Due to the proliferation of disc-type brakes on passenger vehicles, I must assume that they are superior to drum-type brakes.
But I can’t figger out why, because by my calculations (which, as in any time I and math cross, should be double-checked) drum brakes have more surface area. Logically, more surface area = more friction, and more friction = more drag = faster stop.
Brake drums might run from 9.5 to 11 inches inside diameter, with a common width of two inches. Using the smaller size, a 9.5" drum has an inside circumference of 29.8". Allowing a couple of inches each side for the joint and spread, we assume a 25 inch frictionable circumference x 2 inch width to get 50 square inches of friction.
Brake pads might measure 5 x 2.5 inches. They are irregularly-shaped, but I’m both too lazy and too stupid to take that into account, so I’ll err in favour of the pads: 5w x 2.5h = 12.5" x 2pads = 25 square inches of friction.
So, unless my assumptions are way off or my calculations are screwy, drum brakes offer twice the friction area of disc brakes.
Is there some other reason that makes discs preferable to drums?
I kind of thought the advantage of disc brakes was that they were less likely to lock up and cause a skid. Perhaps the greater area of a drum brake cause it to grip too effectively, causing the wheels to quit rotating more quickly, breaking them free from the pavement, and you know the rest. This does seem like it would make disc brakes heat up more quickly, though, especially if they have a lot less area to disperse the heat. Hmmm…
Good brakes can be made either way but disks have some advantages. They can be lighter for the same surface area and heat expansion won’t cause fade as it can with a drum. A big disadvantage is that it’s difficult to design a mechanical parking brake into a disk. Subaru has this but it;s an exception. BMW uses a tiny drum inside the disk but it sin’t very positive.
Disc brakes are a simpler design than drum brakes. They use less parts and are easier to set up. This makes them cheaper to produce and reduces the risk and cost of under-warranty repairs.
Thinking about your comparison of surface area, remember that you’re assuming that the brake shoes have the same radius as the drums. Typically, shoes have a smaller radius. Only a few people radius their brake shoes, and I doubt you could find a mechanic who does it as part of their regular brake job.
Brake discs have more exposed surface area, meaning that they (will probably) cool more easily. So, even if a drum brake might stop as effectively as a disc on the first application, it probably will fade more quickly.
One good example of this advantage is that Jaguar dominated some forms of racing in the early sixties because their cars used disc brakes and could go deeper into turns before braking.
The simplicity of disc brakes means that they can also reduce unsprung weight. I doubt that this actually improves handling much on vehicles run on the street, but it has the reputation of improving performance and is desired as a result.
They have a reputation as being the superior type of brake. Hence, they are marketed as such.
If you don’t mind, let me turn the question. If disc brakes are superior, why do heavy trucks and semis use drums?
Actually that’s a common misconception. Friction is not dependent on surface area. It is only dependent on the force applied to the surface. For example, if you have a matchbox on a table, the force needed to push it across the table is the same regardless of whether the box is sitting flat on the table or on its end.
Of course, this is one of those idealized “laws” you learn in high school physics and doesn’t apply to sticky surfaces like rubber, for instance. But brake pads aren’t sticky so it should be a good approximation.
Note that surface area is relevant. The operative model here is the tranference of kinetic energy to heat, which will be proportional to the coefficient of friction and the surface area for energy transfer.
Time flies like an arrow. Fruit flies like a banana.
One other thing. Even assuming that discs and drums worked equelly well, discs have the advantage because they are much less complex. This means there is less to go wrong. It is also LOTS easier to change the pads on discs. I HATE changing drums, it’s usually worth it to pay someone else to do it. On my car,( 1989 VW Cabriolet ) it takes me less than 5 minutes per wheel to change the pads on the discs. (did it 2 weeks ago :))
It’s not a misconception, its counter-intuitive and counter to my experience.
For example, I have a 9x12’ oriental rug laying on top of wall-to-wall carpet. If I want to drag that rug into another room, it is much easier to pull if I fold the rug so that there is much less rug in direct contact with the carpet. Pulling the rug unfolded is nearly impossible for a weakling such as myself, but folded over about 3x, it is considerably easier.
Friction is dependant on both surface area and pressure given a constant coefficient of friction. That’s the rub, so to speak, the coefficient of friction may not be constant with changes in pressure. If your rug and floor were made of the magic material only found in physics text books folding it would decrease the area but raise the pressure per unit of area by the same factor and the total force would be the same. In the real world of Persian carpets you found a different combination of force and area that made it much easier to rearrange your living space.
The friction of brake systems is more linear but there are disadvantages of going to extremes. Making huge brake parts allows for less force but it’s heavy and wasteful of material. Making the parts small but with higher pressure works but the thermal loads are concentrated in such a small area that metal and pads wear out too quickly.
Ah, gotcha…I get the theory, and it makes sense. Likewise, it should take the same force with the carpet folded (less friction area but more weight on top of that area), but in the case of the rug the “friction factor” is much more intense than the “weight factor”, the rug not really weighing all that much. And I wish it were Persian, especially for what I paid; but its a nice-looking Kashmir.
Meanwhile, back in the world of brakes, what I think I am hearing is that while disc brakes are smaller in “frictionable” area (my term), they more than make up for it by a much higher force behind that friction. If that is true, then that certainly is a simple enough explanation (one that even I, a liberal arts major, can grasp).
Drum brakes don’t work well when wet. I don’t think you can get a new car with 4 wheel drums, but if you have one, you should test your brakes soon after you go through any standing water. It happens occasionally in my 68 firebird. They may fail completely. Go slow and keep both brake and gas pedals down to dry them out. Another thing to claify: disk brakes generally weigh more, and have a higher rotating weight.
OK, I probably shouldn’t have seid “misconception.” Friction is a complex business. For the same force, if you increase surface area, friction does increase a bit. How much it increases depends on the material. If the surfaces are sticky like velcro, friction is proportional to surface area. If the surface is flat and smooth, there is hardly any increase - friction is independent of surface area. Somewhat rough surfaces like wood or carpets fall somewhere in the middle. I’m fairly sure brakes are not sticky and can be approximated as a flat, smooth surface.
Almost. The force is the same - i.e. you press down on the brake pedal with the same strength. They make up for the smaller area by having higher pressure - that is, more force per area. In other words, in a drum brake the force is distributed over a large area. If you concentrated that same force onto the smaller pad of a disk brake, you get the same stopping power.