Question, what happens when you pull handbrake on RWD while being on gas?

[carlosy]

Yes, the rear steps out.
But that's not the point. My point is what happens mechanically in a RWD car.

The power from the engine is going to the rear. But when I pull the e-brake while I am still applying gas and the e-brake slows or even stops the tyres for a short moment, what happens to all the engine power? It's not like the gears are disengaged or the engine shuts off automatically whenever the e-brake is active, is it?

So where does the power go in that moment?

Probably a stupid question.

 

[Famine]

So where does the power go in that moment?

The brake shoes.

 

[carlosy]

Not sure we are talking about the same.
The gaspedal signals the engine to rev. Revs spinning the drive train and drive shaft. Drive shaft is spinning the tyres. But the brake is slowing tyres down. Is it the differential that disengages between drive shaft and tyres in that moment?

If the engine says "spin harder" but the brake says "slow down", something has to give. Well, the drive train does not break, so the power has to go somewhere.

Or does the e-brake override the throttle input - sort of backwards? So that throttle input is getting "ignored"?

 

[TexRex]

I suspect you're imposing a level of intelligence that simply isn't present among the majority of vehicles.

Here's some light reading:

https://www.autoblog.com/2016/06/01/what-happens-when-you-drive-with-the-parking-brake-on/

The previous response will likely make more sense after reading the article.

 

[carlosy]

Ok, I am sure I am being a blockhead here, but I still can't see it. ;-)

So let me break it down further.
I open the throttle (literally/mechanically not just by wire):
More air and fuel is getting into the engine = More revs
Open throttle, more revs vs brake applied, less revs.

Let's just use a pure mechanical car, no electronics.

 

[TexRex]

Ok, I am sure I am being a blockhead here, but I still can't see it. ;-)

So let me break it down further.
I open the throttle (literally/mechanically not just by wire):
More air and fuel is getting into the engine = More revs
Open throttle, more revs vs brake applied, less revs.

Let's just use a pure mechanical car, no electronics.

The handbrake engages the rear brakes. Brakes impede forward progress. Because only the rear brakes are engaged during handbrake use, only half of the car's braking capacity is utilized and forward progress is impeded less than if all four brakes are engaged as with the brake pedal. Still, half of the car's braking capacity increases resistance and forces the mechanicals to work harder for the same result.

 

[kikie]

If the power of the engine is bigger than the friction applied on the rear brakes by the (mechanical) e-brake, you will be able to drive the car, slowly (depends on how powerful the engine is compared to the e-brake). In this case, part of the engine's power will be transferred to the rear brake shoes, generating a huge amount to heat that will damage stuff in and around the wheel hub.

If the e-brake is generating more friction than the engine can produce power, you won't be able to move the car and rev the engine. I think that it is even so that when you don't use the clutch in this case, the engine stalls.

That's how I see it. I could be wrong though.

 

[TheCracker]

I would hazard a guess that it all boils down to the transfer of energy and what the comparative ratios of how much energy the engine is producing and how much energy the hand/parking/emergency brake can disperse as heat. Parking brakes are designed to hold the weight of a stationary car and are either on or off. With an engine continuously supplying energy the brake pads or shoes are just going to wear quickly and overheat through friction (which decreases their stopping power) The best you could hope for is it to destabilise the rear axle and for that to trigger some form of power reducing stability aid. Otherwise you are just going to sooner or later wear the brake material down to the backing plate and start destroying the disc or hub.

 

[_ApexPredator]

This has gone on far too long

Or does the e-brake override the throttle input - sort of backwards? So that throttle input is getting "ignored"?

No. The only thing that disengages drive in the simplest mechanical example is the clutch, not the brakes.

 

[KinLM]

So many complex answers in here. Let me try and word it simply.

It’s no different than if you were going up a big hill in a high gear and didn’t have enough power to make it. The engine will stall out. No matter how much power you give it, the gravity is acting as a “brake” and will slow the wheels down (provided there is enough friction to where the tires don’t spin) and the engine’s rpm’s will dwindle until it stops.

This same thing happens if you slam on the “normal” brakes while still holding the throttle too. All that power is turned into heat in your brake pads, and your engine turns slower and slower until it stalls out.

When you rip the E-Brake, again this sake thing is happening (albeit much more quickly). The engine is instantly “killed” as soon as the rear wheels lock up. You can watch this phenomena happen on occasions with older rally cars, if the driver isn’t able to time the clutch just right, he/she pulls the e-brake and kills the engine on accident, stalling it out.

You are correct that it puts a lot of strain through your drivetrain, though it doesn’t put anything more through it than if you were flat out topping your car, as 100% of your car’s power would still be getting used, yet the car’s speed would remain unchanged.

The only difference is that instead of (mostly) air friction holding your car at a certain speed (or slowing it down) you’re instead holding your engine RPM/slowing your engine RPM through heat in your brakes.

