Car & Driver: Unintended accel tech

Yes, interesting tests. But then, why didn't that Lexus the CHP was driving stop or slow down by the breaks? I sure hope he didn't confuse the gas and brake pedals. But then, he could have been suicidal maybe.

 

Repost of http://priuschat.com/forums/other-c...w-deal-unintended-acceleration-tech-dept.html.

 

He probably didn't push down on the freaking BREAK pedal. Or the Brake pedal either.

No matter how many tests and technical studies are made, there will always be someone who blithely asks "Well, then why didn't the car stop? There must have been something wrong!"

Of course there was something wrong! And that something is usually the driver.

 

There's one thing they might have said:
If you think you are pressing the brake,
THINK AGAIN.
Start over--Pick up that right foot and move it left.

The thing is, when we panic, we don't always think straight, which is why we don't think again. I think they have a point when they suggest all manufacturers of push button cars allow for the "natural" panic reaction of pushing a button repeatedly, rather than pushing and holding.

 

I know this is old news but that Lexus incident seems so odd to me. How is it possible to get to speeds of 120mph and not be able to slow a vehicle? This is a police officer we're talking about! They have much more training than an ordinary civilian. Why didn't he shift to neutral? Why didn't he brake? I know there was loss of life and it was a tragic event but I am very curious as to the circumstances surrounding this case. I'm not convinced there was a problem with the vehicle.

 

One possible factor in the Lexus crash: A previous customer had reported to the dealership that the floor mat had jammed the accel pedal. It could be possible that the brakes were already damaged from that previous event. That would explain why the CHP officer wasn't able to stop the car with the brakes. I still don't understand why he didn't shift to N though.

 

I don't think we'll ever know for sure. When I initially heard about the Lexus, I thought this was a suicide (also). Too many systems had to simultaneously fail for there not to be something suspicious with this incident.

That said, a very common mistake that many drivers make while braking hard is to repeatedly "pump" their brakes. It's been mention by me, as well as others - this is a bad idea! Before the advent of anti-lock brakes, most of us were taught to pump the brakes to avoid wheel lock-up. For over 2 decades now, most cars have come standard with anti-lock brakes, so there is no need to pump the brakes. In fact, by repeatedly pumping the brakes HARD, you can:

(1) Overheat the brake pads, which will cause brake fade, and you may then not be able to bring the car to a complete stop.

(2) A human pumping the brakes will increase stopping distance.


The person driving this Lexus may have severely compromised his brakes while going down this hill at a high rate of speed, as he continued to pump the brakes. Although, even so, it does not explain why he (apparently) was not able to even slow the vehicle? Even with severely faded pads, you will be able to slow the vehicle, although you may not be able to bring it to a complete stop.

While I do not think the Lexus crash was a hoax (like with the Prius), I do, however, think there are a number of suspicious facts that make me wonder about the whole incident. Color me as someone who's not convinced this was all mechanical/electrical.

 

All brakes will fade when heated. To stop a runaway car with the brakes, it's important to apply the brakes fully at the onset, to stop the car with the least amount of braking. If you toy with it, you will heat the brakes to the point where they are no longer effective.

The Lexus involved in the crash was a loaner car. The previous driver reported trouble with pedal entrapment, where heavy braking was required to stop. Undoubtedly this was bad for the brakes. This means that the CHP driver started with less than perfect brakes. The combination of bad brakes and initial braking reaction probably lead to useless brakes.

Tom

 

strange that this repost is getting activity and Lexus ES350

Interesting that this repost is getting a bunch of replies. The article isn't Prius specific at all and "Prius" is found nowhere in the article. My post of the article at http://priuschat.com/forums/other-c...w-deal-unintended-acceleration-tech-dept.html almost 2 months ago has gotten 0 replies as of this writing. These threads ought to be merged and moved to Other Cars (where mine is).

To rehash the Lexus ES350 incidents and some of the questions that have come up, see some of the links I posted at http://priuschat.com/forums/other-cars/76719-gaspedel-lexus-crash-real-fake.html#post1069802.

 

Re: strange that this repost is getting activity and Lexus ES350

I agree 100% that most of these threads regarding unintended acceleration, how to stop your car, etc, etc could all be merged, and should all be merged into one or two threads.

I've found the quickest way to kill a discussion on a topic, is for multiple threads to start up on (essentially) the same topic.

 

That would make sense or suicide.

While possible, I doubt that CHP officer panicked, at least not initially. He is not an ordinary, untrained driver.

Well, if that was an ordinary driver, that would most likely be the case, but a CHP officer?

