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

Discussion in 'Cars in General' started by carlosy, Oct 14, 2018.

  1. carlosy

    carlosy

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    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.
     
  2. Famine

    Famine Administrator

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    The brake shoes.
     
  3. carlosy

    carlosy

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    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"?
     
  4. TexRex

    TexRex Premium

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  5. carlosy

    carlosy

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    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.
     
  6. TexRex

    TexRex Premium

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    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.
     
  7. kikie

    kikie Premium

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    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.
     
  8. TheCracker

    TheCracker Premium

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    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.
     
  9. _ApexPredator

    _ApexPredator

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    This has gone on far too long :confused:
    No. The only thing that disengages drive in the simplest mechanical example is the clutch, not the brakes.
     
  10. KinLM

    KinLM

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    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.
     
  11. Eunos_Cosmo

    Eunos_Cosmo

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    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.
     
    KinLM likes this.