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Old 11-07-2013, 07:55 AM   #11
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Originally Posted by nealmccarter View Post
When the temperature drops, we get a lot of black ice here. Several years ago, the highway patrol issued warnings to 4x4 drivers that they are more apt to spin out under these conditions.
Hi: nealmccarter...Twenty years ago I used to drive for an "Airporter" service. We get a lot of black ice too!!! One early morning while watching cars ahead spin out of control @ 60kph. a passenger asked me "How come we're not". My reply was simple..."We're in Neutral". Alf
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Old 11-07-2013, 09:19 AM   #12
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I have driven in severe winter road conditions my entire adult life with drive options including 1WD (motorcycle), 2WD, AWD, and 4WD. In my opinion, it is more often than not the lack of skills of the driver that leads to problems in icy or snowy conditions, not the vehicle. You have got to have the ability to recognize poor driving conditions and to adjust your driving accordingly. Poor drivers are still going to be poor drivers, whether they are in a 4wd, 2wd, AWD, or on a motorcycle.
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Old 11-07-2013, 09:41 AM   #13
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Poor drivers are still going to be poor drivers, whether they are in a 4wd, 2wd, AWD, or on a motorcycle.
Absolutely, but 4WD systems (including those labeled "AWD") are powerful enablers of incompetent driving.
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Old 11-07-2013, 12:00 PM   #14
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Hmmm... I replied to a post, which has since disappeared. Well, I wrote it, so I'll leave an edited version of the content:

Despite comments that I have heard in more than one forum and personal discussion, cars are not one-wheel-drive... unless you drive a child's toy car. Many adult tricycles with two wheels in back are one-wheel-drive as well.

I don't know if all of these comments are serious, but many people seem to genuinely think that their two-wheel-drive car (either front-wheel-drive or rear-wheel-drive) only has one driven wheel, because on a slippery surface only one wheel spins. What's happening is that both wheels at the same end of the car are being driven with the same torque (twisting force), and the one with less traction (on an icy or muddy patch, or with less weight pushing it against the road) breaks loose and spins first. From that point on, since the spinning tire can't get any grip, and the other tire only gets as much torque as the spinning one can take, neither tire does much good.

If you really had one-wheel-drive, the pulling to one side would be very annoying!

The differential is the component which distributes the power between the wheels, and there are various "limited slip", "locking", and "torque biasing" differential designs used to avoid this problem by doing something other than simply splitting torque equally. Electronic traction control systems that apply the brake on only the spinning tire are another solution. For towing your Escape, if you want to get in and out of those challenging campsites, traction control and/or a limited slip differential are a good idea.

Just to expand on my earlier comments about my own commute... for the first 14 years of living here, I didn't even have traction control or limited slip on my two-wheel-drive cars, and our van (the travel trailer tug) still doesn't have either. These features are good, but certainly not necessary for normal roads, even in the winter in rural Alberta. We don't need no stinkin' AWD!
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Old 11-07-2013, 12:38 PM   #15
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NPR did a story one day with a guy from Motortrend. He said that one reason so many AWD vehicle drivers end up with problems is that the AWD is very good at pulling, ie starting out on slippery conditions. People back out of their drive and start off driving and it gives them a false sense of security. His suggestion was that they immediately try a full stop and see if they slide then.

When I was still commuting here in ice and snow country, I always had Blizzak ice/snow tires on all 4 wheels of my Corolla, traction control and electronic stability control but I knew that none of those things can change the laws of physics, so I would be driving at no higher speed than conditions would allow. People used to fly by me seemingly thinking that they could drive at or above the speed limit regardless of conditions. Frequently I'd see those same people in the ditch a mile or so up the road.
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Old 11-07-2013, 11:28 PM   #16
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When you go around a corner with a 2 wheel drive car essentially only one wheel is driving as the purpose of the differential is to allow one wheel to turn faster/freewheel relative to the other, otherwise the drive tires would wear out very quickly. On a very slippery surface the same thing happens, essentially only one wheel has power to it. Jack up one drive wheel of a two wheel drive vehicle and functionally no usable power is transmitted to the one on the ground, the car won't move. Don't do it but with both driving wheels in the air at a slow speed you can stop one tire with your hands and easily hold it while the other merrily spins.

