Is there a reason why automatic cars roll forward slowly when you take your foot off the break (like so you don’t roll backwards down a hill) and stick shifts don’t? Or is this just a byproduct of automatic transmission?
Do you mean while idling in gear, or in neutral?
While in gear…now that I think about it stick shifts also roll forward while in gear when you take your foot off the clutch & brake, right? What is the reason for this?
The automatic is always in gear, as long as you never move the selector lever. The engine is connected to the drivetrain through the torque converter, which is a hydraulic coupling. As long as the engine is turning, torque is transmitted through the converter.
At higher engine speeds, the torque converter locks up and acts basically like a direct mechanical linkage—like an engaged clutch. But at low rpm, the TC allows slippage. The brakes can hold the drivetrain still, while the engine turns on the other side. If you take your foot of the brake, the force transmitted to the axle isn’t counteracted by the brakes, and the car can roll forward.
In a standard, the car only stays motionless with the foot off the brake if the clutch in depressed or if the transmission is in neutral (but shift your auto to “N” and it will act the same). The clutch allows you to break the connection between engine and drivetrain.
In short, the engine is never completely separated from the wheels in an automatic that’s in gear, while it can be in a standard.
I hope that’s clear. If not, let me know.
Bob, the torque converter in an automatic transmission takes the place of a conventional clutch. The torque converter by design, allows slippage between the input rotation of the engine and the output to the transmission. There is some torque transfer that occurs at idle through the converter, so if the torque required to move the car is low enough, the converter will provide the power.
Prize goes to first doper to find the cite to answer the question.
The answer is: because its engine is running and it is in gear!
Sorry Kid, you were born early. Our cars are not always smarter than we are quite yet. Close, but not quite. Maybe you’re kids.
I just assumed it was because nothing was stopping it and the front end was heavier. Isn’t the front end heavier, with the engine block and all?
ronbo is right. With the engine running and the car in gear, what the heck did you suppose it might do? Stick shifts do this also, as the OP has noted.
The car is in gear, and the idle speed is high enough to make an engaged drive train move it. What is the question?
The auto trans in my car will definately let it roll back while in gear, even on a slight incline. Important to know in San Francisco. I don’t know about when it’s level. I’ll give it a try.
Actually, if you take your foot off of the brake and the clutch on a standard automobile, it’ll most likely stall instead of rolling slowly forward. (At least, it always does when I forget myself and do so when driving a standard.) Do automatics generally idle at a faster speed or something?
No. Although the automatic’s torque converter serves some of the same functions as the manual’s clutch, they are quite different mechanisms.
The torque converter is a fluid drive device. For a visual, thinkof two electric table fans, facing each other. Turn one on, and its spinning blade causes air to move, and that air then causes the other fan’s blade to rotate. The second fan is being driven by a moving fluid (in this case air – remember, fluids include both gasses and liquids). In a like manner, a torque converter uses two facing blade assemblies called turbines, and the fluid used is automatic transmission fluid.
There is inherent slippage in fluid drives. In the table fan example, you can hold the driven blade still with a finger. This is the equivalent of a car idling in gear with the brake on; while things are held still, there is 100% slippage. Remove your finger/release the brake, and now the driven blade/turbine starts to move. In the case of the car, there is still some slippage, but it’s less than 100% now. Torque converters have further design features so that the faster they spin, the less they slip. At highway speeds the slippage may be down to a few percent.*
Clutches also allow slippage, but only when the pedal is depressed. When your foot is off the clutch pedal, you have zero slippage, and if you’re in gear but standing still either the car will lurch or the engine will die. With the automatic, you still have a lot of slippage at idle speed, so the engine won’t die. The small amount of “grip” in the converter is enough to ease the car forward on level ground. You can get a manual to act this way by letting the clutch pedal partway up. Unfortunately, you can also wear your clutch out in short order by letting it slip like that. Clutches last longer when treated almost like “on/off” switches.
Here’s a little more on torque converters: http://www.bankspower.com/Tech_understandtorqueconver.cfm
*With a lock-up converter, slippage is zero when the lock feature is engaged.
No. Although the automatic’s torque converter serves some of the same functions as the manual’s clutch, they are quite different mechanisms.
The torque converter is a fluid drive device. For a visual, thinkof two electric table fans, facing each other. Turn one on, and its spinning blade causes air to move, and that air then causes the other fan’s blade to rotate. The second fan is being driven by a moving fluid (in this case air – remember, fluids include both gasses and liquids). In a like manner, a torque converter uses two facing blade assemblies called turbines, and the fluid used is automatic transmission fluid.
There is inherent slippage in fluid drives. In the table fan example, you can hold the driven blade still with a finger. This is the equivalent of a car idling in gear with the brake on; while things are held still, there is 100% slippage. Remove your finger/release the brake, and now the driven blade/turbine starts to move. In the case of the car, there is still some slippage, but it’s less than 100% now. Torque converters have further design features so that the faster they spin, the less they slip. At highway speeds the slippage may be down to a few percent.*
Clutches also allow slippage, but only when the pedal is depressed. When your foot is off the clutch pedal, you have zero slippage, and if you’re in gear but standing still either the car will lurch or the engine will die. With the automatic, you still have a lot of slippage at idle speed, so the engine won’t die. The small amount of “grip” in the converter is enough to ease the car forward on level ground. You can get a manual to act this way by letting the clutch pedal partway up. Unfortunately, you can also wear your clutch out in short order by letting it slip like that. Clutches last longer when treated almost like “on/off” switches.
