Suppose I park my stick shift car facing down a steep hill, set my parking brake, and leave. The brake fails, the car rolls away and chaos ensues.
If I left the car in reverse gear, would the engine be damaged by running backward as the car rolled down the hill?
If the car is left in any gear (including reverse) your car won’t roll down hill unless it “slips” out of gear, So no, there won’t be any engine (or transmission damage) done and leaving your trans in reverse is safe.
In theory, if the hill were steep enough, the tire traction good enough, the car could conceivably roll. Were this to somehow happen, the engine still wouldn’t suffer any damage. Without power being delivered to the pistons, the engine will turn in either direction just as easily.
That said, it would be damn hard to get a car into a situation where this was possible. But given the above, you should conclude that the pressure on the engine & tranny if the brake slips won’t hurt anything either.
Always leave your transmission in gear when you park, whether you’re on a hill or not, in addition to setting the parking brake. This is a ‘belt and suspenders’ method of guaranteeing the car will stay where you left it … first or reverse, doesn’t matter, but never leave it in neutral.
I would also hope you would curb it too…
I don’t know about other cars, but my last two cars have been stick shift. Either of them would roll pretty easily on a very slight slope unless the brake was engaged. And this is while the car is in gear. My present car is almost brand new (a 2002 Mazda), and it’s the same. It will roll in the garage, and there’s only enough slope on the floor for drainage.
This is opposite of my experience with hundreds of different cars. Unless the clutch slips or the engine has significant compression loss, it will keep the car from rolling even on very noticeable slopes when the vehicle is indeed in gear. This static engine braking effect is more pronounced in lower gears, so it’s far better to leave it in first or reverse than in fourth or fifth. Frankly, I find it simply unbelievable that your car will roll (more than a foot) on such a slight incline if it actually is in gear.
As to the OP, it is conceivable that if the car did roll in reverse and thus turn the engine backwards, it might result in the timing chain or timing belt jumping a tooth IF said chain or belt had a large amount of slack.
And naturally, when you really park the car (rather than construct a scenario for an engine question), you turn the wheels to the curb side to limit how far it could roll or be pushed if something were to happen.
I don’t know the answer to how far it will roll, because the couple of times it has rolled, I hit the brakes before it rolls more than a foot or two.
Frankly, I find it hard to believe it wouldn’t roll without brakes on. What’s to stop it? Cylinder compression? Well, if one is compressing, another is decompressing. Seems like it would be a wash. What else?
My previous car was a Corolla. I know it rolled on very slight inclines because one time the dealer really fucked up a brake job and left the emergency brake entirely disconnected.
Whatever the case, don’t forget to turn the front wheels such that if the car were to begin rolling, the car would roll off the road.
Don’t they teach this in Driver’s Ed 101 anymore?
all of you, go outside, put on your parking brake and put your car in neutral, try to grab a pulley, any pulley, on the engine and turn it by hand. it is not easy.
get on level ground, say a parking lot, put your car in 1st gear and take off the parking brake. get behind it and try to push it.
come back and let us know how far you make it before you get tired.
Really, this thread has nothing to do with HOW to park a car on a slope. I know how to turn into the curb just fine, thank you.
The question is, if the car rolls with the engine turning over backward, will the engine be damaged?
The cylinders operate independently; there is no way for the air being compressed in one cylinder to escape to another. On the compression stroke - and with 4 or more cylinders, at least one cylinder is always on the compression stroke - both valves are closed and the cylinder is going up. If you’re in 1st or reverse with a manual transmission, there’s no way your car will roll down a hill unless your compression is so bad you’re pouring a cloud of blue smoke out the back when the engine’s running. It will roll a bit, usually about a foot, before the engine braking holds it in place. Try leaving your car in gear without the park brake on a level surface, and push it back and forth. You’ll find it has a small range of movement, after which you cannot move it. This is why push-starting requires you to get the car rolling at a few mph before engaging the transmission.
What’s to stop it? Cylinder compression? Well, if one is compressing, another is decompressing. Seems like it would be a wash.
