Re: RE: Part Time 4wd

From: fawcett@uism.bu.edu
Date: Wed Sep 23 1998 - 16:56:03 EDT


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     A few thoughts here David,
     
     <<Brian, you've got it all backwards.>>
     True!
     
     <<4wd Part time is the original system, used on the jeeps in ww2 (and
     I think 1 as well, but i'm not a history buff). It locks all 4 wheels
     to go the same speed all the time, which is great for off reading,
     because you'll never be stuck with 1 spinning wheel and being unable
     to move.>>
     Could be, but I'm not sure... I think the old Willeys were a true 4WD
     system that would not be considered part-time... Worked more like the
     lockers that are sold aftermarket today or perhaps even more rigid?
     
     <<However, on dry pavement, it adds a lot of stress to the driveline,
     as the outside wheels need to turn faster when taking a corner and
     cannot with 4pt.>>
     This is kinda' true but not with the standard (or open) diffs most
     4X4's come with... The tension or "winding up" of a part-time system
     comes from the fact that the torque provided to each axle is equal
     and, therefore, does not let the axles turn at different speeds in
     high traction conditions (on the road). This is what causes wear and
     expensive repair bills with part-time systems with open diffs!
     
     <<4wd full time is a system that has 3 differentials (pt has only 1,
     the standard rwd unit), so all 4 wheels can turn at different speeds
     as conditions require. This is perfectly suited to dry and wet road
     conditions, but can cause a loss of traction in severe conditions
     (stuck in mud, etc).>>
     True... But each axle has a diff so the part-time system has 2 diffs.
     While all 4 wheels can spin at different speeds with open diffs on
     each axle, the keynote here is that the full-time system has a diff
     type system in the transfer case which allows the axles to turn at
     different speeds.
     
     <<This is why manufacturers often include either an automatic or
     manual locking center differential, which prevents all the engine's
     power from going to one spinning wheel.>>
     I think we are mixing things here... Shift on the fly is basically a
     convenience item to engage the front axle... The only way to prevent
     wheels from spinning is to lock the differentials on the axles...
     Great for off-road -- very bad for street.
     
     <<AWD, or all-wheel drive, is a lot like 4ft except you cannot turn it
     off. often times these systems are designed EXCLUSIVELY for road use
     and are far less suitable for off-roading.>>
     Basically true but I've heard that Mercedes (I think) puts out an
     impressive AWD rig that uses the ABS brake system to keep power coming
     to wheels with traction... So they keep open diffs for great highway
     performance/manners while maintaining off-road traction that would put
     alot of 4x4's (including my Kota) to shame!
     
     T.



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