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  #1  
Old 10-17-2001, 03:22 PM
xash xash is offline
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so is it better to park in first gear or in reverse gear on a slope that is :

a.) downwards with car facing downward ( i.e. car will roll downhill head first if left in neutral )

b.) upwards with car facing upward ( i.e. car will roll downhill tail first if left in neutral )

assumptions: manual transmission, handbrake not used.

is there a difference ? and why ?

which option ( within each scenario ) is more likely to damage the gear box ( or engine ? ) if pushed downhill by a large force ?
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  #2  
Old 10-17-2001, 03:33 PM
mancunian mancunian is offline
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Best thing to stop you rolling is to turn the front wheels towards the kerb ( sorry , don't know the US word. Maybe the same)
This stops the car rolling anywhere.

Whatever gear you are in will not effect if the car rolls or not. I don't think anyway.
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Old 10-17-2001, 03:59 PM
xash xash is offline
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thanks jefnixon... that would indeed be a good way to stop the car from rolling easily...

but i am interested more in the effects of force on a parked car in different gears rather than on practical ways to prevent a car from rolling down a slope...
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Old 10-17-2001, 04:02 PM
Robb Robb is offline
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xash, here is a thread with a similar question.
While parked, what gear?
It seems to be more of a contentious question than you might guess.

jefnixon, I bet kerb = curb in the US.
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  #5  
Old 10-17-2001, 04:03 PM
Tony Montana Tony Montana is offline
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first gear both scenarios. IMHO or more correctly in my experience,it tends to pop out of reverse easier. why first? its harder to get a car moving while it's in first. the engine,acting as a brake, would tend to have to spin faster while in first.
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  #6  
Old 10-17-2001, 04:22 PM
kniz kniz is offline
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I agree with Tony about 1st gear, jefnixon about turning the wheel towards the kerb (that's a nice word) and wish to add pull-up the emergency brake since you will need it while getting started again.
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  #7  
Old 10-17-2001, 04:25 PM
scampering gremlin scampering gremlin is offline
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Xash, I'd rely on the hand brake and wheel angles to keep my car parked in place on a hill. Gears aren't designed to manage that sort of stress except possibly as a last resort.
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  #8  
Old 10-17-2001, 04:41 PM
xash xash is offline
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Quote:
xash, here is a thread with a similar question.
While parked, what gear?
It seems to be more of a contentious question than you might guess.
thanks Robb...it's always difficult to search through the seemingly infinite space of the SDMB...

contentious is correct... that thread seems to be inconclusive in reaching a consensus... there are 3 teams playing there... one says leave it in a gear against the roll...one says leave it in a gear along the roll... and one says leave it in a specific gear always... and each seem to say it with as much conviction as the other...

i stress again that i am not looking for practical solutions for keeping a car from rolling... i am interested only in the specific case of which gear to leave the car in, with the assumption that it is unassisted by the emergency brake, to try and figure out which has more ( or less ) damaging potential for the engine/gear box.

and is it the gear box or the engine that shall sustain damage ( or both ? ) if forced downhill in the given scenarios ?
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  #9  
Old 10-17-2001, 05:19 PM
Robb Robb is offline
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xash, the last thread left an impression. I too wonder about the answer.

I have a hard time believing that leaving a parked manual transmission in a gear causes damage to a transmission. Back when I studied things such as dynamics, I learned that dynamic loads tend to be much higher than static loads. Therefore, I conclude that a transmission in motion is under greater stress than a transmission parked in a gear. I also think that my transmission used to suffer more when I revved up the engine, dropped the clutch, and tried to see how much tire I could leave behind. (mind you, the police take offense when you do this right in front of them.)

My guess, is that if the car acutally rolled, the transmission wouldn't be in too bad shape. I think that it might be bad if the roll was due to the clutch slipping. That can't be healthy for the clutch. I don't think that the engine would be damaged. It would just be spinning like it normally does, just not under its own power.