Yes, it’s engine compression that prevents the rolling. There is no “decompression.” The compression from cylinder to cylinder is sequential–about the time one cylinder’s compression stroke ends, the next cylinder’s begins.
The car may roll a foot or so as the drivetrain takes up slack and a bit of compression builds up in a cylinder. Other than that, it’s not really possible for a car to roll on nearly level ground when in gear.
Heck, one person can’t even PUSH a car on level ground if it’s in gear. And you can STOP a car that’s already rolling (not too fast) by putting it in gear (engine off).
I assume you understand that in gear includes the clutch pedal being up? And the gear needs to be FULLY engaged? (Sometimes the shifter won’t move the full distance, and can fool you.)
I don’t know why your cars rolled, but I do know that cars don’t roll if they’re in gear, the clutch doesn’t slip, and the engine has normal compression.
Mine has. There is nothing wrong with it.
Just like it will continue to roll if you turn off the ignition while going down a steep enough hill. The wheels don’t lock up or anything drastic when you kill the ignition so I don’t know why you find it so hard to believe a car will roll on a steep hill from a stop.
billy,
In the case of turning the key off while driving down the hill, inertia helps the car continue to roll. In the case of the parked car, inertia has to be overcome.
I park my sidecar rig in 1st gear (no parking brake). If I attempt to push it while in gear, I can make the drive wheel skid on the pavement, but it remains locked.
Give it a try from a dead stop.
Should it not be more unlikely to roll if the gear ratio is lower (i.e., parked in fourth or fifth gear) rather than first or reverse? It would seem that a 4th gear ratio of 2.73:1 (4th gear 1:1; rear-end 2.73) would offer more resistance to a roll than a 1st gear ratio of 10:1.
An exception might be an old Saab with the transmission freewheel engaged, but that’s an odd case (Saab continued to have the freewheel feature up into the early years of the model 99, long after they had stopped using two stroke engines).
(The freewheel was this feature that allowed the car to outrun the engine speed while still in gear - you could coast down a hill with the engine idling just like you were in neutral, but you could rev up the engine to “catch up” and start accelerating if you liked. It stemmed from the two stroke engines which you weren’t supposed to engine brake with for any extended length of time.)
Insert “less resistance” where “more resistance” apppears.
Boyo Jim–to elaborate on the damage question, the only thing that I can think of that could go awry in the engine is the remote possibility of the timing chain/belt jumping a tooth (or teeth). Typically, if it jumps one or two teeth, the car will run poorly. If it jumps several teeth (which I seriously doubt could happen from one instance of turning backwards), it won’t start, and if it’s an interference engine it could bend the valves. I’ve never actually seen this type of damage, but it’s not impossible.
billy–what I find unbelievable is a car rolling on a very gentle grade such as Boyo Jim described in his 2nd and 3rd posts (“very slight slope,” “in the garage,” “on very slight inclines”). I didn’t specify that condition in my post you cited, as it was continuing discussion from posts where it had been specified.
As for a car that’s in gear rolling from a stop on a steep hill, as the OP suggested, that is certainly possible. It varies with the particular car and the steepness of the hill.
Ringo–in lower gears (higher gear ratios), it’s easier for the engine to turn the wheels than it would be in high gears. In higher gears (lower ratios), it’s easier for the wheels to turn the engine than it would be in low gears.
Here’s a very simplified way to visualize it: in low, it takes 4 engine revolutions to achieve 1 wheel revolution, while in high it takes 1 engine rev to get 1 wheel rev. The engine gets four times as many power pulses to move the car in low than it does in high, making it easier to take off (because the engine is driving the wheels). By the same token when you’re pushing/rolling it, you have four times as many compression strokes per wheel rev to fight against in low than in high, making it harder to roll (because the wheels are driving the engine). (Your correction is right, but it negates your assumption.)
yabob–the Saab freewheeling feature was activated by a vacuum switch. It should not freewheel with the engine not running.