Center diff and icy roads


#1

On the last Cartalk a lady had I think a Rav4, or similar. She had to come down an icy road, and was told that locking the center diff is seriously bad under these circumstances. Unlike our guys I’m no mechanic, but I thought locking the center diff sent power equally to both axles, making the car a true 4WD. The individual wheels on each axle would still spin according to available traction, but one end would not recieve all of the power. Isn’t this the idea when using a 4WD or AWD on slick surfaces, rather than letting one slipping wheel take up all the power? I thought that’s why these vehicles had a center diff, so they could function as 4WD when necessary, and 2WD at other times. Did C & C get a locking axle diff mixed up with a locking center diff? After all, automatic Rav4s and similar vehicles automatically lock the center diff in slippery conditions to send power to both ends; it’s only in manuals that you have a locking diff to achieve, I thought, the same end.


#2

If you were stuck on ice going uphill you might want to lock the center differential but going downhill wouldn’t accomplish anything good. Best to just take their word for it and stop the reasoning process. You could go the other way and assume that they don’t really know.


#3

Neither 4WD and AWD do much of anything to keep you safe. They both may help you get back on the road after you slide off, but they do zip or less at keeping you from sliding off to begin with. They also don’t help you stop on the ice or snow.

Under some conditions some versions can cause wheels to spin that can increase your chances of loosing control and sliding into something or off the road.

In short, you are likely safer in without AWD or 4WD in the snow and ice, but not by much. You are likely a little more likely to get were you are going however.


#4

I agree that AWD won’t help you stop faster, or better. However, in all ski areas when roads are dicey the warnings advise use of chains or 4WD. Surely they know something, and the number of Subarus in Anchorge must also be saying something. One of the basics of 4WD is being able to use it immediately; so if coming down a hill and you do lose control and start to slide, 4WD will surely help you regain control quicker than 2WD. So even if the locked center diff won’t help her stop, wouldn’t it perhaps be better for control, being more able to take advantage of what traction does present itself, and then if she does start to slip towards the edge of the road, where there is often more traction than in the middle, be able to pull back onto the road before ending up in a ditch. 4WD shouldn’t normally be used on roads due to the need for the axles to rotate at different speeds for cornering, but on slick ice that shouldn’t be an issue, same as on any other loose surface. I guess my main question would still be, why was it vehemently recommended against, rather than just being indictated that it may not be of much use? As far as I know these center diffs are not limited slip, which would make a difference certainly. Or perhaps they are.


#5

Life is not simple drivimg an AWD/4 wd vehicle. Not being familiar with what it can and can’t do can be dangerous. Added braking of locking diffs. central, front or back may add some security off road, but trust me as someone who drives and plows icey roads EVERY day during the winter. Going down hill on ice is NO fun and is better left to no braking in any way that can’t be instantly controled (pumping brakes or ABS). I have at time hills so slipper, I had to take the manual hubs out as the drag brake a steering wheel enough to loose control.

Don’t lock on slipery roads…that feature is for high drag situations that require traction but induced spin like heavy snow and mud and sand.

Buy the way, even ice chains can turn to “skates” on hard ice. and a one ton sand truck becomes and uncontrolled bowling ball. That’s why we keep the sides of the road high and hard on hills…the “toboggan” effect.


#6

AWD and 4 wd do add steering control if judiciously used. Even and balanced traction under controlled speeds and they are much better than 2wd. Over extend their capabilities which many often do, and you’ll be lucky if your roof is reinforced. Awd/4wd can get you into deeper trouble , faster than a 2wd. Used properly, the performance capabilities are ALWAYS greater.


#7

You’ve got a conflict here between avoiding spinning wheels and avoiding spinning the whole car. 4WD’s problem is that taking a turn on a slippery surface can cause one or more wheels to lose traction because it’s being forced to go faster or slower than it needs to. 4WD has differentials to split the power (at different speeds) left and right, but no center differential to split power and speed front and rear. AWD has a center differential, so that all 4 wheels can turn at different speeds, if need be, in a turn. The drawback is that on a slippery surface, one spinning wheel could leave you immobilized as all the power goes to that wheel (just like a 2WD when one wheel is on ice). With 4WD, at least you have a chance that the other axle can get you going. The upshot is that 4WD (or the equivalent, a locked AWD center differential) should always be disengaged once you’re going, to reduce the chance of spinning out on a curve.