1968 4 piston front disc brakes

[gt289]

Stopping power is a function of pressure applied, and the coefficient of friction. Increase either one of those things and you'll increase stopping power.

Now it's a correct statement. Swept area, contrary to popular opinion, does not have a bearing
on stopping power. (period, end of sentence)

Brake pad geometry (height, width, surface area) does not influence the gain in a brake system.
(gain relates brake system force input to brake system force output)
Bigger pads do not stop any better than smaller pads.


Don't believe it? As Randall Shafer (StopTech) likes to say "pad material coefficient of friction
is independent of the size of the pad itself. Installing larger pads may increase the life of the
friction material, but there will be no difference in brake system gain." (page 91, "High Performance
Brake Systems" by James Walker Jr.)

The reason why drum brake systems stop so well is not a function of their large swept area
(when compared to most disc setups), it's mainly because most drum systems are self-
energizing, so you actually get mechanical assist that is not present in a manual disc setup.
The drum system's downfall is in hard, back-to-back repetitive stop situations, where thermal
issues become a really big problem, really fast.

Before I worked in the brake industry, I used to think swept area was king, myself. I got
schooled pretty quickly, much to my chagrin (and embarrassment).

 

[AtlantaSteve]

gt289, thank you for explaining that.

I have heard time and again about changing pad sizes to affect stopping force and could never figure out the math of how that would work. In my physics classes we always calculated friction as being a function the coefficient of friction and the force applied, but surface contact area never was part of the equation.

It was a law of the universe.

And then I got into cars and would hear of people cutting down pads to make them stop better and was like "Is physics wrong? Or does it take a break in a braking system?"

But it was such common knowledge that I assumed there were additional variables that I didn't understand.

That said, I'm sure there will be disagreement. I believe this might be as tightly held a belief as the notion that .99999... Does not equal 1....Mathematically, they are identical, but people refuse to accept this, and will fight you over it.

 

[Laurie S.]

While we're on this subject, I just removed the 1968 spindles and 1967 4-piston disc brakes from Trouble and replaced them with Baer brakes. I want to sell these but am wondering what they are worth? They come with stainless brake hoses, also, and there is probably less than 5 miles of use on them.

 

[Starfury]

Hrm. Interesting.

 

[SAC69]

So, if sweep area is irrelevant, (if I am understanding the logic here), it does not make sense for someone to upgrade the rear drumns from 1 3/4" to 2 1/2" size?

 

[gt289]

A direct swap, like Mustang 10x1.75 or 10x2 to a Econoline 10x2.5 will mainly yield increased shoe
longevity. There is some increase in braking because the "wedge" is more effective.
(self-energizing "wedge"- the brake’s anchor pin acts as a brake shoe stop and prohibits the brake
shoe from its tendency to follow the movement of the rotating drum. This results in a wedging
action between the brake shoe and brake drum. This wedging action combined with
the applied brake force creates a self-multiplied brake force.)
It will be a slight difference.
For greater effect, increase the rear wheel cylinder size to 1" or 1 1/16. That'll get your attention
pretty quickly. Small changes in bore sizes = massive changes in force. (inverse square law)
The Shelby's all ran an improved rear shoe compound. Velvetouch semi-met or something like that-
large segment blocks in the shoes too to control cracking during heat cycling.
A change in Mu (coefficient of friction) like that is all that's really needed in the already light
Mustang rear end & the increase in shoe & DRUM size tends to greatly improve thermal issues.
(My wife's '68 runs the 10x2.5 stuff in the rear, 1" wheel cyls and Praise DynoBrake shoes. It's
a good match for the Boss 302 Trans Am front brakes she has.....)
If you really want the effect, use the 11" rear stuff like Cobra Automotive recommends- even
more wedge effect, etc.

 

[SAC69]

Thanks for the insight. I'm about to redo my rear brakes, where could I get 1" wheel cylinders? Is there any adaptive issue if they are integrated into the existing stock setup?

 

[gt289]

Shouldn't be any problems. It does get interesting when you get above 1 1/16" because not
a lot of vehicles came with wheel cylinders that big that'll fit the rear backing plates in an original
manner.
I hit up Autozone for a Bendix 1" rear wheel cylinder, part number 33636, $9.99 ea plus tax.
(I think it's also Wagner F78978 and Raybestos WC37271, but don't quote me on those)
My notes are sketchy but I think '76 LTD is the application.....
Going to the 1" rear wheel cylinders gets you another 184# of force against the shoe vs the
stock 15/16" cylinder.
You may have to dial out some rear bias with a proportioning valve, that's a big jump up in force.

 

[SAC69]

Thanks for the good info.

 

[sigtauenus]

If its a SSBC kit then its the KH calipers and bracket like what was used on 65-67. I had a kit like that on my car before I switched spindles and put single piston calipers on.