Simultaneous Application of Gas and Brake Pedals

I don't like hydroboosts, for the simple reason that it's a doglick,
failure-prone, needlessly-complex system that gives rotten pedal feel.

 

A simple machine design textbook proves you're right nate. The self
energizing feature is the advantage of drums.

 

True, but requiring more line pressure isn't the same thing as requiring
power assist. This is only true if the additional line pressure
required increases the pedal effort so much that a typical human hasn't
enough leg power to make them perform adequately. I don't think this is
the case across the board with disk brakes. I think that is the other
person's point.


Matt

 

True, but a lot of them involve tradeoffs, usually in the "stroke" of
the master cylinder. A smaller bore in the MC will provide good pedal
feel and acceptable pedal effort, BUT is not often workable in modern
braking systems as the pedal will eventually "bottom out" on a
full-effort, high-speed, ABS-active stop - definitely something you
don't want to happen. Likewise, there may not be enough room to leave
enough pedal left in a one-circuit-failed condition. Yes, it *can*
work, but there's plenty of reasons why it doesn't.

That said, in a light car (<2500 lbs.) I agree that a power booster can
probably be optional, although you probably won't see it with ABS for
reasons given above. Additionally, a litigation-sensitive corporation
isn't going to release a vehicle for production requiring *any*
higher-than-normal pedal effort unless it's a specialty model that
they're fairly confident is going to be bought entirely by enthusiasts.

nate

 

That's an honest question. So you are telling me that, in the otherwise
same vehicle, a non-vacuum assisted disc brake will take no more pedal
pressure and at the same time no more pedal travel than a non-vacuum
assisted self-energizing drum brake? If you answer "yes", then I will
defer to your experience and knowledge, though I was sure that the disc
brake in that example would require either a lot more pedal pressure or
a lot more pedal travel or a combination (again, depending on the games
played with the master cylinder design).

What say you?

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

 

You've engineered more brake systems than I have, to be sure, but I'm not
sure I believe that either of these must necessarily be the case. You
might be able to convince me.

Sued for installing airbags
Sued for NOT installing airbags

=

Sued 'cause the booster fails
Sued 'cause the pedal effort was "too high"

Remember, there are Federal standards for pedal effort.

DS

 

Actually, not really. I never engineered a brake system from scratch,
unless you count mixing amd matching parts on my own personal vehicles.
But I have seen what happens when you get something a little off, like
the MC bore size and your 70 MPH panic stop turns out to be a little
more interesting than you planned on when the ABS' hydraulic
accumulators suck all the fluid out of the MC. Whoopsie!

Keep in mind, however, that most of the stuff I worked on professionally
was light trucks - these issues may not actually surface for passenger
cars. The point that I was trying to make is, simply, that there really
are good engineering reasons to use a power booster, and that they
become more compelling when using disc brakes as opposed to drums.

Yeah... really, it boils down to how much room do you have between the
top of the pedal stroke and the floor of the car (don't want to have the
pedal too high, that would be bad ergonomically - you should be able to
get your foot on it without taking your heel off the floor) and can you
juggle your bore sizes and/or linkage to get a reasonable (or Federally
mandated) pedal effort within those parameters while not having the
pedal go to the floor either with one circuit failed or during a full
ABS stop from whatever your maximum design speed is. If you can't, then
you go to a power booster. I'm sure that someone could probably give a
rough, hand-waving estimate (like I have above, but a more informed one)
as to appx. what vehicle weight that starts to happen at... but the
point is, that weight would be significantly higher for a drum brake car
because a) they require less fluid movement (or should, if the shoes are
properly adjusted - since they have positive return springs you really
can't count on that though unless self adjusters are included) and b)
they require less line pressure.

nate

 

And you still haven't. 8^) I was told that the trannies used in AMC
vehicles (in my terminiology "AMC trannies") were what was used in the
subject vehicle. At least two people reading did not take that to mean
that AMC built those trannies. I wasn't intentionally being ambiguous,
but I figured that anyone who knew the score would take it to mean what
I had intended.

So let's see. To apply this rule, no one can ever refer to a wheel that
fits Chevrolet vehicles (as well as certain other brands) as a
"Chevrolet wheel", or a wheel that fits a Chrysler vehicle (as well as
certain other brands) as a "Chrysler wheel"? Is that what we're saying
here?

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

 

One more thing I meant to add...

yet another reason why the "my '67 whatever had manual brakes and
stopped just fine" does not translate over to a similar weight modern
car *not* requiring a power booster... I've been told, although I have
no hard numbers to back this up, that old asbestos brake linings
actually have a higher coefficient of friction than most modern linings.
I don't have enough experience to make a definitive statement on this
but I will say that my '62 Stude with stock brake linings (still
original, only 24K miles give or take a few) stops just fine without the
power booster. However, I've heard people with near identical cars
switching over to power because they claimed that the brakes required
"too much effort." I suspect that that is because the brake shoes have
been replaced with modern linings that work, but require more pressure
to work *well.*

I can tell you that trying to stop a '64 Stude with *disc* brakes and a
failed power booster is a firewall-bending exercise, but that's not
entirely relevant to this discussion, other than the single data point
that Studebaker apparently felt it necessary to mandate a power booster
for all disc brake cars, for whatever reason.

