Brake Assist - Panic Stop (Tire Pressure)

[2nd Chance]

Okay guys needs some advise here.

I have a 2015 Limited with XREAS and my wife got to Experience the Brake Assist first had. I actually think she Pea'd herself. What really scared her is the fact that the rears actually lock up and skid. Which shouldn't happen since the truck has ABS

So this got me thinking.
1. Is it possible that the factory rake of the Limited may be making this worse

or

2. Could it be that I swapped out the tires and put on the TRD/BFG combo. What is different is I am running slightly more tire pressure ( 37psi vs 32psi ) then what is recommend.

Could this actually be causing my rear tires to lock up.

ps I am going to lower the tire pressure tonight and see what happens.

Any thoughts would be greatly appreciated.

 

[Runn4est]

My wife has also experienced the emergency assist however, I don't think it was a full on brake masher, I think she let up pretty quickly. Still surprised the hell out of her. There was no mention of the rear locking up and we're riding k02s @ 46psi. It could have been closer to 40psi at the time it happened since I may have upped the psi after the incident. It was definitely before our lift was installed though.

Sorry that's probably not much help, just sharing our experience.

 

[midnight runner]

How do you know the rear tires locked up? Did someone see the rear wheels stop spinning and for how long? Was tire squeal heard? Tires will make noise at the edge of tractional limits.

ABS cycles around the point of lock up, so yes they could have locked up for a bit, and then the abs module will relieve pressure to start a new cycle. Then it applies pressure until it locks up again. ABS measures the acceleration/deceleration of wheel speed. All of this happens very quickly.

I highly doubt the factory rack is the cause since that's how these vehicles are designed and tested around.

Tire changes more likely. Increasing μ of the tires will cause more weight transfer, unweighting the rear. This is only likely in significant increases in μ, like when a sports car moves to R compound or slicks.

 

[keph13]

What are you considering a skid - squealing noise or actually sliding out sideways? Last time I activated mine (stock everything except Kumho Road Ventures) it definitely squealed. I wonder if, on road surfaces, some amount of locking up is useful to stop? It may be doing more advanced logic than just "pulse the brakes if the tire stops spinning."

 

[1engineer]

Okay guys needs some advise here.

I have a 2015 Limited with XREAS and my wife got to Experience the Brake Assist first had. I actually think she Pea'd herself. What really scared her is the fact that the rears actually lock up and skid. Which shouldn't happen since the truck has ABS

So this got me thinking.
1. Is it possible that the factory rake of the Limited may be making this worse

or

2. Could it be that I swapped out the tires and put on the TRD/BFG combo. What is different is I am running slightly more tire pressure ( 37psi vs 32psi ) then what is recommend.

Could this actually be causing my rear tires to lock up.

ps I am going to lower the tire pressure tonight and see what happens.

Any thoughts would be greatly appreciated.

Some facts:

1. Locking Up During Braking
With ABS, no tires will "lock up" in any braking scenario. Now, what WILL happen is the tires will cease to rotate (there's a difference lol) for a few milliseconds and then rotate again. Basically what happens is the ABS will continue to brake all the way to full lock up and then release... then lock up and then release... then lock up and then release, until the vehicle is stopped or brake pressure is released. I use the term "lock up" here but in reality we are talking about a few milliseconds. It takes 300 milliseconds to blink your eyes... So "lock up" is really just one tire ceasing to rotate for a few milliseconds. Also it might be only one tire or three tires or two tires. Just depends on the tractive condition of each tire at the time.

2. Tire Squealing
In the right conditions, you WILL hear tires squealing. Tires do not have to be locked up for them to squeal. To prove that just go around a corner too fast. The tires are rotating just fine but you are pushing the limits of their coefficient of adhesion and rubber on dry concrete or asphalt will squeal.

3. Factory Rake or Tire Pressure
The factory rake will not make the tires lock up. Different air pressure will not make the tires lock up. Each wheel has its own ABS channel of communication so each wheel is being monitored for rotation. You could have 18 psi in one rear tire and 40 psi in the other and 32 psi in the front tires and none of them will lock up or be affected by the performance of the other three. I will say you will get a shorter stopping distance with a lower tire pressure due to a larger contact patch. You won't get any lock up with a higher pressure though. Remember that the electronics are looking at tire rotation and when it gets to a certain point it releases and grabs really fast, like a pulse. It doesn't matter if the tires are bald, low on air, mud tread or anything else. Those affect stopping distance not performance.

Hope this helps.

 

[Seattle206]

Is this "panic brake" feature in the SR5? I never experienced this before since I bought it in 2014.

 

[T5RUNNER]

Heavy all terrain tires can only hurt your braking performance

 

[2nd Chance]

OK so I just went to a nice country Road 40 miles an hour stab the brakes and I was able to get the rear tires to lock up and slide the truck to the left.

I never took my foot off the break just like a panic stop and there were black marks on the road from where I just did my stop test.

So the rear wheels are actually locking up


Marty-MJ

 

[2nd Chance]

This is a 20mph parking lot test I just did.

Marty-MJ

 

[1engineer]

This is a 20mph parking lot test I just did.

