Question - Unsettling experience while braking

[westwind77]

def not tires or rotors, I am very familiar with those during a failure or issue (these were at 1200 miles on new rotors and new BFG 285 A/T KO's.

Ahh, well there you go! I assume you are lifted? Others on here have had issues with their VSC system after being lifted, had to have the dealer 're-zero' it as it would activate around on/off ramps, etc. Just a possibility.

 

[Jtraskell]

I am thinking now it is a combination of things. Like Westwind said, I doubt it was VSC, you have to be skidding sideways or close to it for it to kick in. But what about "Brake Assist"? That, in connection with ABS and possibly squishy tires? knc169, I did not mean to insinuate that the OP's tires failed, they did what is expected for what the tires are, 4 ply passenger car tires on an almost 5lb truck, traveling down hill at 85 to 90 mph! The sidewalls will absorb a lot of downward force from braking. (And "squish")

Now, brake assist is supposed to work, but only in a "panic" type braking situation. (There are multiple threads here which discuss this and to those who have experienced it, very unnerving...). Here is a link which discusses this system. Brake Assist – Vehicle Safety Basically, the vehicle ADDS braking power, notwithstanding what the operator does or does not do.

The question is, how does the vehicle know when it is a panic brake? As far as I can tell, there is no "sensor" on the brake pedal itself, (nor would that work for various reasons). I am thinking there is an ECU algorithm which looks at a ratio of speed vs braking or the like. Something like, the faster you are going, the LESS braking amount it takes for the brake assist to kick in. Slower you go, the more braking force is needed for it to kick in. This kinda makes sense, at least computer sense.

In the OP's case, going fast, brakes (even though he felt is was light or moderate braking), but enough for the computer to kick in due to speed, the system adds (unwanted and unexpected) emergency braking power, ABS kicks in? And then possibly exacerbated by the squishy tires, resulting in poopy pants!

All of the above makes sense and none of it is really a failure of any specific item or system. Just a bad combination. Again, just trying work it out in my small little brain. Be safe.

 

[westwind77]

. But what about "Brake Assist"? That, in connection with ABS and possibly squishy tires?

Ha, if you ever 'tripped' brake assist you would know! Anything on any seat will be shot forward like a missile!

"Brake Assist is designed to detect sudden or “panic” braking and adds the full pressure needed to help prevent a collision.

If something rolls out into the street in front of a vehicle, there’s only a split second to react. (Figure 1)

Because the reaction time is so short before the driver hits the brakes, not enough pressure may be applied. As a result, the car may not stop in time.

But in a Toyota, if a driver fails to apply enough pressure to the brake, the Brake Assist sensors will detect this sudden or “panic” braking and add pressure. (Figure 2) This additional pressure can help the driver avoid hitting the object."

Brake Assist – Vehicle Safety

I have activated it more times that I care to count. How exactly it 'senses' I am not sure, but if you drive normal, release the gas and quickly/sharply tap the brake (you don't even have to push it down far) the system can/will activate. If you give a quick acceleration then quick to the brake it will trip it as well. Of course, if will activate like Toyota says above (the true intended way it should). If the OP hit the brakes in the manor to activate BA I would be shocked, and besides, he would have had a different reaction than just some vibration, lol.

 

[Jtraskell]

Fair enough. When you kicked in your brake assist, did you notice if your ABS kicked in? Or if you did not notice, did you leave braking skid marks...(on the street, not your underwear...) It was my impression that the OP felt is was a violent vibration, if not brake assist, then what is left is heavy downward force with squishy tires and ABS. (Due to speed, inertia, slope and braking.)
I just hope I do not ever experience something like that.

 

[westwind77]

Fair enough. When you kicked in your brake assist, did you notice if your ABS kicked in? Or if you did not notice, did you leave braking skid marks...(on the street, not your underwear...) It was my impression that the OP felt is was a violent vibration, if not brake assist, then what is left is heavy downward force with squishy tires and ABS. (Due to speed, inertia, slope and braking.)

You can physically feel the brake peddle being pulled away (further to the floor) from your foot when it activates (if you release, obviously if you are in a true panic you are going to be pushing hard/fast and will not feel it). It does make a 'slight' clicking sounds, now if that is the BA system and hydraulic pumps pushing more force to the brakes or if that is the ABS system I cannot tell you. No, no skid/brake marks on the road (or pants, but close the 1st time it engaged, lol) There are a number of threads, mostly older, about the BA system and peoples reactions to it. I think someone even asked their dealer if they could deactivate it (of course no).

 

[Jtraskell]

Yea, I remember those previous threads and I referred to them in my current post above. No skid marks, emergency braking, ABS was active even with BA. Ok, that being the case the mystery continues. Just for s...s and giggles, when you activated your BA, were you driving at about the same speed each time? Say 40 to 50 or the like, on relatively level ground....

 

[westwind77]

Just for s...s and giggles, when you activated your BA, were you driving at about the same speed each time? Say 40 to 50 or the like, on relatively level ground....

I have activated it at speeds from 20ish to 50ish, down hills, up hills, and flats. We seem to have a lot of !diot drivers and animals running around so it gets tested frequently (although I will say I feel like it has been a while since it has been 'tripped', which is a good thing).

 

[Jtraskell]

Alright then. Since you have had basically the same experiences as the OP, and I assume you are running OEM tires, just a guess since your detailed signature does not indicate otherwise, do you have a theory as to what happened? I am not being critical, just trying to figure it out......

 

[knc1us69]

Yes I am lifted but by the dealership. I bought my TE new already lifted and recalibrated and balanced by the dealer. Again I am not saying system is wrong or failing just that I don't like that particular system feels compared to other vehicles I have had. I love my TE so don't mistake my strong will to mean I am dissatisfied with my T4R

 

[1000MPH]

There is definitely some sort of brake boost happening during moderate to heavy braking on downhills at speeds above 60 or so. I was activating it at will on HWY 178 (not far from Death Valley). Don't know what system it is but it downshifts the trans and makes the brakes feel overboosted. There are several levels of this. The worst so far downshiftsto 3rd for a sharp downhill turn and won't up shift with throttle until about 3k rpms or my speed drops to about 30mph.

 

[westwind77]

Alright then. Since you have had basically the same experiences as the OP, and I assume you are running OEM tires, just a guess since your detailed signature does not indicate otherwise, do you have a theory as to what happened? I am not being critical, just trying to figure it out......

Actually, no, I have NO issues like the OP. I have had my 4R at higher speeds and applied the brakes and she slows perfectly, smooth as glass. No clue what is happening with the OP 4R. Just was saying he (the OP) would have to be like oh sh!t, i'm doing 90 and hit it brakes fast and hard for BA to kick in, which I doubt anyone would do at that speed. Just trying to help eliminate things given my experiences with my 4R, that's all.

 

[Jtraskell]

I understand and agree. That is what we are both trying to do. Oh well, guess it is going to be one of those non-life changing mysteries that may keep me up one night, when I have nothing else to keep me up that night! Be Safe.