Unintended acceleration

Huw Powell audi at humanspeakers.com
Wed Oct 21 20:01:40 PDT 2009

Brett Dikeman wrote:
> On Wed, Oct 21, 2009 at 8:52 PM, Ed Kellock <ekellock at gmail.com> wrote:
>> An interesting read... it discusses vacuum assisted braking systems and the
>> inability of a car's brake system to stop it at full throttle, regardless of
>> assist.
> Except it's a load of crap; they all have check valves, and brakes are
> MUCH stronger than engines. 

Brett, have you ever experimented with vacuum assist brakes with an open 
throttle?  I have, and you get two or at best three presses of the pedal 
before it goes hard.

> Think about the kind of torque you'd need
> to get from 0-60 in 150 feet.  That's barely across a large traffic
> intersection.  Yet, your brakes can do the opposite handily...

The article mentioned needing 225 pounds IIRC of force on the brake 
pedal with no vacuum available to assist it.

> My 'R has vacuum-assisted brakes.  I assure you, they work very well,
> including full-throttle-to-hard-braking transitions.  Even if the
> engine shuts off, there is an auxiliary vacuum pump which kicks in any
> time the ignition is on and vacuum pressure drops.  I was bored one
> evening waiting for someone and played with it- you have to work
> pretty hard to use up the vacuum faster than the pump can replace it
> and exhaust the vacuum reservoir. 

Maybe the Lexus didn't have a back-up pump?  I don't know what an 'R is, 
but I suspect it comes from a more race- or sport-oriented heritage than 
a Lexus?

> It's also curious why the car, which surely was throttle-by-wire,
> didn't shut off the throttle when the brake was applied.  That's very
> common behavior, has been for nearly a decade.

Apparently Toyota is behind the curve on that?

> Why didn't the driver
> attempt to put the car in neutral, switch off the ignition, etc?

Unfamiliar car (in the lead example of the story) that required a 3 
second push on the "start" button to turn it off; a complex 
manually-shiftable automatic shift lever pattern.  Not as easy as 
pushing the lever from "D" to "N".

I liked how, just as back in Audi's day in the flames, the floor mats 
are cited as a prime culprit.

Huw Powell



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