General disc brake question

[FISHBREATH]

Hey guys,

When considering retro-fitting new disc brakes on an old drum brake car, what is the importance of the number of cylinders/pistons in the calipers? I have heard of benefits of "one is best" and "the more the merrier."

Any opinions?

 

[6pk2goDemon]

I agree with the "one is best" theory. Those calipers are much more readily available including parts. Less to go wrong and easier to rebuild.

The 4 piston KH calipers like they used on the a-bodies are ok if you have power brakes, but they're tough to stop w/ manual brakes. Harder to find parts for.......and more parts needed.

Just my opinion.....

 

[rmchrgr]

Hey guys,

When considering retro-fitting new disc brakes on an old drum brake car, what is the importance of the number of cylinders/pistons in the calipers? I have heard of benefits of "one is best" and "the more the merrier."

Any opinions?

What kind of driving are you planning on doing?

The regular single-piston factory calipers from the '73-up cars work just fine, I have them on my car. If it's any indication, the factory disco'd the KH 4 piston calipers in '72 and went to a single-piston caliper in '73. Less parts to go bad, similar amount of surface area.

If you want to be able to stop and feel like you could be thrown through the windshield, maybe get the multiple-piston calipers and huge rotors. Really though, as long as the rest of the brake system is up to snuff and dialed in, I personally wouldn't think there's much difference between single and multiple for normal driving.

 

[FISHBREATH]

Thanks for the input, guys. It is a '68 'Cuda, soon to have a 440 & 5 speed. I want it for street use, so it will not see a 1/4 mile or road course track, but I want it to stop like a newer car. I noticed that the aftermarket multiple piston brakes are much more expensive than single piston brakes.

How about rotor size for this application? Is bigger better for the street?

 

[Bigmacdak]

Good question! Im fixing to start swapping my front drums for discs' as well. I figure the single will do just fine, both are gonna "lock em up" so to say if you stomp it too hard.

 

[rmchrgr]

Thanks for the input, guys. It is a '68 'Cuda, soon to have a 440 & 5 speed. I want it for street use, so it will not see a 1/4 mile or road course track, but I want it to stop like a newer car. I noticed that the aftermarket multiple piston brakes are much more expensive than single piston brakes.

How about rotor size for this application? Is bigger better for the street?

Larger rotors would offer more friction surface, theoretically allowing more braking force. Would also add more mass though, something you don't need on the front of a big block A-body. As you may know, the 440 'Cudas from the late '60s were notoriously nose-heavy and supposedly handled terribly due to the poor weight distribution.

Whatever pieces you choose for the swap, you'll want to make sure the rear brakes are in good working condition too and that the proportioning from front to back is dialed in correctly. Most of the braking is done by the front brakes, so with the added weight, it might take a little trial and error to prevent lock up. I'd consider new strut rods and bushings also (to handle more braking force) and heavier duty torsion bars with the extra weight up front.

The extra weight will also change the alignment specs, which can affect braking if incorrect.

 

[BrianT]

rmchrgr makes some very good points. It all boils down to how much do you want to spend?

I am building a fun weekend warrior stroker small block car. I am putting on single piston brakes from a 73 dart up front and 11" drums off of a 78 van on back. I am using all new or rebuilt components and that should stop fine.

I have had 9" drum brakes on the front for the last 2 1/2 years and that has made for some interesting driving situations. I can't wait to make the conversion. Plus I get to add new wheels and tires when I convert to the big bolt pattern!!!

 

[Vs29H1B]

Anytime a conversion is contemplated, i.e. drum to disc, there are a number of factors to consider, including what you have readily available to you for the conversion.

My 1971 340 Duster came from the factory with the K-H 4 piston calipers and they work great. Rotors are available for $69 or less, you do have to look though and there are numerous threads in this specific forum with the availability info. The calipers can be rebuilt one time --- stainless steel inserts are available --- or you can just buy the Raybestos kits for rebuilding the piston bores, since these are the same as the early Mustang K-H calipers.

Remember, when you switch to the larger bolt pattern, i.e. the '73-'76 A body setup you get advantages and disadvantages. Advantages = greater availability of rotors at lower prices, same for calipers, pads, lines, bearings, etc. But you also have to factor in the cost to buy new wheels that are of the 4.5" bolt circle. Also a big advantage to the 4.5" is that the number of aftermarket wheels is enormous too.

I like my manual brake K-H setup; it works great and most of the parts are available via mailorder.

 

[FISHBREATH]

Thanks for the info, guys. Good basic knowledge of the systems is the way to begin and I appreciate your input.

It is good to know that the single-piston calipers will perform about as well on the street as the megabuck 4 piston sets. I can now see how gigantic rotors and calipers would help a car on a road course with constant braking from high speed.

Now, all I have to do is figure a plan. I will base everything on a beefed up suspension (T-bars, HD leafs) and 8 3/4 rear (if I can get my hands on one), not to mention stiffer front end components.

