Simultaneous Application of Gas and Brake Pedals

As strange as it might seem, the potentially disasterous application of
throttle and brake is not limited to old ladies pulling into a parking
space in front of a beauty shop.

Examine and quantify the geometric positions of the brake pedal and
accelerator pedal in a current series production car. Do the same in a
1950 model. You will note a striking difference.

In early cars, the brake pedal was a considerable distance to the left and
more importantly, much HIGHER than the accelerator pedal. It is virtually
impossible for the right foot to press on both at the same time. The shoed
foot is simply not large enough to accomplish this.

Now look at a modern car. The brake and accelerator pedal have little or
no difference in height. This was necessitated by the low seating
arrangement and also the narrower space available due to the center
console. Is is entirely possible for the right foot to press on both
pedals at the same time. Try it.

The reason why the brake does not overpower the engine in many cases has to
do with the angle of application. The driver impulse is to press harder on
the "brake", but in doing so he opens the throttle further. As the
throttle opens further, the angle of the foot on the brake pedal becomes
more and more acute, lessening the force of application. You can try this
out safely on a large parking lot. Try various wrong applications and you
will soon find one which causes the accident causing lurching.

Auto companies should address this issue with more rigor.

 

The heavy duty floor mat (Mopar brand, which I use in the winter) in
my 300M tends to creep forward and up against the center console -
which means it gets up and behind (and to the right) of the
accelerator pedal. This reduces the amount of foot-space to the
immediate right of the accelerator pedal and moves the right foot a
little to the left instead of being centered on the accelerator
pedal. I've found that in this position I brush against the *back* of
the brake pedal when pulling back on the gas.

All in all I'd have to agree that there should be more spacing between
the gas and brake pedal.

But I think that it's a manditory design criteria that the brake
system of any car is supposed to be able to over-power the engine in
all situations. Back in the days when you had a spring that pulled
back on the throttle plate, if that spring broke you could have WOT
(wide-open-throttle). I can't imagine the braks system of any car not
being able to stop the wheels from turing - even in that situation.

 

Couldn't in my 1967 GTO. The engine torque far overpowered any brake
pressure I could place on the brake pedal. Now the car wouldn't actually
move (the front brakes kept it in place)...but it would sure billow plenty
of smoke from the spinning rear tires!

 

And if it had been front wheel drive?
That's the rub with many of todays high powered vehicles. You have
antilock brakes that are made as small as they can get away with to
keep the weight down (and since they have antilock, it is hard to
overwork them anyway) and now we have cars with more horsepower than
the old muscle cars. The power brakes are engine vacuum operated, and
the vacuum goes for a dump when the engine is under load.

So, yes, there are MANY cars on the road today that would have a hard
time restraining the engine with the brakes even well below full
throttle.

 

Isn't more braking power put to the front wheels of a car due to the weight
distribution properties during stoping? I don't know the ratios, though.

How do you figure? Antilock does not help with heat tolerance or
dissipation.

Yes, but you should still have pressure for at least a couple brake presses
stored up in the system - same as if the engine stops while driving.

 

Well, sure, but that was when Deet-riot was still selling cars with 300
horsepower and 9-1/2" drum brakes at all four corners.

 

Yup, the ratios are appx 85/15 for FWD and appx 60/40 for RWD

Correct, there is a vacuum check valve in the booster inlet that
should prevent the vacuum from dumping out when the engine is
under load. It usually takes between 8 and 10 (and sometimes
more) pedal pumps to deplete the stored vacuum in a brake booster
with the engine not running.

 

Well said.
I would also submit that this mysterious sudden application of
throttle is likely only reported in cars with automatic transmissions,
since the resultant crash would probably be avoided if one foot was on
the clutch--i.e. the person backing up (in this case) and experiencing a
sudden racing of the engine would instinctively stomp the clutch and get
the vehicle stopped, perhaps more slowly since some of the pressure is
on the throttle as well as of the brake...you get the idea.

Then the driver would look down and realize what was happening, and
take their foot off the accelerator, hence there'd be no accident to
leave them all dazed, confused, and fully convinced that the "engine
just raced unexpectedly" --since after an ordeal like that many people
are often somewhat traumatized and don't really have a clear idea what
the hell just happened.
Just my 2ç.

BTW I'm new to this forum, very interesting & informative.

 

Horsepower doesn't matter much in this case, it is torque that matters
and only a few cars today have torque ratings above the muscle cars of
the 60s.

I guess it depends on how you define many. I don't think any four
cylinders and probably precious few V-6s can do this. Sure, the large
V-8s probably can generate enough torque to overcome the brakes on the
drive wheels, but I'd have to try it to be sure.

The logic that suggests that few cars can do this is simple. Look at
how long it takes (in time, not distance) to accelerate a car to 60 MPH.
That tells you how fast energy is being put into the motion of the
car. Most cars take 6 or more seconds. Now look at how long it takes
to stop the same car from 60 MPH. It will often be half this time or
less. This tells you that you can remove that same amount of energy
with the braks about twice as fast (or more in most cars) as you can put
it in with the engine. This gives you a rough suggestion that the
brakes are substantially more powerful than the engine.

