Unintended acceleration

LL - NY larrycleung at gmail.com
Wed Oct 21 20:27:17 PDT 2009

I'm also overall, with Brett on this, with the exception of the vacuum
assist issue.

As an experienced autocrosser running, for all intents and purposes a
Scoobie WRX (Saab 92X Aero), I curse Subaru for not either including a
vacuum pump for the brake boost, or going to hydraulic brake boost (both
adding to cost that Subaru I guess "chose" not to add). In going from near
WOT or WOT to brake I can often run out of boost. The tighter the course,
the worse the problem. It is worse with ABS active than not. I have checked
out the entire braking and boosting system, all meets the Service Manual
requirements, including check valve and booster function.

Extrapolating to a full road course, I would venture I wouldn't have the
problem as much, as there is greater recovery time between brake
applications. Of course the Lexus and Camry's mentioned in the article don't
run boost but, extrapolation makes this "plausible".  But, in terms of HP,
auto brakes WAY overpower their (assisted) braking, just as in the old Audi
days. Guess jounalistic sensationalism dies hard.

(and as an occasional driver of one of those VAG products with the DBW
cut-off of throttle and simultaneous brake pedal application, I find it
infuriating. Glad the Saabaru at least doesn't have that).


On Wed, Oct 21, 2009 at 11:01 PM, Huw Powell <audi at humanspeakers.com> wrote:

> 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
> http://www.humanspeakers.com/audi
> http://www.humanthoughts.org/
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