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Drum Vs. Disc brakes

So, I have a question about the more effective braking system, several in fact.

So, brake fade, what exactly is that? and do disc brakes eliminate that? I have heard about "spining" the pads of the drums ("breaking" the brakes) under heavy stress. Under excess stress how do disks usualy fail? I understand that disk brakes are exllent for racing sports and suchlike, but if they put more engneering into drum style brakes, would they add up? or is their only so far that style can go?

Comments

  • edited September 2009
    Are you living in the 60s? If drum brakes could match disc brakes, there'd be drum brakes on all modern vehicles. Check out a few modern vehicles. Find any drum brakes? On the front axle?

    I didn't think so.

    Break fade is a measure of how the brakes respond to repeated heavy use, such as down a mountain at speed. If brakes "fade," it means they become less effective with repeated use. Brake fade is a matter of heat build-up within the brakes. Drum brakes will fade sooner than disc brakes, because drum brakes don't shed heat as quickly as disc brakes.

    Never heard of "spining" the pads or "breaking the brakes." Please elaborate.
  • edited September 2009
    Basically, disk brakes work better and work when they are wet. Drums don't. On the other hand, it's hard to mechanically clamp pads to a disk when the car is parked whereas it is easy to pull drum brake shoes into solid contact with the drums using just a cable. That's why many vehicles have drums on the rear and disks on the front -- disks to stop the car, and drums to hold it once it is parked.
  • edited September 2009
    Brakes work by creating friction, the friction converts the energy of motion into heat energy. The breking system needs to dissipate all the heat quickly and effectively to function. Once a braking system is so hot it can't get rid of more heat then it "fades" meaning it can't brake any harder because it is at it's heat capacity. Disc brakes can dissipate a lot more heat faster than drum brakes. Therefore disc brakes don't fade in general.

    The power of a braking system is proportional to the amount of surface area of the "friction material" The more surface area where the pads contact the rotor, or shoes contact the drum, determines how much stopping power you have. Drum brakes have a lot more surface area than disc brakes. Therefore if you need to stop a heavy vehicle faster you can engineer more braking power into the same size wheel with drum brakes. This is why most heavy trucks use drum brakes. The trend to larger wheel sizes in cars is driven by getting more braking capacity on the car using disc brakes.

    Another difference is how the brakes work when wet. Discs brakes are an "open" type system. They get wet easily but the water spins off the disc instantly and the friction of applying the brakes creates heat quickly that burns off any remaining moisture instantly. Therefore wet braking performance is almost the same as dry braking performance.

    Drum brakes don't get wet easily because the shoes are enclosed in the drums. However if you submerge the brakes going through an area of deep water braking power is severely impacted. Once water is in a drum brake it takes a while for the water to drain out. During this time little heat can be dissipated and therefore the braking power is only a fraction of the normal braking power. Eventually they will return to normal but this could take minutes instead of seconds.

    Since large vehicles have bigger wheels they are less likely to get wet. Proper engineering can reduce the brakes exposure to moisture as well. Therefore drum brakes can be excellent systems in suitable applications.

    So if you need more braking power, advantage drums. If you need to reduce brake fade, advantage discs. If you are driving through areas of deep standing water frequently, advantage disc brakes.
  • edited September 2009
    just asking the question, I never was informed of the better performance of the two in a side by side comp.

    What I was saying about spinning the pads is this. Some person takes their drum front 1/2 ton truck and loads it up WAY too much weight (2500-3500 pounds). He's on his way home, at 60mph (dumbass) and a deer runs in front of him. BAM he slams on the brakes, they lock up. With the extra weight pressinf down, it gives too much traction to the tires and they rip the pads loose inside the drum.

    Fact or fiction?
  • edited September 2009
    The real reason drum brakes faded and lost braking when wet was because drum brakes relied on a percentage of self actuation to provide the brake shoe force against the drum.
    100% of the brake pad force on a disk brake comes from the hydraulic pressure on the piston.
    On drum brakes, a lot of the brake shoe force came from the pivot point of the brake shoe.

    When overheating or wetness reduced the brake shoe's coefficient of friction, the effect was compounded by the loss of self actuation.

    Some older race cars had twin trailing shoe drum brakes that don't self actuate. These needed a lot more brake pedal force or power assist but the braking was more linear and the brakes did not completely go away when they overheated.

    The lack of self actuation is also why disk brakes almost always use power boost while many drum brake systems were completely un-boosted.
  • edited September 2009
    My 1953 Chevrolet's normal braking mode was fading, heh, heh. Lousy brakes. Considering the way I drive, I am lucky to be alive today.

    It has been quite a while now since I experienced fading.

    However, several years ago, I did on my 2002 Sienna.

    In Mexico, the only effective way to control driving speeds is to put topes (speed bumps) wherever they absolutely completely have to go slow. In one 51 mile drive from my house to the Sam's Club in Tehuacan, there are 63 topes, more than one a mile.

    Between Tlaxcala and Puebla City, there is a low speed highway, four lane divided. Every half mile or so for much of this highway there is a tope.

    I had driven in from the border in my 2002 Sienna, and I was getting tired of constant topes. So, I started doing what everyone else was doing. Go over the tope, and accelerate full throttle to the next one, and brake heavily at the last possible instant.

    Suddenly, with no warning, I had almost no braking power and actually went over the tope at too high a speed. It has been so many years since I had experienced fading, that it took me a while to understand what had happened.

    That highway would be a darned good brake testing course.

    Need I say I backed off a bit on my speed? It has never happened again.
  • edited September 2009
    I have two vehicles that have factory disk/drum combination on the rear. The hydraulic brakes operate the disk brake portion, and the parking brake system works through the much smaller drum. This gives me a very reliable rear brake system.
  • edited September 2009
    Never heard that one before. I'd say the most likely outcome in the scenario you propose is the brake shoes (pads go with disc brakes, shoes go with drum brakes) would overheat and not provide sufficient stopping power.

    If the brakes were applied hard enough to lock the wheels the tires would skid and the brake shoes would suffer no damage at all. Once you slam on the brakes the weight in the truck tries to move forward (momentum) and does not push down.

    The weight will be transferred to the front brakes, which do most of the work anyway, but it might not be enough to prevent them from locking. Either way, the brake shoes will not be torn off their backing plates.

    If you don't overload the vehicle you really shouldn't have anything to worry about, regardless of the brake type. Drum brakes are still used in lots of heavy-duty applications.
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