“Ron Plantan, principal engineer for Bendix air disc brakes, said, “During the side-by-side testing, data showed the stopping distances for the Bendix air disc brake-equipped truck was within the range of 305 to 325 feet. The drum brake-equipped truck stopped in the range of 450 to 518 feet initially when cold, but as the drums heated up, the stopping distances became progressively longer. Stopping distance for the hot drum brake-equipped vehicle exceeded 750 feet, while the air disc brakes consistently stopped at around 320 feet.”
So the claim that semis use drums because they’re better (the foundation for all this nonsense) is FALSE. End of story.
I have to admit, though, that I did enjoy the video. It was like seeing a new edition of “My Name is Earl”, which I always liked. Except Earl was smarter than the guy in the video. {
The most efficient brakes in terms of energy use are the regenerative brakes used in some hybrid and electric vehicles. The brakes are basically a generator that charges batteries so that energy can be used later. No frictional heat and nothing wears out unless you get the system too hot but I guess you would switch to conventional braking systems before that happens.
Or a compromise, like the Veyron and McClaren. A retractable huge spoiler that tips up to act as an air brake. It acts in two ways; (1) it creates drag, and (2) it loads the rear end so the rear brakes can help more.
Mercedes had a large car width flap mounted just behind the cockpit of their F1 cars in the mid 50s to help haul them down from high speed. Jaguar won a Lemans in the 50s beating much faster cars because of their disc brakes when everyone else was using drums.
Everything said in the video was correct but misleading. Drum brakes are more efficient ,but disc brakes are much better in all other ways, they don’t fade like drums, their stopping power doesn;t disappear when the friction material gets wet and when properly designer they can generate more absolute stopping power.
His cautions about longer stopping distances with discs appliy only to converting manual drum brake cars to manual disc brake cars.
And “efficient” is the wrong word, anyway. That refers to the the amount of work out for a given input. Applying pressure is not work, otherwise I’d be getting a good workout when I apply pressure to the barstool at the local pub. “Grabby” would be better.
Texases, from a physics standpoint applying pressure to the brake pedal is “work”. It’s using energy to accomplish something. Applying pressure to the barstool is simply gravity. {
Holding pressure does no work. Pushing the pedal through a distance does some work. One could rig a weight to apply pressure to the brake pedal, it’s doing no work. The claim that drums are more efficient than discs because it takes less pressure isn’t about efficiency, it’s about sensitivity.
I see your point. I respectfully disagree, because you the driver are expending energy to do so, but I see your point. A weight hanging statically against a fixed resistance is expending no energy. It contains inertial energy, but is expending none. If the weight were used to also apply the brakes, as your leg would be, it would during that process be doing work.
The difference is that you need to continue to expend energy to keep your muscle contracted to keep pressure on the pedal. The weight needs do nothing once the pedal is compressed.
I would argue that drum brakes are more efficient technically because they have the ability to supplement the pedal input with some of the vehicle’s inertial energy via their self-actuating ability. That does not mean they’re better, just technically able to create more friction with a given pedal input than discs.
Leverages within the system are irrelevant. All they really do is balance pedal travel with force. They don’t actually multiply force, they simply cause the total force it to be applied over a longer distance and the final result to be applied to a smaller area. It creates greater force per area to the slave cylinder piston than is being applied to the master cylinder piston, but the total force remains the same. Note that I’m ignoring heat and frictional losses to the system, because I don’t believe they’re significant or relevant to the point. The same applies whether the brakes are drums or discs.
For small light inexpensive cars, discs on front and drums on rear which can double as a large parking/emergency brake is perfectly adaquate. The drivers of these lower performance cars will never likely notice the difference in performance and save money on purchase while arguably, getting a better/ larger all in one parking brake. If you want the best bang for the buck, having drums on the rear of these cars is no more a concession to poor braking then having smaller disc brakes then larger models on front.
With 70% of the braking done on the front wheels, the only way you would notice the inferiority of these low performing cars, would be if the drums started to fade while making repeated stops from 60 mph while driving backwards. If you do that stuff, yes, 4 discs would be better on small compact cars where purchase price is an issue. My truck has drum brakes on the back and in that year of manufacture, it has class leading low stopping distance and resistance to fade. It’s also a low performing truck not ment for high speed racing too. I would think that all my good, fellow cheap skate buddies would appreciate this “good enough” engineering. During off roading, they are much less suseptable to damage while being enclosed.
The discs are shielded by a thin metal shield that can easily be bent. I had to trim lower half of a shield off after smashing one of them up on one excursion. Never had that damage problem with drums !
I’m sure self actuation could be designed into disk brakes if it was that important. You could have a double caliper, a primary caliper powered by hydraulic pressure from the master cylinder and a secondary caliper powered by hydraulic pressure created by the reaction torque of the primary caliper.
But, I believe that self actuation is responsible for a lot of the drawbacks of drum brakes. If the coefficient of friction of the brake shoes changes, from overheating or from getting wet, the loss of friction is compounded by the loss of self actuation, and that’s why drum brakes just completely go away when the shoes get wet whereas with discs, the loss of braking is only proportional to the change of the coefficient of friction.
Also drum brakes don’t necessarily have to have self actuation. I had an old motorcycle that had twin leading shoe front brakes. Lots of self actuation when the bike is going forwards, but when the bike was rolling backwards, it took forearms like Popeye to make that front brake work because it became a twin trailing shoe brake while going backwards.
Drum brakes make much better parking brakes because they have more surface area and can be applied more easily by hand. That makes them more efficient…for a parking brake.
"Also drum brakes don’t necessarily have to have self actuation. I had an old motorcycle that had twin leading shoe front brakes. Lots of self actuation when the bike is going forwards, but when the bike was rolling backwards, it took forearms like Popeye to make that front brake work because it became a twin trailing shoe brake while going backwards. "
LOL, an excellent illustration of the value (and weakness) of self actuation.