On the road under normal conditions in a straight line 2 wheels are driving so your choice, call it one wheel drive or two wheel drive both are correct depending upon conditions. The only true 2 wheel drive vehicle under all conditions would be a solid axle.

A 4 wheel drive with standard differentials and a standard transfer case under slippery conditions may essentially have only one front and one rear wheel with power. If the the 4 wheel drive is full time 4 wheel drive/AWD and standard axle differentials slippage may occur in the transfer case under slippery conditions and essentially only one wheel of four can get power hence the reason for the "lock" position on some transfer cases.

I've read that due to the tooth geometry of some old (e.g. 1930's, 1940's etc.) differentials a standard differential acted a little like a limited slip and did transfer some power to the stationary wheel. Limited slip can be a pain on a corner as both drive wheels can start spinning and that end of the car will slide out far easier (don't ask how I know). That is why a positrac or similar automatic traction differential is not put on a front axle by the manufacturer unless it is a true off road vehicle (slow speed). An air locker differential is different as you can (and should) choose not to engage it on the highway.

The old adage 2 wheel drive in and 4 wheel drive in reverse when you get stuck usually works. I agree with thoer, AWD or 4 wheel drive does not change the laws of physics for braking and steering.

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Old 11-08-2013, 12:18 AM   #17
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When you go around a corner with a 2 wheel drive car essentially only one wheel is driving as the purpose of the differential is to allow one wheel to turn faster/freewheel relative to the other, otherwise the drive tires would wear out very quickly.
Keeping the two tires on the same axle from fighting each other is the purpose of a differential, but a normal differential (anything in a production road vehicle) is not a pair of freewheels (overrunning clutches). Even while going different speeds (or the same speed) each wheel gets the same torque as the other - neither gets zero or "freewheels".

See any basic auto mechanics description... such as this one by GM from 1937, which I think is very well done, even though it doesn't mention torque. If you watch this video, note that both wheels are always being driven. It also explains hypoid ring-and-pinion gearing, calling it "low drive".
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Old 11-08-2013, 12:31 AM   #18
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Originally Posted by Dave D. View Post
Limited slip can be a pain on a corner as both drive wheels can start spinning and that end of the car will slide out far easier (don't ask how I know). That is why a positrac or similar automatic traction differential is not put on a front axle by the manufacturer unless it is a true off road vehicle (slow speed).
That's not the only reason for avoiding limited slip in front. Another and more important reason is that the unequal and changing torque split fights the steering. Because of this, only the smoothest-acting limited slip designs are used in the front for production road vehicles, and even then they're uncommon. Worm gear designs (Torsen or Quaife) and viscous coupling systems are the most common... and they're on the sporty front-wheel-drive cars with the highest power-to-weight ratios, such as the Mazdaspeed3. Low-speed applications are more likely to use a simple locker.
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Old 11-08-2013, 12:41 AM   #19
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I remember my 1987 Subaru wagon. If you put it in four wheel drive and tried to turn a corner on pavement, the engine would die. Didn't have the oomph to turn the wheels. Then you had to straighten the wheels and back up several yards to unlock the 4X4.
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Old 11-08-2013, 12:44 AM   #20
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Jack up one drive wheel of a two wheel drive vehicle and functionally no usable power is transmitted to the one on the ground, the car won't move.
Right, no useful power is transmitted is transmitted to either wheel, because the are both getting the same minimal torque: the one in the air can only take enough to overcome bearing friction, so the one on the ground gets the same minimal amount and thus can't move the car.

Quote:
Originally Posted by Dave D. View Post
Don't do it but with both driving wheels in the air at a slow speed you can stop one tire with your hands and easily hold it while the other merrily spins.
Again, your hand is applying just a bit of resistance, matching the bit of spinning bearing resistance (and brake drag, and tire and wheel aero drag) of the spinning side. Both are being driven, each by a torque limited by the one with the least traction (the "free" spinning one).

Quote:
Originally Posted by Dave D. View Post
The old adage 2 wheel drive in and 4 wheel drive in reverse when you get stuck usually works.
Same thing for winches: use them to pull back out when you get stuck, not to pull yourself into being so stuck that you need a bigger vehicle and winch to get you out!
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