Finally, stainless braided brake hoses are badass. This has *no*
relevance whatsoever to this discussion, but I just felt like throwing
that out there. I just replaced the front hoses on my 944 yesterday as
the originals were well past their "best before" date (and the stainless
ones were half the price of factory, even better) and the pedal feel is
astonishingly good - and I haven't done the rears yet! Now the sludge
that came out of the clutch slave when I bled it... that wasn't so
badass. Hope that's not a sign of an impending future repair, 'cause it
looks like a royal PITA, unless you pull the starter first. Why Porsche
in its infinite wisdom didn't put the starter and the clutch slave on
opposite sides of the bellhousing is beyond my ken...

nate

 

So you would *never* refer to certain wheels in a junk yard as
"Chevrolet wheels" or "Ford wheels". You would first find out who
manufactured the wheels and reference them that way. Or how about cell
phone chargers? If I wanted to buy a charger for my Motorola cell
phone, I would walk into a store and ask for a Kikosuki charger because
they are the company who made it, and the store owner would immediately
know it was for a Motorola phone even thought Kikosuki makes chargers
for 3 other brands of cell phones. Let's get real.

Well - I agree with you on the under-engineered part - except for that
tranny. It was ridiculous to put that tranny in that heavy a vehicle
with no cooler. The proof in that pudding was having to replace the
fluid every 20k miles in a time when no one ever changed ATF.

The over-engineered aspect of the IH vehicles was also their demise -
their trucks and Travelalls could not compete with the lighter vehicles.
Nor could the Scout compete witht he lighter more nimble competition.
Not saying over-engineering is necessarily bad, just not competitive
in a very competitive consumer market.

BTW - I did things to that Travelall that would have totally done in
many other vehciles, so I hear you on the over-engineered thing. It
was heavier than anything. IH dealers used to brag that "our engines
are 600 pounds heavier than the equivalent Ford engine!" And that's
what did them in. Same with the 1942 Gravely tractor that I used to cut
the grass while growing up.

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

 

Ignoring the fact that I never said they were under-engineered, though I
did point out that they put an automotive tranny in a tank with no
cooler on it that needed fluid replacement every 20k miles. Yeah - I
would call that particular feature under-engineered, though I had not
called it that. Grossly under-engineered here, grossly over-engineered
everywhere else - maybe the correct term for them would be "very
unbalanced" (which is by definition "not competitive" as time proved).

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

 

My dad had a whole fleet of their trucks in his mechanical contracting
business. One small pickup truck had a four cylinder engine. It was
obvious that they had taken the mold for casting a 304 CI V-8 and
blocked off one whole side of the mold to create the 4-banger. We used
to laugh at that.

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

 

There you are. Throw in your intial tolerance
stackups/vehicle-to-vehicle variations on all that stuff, and then add
the variations as things age, and you have no more design safety margins
left, and you're eaten up with recalls and bad publicity.

So can anyone say how many models of mass-produced passenger vehicles
sold in the U.S. in the last 10 years have disc brakes without boost of
some sort? Is it more than zero?

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

 

Yeah, eh? I try to avoid it, and I figure I'm about 80% successful.
Nowtimes, I usually just killfile jokers who spoil for a credentials
pissing match. "KaWALLski" doesn't really need to know why it's so
laughable for him to try and argue headlamps with me, and "Loathesome
Pawn" doesn't really need to know that I'm an appointed member of the
National Academy of Sciences Transportation Research Board (Oops! Oh well,
add that to the 20%!)

-DS (...he also doesn't need to know he misspelled "loathsome"...)

 

Sure you would. But you wouldn't refer to an "AMC starter" (it's a Delco)
or an "AMC alternator" (it's a Motorola) or an "AMC carburetor" (it's a
Carter or a Holley) or an "AMC transmission" (it's a Hydramatic or a
Torqueflite).

You *would*, however, refer to an "AMC engine".

I'm sure you can understand the difference. The question is whether you'll
choose to admit it.

 

Well, you'll have to tell the guy who was knowledgeable enough to know
the different applications and conveyed that to me - except he's been
dead probably 15 or 20 years. anbd which I understood what he meant -
just as people here understood what was meant even though it was not
quite up to the Stern standard for truth and excellence.

So is it because wheels are round and transmissions and alternators are
irregularly shaped that there's a difference in how they are referred
to? Just trying to figure out the rules. 8^)

Truth is Daniel, knowledgeable people all the time refer to such devices
by application even though technically it's not up to your standards,
and people all the time understand the information that is being
conveyed (which is the goal most of the time. No different I guess in
correcting someone when they refer to a "lash adjuster" as a lifter, or
talking about torque in pounds, both of which I joke about all the time.
Just depends on how anal we all want to be.

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

 

Hmmm - speaking of being anal - looking back at what I just posted - was
a time I would have been mortified at the typos I made in that
masterpiece. Oh well. In too much of a hurry. (and look at that - two
incomplete sentences in a row! Wuff!

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

 

I'll agree to an extent on disks needing more LINE PRESSURE. But there
are ways to get line pressure without "needing" power assist.

And as for the amount of extra line pressure needed, I think that is
minimized by the fact that disk calipers typically have a much larger
piston area than do similar capacity drum brakes, which is part of the
reason that the drums need to be self-energizing.

 

My comment was in reference to a '69 (so it does have a split braking
system just like a modern car- which I can also see making quite a
difference). I should also point out that it performs just fine with
*modern* lining materials- I'm not anal enough about it to dig up NOS
brake linings from 1969! And besides, I like to DRIVE it not just have a
trailer queen, which is what happens if you insist on NOS service items
like brake linings, belts, and hoses.

 

Not at all- I'm saying that it will take MORE pedal travel, but that
more pedal travel is a GOOD thing because it allows finer modulation of
braking pressure. The pedal effort will depend on the amount of increase
in the pedal stroke, and will typically be a little higher than a
non-boosted system. But I think its quite safe to say that there are
non-boosted cars out there that have less pedal effort than some boosted
cars. There's a lot of overlap. I'll say again that people equate "power
brakes with a failed booster" to "non-power brakes" and that's just flat
WRONG. A failed booster makes pedal effort FAR higher than non-power brakes.