Thanks again for the help.

P.S. I hope you don't mind me asking more questions along the way.

 

[mvh]

Tom Condran's book Performance Handling for Classic Mopars is a very complete source of information on brake swaps using Mopar factory parts (as well as many other things).

Then you could also investigate the modern aftermarket kits from Wilwood and Scarebird.

Thanks for the info, guys. Good basic knowledge of the systems is the way to begin and I appreciate your input.

It is good to know that the single-piston calipers will perform about as well on the street as the megabuck 4 piston sets. I can now see how gigantic rotors and calipers would help a car on a road course with constant braking from high speed.

Now, all I have to do is figure a plan. I will base everything on a beefed up suspension (T-bars, HD leafs) and 8 3/4 rear (if I can get my hands on one), not to mention stiffer front end components.

Thanks again for the help.

P.S. I hope you don't mind me asking more questions along the way.

 

[Jim Lusk]

The K-H discs are great, BUT probably the weakest point is the THIN rotor. No way to fix that either. They also don't need power brakes. My son was autocrossing his 383/4sp Barracuda with them until he cracked a wheel center on the home-built 15x10 SBP wheels. That was the only reason we switched to big bolt. Rotor size has nothing to do with clamping force, but with cooling ability.

For what it's worth my younger son's 67 Dart convertible has 73-76 manual discs with a 440 and four speed. I blew the left front tire at 70mph around a sweeping right hand turn on 101 heading into San Luis Obispo a few years back. The car dove left immediately, but I was on the brakes HARD (too much adrenaline to do anything else but lock them up). I was able to keep the car on the roadway until I was going slow enough that the shoulder didn't scare me anymore. The brakes never faded or gave up. The rears are 11x3" drums.

OH, one other thing. The myth about big block a-bodies not being to handle was based on the junk that passed for tires in the 60s. Had no dip or dive in that panic stop.

 

[DionR]

Rotor size has nothing to do with clamping force, but with cooling ability.

And more leverage. That's why many of the super car brake systems have less piston area than the single piston caliper that came from the factory.

Additionally, most multi-piston calipers (depending on design and material) are substantially stiffer and weigh less. A great example would be when Mopar Action ran a first gen Viper caliper on one side of a car and a stock single piston caliper on the other side. The car stopped straight and they saw no pull when the brakes where hammered. The Viper caliper had less piston area, and thus didn't have the clamping force of the stock caliper, but because of flex in the stock caliper they both exerted the same clamping force on the rotor.

 

[mvh]

Hi Jim

Glad you kept the Dart on the road...

I thought I'd point out that Summit Racing now stocks slotted crossdrilled rotors that are a direct replacement for the 67-72 style. So at least you can get better cooling, even if they aren't any thicker. And they are way cheaper than the previously available rotors -- bonus!!

The K-H discs are great, BUT probably the weakest point is the THIN rotor. No way to fix that either. They also don't need power brakes. My son was autocrossing his 383/4sp Barracuda with them until he cracked a wheel center on the home-built 15x10 SBP wheels. That was the only reason we switched to big bolt. Rotor size has nothing to do with clamping force, but with cooling ability.

For what it's worth my younger son's 67 Dart convertible has 73-76 manual discs with a 440 and four speed. I blew the left front tire at 70mph around a sweeping right hand turn on 101 heading into San Luis Obispo a few years back. The car dove left immediately, but I was on the brakes HARD (too much adrenaline to do anything else but lock them up). I was able to keep the car on the roadway until I was going slow enough that the shoulder didn't scare me anymore. The brakes never faded or gave up. The rears are 11x3" drums.

OH, one other thing. The myth about big block a-bodies not being to handle was based on the junk that passed for tires in the 60s. Had no dip or dive in that panic stop.

 

[Vs29H1B]

I saw the slotted, cross-drilled rotors in the Summit catalog for $69 each, before applying the discount code. That's a reasonable price. Does anyone sell the hats that must be pressed onto the rotor? And if available, how much for the hats?

 

[2 Darts]

Hey guys,

When considering retro-fitting new disc brakes on an old drum brake car, what is the importance of the number of cylinders/pistons in the calipers? I have heard of benefits of "one is best" and "the more the merrier."

Any opinions?

The single caliper system is simpler to maintain. When the caliper "floats" it equalizes pressure on both sides of the rotor. When it does not float, typically the inner pad wears out first.

The KH style caliper allows the pads to be removed without removing the caliper. This is why they used to be favored by endurance racers.

The big variables are clamping force, swept area, rotor mass and surface area, and the coefficient of friction of the pad material. Some pad materials are so hard, they need a little heat in them to work optimally.

I don't think running ¼ mile at a time will really stress a properly working disk brake system on a largely street driven car. If you have a terminal speed of greater than 140 mph, then I think it's time to look for something more substantial in the way of brakes.:read2: