The RWD thing is definitely important here. Given that the majority of cars and a significant number of SUVs are now FWD, this doesn't hold water. Given it is the front brakes that are more powerful than the rears, and if your brakes are maybe a tad worn out, you might not be able to take your brakes from a dynamic friction situation to a static friction situation. In the mean time, your engine would just continue injecting heat into the brakes, which could easily overcome them. It might also be that he was not braking with the full force initially, meaning some degree of heat saturation may have already been present.
That's really not true at all. Generally, as braking systems pick up heat, they become continually less and less efficient at slowing the car down and will eventually stop working entirely. This will be especially true when fighting against an engine running at wide open throttle. It's called brake fade.
While it is the case, by design and typically by regulation, that brakes have more braking force on paper than the engine and drivetrain can produce, in the real world it is not so straightforward.
ICE engines have active heat rejection and conventional brakes don't. So ultimately in any prolonged fight between an engine and brakes the engine will win.
Hot brakes have a lower coefficient of friction. This is the brake "fade" experienced on long downhill runs or heavy use, such as while towing heavy loads. In extreme cases brake pad outgassing, brake pad glazing and boiling brake fluid, all a consequence of heat, will degrade brake power.
Traditionally vacuum assist is used to amplify brake force in passenger cars. The vacuum reserve is, however, finite and little vacuum is available from an engine with a stuck open throttle. When the throttle is even partially open and the engine RPM is kept low (such as when fighting it with the brakes) the vacuum drops severely. When boost runs out brake force is greatly reduced. This all changes in heavier vehicles where more robust systems are employed.
Some vehicles have enough power to overcome the brakes on driven wheels. Some vehicles have "low range" gearing that can also easily overcome the brakes.
The situation described by helsinkiandrew was probably a combination of brake fade that emerged while fighting the engine and lack of brake boost due to low vacuum because the throttle was somehow stuck. But there isn't enough information to say for certain.
> However, one thing that i don't think many people appreciate is that in some of these scenarios if the driver begins braking normally (i.e. to slow their speed rather than a full-on emergency stop) and they continue to do so for a little while they will very rapidly burn up their brake pads, and then they won't have the ability to use the brakes.
I don't think that brakes burn out that quickly.
I once had a car with a brake problem such that the brakes were always somewhat on. They lasted for quite a while. (That car had a lot of horsepower so I didn't notice the extra drag. However, even it couldn't overpower its brakes, which were nowhere near as good as today's brakes, when I stomped on both pedals.)
That's not how it works. The brakes are more powerful than the engine, sure, but only at full braking power. Ask anyone who owns a rear-wheel-drive sports car what happens when you push both pedals at the same time and they will literally leave you in a cloud of smoke.
AFAIK the brakes on most if not all vehicles are more powerful than the engine, and the front ones more so than the rear. To have the rear brakes locked up while the front gets presumably overpowered by the motor is a very odd situation, perhaps best described as a "reverse burnout".
This happened to a friend of mine driving through the rocky mountains in Canada; brake system overheated and he had no breaking ability until it cooled down.
it is important to know if your car is FWD/RWD/AWD. you don't want to do this if you have a FWD or AWD car. the brakes will be applied to a powered axle and the brakes will just roast. (front brakes = majority of braking power, or all of it depending on car, when you hit the brake pedal. rear brakes sometimes reserved for e-brake only)
that being said - applying brakes and full-throttle to a RWD car will either bog down the engine/transmission and do nothing if under-powered, or if you have some power it will break traction while the car stands still, effectively "burning out" your rear tires and smoking them. then you can do fun things like donuts :D
Engine braking is completely irrelevant on a RWD car during hard braking. The weight transfer means barely any braking is needed on the rear wheels, aside from a token amount to keep the car going straight.
Weight and ground pressure cancel each other out in the friction equation. The issue is where all that heat goes. Heated brakes will fade, glaze, fail, break, burn or a combination of the above, so there are limits on how much braking force you can apply to wheels
> No, they do not. Any car has brakes that can overcome its tires.
Well yeah, that's why we have ABS right?
But there's more to it than that... having a good way to engine brake (and in this case regen) keeps your brakes cool and prevents them from overheating. Regardless of stopping power in ideal conditions, overheated brakes are not safe.
Things got weird out back for RWD vehicles with the advent of brake-based "electronic limited slip differentials". Now the rear brakes have to potentially soak up gobs of energy to prevent a one tire fire vs. an otherwise open diff.
I remember helping a friend do brakes on some blown rwd v8 AMG he had (sl55 perhaps?) and the rear brakes were just as beastly as the front in every way because of this.
Since the Tesla's don't have a motor per rear wheel I wouldn't be surprised if they similarly rely on the brakes for limited slip.
I believe the reasoning here is that braking all the way down a steep grade can cause your brakes to overheat and become temporarily inoperable. I don't know whether this is a real effect or not, but I have heard it given as a justification for engine-braking.
The reason generally given is that in at least standard cars the brakes can apply far more torque than the engine can. It's telling how naive people are, they think the problem is 'unintended acceleration' A mechanical engineer thinks the problem describe is 'unintended brake failure'
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