A while back we had a nice, insightful debate on the merits of drum brakes vs disc brakes. Most people felt that disc brakes were better for today’s maniac hurried drivers, which I will agree. However I did concede a bit prematurely in regards to my defense and promotion of drum brakes and the strong heritage they have in this country and throughout the world.
I felt as if I was alone in my thinking on drum brakes, but I found a video today which explains their merits in such a way that I cannot.
I will argue that the EFFICIENCY of drum brakes is much greater than discs. If you were enter in an engineering contest with the task of designing the most efficient friction brake for the lowest cost possible, in the smallest lightest package possible, you would go with a drum design. Would you risk a million dollar prize due to head strong notions about the efficiency of disc brakes? Not me. The proof is in the pudding, Drums are more efficient. Not better, just more efficient.
I will say that for today’s hurried unskilled drivers that disc brakes are better for everyone, even though they are notoriously inefficient. They are great at dissipating heat and have an linear application rate. As far as efficiency goes, well they are blown out of the water by drum brakes.
Drum brakes are self energizing and they are not linear, once you reach the wedge effect of the self energizing action of drums, its all or none. They are poor at dissipating heat and if you get water in them your in trouble. Most drivers today use the brakes excessively and would quickly overheat drum brakes, thats why semi trucks are still able to use the cheaper more efficient drum brake, because skilled drivers are behind the wheel.
The great John Muir of the VW owners bible fame once said that " I try to drive in such a way that I don’t need to use my brakes at all " But he was a relaxed kind of guy (possibly due to marijuana use, no proof just speculating), in no particular hurry to get anywhere. I am the same way in my vehicles, I try to drive in such a way to anticipate traffic flow and maintain a constant speed vs constant acceleration and braking.
Those of us who grew up driving non power drum brakes and know that they could be locked up somewhat easily know that that disc brakes are not more powerfull. VW and a great many other automobile manufacturers used twin wheel cylinders to eliminate the ‘self servo’ action and reduce brake performance to prevent locking up and of course there were no power brake boosters on those old buses. A long downhill run with an automatic transmission can get the drums so hot that they are nearly useless though.
" Drums are more efficient. Not better, just more efficient."
What, exactly, do you purport to mean by “efficient”? A (friction) brake is a device designed to change kinetic energy into heat energy. Heat energy–the random motion of molecules–is the highest-entropy form of energy possible; consequently, converting energy into heat is almost always (near) 100% efficient. (Granted, there may be a SMALL amount of energy that gets converted into a squealing sound instead, but 99.44% or better is directly to heat).
Where "efficiency’ is more meaningful is when “swimming upstream,” entropy-wise. Changing heat energy into kinetic energy is something that doesn’t tend to happen spontaneously, as evidenced by the (comparatively much lower) efficiencies of the otto cycle, steam engine, turbine, etc.
A more meaningful concept of “braking efficiency” is: what does the brake turn the (momentarily undesirable) kinetic energy into? Friction brakes turn it into heat, which serves zero useful function to the vehicle, and can be a PITA to dissipate. Hydraulic/regenerative/flywheel braking turns it into a form that can be subsequently re-used, and thus is more “efficient” at recapture.
Technically, their EFFICIENCY is better because by virtue of their self-actualizing feature they convert some of the inertial energy of the vehicle into friction (it actually gets converted into heat energy, but I’ll spare you the lengthy dissertation).
But, in addition to Rod’s comments, you don’t know the meaning of fear until you drive through a puddle with drum brakes and discover that your brakes have disappeared because of the water entrapped in the drums and they don’t easily dry out. Disc brakes spin dry. Drum brakes do not.
Having grown up on drum brakes, I don’t miss them at all. Disc brakes are definitely superior for cars.
By the way, if this guy really WERE one of the world’s experts on brakes he’d tell us his credentials. But he doesn’t. Sigh.
I should point out that some of his points are loony; drum brakes are also terrible without hydraulic assist too, and vacuum assist is also a true asset. Doubt me? Drive a Model T.
This guy is a nut. And, frankly, a joke as a teacher. He hasn’t arranged his argument into an intelligent organized dissertation, he simply gets up, says “I’m a world expert” and babbles on repeating that drum brakes are much better than disc brakes. I can guarantee you he didn’t attend the college I retired from. I doubt if he’s ever attended ANY college.
When I was an undergrad, a professor of mine gave us a one-question quiz, worth points toward our grade. The question was “who wrote your book and what were his qualifications”. His point was to always be skeptical of whoever you’re learning from. Be sure they have the credentials to know what they’re talking about. It was good advice.
By efficiency I mean stopping power relative to pedal pressure. If power brakes were not an option would you want 4 wheel disc brakes? I think not.
