There is nothing inherently wrong with drum brakes in the rear. Except they are a PIA to replace. My 97 F150 has rear drums, original shoes after 120K miles. It could be that disks are simply an expense unwarranted in this vehicle, especially considering the extra regen braking afforded to the front.

And it is true that disk brakes do NOT have a return mechanism. They only know how to apply pressure, not retract. Anyone who has ever replaced pads on a disk brake system knows you have to use C clamps to push the pistons back into place in order to put new pads in place, to compensate for the larger "meat" area on new pads.

 

Why would drums be cheaper? From what I can tell the drum brake is far more complicated.

 

In a drum brake it is much easier (cheaper?) to incorporate the parking brake into the existing mechanism.

 

I agree that drums in the back are not a major defect. I'm sure they work fine. However, disk brakes do have a return function built in. The idea that they drag a little all the time is not correct. If even a little constant friction was present, they would wear quickly. They are part of a hydraulic system and when the brake pedal is released, the pads are "sucked" back away from the disk. In addition, I'm pretty sure that, at least on some systems, the retainers double as return springs. Only a very light pressure is required.

The reason the drum brakes have such large return springs is the self actuating design where the brake leading edge is "jammed" into the drum. If you ever tried to stop an old car (without power brakes) going backward down a steep slope, you found the weakness of this system.

The reason you have to compress the new pads is that, as the old pads wore, the system pulled more fluid in from the master cylinder and the "disengaged" position of the caliper is now too close to the disc.

 

When the brake pedal is depressed upwards of 3000PSI of hydraulic pressure is generated to PUSH the brake pads against the rotor. When you release the brake pedal there is only a "relaxation" of the pressure, there is no brake fluid "reverse" flow that would "suck" the pads back off the rotor.

The "high spots" on the rotor serve to drive the pads backward and in some cases the piston seal has a spring which serves as a small amount of "return" force.

Absent any force, hydraulic pressure, applied to the pistons there is NO forcing of the pads against the rotor and thus no wear.

 

Quote from "HowStuffWorks"

"The single-piston floating-caliper disc brake is self-centering and self-adjusting. The caliper is able to slide from side to side so it will move to the center each time the brakes are applied. Also, since there is no spring to pull the pads away from the disc, the pads always stay in light contact with the rotor (the rubber piston seal and any wobble in the rotor may actually pull the pads a small distance away from the rotor). This is important because the pistons in the brakes are much larger in diameter than the ones in the master cylinder. If the brake pistons retracted into their cylinders, it might take several applications of the brake pedal to pump enough fluid into the brake cylinder to engage the brake pads."

 

"This is important..."

No, if the brake pistons retracted farther into the cylinders the pumping volume of the master cylinder would simply be increased.

 

I dunno about that Willard. Every time I have done a brake job and had to press the piston back (to fit the new pad), my first pump on the pedal was to the floor.

 

As risky as it is to disagree with "How Stuff Works", I'm going to. Brake fluid is incompressible. While driving the brake pressure is essentially atmospheric. When the pedal is depressed the master cylinder increases the pressure, When the pedal is released, the system goes back to the original state. If it doesn't, a vacuum is created and either the pad is pulled back or more fluid is sucked in from the master cylinder reservior.

Looking around on the Internet, I found this paper on finite element modeling of a disk brake system (which was submitted to the Society of Automotive Engineers):
http://www.simulia.com/download/solu...c03_delphi.pdf

In the abstract it says:

"The rubber seal performs the dual function of sealing the piston bore
and retracting the caliper piston after a brake apply."

In the standard disk brake system the seal does act as a return spring. If the piston bores become corroded/dirty, this may not be enough and the brakes drag. If you do an internet search you will find many patents for supplementary return spring systems to minimize this drag. My recollection is that some cars have implemented some of these systems and it was advertised as an improvement a few years back, but I can't seem to find that. One of the features of the 2008 Dodge trucks is "reduced drag brake caliphers" but I don't know the details.

 

You can look up and theorize all you want but when you have a disk brake system up in the air on a lift the brakes drag as you cannot get the wheel to spin free like on a drum brake system. At best I have seen a 1/2 turn on a disk system and several turns on almost all drum systems. When I was drag racing we used to remove disc systems and refit with drums if allowed because of the parasitic drag.