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Leaving car in gear when parking on a hill

  1. Jul 27, 2011 #1
    Just something I started to wonder about. It is said that if you park on a hill, put the hand brake on and leave the car in gear. First when I heard about it, I figured its pretty clever, giving the wheels more friction to overcome.
    Now shortly after I heard that when parking on a hill, facing uphill, you would have to put it in first gear. However, when the nose is pointing downhill, you would have to put it in reverse.
    This confused me a tad, since I thought there's really not much difference, except for the difference in gear ratios between 1st and reverse gear. As far as Im concerned, setting the gear opposite to the car-rolling-down-hill direction doesnt really matter. I thought it was only about the friction. Or is it harder for the wheels to make the engine run backwards if the gear is set opposite to the rolling direction? Or is it even possible to run the engine backwards this way?

    I did some research concerning this question and it seems there are mixed opinions. What do you guys think?

    Thanks in advance,
  2. jcsd
  3. Jul 27, 2011 #2
    An engine is more than just a giant clockwork driving the output shaft.

    There are many auxiliary devices that may be mechanically driven by the engine rotation; water pump; fuel pump; distributor; alternator; compressor; oil pump to name but a few.

    What happen to the system if you are able to drive these backwards?
  4. Jul 27, 2011 #3


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    Don't you have a park position for the gear shift?
  5. Jul 27, 2011 #4


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    You've clearly never driven a manual transmission.

    Honestly, I'd say you're probably OK in either direction. First and reverse are definitely the two best gears, since they have the most mechanical advantage, but I don't think there's a whole lot of benefit to be had from choosing one over the other regardless of angle.
  6. Jul 27, 2011 #5
    As much as I find truth in this post, I think most engines are designed in a way so they can rotate both ways, including all the gadgets they are connected to. Cant be sure though.
    Quoting another guy from another forum:

    "As far as the engine being harder to turn in reverse direction than forward, a little test.

    Flat road, engine off, emergency brake off,
    top gear to make it easier to push.
    Push it forward (engine rotates in normal direction).
    Push it back (engine rotates in reverse direction).

    Feel any difference?"

    (From this forum/thread: http://forum.miata.net/vb/showthread.php?t=371055)

    I dont own a car. Its a theoretical question. Plus Im talking about manual transmission.
  7. Jul 27, 2011 #6

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    Combining the hand brake with the gear is safety 101 as far as I'm concerned (always make sure you have a safety backup).
    The hand brake might give, so you want the gear as backup.
    And depending on the slope you may not want to put to much strain on the gears, which may give as well.
    Btw, this reasoning applies to a park position for the gears as well (combine it with a hand brake if you have one).

    As far as forward or reverse is concerned, I think it's best to put the load on the gears in such a way as they are designed to take it.
    This would mean first gear when parking upward, and reverse gear when parking downward.
    Last edited: Jul 28, 2011
  8. Jul 27, 2011 #7
    Have to admit, didnt actually think of the direction of the load on the gear teeth. This is right though. Yet putting the load "correctly" on the gears makes the engine spin backwards (incase the car does start rolling down the hill), which many have said and considering that its not the natural direction for the engine to spin, cant be good either. As many textbooks support your way, I think its assumed the car wont start rolling, and considering the direction of load on the teeth, its just better to have it this way in the long run.
    Think this could actually sum it up. So thanks for that.

    As for the theory of having the car in gear opposite to the rolling direction being more effective in holding the car in place, can anyone proove the machanics behind this?
    Because I, and as I can see many others on different boards dont think this is correct.
  9. Jul 28, 2011 #8


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    Just for the record - I have driven manual transmission cars. However, it has been over fifty years, so I'll simply say I forgot.
  10. Jul 28, 2011 #9

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    Thanks! :smile:

    My reasoning does not hold only for the gear teeth.
    It holds all the way to where the connecting rods are connected to the pistons.
    As for the pistons themselves, they move the same whether the engine is forced backward or not.

    As for which direction would give the most friction.
    I can't say, but I suspect it won't make much of a difference.
    What does make a difference, is that if you put forces into the system that it is not designed for, you should not be surprised if something breaks.
    I think it will provide less friction that way! :wink:

    I am curious though.
    Can you give reasons mentioned why one way or the other would give more friction?

    Btw, if the car starts moving regardless, I think you shouldn't worry any more about your engine or gears.
    Obviously it didn't provide enough friction, and that won't improve.
    That and you'll have bigger problems! :surprised
  11. Jul 28, 2011 #10
    There are a lot of advantages to leaving a car in gear when parking on a hill. Most modern cars are front wheel drive. The parking brake engages the rear brakes. By putting the car in gear, you have locked both the front and rear brakes.
  12. Jul 28, 2011 #11
    It's best to leave it in reverse because it has the lowest gear ratio. For example, a T-56 transmission has a 2.90:1 ratio in reverse compared to 2.66:1 in first gear. The friction of the drivetrain/engine, gear tooth pattern, and accessories are all insignificant. With the car in gear, the drivetrain is "connected" to the engine, and you would have to overcome the compression of the engine for the car to move. It should only be used as a backup in the event your emergency brake fails.
  13. Jul 28, 2011 #12
    Do they teach turning the roadwheels into the kerb any more?
  14. Jul 28, 2011 #13
    actually, if you park a manual vehicle facing up hill, you want to put the gear selector in reverse. you don't want to take any chance of turning your engine backward. some engines have hydraulically controlled timing components that keep tension on the belt or chain that synchronizes the crank shaft with the cam shaft/s. when the engine is running, oil pressure is directed to them, but with the oil pump not being driven, these can bleed off leaving some slack in the chain or belt.

    looking at the front of the engine the crank shaft turns clockwise. the tensioner that keeps the belt or chain tight is located on the left hand side. the pulling force is on the right hand side of the chain, and the slack is on the left. therefore turning the engine backward can re-direct the slack to the right and in between this faze, it can cause the belt or chain to jump teeth throwing off the timing. this can cause engine damage when the engine is started again. the piston can come into contact with the valves that are controlled by the cam shaft "in some cases" causing them to bend. or best case scenario, it will run poorly or not at all.