Now, of course, you have to factor in that the engine is working on
typically only two wheels and thus may be wheel spin limited initially,
but that only applies to cars that are fairly high performance. The
brakes are working on all four wheels, however, mostly on the front due
to weight transfer. Even so, I'll bet that only a few vehicles have
engines with sufficient torque to overcome the brakes on even two
wheels, and certainly won't overcome all four as the Audi proponents
originally claimed.

Keep in mind that most torque convertors stall at less than 2,000 RPM so
you can't consider the engines peak torque, but must look at the
torque available at whatever the stall RPM is for that car's TC. This
will typically be much less than the peak torque.



Matt

 

300 HP not necessarily at the wheels. Even 300HP was exaggerated.

Drum brakes are more efficient (hydraulically speaking) at braking
than disk brakes. Way more surface area too. But more prone to fade
(which does not come into play in the current context).

 

No, but because antilock brakeswork smoother if they don't lock in the
first place, manufacturers tend to install smaller less effective
brakes on cars with antilock as standard.

 

And the torque at the wheels is typically in the range of 9 to 15
times crankshaft torque with a standard transmission, and higher with
a torque converter equipped car.
From a dead stop not many cars can overcome the brakes, but when
attempting to stop at speed, even relatively low speed, and hitting
the accelerator at the same time, it is a bit different story.

 

Huh??? If anything brakes have gotten bigger, not smaller. Especially
since larger wheels are so popular nowadays. You end up with a lot of
room under the rim to put in bigger brakes. Also, the anitlock system
does not give the brakes more braking capacity. If the brakes heat up
to the point they start to fade, anti lock systems are not going to help.

No there are not. You can easily test the theory out. Mash the gas pedal
to the floor. Then mash the brake as hard as you can. I guarantee you
the car will eventually stop.

 

Also, of course, there's a check valve in the brake booster, so even
if you had no vacuum whatever in the intake manifold you'd have
several stops before you lost power assist (not arguing with you,
pointing out another reason you're right).

 

I've yet to drive a car or truck that couldn't stop from any speed up to
60 MPH with WOT. I've never tried faster speeds, not even in my teenage
days.


Matt

 

Both true. I've always found manual drum brakes quite capable. Detroit used
to make some cars with 300 gross hp, but not very many. Not like now! I
think cars today, on average, have more power than they ever had before, on
average. If you look at the big performers, of course it's far more obvious.
Several on the market with 400 hp, NET. Find a muscle car with that.

 

That's because of the "self-energizing" property of drum brakes. The
geometry of the pivot point is designed such that the small amount of
friction applied due to the pedal pressure gets amplified by rotating
the shoe into the drum harder (a multiplication effect, a mechanical
"power brake").

The downside of power brakes, which is a necessity with disk brakes
because they do not have the designed-in mechanical amplification, is
that after one or two pumps of the pedal while under throttle, your
vacuum reserve is depleted, and you essentially have no (or extremely
weak) brakes - this could become critical if a sudden acceleration
situation arises (due to driver error, floor mat jam, or vehicle
controls failure). The mechanics of the drum brake is trotally immune
from that loss of amplification.

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
adddress with the letter 'x')

 

I thought it was because the 100% of the pressure in the brake line is
transfered to pushing both brake shoes into the drum (because the
wheel cylinder is pushing 2 pistons outward towards the drum out of
both sides of the cylinder) vs disk brakes (where half of the pressure
is wasted by trying to force open the calipers and the other half is
used to push the pads into the rotor).

--------------------------------------------------------

A condition I can't imagine happening in the field, unless it's one of
these hypothetical mysterious run-away
full-throttle-while-standing-still cases. Even in that case, you're
not going to be pumping the brakes several times (and depleating the
vacuum reservior) - you're going to plant your foot on the brake pedal
and keep it there. In that case, you're not going to depleat the
vacuum.

 

Joe wrote:

The standardized methods by which horsepower is measured have changed
MANY times over the years, not just the one big change (from SAE gross
to SAE net) in 1972. The actual definition of SAE Net has been revised
many times in the intervening years, as had SAE gross before that. Its
meaningless to compare a 1969 "375 horsepower" 440 to a 2005 "350 HP"
5.7 Hemi, except to run the two cars on the same dynomometer (or weigh
them and run them down the dragstrip and calculate based on ET or trap
speed- a surprisingly consistent and accurate method). Its quite popular
now to dismiss the 60s ratings as over-optimistic, but having driven
both old and new and having seen both old and new run on the same dyno
is a real eye-opener. A lot of the 60s engines were actually
deliberately under-rated because the insurance companies were trying to
avoid covering high-horsepower cars.

 

Bill Putney wrote:

Why do people keep saying this? Disk brakes DO NOT "require" power
assist at all. I much prefer the feel of manual disk brakes to any other
braking system out there. My '69 Dodge currently has stock Kelsey-Hayes
front disks and stock rear-drums, activated by a MANUAL disk brake
master cylinder and a MANUAL pedal linkage. The feel is just wonderful,
and really only slightly higher pedal effort than when it had a power
booster, MC, and pedal setup. There is much more pedal *travel* which
allows finer control over braking with the manual setup. The car stops
on a dime.

And if that streetable example weren't enough, how do you explain the
fact that no NASCAR race cars have a power booster, but they all have
4-wheel DISK brakes???