 

I understand that the Lexus had the mat issue. It was a heavy all weather mat that was for an SUV so too big for the Lexus. Also, it was installed over the carpet mat and it was not hooked into place.

The mat goes over the gas pedal and under the brake pedal. When you press on the brake pedal, you are essentially pressing on the gas pedal.

He thought he was braking, which he was, but he was applying pressure to the gas at the same time, which he did not know he was doing.

The reports indicate speeds in excess of 120 MPH were reached very quickly. People witnessed sparking and flames coming from his tires (his brakes).

He was unfamiliar with the car. And as others have said, the brakes were used heavily 2 days prior when a gentleman who had the car had the same problem. He knew what was happening quickly and pulled the mat off the pedal. He told the dealership what happened but apparently, they did nothing about it.

Why not shift to N.....who knows? No matter who you are, panic is a terrible thing.

 

The NHTSA initial site report indicated considerable brake damage from heat and high pressure, so he was putting considerable force on the brake pedal after brake fade began.

But we don't know the details of how it faded and what condition the brakes were in when he received the car. The dealership did not perform due diligence after the previous customer reported problems.

I need to go find that older report (NHTSA? attorney?) of a certain Lexus with WOT needing 225 pounds of pedal pressure to be stopped. It needs to be compared to this case.

 

Wow that is a lot. I used to work for a heavy truck manufacturer and I designed a brake pedal system for a truck. It was an air brake system, but I recall that my calculations were that ~ 80 lbs force on the pedal would provide full pressure from the foot valve (air valve) and somewhere around 125 lbs force would push the actuator to full stroke. I also recall from the human factors handbook that the 95% percentile man could apply ~ 475 lbs force in a sitting position using both feet. I designed the pedal to be able to withstand over 2x that amount. The brake pedal is a part that should never break under any conditions.

 

Are your design pressures with the power assist system operating normally?

That WOT Lexus number was with the booster reservoir depleted. And with the engine at Wide Open Throttle, the vacuum was nearly gone, leaving almost nothing to drive the power assist, so braking was almost entirely manual.

While numerous readers here have boldly stated that all brakes can always overpower the engine every time, I believe that a significant fraction of the driving public cannot apply 225 pounds of pedal pressure. And if the San Diego Lexus started that day with already damaged brakes . . .

 

Air brakes don't have a power assist like hydraulic brake systems. The multiplication of force is a function of the air pressure applied to a diaphragm inside an air chamber at each wheel. The diaphragm pushes the lever to actuate the drum brake or disc brake (drums are more common with heavy trucks). The air pressure is supplied by 'dry' tank(s) with a nominal pressure of 120 psi.

The foot valve (attached to the pedal) can only supply as much air pressure as is available at the dry tank minus losses in the air piping. For my design, ~ 80 lbs force on the pedal would deliver full pressure to the system. A typical non-emergency brake application with nominal tank pressure would only require about 15 - 30 lbs force IIRC.

I agree that 225 lbs is a lot; many people would struggle to provide this much force on the pedal. I could do it because I weigh 230 lbs, but I wouldn't expect a 105 lb person to be able to do it.

Air brake (road vehicle) - Wikipedia, the free encyclopedia

 

225 lbs pushing with ONE leg is not that hard to do. brace yourself against a wall (or car seat) and see how much weight you can push. i think you will be surprised. put them in a panic situation and adrenalin easily doubles their ability i think.

in the San Diego crash, i think terrain and elevation changes played a big part in it as well. cant help but think the driver simply panicked. i know, that his background, training, etc. made most discount that possibility right off the bat, but i think its easy to panic when you have your family screaming during all this.

its too bad that we will probably never really know what happened

 

Yes . . . for average size, able bodied males. But that is a minority of drivers.

While most drivers who meet only 2 of those 3 criteria can still apply this pressure, a noticeable fraction cannot. At 1 of 3, a larger fraction will fail. At 0 of 3 -- which is still a lot of drivers -- many will fail.

As someone with first hand experience with pilot-induced SUA (but resolved by reflex thanks to a manual transmission), I strongly believe that most cases are pilot error. But it is a serious stretch claim that all are. The San Diego crash seems to be among the best candidates to investigate for other causes.

 

I agree totally. Having passengers shrieking "Oh my God, Oh my God, we are all going to die", really disrupts ones' thinking abilities.

I eventually trained both wife and mother that the best thing they could was point out possible incidents and then be as quite as possible so as to allow me to figure out what to do.

Cheers
Warwick