    1st gear facing down hill and reverse facing up hill to be completely safe. the lower the gear ratio the better. if you have a 4x4, putting the t-case in low4 is even better.

    with an automatic there is a rod called a park pawl that engages when put in park. this locks the transmission under normal circumstances. but, if you have accidentally put the transmission into park while rolling one too many times, and heard the annoying ratcheting sound it makes, the pawl could be damaged and not hold as well. so setting the e-brake is always a good idea when parking on a hill.
  15. Jul 28, 2011 #14
  16. Jul 29, 2011 #15

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    I'd like to retract my reasoning on the load on the gear transmission.
    I will have to admit to have insufficient knowledge of car engineering.
    In retrospect I suspect the gear transmission is not the most significant possibility for car failure.
    I think people living in hilly country that drive cars with manual transmissions should have experience with the most common cause of car failure when parking....
    Living in about the flattest land of the world, I do not have this experience.

    What dyce says sounds very plausible.
    @dyce: is your explanation based on experience?
    And as Studiot remarked, when the car starts rolling regardless of being in gear, it should still be stopped by the kerb (if parked correctly).
    @jtherr0: does the reverse gear have the lowest transmission ratio on all cars? Or only specific types?

    Still, I think the problem is not which gear provides maximum friction (which is what jtherr0 provided), but which gear minimizes the chance on damage to the car (which is what dyce provided).
  17. Jul 29, 2011 #16


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    The transmission has to deal with some heavy reverse loads loads when you change down at speed so I don't think you need to worry at all about stress whilst stationary on a hill. The static load in either direction is resisted by the compression, which is pretty much the same in either direction of engine rotation - only depending on valve timing and compression ratio- and, also the actual gear ratio, of course.

    I remember having an old vehicle with a two cylinder engine with very poor compression. If you put it in gear on a hill it would make its way slowly downward in jerks as each cylinder pressurised in turn and then the pressure leaked away. Very disconcerting as the handbrake was pretty poor too. I had a wooden wedge for peace of mind.
  18. Jul 29, 2011 #17
    I'm an Ase certified technician with only 4 years experience, although I'm not what is considered a veteran technician, knowing not to rotate an engine backward is common knowledge and one of the first things taught in engine repair.

    not all transmissions provide a lower reverse gear. some are the same as first gear.

    here are some of the timing components for the Chevy 2.8L, 3.0L, 3.2L, or 3.6L. as you can see, it is quite the complex timing system.

    this is a hydraulic tensioner. it uses oil pressure against a small piston that moves out to apply tension on the chain.

    this is the tensioner shoe that guides the chain. the hydraulic tensioner presses against this which in turn presses against the chain, taking up any slack in the chain.

    this system uses three chains and three tensioners. so allot can go wrong.

    there are 4 cam shafts. 2 per side. on each side of the engine, 1 controls the timing of the air and fuel intake, and 1 controls the exhausting of burnt fuel vapors.

    (1) Camshaft Actuator Vane
    (2) Timing Chain Sprocket
    (3) Engine Oil Pressure-For retarding the camshaft
    (4) Camshaft
    (5) Input Signals from Engine Sensors
    (6) Engine Control Module (ECM)
    (7) Camshaft Actuator Solenoid
    (8) Engine Oil Pump
    (9) Engine Oil Pressure Supply
    (10) Engine Oil Drain
    (11) Engine Oil Pressure-For advancing the camshaft
    (12) Camshaft Actuator Rotor
    (13) Camshaft Position Sensor Reluctor
    (14) Camshaft Actuator Lock Pin
    (15) Camshaft Actuator Housing

    this engine also has the ability to control the timing of each cam shaft hydraulically in order to control:

    • Engine power output

    • Fuel economy

    • Lower tailpipe emissions

    NOTE: the hydraulic tensioners in this engine have a ratcheting ability which is sort of a fail safe. as the chain starts to age and stretch they ratchet out one click at a time. this holds them in place when oil pressure bleeds off. but they don't always do their jobs. oil and sludge build up can keep them from working. also, not all hydraulically controlled tensioners work in this way.

    you seem interested enough that i figured you would appreciate this info.
    Last edited: Jul 29, 2011
  19. Jul 30, 2011 #18

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    Thanks for the pictures!
    They look very impressive.

    I'm also interested in the "ratcheting ability which is sort of a fail safe".
    As with everything in safety engineering, things need to be equipped with a fail safe.
    How does it work? And does this help against problems if the engine were to turn in reverse?
  20. Jul 31, 2011 #19
    The ratchet mechanically takes up some of the slack in the chain(s), making the engine less likely to incur damage due to piston to valve contact from out of synch timing events.

    Engines that also have provision to significantly alter the cam timing usually have a considerable amount of clearance built in and normally wouldn't have an issue with being turned backwards. However, as noted a malfunction of the ratcheting mechanism would leave the engine susceptible to skipping of a chain when rotated backward.
    Last edited: Jul 31, 2011
  21. Jul 31, 2011 #20

    mender is right.

    in short the tensioner kinda works like a zip tie. once it clicks in a notch, it cant be pulled back. only the tensioner does have a small button that can be pressed to release it so that it can be compressed and a pin inserted to hold it in the compressed position for installation. once the chains are in place, the pins are pulled out and they ratchet out with light spring force. oil pressure is used to push them out further and take up more chain slack over the life of the engine.
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