I think that your better bet for finding an answer is the cartalk.com people. They seem to have some experience with off-beat car questions.
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  #10  
Old 10-17-2001, 07:35 PM
Kalashnikov Kalashnikov is offline
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Sheesh.

Haven't you guys ever push-started a car?

Kids these days!
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  #11  
Old 10-17-2001, 09:01 PM
El Zagna El Zagna is offline
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I am speaking from experience here. I had a '63 VW that needed to be push started more times than I can count. If you tried popping the clutch in 1st gear it would come to a sudden, jerky stop. Same thing for reverse (sometimes I was stuck on a hill and gravity dictated my options). 2nd or 3rd was much easier.

For most cars reverse is actually a lower gear than 1st.

My car knowledge ends here.
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  #12  
Old 10-17-2001, 09:28 PM
TBone2 TBone2 is offline
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If you refuse to employ the parking brake or turn the steering wheels toward the curb ('kerb,' whatever) or resort to any other sensible precaution, reverse is your best bet for getting the maximum braking force from your engine. As I think bnorton was attempting to point out, in most transmissions, reverse is the selection with the lowest reduction ratio, that is the engine must turn more times per foot of travel in reverse than in any other gear. Working this mechanical advantage backward, it is clear that reverse will get the max out of the braking power of the engine.
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  #13  
Old 10-17-2001, 09:34 PM
Telemark Telemark is online now
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Whichever gear is lower ratio on your car, first or reverse. It really doesn't matter which direction your car is pointed, engine compression won't change.
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  #14  
Old 10-17-2001, 10:19 PM
Ringo Ringo is offline
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The braking power of the engine will come from both friction of the moving parts and the (unfired) compression in the cylinders. That's X amount of resistance that will be transferred to the drivetrain via the transmission. The gear ratio is the key component here and what I've been able to gather from the few sites that actually include reverse gear ratios they are generally equivalent to first gear (~3.5:1).

Ultimately then, that does nothing to settle the question. So in an unfired cylinder, does your exhaust valve open early or late, or on time? Another question to address is whether the helical (forward) gears of your transmission disengage more or less easily than the straight-toothed (reverse) gear when subject to (the) stress (of gravity tuggin' your ride downhill).
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  #15  
Old 10-17-2001, 11:12 PM
Princhester Princhester is offline
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The people making the valid points in this thread and the other one linked by Robb, are Robb, brad_d and sewalk.

Anyone who suggests that you should put the car in the gear that opposes the slope (i.e. reverse when facing downhill and first when facing up) is working on an intuitive but dead wrong assumption that engines are harder to turn backwards than forwards, which shows they know nothing about engines and have probably never worked on them extensively. Quite simply and as others have pointed out, car engines do not contain ratchets that prevent them turning backwards.

The only reason for preferring reverse over first is that (according to some posters) reverse is a lower gear. If it is on some cars, (I'm not saying it isn't) it's not lower by much.

Personally I would be much more worried about driving an engine backwards than the possibility that I am on a slope sufficiently steep to turn the engine over in first but not in reverse. Sewalks points about the possibility of timing gear damage in a reversed engine. I would also be worried about various pumps pushing oil the wrong way.

As to damage to the tranny due to the engine being forced to turn by the slope, forget it. What Robb said. How can the forces created by the slope and gravity be greater than the forces created by an engine under normal driving conditions? Or to put it another way, by driving up a hill, you are clearly applying a greater force through the tranny than gravity and the slope of the hill can create, otherwise you couldn't go up it, could you? And that's before you even begin to consider dynamic forces.
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  #16  
Old 10-18-2001, 07:31 AM
pulykamell pulykamell is offline
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Isn't reverse a lower gear in most cars (if not all?) At least with the older cars, you still occassionally (very occassionaly) in Eastern Europe (Romania) see cars going up hills in reverse, when first won't do. Just wondering. I've been taught that reverse is almost always geared lower than first.
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