Simultaneous Application of Gas and Brake Pedals

On Fri, 28 Jan 2005, Steve wrote:

> Bill Putney wrote:
>
> > The downside of power brakes, which is a necessity with disk brakes
> > because they do not have the designed-in mechanical amplification,
>
> Why do people keep saying this?

The parrot effect, I'm guessing.

> Disk brakes DO NOT "require" power assist at all.

You are correct. They don't.

> I much prefer the feel of manual disk brakes to any other
> braking system out there.

Well, to be perfectly semantic about it, the only "manual brakes" out
there are the ones on vehicles specially modified for the handicapped. But
yes, I agree with you, a properly set up and dialled-in unboosted disc
system cannot be beaten in terms of pedal feel.

> 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.

Unboosted discs are terrific in slick winter conditions, too. We're
looking at deleting the brake booster from my '92 Spirit R/T clone. It'll
doubtless take a different master cylinder, but there's an enormous
variety of MCs that'll fit.

I think the mistake most people make is in thinking that power brakes
without boost are the same as unboosted brakes. They're very definitely
not. The mechanical advantage of the brake pedal over the cylinder is much
greater in an unboosted setup than it is in a power setup.

DS

Daniel J. Stern wrote:
> On Fri, 28 Jan 2005, Steve wrote:
>
> > Bill Putney wrote:
> >
> > > The downside of power brakes, which is a necessity with disk
brakes
> > > because they do not have the designed-in mechanical
amplification,
> >
> > Why do people keep saying this?
>
> The parrot effect, I'm guessing.

Probably because the vast majority of disc brake cars do have power
assist - dating back to the first installation of modern discs on
American passenger cars in the '63 Studebaker models, where the disc
brake package came with a mandatory power booster.

>
> > Disk brakes DO NOT "require" power assist at all.
>
> You are correct. They don't.
>
> > I much prefer the feel of manual disk brakes to any other
> > braking system out there.
>
> Well, to be perfectly semantic about it, the only "manual brakes" out
> there are the ones on vehicles specially modified for the
handicapped. But
> yes, I agree with you, a properly set up and dialled-in unboosted
disc
> system cannot be beaten in terms of pedal feel.
>
> > 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.
>
> Unboosted discs are terrific in slick winter conditions, too. We're
> looking at deleting the brake booster from my '92 Spirit R/T clone.
It'll
> doubtless take a different master cylinder, but there's an enormous
> variety of MCs that'll fit.
>
> I think the mistake most people make is in thinking that power brakes
> without boost are the same as unboosted brakes. They're very
definitely
> not. The mechanical advantage of the brake pedal over the cylinder is
much
> greater in an unboosted setup than it is in a power setup.
>
> DS

True, although with a reasonably heavy car it does become almost a
necessity to have the power booster because you are trading off pedal
travel against line pressure, and both need to be kept at a reasonable
level.

Personally the only car I've driven with unboosted discs was my '71
914/4, and the feel is spectacular...

nate

Daniel J. Stern wrote:

>
> I think the mistake most people make is in thinking that power brakes
> without boost are the same as unboosted brakes. They're very definitely
> not. The mechanical advantage of the brake pedal over the cylinder is much
> greater in an unboosted setup than it is in a power setup.
>
> DS

Absolutely, the pedal LINKAGE is often different in addition to a
different bore size in the master cylinder when comparing power brake
and non-power systems for the same model car. In the case of Mopar
B-bodies like my '69, the power brake cars had a bellcrank mechanism
that moves the master cylinder piston FASTER than the pedal moves. The
loss of leverage is more than offset by the power booster... UNTIL the
booster quits working and mashing the brake pedal is like stepping on a
block of lead. The manual brake pedal linkage is a direct connection
from the pedal to the MC piston and has much more mechanical advantage,
at the cost of more pedal travel. All other power brake cars have a
similar mechanism for reducing pedal travel, or simply attach the
pushrod closer to the pedal rather than closer to the pedal hinge
(another way of changing leverage)I LIKE the added pedal travel- for one
thing it makes "sudden acceleration" a lot less likely (just to tie two
threads in a knot... ) ;-p

Daniel J. Stern wrote:

> On Fri, 28 Jan 2005, Steve wrote:
>
>
>>Bill Putney wrote:
>>
>>
>>>The downside of power brakes, which is a necessity with disk brakes
>>>because they do not have the designed-in mechanical amplification,
>>
>>Why do people keep saying this?
>
>
> The parrot effect, I'm guessing.