Marty-MJ

Those are ABS marks. Sure, they left some rubber but see the intermittent marks in your pic? Not locked up. Here are locked up skid marks:
You will get those marks under the right conditions. The main advantage to ABS is you can still steer during heavy braking as your brakes pulse. When fully locked up (non ABS) you are just along for the ride and cannot steer.
It's normal.

 

[philsey]

Is this "panic brake" feature in the SR5? I never experienced this before since I bought it in 2014.

You probably don't text and drive like the OP's wife. j/k (sorta).

 

[philsey]

@OP. You should hear the ABS working during these 'lockups'. Mine chatters like hell - no mistaking what it is.

 

[Koddo]

The panic assist is pretty ridiculous on the 5th gen...at least my 2010. I recently was coming up to a stop light going about 50 and it turned yellow. This intersection is a regular in my daily commute and the lights turn very quick. So I had to make a more abrupt stop but I apparently pushed the pedal a little fast and the panic assist went apeshit and there was all sorts of ABS commotion for what would have been a normal stop in any other vehicle. I personally don't think they got it dialed in very well for the 4R (2010 may be worse -I did read somewhere they changed the brake feel in 2013+).

 

[nb rob]

after my first brake assist surprise , I tapped the brake pedal a few times on a gravel road to get used to what it takes. As advised on T4R, it was the speed of the actuation that kicked BA in.

Make no mistake, on pavement it'll make your tires roar, but it isn't locking up.
I think the rear tires have much less purchase once the nosedive of full braking occurs.

Steering is still very good, and stopping distance is amazingly short. As a safety feature, I think it's incredible. I've driven with people that chronically hop on the brake pedal. I'm sure they would hate brake assist with a passion

 

[2nd Chance]

@OP. You should hear the ABS working during these 'lockups'. Mine chatters like hell - no mistaking what it is.

I am going to have to a few more stops on the way home from work tonight to see if I hear the ABS. I can tell you the Nose Dive that happens in the truck makes you feel like your looking at the ground directly in front of the truck.

 

[Elite130]

I have experienced the brake assist, or whatever it is called on my 2016 4Runner trail. I've done some hard braking and the brake assist actually made the braking situation worse. It all but locked up the wheels when it didn't need to. I personally hate it.

 

[Watchdogg]

I have experienced the brake assist, or whatever it is called on my 2016 4Runner trail. I've done some hard braking and the brake assist actually made the braking situation worse. It all but locked up the wheels when it didn't need to. I personally hate it.

I also hate it on my 2015 trail. Only had it happen twice but both times scared the hell out of me because there was no reason for it.

 

[00Sebby]

The Panic Assist feature is the only complaint I have on my '10 TE. Just because I need to stop a little quicker for an unexpected light change or someone pulling out in front of me doesn't mean I need "assistance", especially in the form of nose-diving, tire squealing, stupidity that makes me look like an idiot and/or causes the tailgater behind me to run into me. I can just push the pedal harder if I need to stop quicker... been doing it for years. If I really need to stop, I can step on the brakes and let the ABS do its job. Either way, it should be my choice and my input should get an expected output. I don't need the truck guessing what is going on because I moved my foot from the gas to the brake quickly. My wife would also freak out and refuse to drive it if it did it to her. Of the dozens of vehicles we have owned and the many more cars that I have rented, this is the only vehicle that has ever had this stupid feature. I have rented several 2015/16 Camrys, Corollas, etc. and drove the crap out of them to see what they do, and they don't do this. Like I saw somewhere else it must be designed for the lowest skill level driver.

 

[Jeremy556]

Have you and your wife driven non-ABS vehicles and experienced actual skids before? The brake assist will stop with 100% force when you move your foot quickly from throttle to brake, which will make lots of noise and may leave skid marks in several places. A good test would be to try it again on wet pavement, and turn the wheel a little. If it is really locked up, you'll go spinning.

 

[1engineer]

The Panic Assist feature is the only complaint I have on my '10 TE. Just because I need to stop a little quicker for an unexpected light change or someone pulling out in front of me doesn't mean I need "assistance", especially in the form of nose-diving, tire squealing, stupidity that makes me look like an idiot and/or causes the tailgater behind me to run into me. I can just push the pedal harder if I need to stop quicker... been doing it for years. If I really need to stop, I can step on the brakes and let the ABS do its job. Either way, it should be my choice and my input should get an expected output. I don't need the truck guessing what is going on because I moved my foot from the gas to the brake quickly. My wife would also freak out and refuse to drive it if it did it to her. Of the dozens of vehicles we have owned and the many more cars that I have rented, this is the only vehicle that has ever had this stupid feature. I have rented several 2015/16 Camrys, Corollas, etc. and drove the crap out of them to see what they do, and they don't do this. Like I saw somewhere else it must be designed for the lowest skill level driver.

It doesn't guess. It measures your speed of actuation of the brake pedal. Try it sometime: Push the brake hard but at normal speed and the vehicle will act just like you envision. Push the brake super quick, like in a panic stop, and not even hard, and brake assist will engage. It took me exactly once before I changed my braking habits.