AWD is almost useless beyond pavement




Glossary


larger tires


ten stupid things



what is ...

part time 4WD ?


full time 4WD ?


all wheel drive ?

difference AWD and 4WD?

automatic 4WD ?





4WD school


locking differentials










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© 1996-2005 Harald Pietschmann





My comment relates to the way most "all wheel drive" vehicles (usually
minivans - some SUVs like Lexus RX300 or Ford Explorer V8) distribute torque
to the wheels and how they manage traction.


why you find me here using the term torque rather than power



Part time 4WD vehicles always distribute torque inside their transfer case
50/50 to front and rear when the shifter is in one of the 4WD positions (Hi
or Lo). Most of the time each of the 4 wheels gets 25% of the torque - that
minimizes wheel spin. This system is very strong and reliable -
unfortunately it can't be used on pavement. Should one of the wheels or one
axle lose traction - the other axle will still receive a reliable supply of
torque (up to 100%) through the transfer case.

Conventional full time 4WD vehicles use a differential inside the transfer
case to make 4WD use on pavement possible. This is done with a ring and
pinion set or a planetary gear set. Each of the 4 wheels constantly gets 25%
of the torque (as long as the ground is level and traction is equal) - that
in itself prevents wheel spin. This setup is also very strong and reliable.
However, this system would be handicapped for off-road use (it is much more
likely to lose traction off-road than on-road) if it would not have some
sort of spin control. Call it traction control, if you will. If one of the
tires or one axle would lose traction - the differential in the transfer
case (often called "center differential") would allow the axle with lost
traction to spin (could be both tires or one). To avoid this some of the
full time 4WD vehicles offer a manual center differential lock (all
LandRover, Mercedes G etc.) or other means of (automatic) spin control. A
mechanically locked center diff acts like a part time transfer case - and is
as strong and reliable as a part time system off-road. Some vehicles do not
manage or lock any of the differentials. Instead they have systems that
slows down spinning wheels.

Now, all wheel drive (AWD) is a completely different "beast". In my opinion
it is useless beyond pavement because it does not have low range like the
other two systems. Low range provides necessary crawl speed for maximum
control during difficult driving situations. And low range range provides a
significant torque multiplication.Low gears in a 4WD are like the low gears
in a mountain bike. Can you imagine a mountain bike without low gears? Main
drawback of AWD is that the transfer case can't be manually locked. Two
different systems are in use - neither one works well for use beyond
pavement: AWD System one (true all wheel drive - or full time symmetric
AWD) has a conventional differential inside the transfer case - each of the
wheels gets about 25% of the torque as long as traction is equal. However,
the center diff cannot be mechanically locked.
To prevent a complete loss of traction when one wheel or one axle would
spin, a viscous coupling or a similar device like a Haldex coupling (see
note) will try to "glue" both driveshafts together to keep enough torque
flowing to the axle with traction. Works kinda OK on slippery pavement when
the vehicle has already sufficient momentum and the connecting device has to
kick in very infrequently. Off-road or in other situations with slow speed
and high demand for torque the glue box (viscous coupling or Haldex etc.) is
overstressed and fails to deliver the needed torque. High torque transfers
and continous use make especially viscous couplings fail. Haldex units are
much more reliable but cannot satisfy the constant high demand for torque at
all wheels either.

AWD System two (automatic asymmetric AWD - and in a way actually only a
sophisticated 2WD system) might not have a differential in the transfer case
(Volvo, Honda, etc.) but some do (Jeep Grand Cherokee). Primary power goes
only to one axle (makes spinning tires much more likely due to inefficient
use of traction - as likely as in any other 2WD car). However, both drive
shafts are joined by a viscous coupling or a similar device (see note) and
as long as all 4 wheels turn at the same speeds the control unit remains
inactive. Once the powered axle or one of the powered tires loses traction,
the powered drive shaft rotates faster than the one that is just rotating
along. The control unit reacts to the speed difference and kinda glues both
drive shafts together. This way the previously unpowered shaft will get some
of the torque and rescue the failing tires. Same story as in system one:
Works kinda OK on slippery pavement when the vehicle has already sufficient
momentum and the control unit has to kick in very infrequently. Fails
miserably when need of high torque arises or when activated frequently.
Cannot satisfy the constant high demand for torque at all wheels when
off-road.

Now you may understand better why I think AWD is not suitable for use beyond
pavement.

The last generation Jeep Grand Cherokees (1999 WJ and 2005 KJ) with
QuadraDrive II are not to be confused with a viscous coupling system. The J
GC also powers only one axle (rear) but the Gerodisc technology and
E-Gerodisc used by Jeep is very strong, very reliable and provides
sufficient amounts of torque to where it's needed.





"Capt. Rob" wrote in message
oups.com...
16 gallon gas tank + 15 mpg = 240 mile range! That's the same as a 1973

Toyota Landcruiser! And the Land Cruiser (FJ-40) is no Boulevard car!


Tribeca gets 18 MPG city, 24 HWY. I get just under 25, sometimes
better.

Tribeca has VDC traction control, full time AWD at all speeds. The
Tundra doesn't.

Tribeca has quieter cabin, 3 zone climate control. Wind noise is also
lower in the B9.

Tribeca is capable of 135 MPH compared to the Tundra's 115 and the poor
Tundra can only do that sad speed in FWD mode!

Tribeca maintains antilock even when traction control activates while
the Tundra must rely soley on it's simple less sophisticated ABS
system.

The Tribeca has 5 speed sportmatic transmission with sport mode and
manual shifting.

Now...would you like to know how badly the B9 clobbers the Tundra on
the skidpad???? You'd look a little smarter if you had a clue that the
B9 is not a truck or even a standard SUV.

RB
35s5
NY