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Originally Posted by tw
Either each wheel operates independently. And then tires do not fight each other. And then one tire that slips is the only wheel turning. So, four wheel drives interconnect all wheels so that all spin together. So that no one wheel spins with other not spinning. Wheels must be interconnected to that all wheels turn even when one slips.
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This is where you're fucked up, 4 wheel drive does NOT interconnect the 4 wheels to all turn together. 4 wheel drive is a misnomer, it's actually 2 axle drive. Each axle has 2 wheels (except dualies but forget them), and between the 2 wheels is a differential, like all cars.
Quote:
The differential has three jobs:
•To aim the engine power at the wheels
•To act as the final gear reduction in the vehicle, slowing the rotational speed of the transmission one final time before it hits the wheels
•To transmit the power to the wheels while allowing them to rotate at different speeds (This is the one that earned the differential its name.)
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So with 2 wheel drive, 1 wheel is actually powering the car, and with 4 wheel drive it's two wheels powering the car. Only certain race cars that operate on loose (dirt) surfaces, or only turn at slow speeds (drag racers), have a locked differential (called a spool) to turn both wheels on the axle together.
Now many (most?) 4 wheel drive vehicles use a limited slip differential on at least the rear axle. Either Eaton's clutch type (like GM's posi-traction), a ratchet type (like Ford's Detroit Locker) or the newer Torsen geared type (developed for front wheel drive cars) so that when the wheel powering the axle loses traction, the power is transfered to the other wheel on that axle. Newer (read high end, more expensive) systems, especially full time all wheel drive systems, use a limited slip differential between the front and rear axles.
Many 4 wheel drive systems, especially in trucks, are designed and built so the front axle turns a tiny bit faster than the rear axle. This is for straight line stability at speed. On loose surfaces it's not a problem, but on dry (good traction) surfaces it can cause increase rear tire wear. This is where that "wheels fight each other" shit comes from. Actually a couple of pounds difference in tire pressure can cause the same condition, but it's not a serious safety issue.
Quote:
Originally Posted by DanaC
I live in a tiny village; with lanes designed for single lane horse and carriage use. When I walk past the village school the place looks like a fucking 4x4 convention. Massive cars, using massive amounts of petrol, to ferry a small child the ten minutes it takes to get to school. Thereby making the roads significantly more dangerous for any children whose parents elect to walk them to school.
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This has nothing to do with 4 wheel drive, it's a social issue. Scandinavia is loaded (dominated?) by 4 wheel drive vehicles that are no bigger than the 2 wheel drive counterparts. Subaru, BMW, VW, Ford, Audi, Infiniti, Volvo, Mitsubishi, are some of the companies making 4, or all, wheel drive cars no bigger than the 2 wheel drive crowd.
Now the newer electronically controlled, viscous coupling, 4 wheel/all wheel drives sound like the perfect choice for being prepared. BUT, the reality is the are more expensive to buy, and to operate. More moving parts to buy initially, more shit to wear out, more systemic parasitic power losses, although the new systems have cut those losses to a minimum. Plus the fact that in more heavily populated parts of civilization, the roads are cleared pretty quickly, usually less than 24 hours (bare pavement in 36) in most cases.
Therefore, the cheaper/smarter choice, unless you do some serious off roading (damn few do), or you're a survivalist, is leave the 4 wheel drives to the EMTs, or people that absolutely-positively-have-to-get-there, and stay the hell home and have orgies, on the few days a year the roads are bad... or learn to drive in bad weather.