Here’s a little more on torque converters: http://www.bankspower.com/Tech_understandtorqueconver.cfm
*With a lock-up converter, slippage is zero when the lock feature is engaged.
Is this a whoosh? The front of my television is heavier than the back, but it doesn’t go sliding around my living room if not tethered down.
Thanks for the great responses. But I’m ashamed to say I need this dumbed down a little bit - I can follow most SDMB threads, but I know nothing about cars. Specifically, what is ‘slippage’? What exactly are the gears doing that cause them to somewhat catch and somewhat slip, or cause you to slowly move forward without giving it gas? Can anyone give a quick overview of exactly what is happening inside the car that causes this? I know basically that the gas ignites and causes the cylinders to move which turns something…this turning thing is linked to some gears that then turn the wheels…I don’t know what a torque converter or clutch even does - you can see at what a low level I’m operating here!
MikeS to clarify I intended to say when you barely take your foot off the clutch - just enough so you don’t stall - and this causes (at least my car) to roll forward.
Here’s the How Stuff Works article on torque converters.
The slippage doesn’t happen around gears… the torque converter is essentially two fans pointing at each other, as others have said.
One fan is connected to the engine, and the other is connected to the wheels. When your foot is on the brake, you’re holding the second fan, preventing it from turning - but the first fan can still move, so the engine doesn’t stall. (OTOH, with a stick, the engine can’t turn in the same situation, because the clutch acts as a physical connection between the engine and wheels.)
But when you take your foot off the brake, you let go of the second fan, and it begins to move because the engine is still turning the first fan. Thus the wheels move.
…and in a stick shift car, you can replace the fans in that model with flat plates that revolve like record turntables. In normal driving mode these plates are physically held together, so the drive from the engine is transferred to the wheels. When you depress the clutch pedal, the plates are moved apart, causing the engine to be decoupled from the wheels. The “biting point” is where the plates are pressed together just firmly enough to transfer motion, but with some slippage. So you can see why the clutch wears out fairly quickly if you make a habit of holding the car on a slope using the clutch - the plates are physcially rubbing against one another.
Do you Murrykins still drive stick-shifts, anyway? When I visited I couldn’t rent a stick-shift for love nor money, and I had never driven an automatic. Imagine my surprise – and the look on the rental guy’s face – when I took my foot off the brake and the car merrily started accelerating towards the wall of the car park. 
It is hard to rent a stick here. But many of us actually do drive them for our personal cars. I’ve only owned one auto, and vowed never to buy one again.
But stickshifts in America generally only end up on three kind of cars: very cheap ones, sports cars, and trucks. And the truck thing isn’t as true anymore. Also, foreign cars seem much more likely to be MTs than domestics.
Assuming the vehicle is on level ground, how do you picture this happening, exactly? Gravity acts downwards, not sideways.
Why, it’s the condition when something is slipping! 
In my table fan example, it’s the moving air slipping around the second fan blade when you’re holding it still with your finger.
In an automatic transmission’s torque converter, it’s the moving transmission fluid slipping around the turbine. Note: as Mr2001 mentioned, this has nothing to do with the gears – this is in the torque converter. There’s 100% slippage when the car is standing still (in gear, brake on), as none of the fluid motion propels the car, and all of the moving fluid slips around the turbine. As you drive, more and more of the force of the moving fluid propels the car, and less of it simply slips around the turbine.
In a manual transmission, it’s the parts of the clutch mechanism sliding alongside each other, which is what happens when you barely take your foot off the clutch and start rolling forward. If you take your foot all the way off the clutch, you have no slippage and the car stalls (and/or lurches violently), or with more gas the car is propelled up to speed.
**
As mentioned before, the gears have nothing to do with this. In Drive, an automatic transmission’s gears are always engaged. It’s the torque converter that lets you creep forward. Likewise, with a manual transmission, you can be in gear with the clutch pedal down while standing still, and then creep forward as you slowly let the clutch pedal up. It’s not the gears, it’s the clutch that lets you do this. In an automatic, it’s not the gears, it’s the torque converter that lets you do this.
**
Check the links provided in this thread. Go to Google and find “How Stuff Works,” or enter “torque converter,” “clutch,” and other keywords. Somewhere out there is an explanation that will click with you.
Thank you, Gary T for your exquisite description of a torque converter: “For a visual, think of two electric table fans, facing each other.”
I’m a devoted manual driver and have always kind of understood in a vague sort of fluid-something-or-other way how an automatic transmission worked. I’d even read How Stuff Works’ description of the mechanism several times without gaining the insight your simple analogy provided. Finally I grok why an automatic has that disconcerting feeling of tension at a stop and why it creeps when the brake is released. Thank you!
Might I suggest a way to make your table fan analogy perhaps a tad clearer, though? Replace one table fan with a pinwheel of the same size. See, when I first read your setup, I expected both fans to be independently powered because, well, that’s what fans do – but everyone expects a pinwheel to derive its spin from a moving fluid (air). Now, when you turn on the table fan, the pinwheel spins about. Analogously, the fan’s power is coming from the engine and the resulting spin of the pinwheel is driving the wheels.