When I back my 4WD subaru into my icy drive way from a full stop, one wheel loses traction on this sheet of ice while all my other wheels are still on solid ground but none of them move.Why is this? Doesn‘t this defeat the purpose of 4 wheel drive?
Interesting. The car lacks a limited slip differential. I don't suppose many people know that. A car with LSD will transfer power to the wheel(s) with traction when one loses grip. Subaru can't have bothered with this. I expect most decent 4WD cars have LSD but it's something to be considered if you live in a snowy area. Some Jeep type vehicles have a transmission lock that will send equal power to all four wheels no matter what traction is lost.
They're pretty much the same. However do not buy one without the temperature monitor on it, no matter how cheap it is.
OK the high $ ones work better but they cost 100$ but if you want straight hair than go for what you want!
It's an issue with the traction control. When one wheel spins the computer backs off the power and gently applies the brake to that wheel, stopping it spinning and transferring power to the other wheels. But then the next wheel starts to spin as well, so it brakes that one. Repeat until it's braked all the wheels, as none of them have traction. Now it has run out of options, so it just sits there. You can turn off traction control, but then you are back to the wheel with the least traction simply spinning again. More sophisticated 4WD systems have locking or limited slip differentials, or computer controlled clutches in the diffs, that do a better job at applying power to all the wheels, but that's actually of limited use as having 4 wheels spinning on ice is still zero traction, and will probably just cause you to slide sideways into a ditch anyway. The system your car has isn't useless, it's just the conditions are beyond it's capacity. If you had some traction on the other wheels it would be able to move you. It's just when you have 4 wheels slipping, it's stuck, and doesn't sit there spinning pointlessly. When conditions are that bad, you need tire chains to give the tires a bit of traction.
Firstly, because there is less weight on the rear wheels their traction is always inferior to that of the front wheels. So leading with the rear will never be ideal. Probably your model of Subaru has an open differential (Open Diff) and lacks a limited slip differential (LSD). With four wheel drive you have power going to every wheel until you lose traction. What a differential essentially does is give power to the wheel with the least resistance. An open differential allows each wheel to spin at different speeds to provide increased handling around corners by relieving tension on the outer wheels. With a locked differential wheels will turn at the same speed making turning more difficult. Open differentials are used because when cornering the inside wheel has less distance to cover than the outside wheel. This is a desirable trait with standard road conditions but not in slippery conditions because when a wheel looses traction the power will be sent to the spinning wheel (wheel of least resistance), thereby reducing overall traction. To overcome this some car manufacturers provide a method to temporarily lock the differentials to ensure that both wheels start churning at the same speed to provide added traction in challenging conditions. Limited shift differentials reduce unwanted violent wheel spin on one side of an axle, and the subsequent loss of drive power, while still allowing for the relatively small differences in rotation while cornering. Let us know the model and year of your Subaru and we may be able to be more specific if you need. Best Wishes and Good Luck!