Semi trucks still use drums. Drum brakes have more surface area than disc brakes, therefore more stopping potential. In conjunction with the airbrake system, they work very well.
The big advantage of drums of course is that they can be designed to be self-energizing and self-adjusting.
There is one more advantage I have not seen mentioned here:
unlike disc brakes, drum brakes have no drag when released, and so contribute to better fuel economy.
Efficiency regardless of how you define it, takes second place to practicality. Just being efficient, does not make it useful.
Semi trucks are designed as engine braking units with friction braking spread out over several axles. . They have multiple gears to take you continuously up and down the speed range while under engine braking constantly, even in auto transmission trucks. Cars at made differently. You can’t equate the two why one is chosen over the other.
Now, as far as ride is concerned, unsprung weight is a big factor. Discs win hands down and dry out and cool off much more quickly. These are practical and not efficiency considerations when you have power assist and efficiency takes a distant second.
Besides, the electric motor/generator is the real braking champ.
Who cares about “stopping power relative to pedal pressure”?? If power disc brakes work better than drum brakes (and they do), bring on the disc brakes! Might drums be the best solution for a Tata Nano? Maybe, don’t care. And if drums are superior, why aren’t they used in racing, where performance per pound is critical?
@WheresRick: I’m not sure if you’re confusing efficiency with leverage. Drum brakes have large shoes moved by pistons that have a fairly long travel. Disc brakes (on cars anyway) have smaller pads for the most part that don’t travel very far and don’t “self-servo” as someone else mentioned. So you have to push the pedal harder (assuming no power assist) to press them hard enough against the rotors to stop the car.
Even if somehow you could call drum brakes more efficient, discs are more linear in their application, fade less, and are less complicated. There are plenty of situations in which I would gladly give up efficiency for convenience. A Prius could be considered as much more efficient than my car, but I don’t want one.
The non power assisted disk brakes on my motorcycle stop my motorcycle just fine, and I don’t need forearms like Popeye to do it either.
I love the linearity and lack of hysteresis inherent in hydraulic disk brakes.
Properly working disc brakes do not create drag when released. That is a misconception.
The piston of disc brakes contain a rubber part that distends when the piston is moved by the hydraulic pressure and returns to its static state when the pressure is removed, pulling the pad on that side away from the disc and, if the caliper is floating free, also allows the pad on the other side to release its friction. If you lift a wheel of your car and the brake pad is dragging on the disc, you need to check your brakes out. That condition can cause serious heat buildup at highway speeds, leading to a warped rotor, boiled fluid, and damage to the caliper (which contains rubbery bits subject to possible heat damage).
"Well maintained 4 wheel drum brakes were more than adequate"
That is a generalization, and like all generalizations, it cries out for clarification.
Yes, 4 wheel drum brakes that were adequately-sized for the vehicle, and which dissipated heat quickly were at least adequate. For instance, the drum brakes on Rolls Royces of the late '70s were extremely good, and disc brakes did not outperform them except when it came to driving in very wet conditions.
Similarly, Buick went from the weakest drum brakes in the industry in the early '50s to truly excellent drum brakes once they adopted aluminum brake drums in the '70s.
My '71 Charger’s drum brakes were truly pathetic and I am tempted to say that their inadequacy was a safety hazard. During that model year, Dodge offered optional heavy duty drum brakes (essentially for taxis) and also disc brakes, but since that was my last car bought from dealer inventory, I had no choice regarding the braking system, and the standard drum brake system was just not up to the task of stopping the vehicle from high speed with any sort of confidence.
I will probably get flagged but I have theory where the OP’s screen name comes from. That is what his co-workers ask each other so they can avoid goofy conversations such as this.
I have front discs on my Corolla, and drums all around on my truck. I have no problem applying the amount of pedal force required to stop on either vehicle. If that’s what is meant by “efficiency”, that isn’t an issue on either. But I do notice two distinct differences when braking between the two.
The drum braked vehicle tends to pull one way or the other or both when braking. More so than the front disc-equipped vehicle.
It’s a little easier to modulate the exact amount of braking force I want to apply on the disc equipped vehicle compared to the drum-equipped. The truck, on hard braking, tends to start skidding a little easier than the Corolla.
I can’t say for certain though whether it is actually the brake systems – drum vs disc – or other uncontrolled factors, such as the weight difference of the vehicles. The truck certainly weighs a good deal more than the Corolla. Just my observations is all.
I’m not qualified to comment but the last drums I had were on the rear of my 86 Park Ave and those are thankfully the last ones I ever worked on. Everything since has been disc all around. I like that just fine.