Why does your car sometimes move forward after taking your foot off the brake and sometimes not?


Why does your car sometimes move forward after taking your foot off the brake and sometimes not?

In: Engineering

The transmission/transaxle. Sometimes when you stop the transmission is between gears. The slight roll happens as gravity pushes the car to the next point of resistance, usually only about an inch or two. You’ll find that electric cars don’t behave the same way because they don’t have traditional transmissions.

Depends on how much pressure you left on the brake pads when you stopped. If you left a lot (meaning you were pressing the brake fairly heavily when the car stopped) there will still be pressure on the part that clamps to brake. When you take your foot off, that little extra pressure is released

If you brake more evenly when you pull into a stop, it won’t happen much

Assuming you’re in an automatic transmission, the engine and wheels aren’t completely disconnected when you apply your brake – there’s a mechanism that allows the engine to continue without stalling from the wheels not turning.

That said, when you release the brake, the wheels will naturally attempt to roll forward at engine idle speeds. However, if you’re facing uphill at all, it can stop the car from rolling forward.

If you were to repeat this in the same car on level ground over and over again, it should pretty much always start moving forward when you release the brake.

You braking stops the car in its place. You then put it into park. There is some play in your drivetrain from inside the transmission to the wheels, and where that parking pin stops the transmission from moving may not be directly lining up with what it catches on. So if you have gravity trying to force you downhill, taking your foot off the brake allows the drivetrain components to move a little until they take up the slack in the system and then use the resistance of the transmission to hold the car.

There’s another factor here, if you were doing this on flat ground consistantly and with an automatic transmission in drive, you should roll forward every time. If you’re in neutral or park, then no power will go to the wheels.

What might be happening is you noticed you won’t roll forward on a slight incline. There will be a certain slope where the slow rolling force will be matched by gravity, or if you’re in a more modern or luxury car, there might be an anti-rollback feature that’s kicking in. On my car there’s a function that keeps the break applied when you lift your foot off the brake pedal until you press the gas, but I have to enable it.

* Most car engines run all the time, even when the car is stopped.
* This is called “idling” and it’s almost the same as pressing the gas pedal down just a tiny bit.
* There is a special device that allows the engine to keep spinning even though the wheels aren’t.
* It reacts to the resistance of the breaks and prevents the engine from trying to over power the force of the brakes.
* However once you let up on the brakes, the device re-connects the engine and if that idle power is enough to make the car move, then it does.