No. The reality of modern consumer vehicles that will be driven by
quite a range of ages, mental quickness, and physical strength.

I used to drive an International Travelall (similar in size to a Chevy
Suburban). Unfortunately the mfgr. figured it didn't need power
steering - but, man, you should have tried to parallel park that thing -
quit4e a feat even for a teenager. I think there would be similar
problems selling a modern vehicle with unpowered disk brakes as selling
ones without power steering just due to human factors.

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

On Fri, 28 Jan 2005, Bill Putney wrote:

> >>>The downside of power brakes, which is a necessity with disk brakes
> >>>because they do not have the designed-in mechanical amplification,

> >>Why do people keep saying this?

> > The parrot effect, I'm guessing.

> No.

Rawk! Yes.

> The reality of modern consumer vehicles that will be driven by
> quite a range of ages, mental quickness, and physical strength.

Right, 'cause women and little old men *never* drove before what you
arbitrarily consider the "modern" age. Pfft.

> I used to drive an International Travelall (similar in size to a Chevy
> Suburban). Unfortunately the mfgr. figured it didn't need power
> steering

No, the original owner decided it didn't need power steering. In any
event, that's irrelevant to the topic at hand, which deals with *brakes*.

> I think

Not hard enough, as it seems.

> I think there would be similar
>problems selling a modern vehicle with unpowered disk brakes as selling
>ones without power steering just due to human factors.

My 1946 Cessna had completely reliable, effective, simple, light, cheap,
unboosted hydraulic disc brakes.

My understanding is that a major problem with disc brakes is that they don't
work well as automotive parking brakes. Discs tend to loosen as they cool
(exactly what nobody wants) while drum brakes tend to tighten as they cool.
More better.
Quent

On Sat, 29 Jan 2005, QDurham wrote:

> My understanding is that a major problem with disc brakes is that they
> don't work well as automotive parking brakes.

This has been solved for decades: A small drum brake exclusively for
parking brake usage in the "hat" section of one or both sets of discs.

>> My understanding is that a major problem with disc brakes is that they
>> don't work well as automotive parking brakes.
>
>This has been solved for decades: A small drum brake exclusively for
>parking brake usage in the "hat" section of one or both sets of discs.
>
Exactly. Problem solved -- at some minor complication. I was told that Jag
solved this problem by "twisting/warping" the disc brakes. Thoughts?

Quent
>
>
>
>
>

> I used to drive an International Travelall (similar in size to a Chevy
> Suburban). Unfortunately the mfgr. figured it didn't need power
> steering - but, man, you should have tried to parallel park that thing -
> quit4e a feat even for a teenager. I think there would be similar
> problems selling a modern vehicle with unpowered disk brakes as selling
> ones without power steering just due to human factors.

I'd have paid extra to get my car without power steering, but that wasn't
even an available option. Power steering is great until your engine
stalls in the middle of an intersection; then even a wrestler will have
trouble avoiding a wreck.

Daniel J. Stern wrote:

> On Fri, 28 Jan 2005, Bill Putney wrote:
>
>
>>The reality of modern consumer vehicles that will be driven by
>>quite a range of ages, mental quickness, and physical strength.
>
>
> Right, 'cause women and little old men *never* drove before what you
> arbitrarily consider the "modern" age. Pfft.

Oh - I'm sorry - I thought we were living in the present. How silly of
me to exclude cars from 20 and 30 years ago from the here and now.

>>I used to drive an International Travelall (similar in size to a Chevy
>>Suburban). Unfortunately the mfgr. figured it didn't need power
>>steering
>
>
> No, the original owner decided it didn't need power steering. In any
> event, that's irrelevant to the topic at hand, which deals with *brakes*.

No - it was a poorly designed standard package that also included an AMC
automatic transmission with no external cooling - fluid needed changing
every 20,000 miles - you knew it needed it when the tranny started
slipping. They quit making them for a reason.

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