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In manual transmission, it is common practice to park a car in the first gear on uphills or in reverse gear on slopes - in addition to park brakes of course, but let's assume it is not being used in this case. My question is, if the engine is not running, what is preventing the transmission from rotating the otherway? Why doesn't a car on first gear roll backwards?

I also read somewhere that the gear doesn't matter as long as it is the lowest gear available. So is it okay to park a car in reverse gear on a hill? Why the lowest gear? Is any of this dependent on the type of engine - petrol or diesel?

Apologies for asking so many questions at once, I don't have enough reputations to post them as different threads. :)

  • Welcome to the site. Asking multiple questions in this context is fine as they are all related to the theme in the question title. – Zaid Nov 15 '16 at 15:19
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Engaging gear increases the effort needed to move the car

Typically (assuming a RWD vehicle):

  • In neutral: car moves when wheels, axles, diff(s), propshaft & gearbox output shaft turn
  • In gear: car moves when wheels, axles, diff(s) propshaft, gearbox output shaft, gearbox input shaft, flywheel, clutch plate, pressure plate, crankshaft, valvetrain, pistons, crankshaft pulley, serpentine belt, water pump, power steering pump, alternator pulley, AC belt & AC pulley turn

It doesn't matter whether the engine is petrol or diesel, or whether the vehicle is FWD, RWD, 4WD or AWD; engaging gears will add a significant requirement on what needs to turn in order for the wheels to move.

Selecting the lowest possible gear has the effect of increasing mechanical advantage, so it helps to select the lowest possible gear to keep the car rooted to the spot (the same angle change at the wheel requires a bigger angle at the crankshaft, less torque available to the crankshaft for overcoming resistive forces).

  • I had thought that the highest gear would require the most turning force and thus hold the car more securely? – Steve Matthews Nov 15 '16 at 16:42
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    @SteveMatthews In first gear, your engine will spin approximately 12 times for each time the wheel rotates. In top gear, your engine will rotate approximately 2.5 times for each wheel rotation. – rpmerf Nov 15 '16 at 16:56
  • @SteveMatthews on second thought, I'm not so sure now. The higher rate of revolution means there is less torque available to the crankshaft to overcome resistive forces – Zaid Nov 15 '16 at 17:25
  • Zaid, your answer and @rpmerfs comment is correct. While torque of a running motor is multiplied by 12 on its way to the wheels, torque from the wheels is divided by 12 on its way to the motor. – sweber Nov 15 '16 at 20:12
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    Gear ratio is called Mechanical Advantage "MA" =equal to ratio of torque applied to torque output. With torque applied being the pistons compression PSI (pounds per square inch) based on ci (cubic inches of displacement) times number of pistons. Actual best gear in many cars is reverse. – spicetraders Nov 15 '16 at 20:36
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The piston compression , if good combined with a low gear ratio will increase the force required to turn over the engine, more than than gravity. A high gear ratio is easier to push. The same is true when moving the car. Torque and gear ratio are ratiometrics from the mechanical advantage.

  • This should also mean that this only works in engines where at least one cylinder has all valves closed at any time. As far as I know 99.9% of all automotive engines fulfill this condition, but one and maybe two cylinder engines might not? – Ives Nov 17 '16 at 8:20

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