Are rear disc brakes worth the time and effort?

[Back N Black]

I have 69 charger with A body conversion front disc brakes and stock drum brakes in the rear. I was thinking about installing rear disc over the winter, is there a big improvement over the drum brakes?

[b5blue]

Only if you race. 

[Kern Dog]

When I added the Dr Diff 11.7" rear discs, there was no improvement at all other than the appearance.

[birdsandbees]

Save your money for good fuel....

[John_Kunkel]

When I added the Dr Diff 11.7" rear discs, there was no improvement at all other than the appearance.

Was there a factory proportioning valve feeding the rear discs?

[Kern Dog]

I have learned from my mistakes....at least from some of them.
I had a factory "A" body disc-drum proportioning valve in place then. Much later, I switched to a drum-drum distribution block. Braking did improve a little but it still wasn't great.
Online forums are helpful but you just don't know if the advice you're getting is legitimate.
One example....Advice on brakes when deviating from a factory setup.
I'm learning that caliper bore sizes and the differences between front sizes and rear really do matter.
I ran 12" Cordoba front rotors and 2.75" calipers and the Dr Diff 11.7" rear rotors and 1.5" caliper pistons. This compares to a factory front disc setup with a 15/16" rear wheel cylinder. I'm thinking that the larger bore size of the rear caliper requires more fluid volume to move. This leads me to think that a rear disc setup may require more pressure and volume to see the benefit of it.
I'm leapfrogging my setup though....13" Cobra front discs and a hydroboost unit are going in.

[375instroke]

Only if you race. 

I say the opposite.  The harder you are on the brakes, the less the rear matter.  We burn through a set of pads at about 28 hours of racing, and rotors at 36 hours, but the rear discs still look good after half a dozen races, and that's with the rear set so the front lock up just before the rear.

[b5blue]

They are just not needed regardless.   

[Mopar Nut]

Unless you have a set up like this. 

[b5blue]

Cool! Emergency brake also?

[Kern Dog]

The term is "Parking brake".
The term "emergency brake" is incorrect. 
In many cases, the second caliper is for drift cars.

[b5blue]

"Bullshit" 100% loss of brake system is an "Emergency" so call it whatever, you know what I meant.

[Mopar Nut]

Cool! Emergency brake also?

Yes.

[Kern Dog]

"Bullshit" 100% loss of brake system is an "Emergency" so call it whatever, you know what I meant.

That is not why the parking brake was designed and installed. It is to hold a vehicle in place while it is parked. Those that call it an emergency brake are incorrect.

[birdsandbees]

All I know is that I wish I had set it !! 

[RTSE440]

I tend to agree with kerndog it's a park brake used when parking but just to throw a spanner in the works in australia we call it a hand brake

[AKcharger]

NO

[b5blue]

All I asked was did that dual caliper disk setup "HAVE an emergency brake?" The name of a stock anything has nothing to do with "rear disk brakes."   

[375instroke]

"Bullshit" 100% loss of brake system is an "Emergency" so call it whatever, you know what I meant.

That is not why the parking brake was designed and installed. It is to hold a vehicle in place while it is parked. Those that call it an emergency brake are incorrect.

One can use the parking brake in an emergency.  Holding the car when parked is normal use of the parking brake.  Unfortunately, one can't use the park detent of the transmission in an emergency.

[b5blue]

All I asked was did that dual caliper disk setup "HAVE an emergency brake?"

[Kern Dog]

All I asked was did that dual caliper disk setup "HAVE an emergency brake?"

[b5blue]

So no? 

[Kern Dog]

To revisit this old topic.....
Here is what I have learned recently:
For a brake system to function properly, the components need to all work together. Think of the brake system like you would an engine.
An 8 to 1 engine with a huge cam would be like a car that has huge discs up front, small drums in the rear and very skinny tires. The system won't work as well as it could if the parts don't compliment each other.
To elaborate:
In the Summer of last year, I saw a video on YouTube where a guy in a '64 Mercury Comet took another guy on a test drive despite the car having a stuck throttle. The brakes overheated and the guy crashed into some innocent lady in a minivan but lucky for her, she lived through it all.
He had a stuck throttle and was riding the brakes to keep speeds down. His car was capable of some ungodly power level but he had tiny front disc brakes.
THIS got me thinking about focusing on making some improvements.
I read Andy Finkbieners book on B body upgrades and was intrigued by his segment on brakes, He stated that an optimal 4 wheel disc brake street car should have a 2 to 1 brake bias front to rear. In short, the front caliper surface area should be twice the number of the rear. If you deviate too far from that number, the bias will be off and the car will not stop as fast. I had 12 inch rotors and 2.75" front calipers, 11.7" rotors and 1.5" rear calipers. This was a 3.3 to 1 ratio. This meant that the fronts were taking on more load and the rears were almost doing nothing.
I looked to Dr Diff. He offers a 13" front disc kit with twin 1.59" bore calipers. This, along with my existing rear setup was a factory arrangement for some 1994-2001 Mustangs and had a 2.2 to 1 ratio.
It still wasn't quite right though.
It wasn't until I swapped in a reproduction 1966-70 B body brake booster that I was able to realize the actual effect of these brakes. From 2001 until a few weeks ago, I had a 1975 single diaphragm A body booster in the car. It worked fine with the disc/drum arrangement but didn't provide enough boost for 4 wheel discs.
I am ecstatic about how the car stops now. I was able to skid the front tires for the first time in 15 years or more. To me, if you are unable to get the tires to skid, your brakes are not working hard enough. I wanted to be able to skid, then make it a habit to know where that threshold is and stay just below it.

[Mike DC]

 
Big disc brakes require big power assist.  Our classics never had very powerful OEM brake boosters because the original brakes weren't that potent.  Hence the popularity of hydro boost retrofits.


It definitely matters about the 'native' proportioning between the front & rear.  The prop valve is more like a fine-tuning pass and it has a bigger effect when you first jam on the brakes than when you hold the pedal down steady.  The baseline ratio between the F/R calipers still needs to be in the ballpark. 

 
As for whether you need rear discs at all . . .
It's a sports car thing.  Sports cars deal in higher speeds, their suspensions hold them flatter when they brake, They do more braking per mile, etc.  The combination of that stuff leads to wanting more brake muscle in the rear.  The average commuter rides around in a soft floppy chassis and he only uses the brakes when he sees taillights lighting up in front of him. 

Discs do give you more brake feel than drums, all else being equal.  There is that.

I like rear discs partially just out of laziness. Pads & rotors are easier to change than drums & shoes. 

[Kern Dog]

I absolutely agree that 4 wheel disc systems need some manner of assist. I tried several different master cylinders with a manual disc setup and NONE of them felt right or stopped the car safely. NONE.

I don't quite agree with the comment about the stock booster being inadequate for a 4 wheel disc system though. It took me a long time to get here but I'm finally satisfied with my brakes. They feel linear, they have proper travel and they finally have enough force to skid. I couldn't skid with a 73-76 A body single diaphragm booster, one that I've had on the car since 2001. The swap to the OEM style 1966-70 B body dual diaphragm booster made ALL the difference.

Hydroboost is an excellent idea if you can make it work. Some people worry that you're depending entirely on a power steering pump and a belt to steer and stop the car. I wasn't worried about that....a stalled car has no vacuum and isn't driving the steering pump either.