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How come in oder to prevent a parked vehicle from rolling down a hill, always place the transmission in low gear for a manual transmission? Thank you!

"To prevent a parked vehicle from rolling down a hill, always set your park brake and place your transmission in park (for an automatic transmission) or low gear (for a manual transmission)."

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  • We appreciate you posting question, but I'm wondering what this question has to do with Motor Vehicle Maintenance & Repair? Commented Mar 1, 2023 at 22:14
  • Sorry, I didn't know questions are only limited to maintenance & repair ...
    – Maurice
    Commented Mar 1, 2023 at 22:27
  • I thought motor vehicle "mechanisms" related questions can also be asked.
    – Maurice
    Commented Mar 1, 2023 at 22:30
  • How does it apply to an issue you have with M&R or how would this create an issue related to M&R. It appears it doesn't. It is very akin to a driving question, which parking would be. Commented Mar 1, 2023 at 22:36

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If the emergency/hand/park brake was to fail on a hill, the vehicle would start to roll down the hill. If the manual transmission had a gear selected, the vehicle’s rotating road wheels would cause the engine to also rotate, this would at least slow the vehicle down.

If the highest gear was selected, one rotation of the road wheels would likely cause the engine to rotate once.

If a low gear is selected, one rotation of the road wheels would cause the engine to rotate several times, adding more friction/load than in a high gear.

What you would actually find in a low gear is that there would be so much friction added by the engine that the vehicle would not move at all even with the brake fully off.

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  • Thank you so much for your detailed answer!
    – Maurice
    Commented Mar 1, 2023 at 22:28
  • And it is more than just 'friction' -- to turn the engine, you have to actually compress air and this can require quite a bit of force.
    – Glen Yates
    Commented Mar 3, 2023 at 21:07
  • @GlenYates I don’t think compression would help much with holding the vehicle stationary on a hill, cylinders are not sealed that well. If compression was that significant, you wouldn’t be able to turn an engine over using a wrench. I think it it will be friction that stops the vehicle moving when in a low gear.
    – HandyHowie
    Commented Mar 3, 2023 at 22:13
  • @HandyHowie I have turned an engine over with a wrench, try it with spark plugs removed versus in place, and you'll see quite a difference. Now in 1st gear multiply the force required by the first gear ratio say ~3:1 and then by the differential ratio say 4.1:1 and the required force at the tires can be 12 times what would be needed at the crankshaft!
    – Glen Yates
    Commented Mar 4, 2023 at 0:15
  • @GlenYates Yes I agree, I have done it numerous times. What I am saying is that as you turn the engine, the compression leaks away, it doesn’t stop movement. At best the compression would just slow a vehicle down, it wouldn’t stop it moving, whereas the friction will stop it moving.
    – HandyHowie
    Commented Mar 4, 2023 at 8:41

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