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Old 01-01-2013, 07:16 PM   #4
NN5I
Carl, nn5i
 
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Join Date: Sep 2011
Location: Tallahassee, FL
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Typically an all-wheel-drive (AWD) vehicle is intended to be in 4WD all the time, and has a transfer case that includes a differential gearset between the front and rear axles so that the front and rear axles can turn at slightly different rotational speeds -- in the same way that the differential gears in each axle allow the port and starboard wheels to turn at different speeds.

Vehicles called 4WD rather than AWD usually don't have this third differential nowadays, and should be in 2WD and not 4WD when they're on dry pavement. Ever-so-slight differences in rotational speed, front and rear (which might be due to tiny differences between tires, for example, or even caused by a heavier load on the front than on the rear) could cause the front wheels to try to roll one mile plus 1/2 inch while the rears were rolling only a mile. With no differential gears between front and rear, that can develop forces in the drivetrain sufficient to break teeth in the transmission and axles, or to cause rapid wear. So 4WD vehicles should be in 4WD only when the road surface is at least a little bit slippery; OK on wet pavement, not OK on dry pavement.

Having said all that, many modern AWD vehicles have complex couplings, sometimes hydraulic, that measure slip at each wheel and vary the power sent to each wheel (sending less power to a wheel that is slipping). Such a system often has a dedicated computer that also includes handling assistance and the like. Too complicated for me.

I once owned a 1978 Chevy 4WD pickup that had what was then called "full-time four wheel drive" -- its transfer case did have a differential gear, and you could drive in 2WD or in 4WD regardless of what surface you were on. It worked well.

As for why they'd say don't use 4WD on snow and ice, I think they're wacky. But what would you expect of people who don't proofread what they write?
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