop, it will release the pressure to the wheels immediately. The immediate signal of a panic stop is rapid release of accelerator pressure. My question above about right or left foot braking gets to that point--if you left foot brake, you may well not release accelerator pedal pressure rapidly, thus delaying full braking. (Not unlike what might happen with any car with traditional brakes if you're still feeding some gas initially.) Any substance to this information? (FYI-I have experienced no brake issues on the Prius, nor a prior RX400h.)

 

I don't have any technical material for the 2010's, but for my 2004, here is a schematic of the braking system



The braking system in one of its failure modes



Hope this helps

 

I gas and brake with my right foot

 

Re: Floating feeling when braking

I too have had this "floating when braking" occur in my 2010 prius and it is terrifying. I am taking my car into the dealership on Sept 14 and will report back what they say. I hope that everyone who has issues like this, and the jerky braking in reverse on a hill... will hound their dealers until Toyota admits that this is occuring and figure out how to fix it.

 

Re: Floating feeling when braking

I too mentioned in a post on here that I experienced it recently with my 2010 Prius V model. I was going over an overpass that had a bad pave job from the overpass to the road. Anyway, long story short it didn't scare me at all, since my 2007 Honda Civic Hybrid acted the same exact way over this same location (it's enroute to work so pass it every day, just depends on how far the traffic is backed up to the light if I have to brake at the same time).

Anyway, when dealing with the Honda Dealership there were no recalls and I was told it was the Regen braking/Friction braking mix that causes this. I never had an accident as a result, and learned to brake accordingly with it. It may just be something your not familiar with and let's hope that is the situation versus some manufacturing flaw.

Good Luck at the dealership and let us know what you find out. I am sure many are interested in the response and result.

 

Re: Floating feeling when braking

My friend's 2006 Acura TSX manual does this too -- he says if he's breaking and hits a sizable bump he can feel the ABS kicking in. I never felt anything as a passenger... It's obviously coming from the tire momentarily slipping and the ABS releasing the brake even though traction isn't really a problem.

The bouncy tires on the Prius probably don't help..... I haven't noticed this on my 2010 but I've only put 100 miles on her so far... No panic stops.

 

Re: Floating feeling when braking

What happens when the electronics fail and it doesn't switch to friction braking ?

What is the 60-0 FRICTION braking specs ?

 

Where's your source of that info ?

Are you also saying the front wheel regen braking is more powerful than the 4 wheel traditional braking system ? That does not sound plausible.

 

momentary is the operative word here. electrical braking or regen is instantaneous and consistent no matter what.

mechanical brakes are MUCH slower to intiate and may have other conditions that must be overcome that the mechanical system can neither anticipate or adjust for like wet pads, low fluid pressure, etc

 

Re: Floating feeling when braking

It can do that without decellerating (instant).

 

Re: Floating feeling when braking

We are having a lot of road re-surfaced in my area, and I too have experienced this odd feeling at least twice now. Quite startling/scary the first time. Now that I know what is going to happen, though...I adjust my situation. I brake until I'm ready to drop down over the unsurfaced area, stop braking until I'm past the drop, and then apply the brakes again. No problems at all. Works for pot holes too.

 

Re: Floating feeling when braking

This just happened to me today and it's very disconcerting to say the least. I was going about 10mph and braking as I approached a stop light when about 25 feet from the light I crossed over a very small bit of uneven pavement. The Prius lurched, engine rpms seemed to race and ABS started kicking in. I stayed hard on the brakes but almost floated through the red stop light and into passing traffic before thankfully stopping.

I can not subscribe to the thought that this is "normal" behavior by any stretch of the definition and based upon the number of posts in this thread alone it seems that Toyota could have a very big problem on their hands.

 

We have this problem on our 2010 prius - about 30 episodes in 4500 miles of "acceleration" while braking and hitting a bump. The theory of braking gap due to cutover from regen to friction braking after a bump would be consistent with our experience during the episodes. Strongly urge anyone experiencing it to REGISTER THE EPISODE(S) WITH THE NHTSA (just Google "report safety defect NHTSA". The floor mat recall is unrelated to this braking problem, and it won't get fixed until the government leans on Toyota. That will only happen when they see multiple posts on the government site with the same symptom (hopefully before a pedestrian gets hurt or killed). We won't drive ours much further if it isn't fixed.

 

I somehow missed this post. Sorry I took so long to respond.

It is true that pressing the brake pedal generates hydraulic pressure. In normal operation, this pressure is sensed by two electronic pressure sensors and used to signal braking intent to the braking ECU. A "stroke simulator" gives the hydraulic pressure something to work against. The stroke simulator is nothing more than a piston and spring. The braking ECU uses the pressure signal to decide how much braking is needed, whether it be regenerative, engine, or friction braking. Friction braking is powered by an electrically driven pump.

In the event of a failure of the power braking system, solinoid valves open that allow the hydraulic pressure to manually actuate the front brakes. Note that this only happens in the event of a failure. The rear brakes can be mechanically actuated with the parking brake lever.

There is also a potentiometer attached to the brake pedal lever. It is used to sense the rate at which the pedal is being depressed. If the ECU senses that the driver has rapidly moved from the gas to the brake and is rapidly depressing the brake, the ECU will invoke brake assist mode. With brake assist, the ECU slams on the brakes for you, actuating them faster than you could. It helps speed up braking reaction time.

Tom

 

this i have experienced several times. i have always reacted to potential emergency driving situations the same and the differences in how braking works in the Prius is not extremely user friendly for me in these situations.

when i see something that might happen, i immediately take foot off the gas and put it on brakes with the intention only to be ready to brake if need be. when i do this i automatically depress the brake slightly. in previous cars this was only enough to just begin to create drag.

in the Prius however, its like slamming on the brakes for a split second. now i personally hate that the Pri does this especially when there is no need to slow down (i HATE losing momentum needlessly) but unfortunately my actions are reactions and i have found it difficult to adjust.

this is something i have noticed several times on both Pri's so nothing new here. i guess i understand why it happens. something i continue to work on.

i guess i probably should look for other solutions like cutting down on caffeine, etc...

 

I've done that a few time too. I use the same technique of covering the brake when I see a potential problem, and there have been a few times where I covered too aggressively. This was generally followed by me apologizing to my passenger and saying "Whoa, I didn't mean to brake that hard!"

Tom

 

Braking problems especially on wet roads

I am still getting used to my 2010 IV. Only had it since Sept 1. Yesterday, it was raining as I approached a stop sign and pressed on the brake pedal to gradually slow me down. All of a sudden it felt as if the brakes were not applied and the car did a "mild" leap forward (I still wasn't stopped when this happened). I did not check my dash to see if any indicator came on at that time. A friend with a 2005 model says that this could be the wheel slipping/losing some traction and power was applied to other wheels to compensate.
Has anyone experienced this situation?

 

Re: Braking problems especially on wet roads

It's been raining here the last couple days, but we've had no problems so far. Yeah, one reason why I hesitated on the package V with ATP was i kept wondering about how safe having all these "safety" mechanisms would be. I trust myself more than I do circuitry and software, and I had this nightmare of the car suddenly braking on the highway for no reason at all because of faulty radar or whatnot