 

[Eunos_Cosmo]

If the car has a traditional automatic, I suspect a lot of energy will turn into heat inside of the torque converter. Some will turn into heat in the brakes. Some will eventually make it to the road and push the car forward. My experience on the matter: The Chevy Silverado rental car I drove for several miles, accidentally, with the parking brake on last week. (Please don't read this Avis)

With a manual, that engine is gonna stall or the clutch is gonna slip or that brake is gonna burn up.

 

[IMightBeRight]

I think I got the genuine answer you are looking for. The separation between the engine and the rear wheels is the torque converter. Torque converter is kind of like two fans facing each other but with fluid in between. When one fan spins (One attached to the engine) it spins the fluid which spins the other fan (attached to the transmission). Additionally, in between the plates is equivalent of a small automatic clutch plates that engage when you are moving at a normal speed to prevent power loss that is created by one fan spinning another (the clutch locks the power output from the engine to the wheels). So when you pull the handbrake in a RWD automatic car the clutch plate disengages and it’s just one fan (engine side) spinning a fluid but the second fan (output/ attached to the transmission ) is being held by your handbrake (in the way of your hand brake holds the rotor and everything from rotor to that output fan is locked out).

Or clutch could just slip wearing it down a bit or a lot depending on how you do it.

You know in an automatic when you stop at a red light? The same thing happens the clutch disengaged and it just one fan spinning (other is being held by your brake. Also the reason why automatics cars creep forward when you let go of the brake. cuz the fan attached to the engine doesn't stop spinning and since your transmission is in gear fluid spins the fan attached to the transmission making your car creep forward). When you start moving its one fan spinning the other until you reach there required speed and balance of the two fans spinning at a close enough rate at which point the clutch engages.

Here is what my knowledge is limited to tho. I cannot for 100% say that your automatic transmission computer doesn’t just disengage and lets the engine spin and rear wheels lock (like it putting it in neutral). Considering that automatic transmissions have gotten pretty fast and that your handbrake probably has a sensor that tells the computer it has been pulled I wouldn’t be surprised if it just drops into neutral by itself.

if you look at the image the "fan" I'm referring to is the turbine. The impeller also has a turbine like structure inside.

 

[wfooshee]

Nothing "disengages." Something slips.

If it's a manual transmission, the brake slips, heats up, eventually burns up if you don't release it. As the car slows, so does the engine. The clutch won't slip, the engine would stall first.

If it's an automatic, the torque converter could slip, unless you're going fat enough to have the torque convertor lock-up engaged, in which case you're back to simply burning up the rear brakes. If the torque convertor lock-up is disengaged, the torque convertor can slip, just like it does while you sit at a light. It won't slip much with revs on, though.

The brake does not override throttle except in the sense that it creates drag the engine will try to overcomes. You're looking for, "Where does the energy go if the brake is dragging and the engine is running?" Well, heat is energy. Dragging the brakes heats them up, and that heat is the energy you think is simply disappearing. mechanically, nothing drops out of the driveline system. The engine is still driving the wheels, but the brake is fighting it.

Edit: DAMMIT!!! I just saw how old the thread is!

 

[IMightBeRight]

Nothing "disengages." Something slips.

If it's a manual transmission, the brake slips, heats up, eventually burns up if you don't release it. As the car slows, so does the engine. The clutch won't slip, the engine would stall first.

If it's an automatic, the torque converter could slip, unless you're going fat enough to have the torque convertor lock-up engaged, in which case you're back to simply burning up the rear brakes. If the torque convertor lock-up is disengaged, the torque convertor can slip, just like it does while you sit at a light. It won't slip much with revs on, though.

The brake does not override throttle except in the sense that it creates drag the engine will try to overcomes. You're looking for, "Where does the energy go if the brake is dragging and the engine is running?" Well, heat is energy. Dragging the brakes heats them up, and that heat is the energy you think is simply disappearing. mechanically, nothing drops out of the driveline system. The engine is still driving the wheels, but the brake is fighting it.

Edit: DAMMIT!!! I just saw how old the thread is!

With a normal cable hand brake yes, the brakes would simply heat up (along with the torque converter generating heat as well) because the factory handbrake is not set up to lock the rear wheels if you are traveling with sufficient speed or even gunning it of the spot. Considering that even some 2022 cars have handbrakes I would highly, highly suspect that the sensor would disengage the clutch and just let the torque converter generate heat. No need for the clutch to keep pushing when you’re trying to get the car to stop. And since torque converters lose about 10%-15% power output even if you pulled the hand brake and gunned it your car would still move forward generating heat both at the torque converter and the brakes.

If the clutch was to stay engaged while you’re standing at the red light it would mean that the clutch is slipping and thus would burn up. Consequently, if you pull the handbrake and gun it, the clutch would be disengaged, and it would be purely the turbine rotation driving you forward. As for when you are traveling with speed, I can’t say for certain the clutch would disengage.

Again I’m not saying that I know 100% that the clutch disengages but I have good reason to speculate it would.