SAE TopTec:
Innovations in Four Wheel Drive/All Wheel Drive Systems
South Bend, Indiana. April 12 – 14, 1999
by Dr. Brad DeLong
Author of: 4-Wheel Freedom: The Art
of Off-Road Driving
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The
Society of Automotive Engineering (SAE) organizes a number of seminars
throughout the country every year, covering various phases of automotive design.
I attended the one on the design of 4-wheel drive systems, put on in AM
General’s hometown of South Bend, Indiana. The seminar consisted of two days
of discussions and presentations, followed by a day on AM General’s off-road
course. We had a chance to drive a number of vehicles demonstrating the new
systems available, including the 1999 civilian Hummer, which uses the new
TracTech4 (TT4) for traction control.
The attendance at the TopTec seminar was around 250 and included engineers,
designers, marketers, manufacturer’s representatives, journalists, and other
segments of the automotive industry. When one of the speakers asked the audience
for a show of hands to indicate how many people had actually driven off-road,
only about 10 or 12 people raised their hands. I was surprised to see the lack
of direct off-road experience among this group who are so heavily involved in
the design, manufacture, and marketing of 4x4 systems.
That was partially remedied on the third day of the seminar. About 105 of the
group participated in the Ride and Drive session.
4x4 Terms
4x4 terms are confusing, because different manufacturers use the 4WD
definitions differently and sometimes interchangeably. Here’s my conception of
what the definitions mean:
4-wheel drive (4WD): A general term for a drivetrain system that can send
power to all 4 wheels, but the 4 wheels are not necessarily under power all at
the same time. It uses a transfer case (T-case), transaxle, or the equivalent to
transfer power from the engine to both the front and rear axles.
Part-time 4WD: A 4WD system that can only be used on slippery surfaces,
such as dirt, gravel, snow, or ice, because if used on dry pavement, there is a
great deal of binding between the front and rear axles and driveshafts. This is
because the T-case transfers the power by using a fixed set of gears. When
turning, a vehicle’s outside wheels move farther than the inside wheels, and
its front wheels travel farther than the rear wheels. The rear wheels cut across
the turn in a smaller radius than the front wheels have made. If the front and
rear axles are locked together through an unyielding set of gears in the T-case,
binding occurs. This doesn’t happen on a slippery surface, because the wheels
slip and slide, relieving the binding.
Full-time 4WD: A 4WD system that can be left engaged on dry pavement all
the time because it has an open differential or its equivalent in the T-case
that relieves the binding occurring between the front and rear axle. It
doesn’t give as good mobility off-road as part-time 4WD, because the open
differential in the T-case allows a set of wheels (front or rear) to spin if
they don’t have traction.
All-wheel drive (AWD): A term implying that all 4 wheels are under power
all the time. It implies that the vehicle doesn’t have a 2-wheel drive (2WD)
option. Vehicles carrying an "AWD" designation usually don’t have a
low range option in the T-case. The term is misleading, because many of these
systems don’t supply power to all 4 wheels all the time.
4WD on demand: A 4WD system that supplies power most of the time to just
the rear wheels in a vehicle that is basically rear-wheel drive (RWD) or to just
the front wheels in a vehicle that is basically front-wheel drive (FWD). The
T-case, or its equivalent, sends power to the other set of wheel when the basic
driving wheels start to slip. The 4WD operates "on demand." Most AWD
systems really provide 4WD only on demand, not all the time. That’s also true
of many "Full time 4WD" systems. There are performance and handling
advantages if a vehicle operates with power to all 4 wheels all the time.
Electronic traction control (ETC):
A system which stops a slipping wheel from spinning and transfers power to
wheels that still have traction. It uses the ABS. In fact, it is the mirror
image of ABS. ABS is designed to prevent a braked wheel from locking, which
increases traction when trying to stop. The ETC system locks a wheel that’s
turning without generating torque because it’s trying to operate on a slippery
surface. This directs the power to the wheels that can still generate torque. In
vehicles with ETC, the ABS has priority. If you step on the brakes, ETC shuts
off.
Hill descent control (HDC): A system that uses the ABS and ETC to
maintain control during hill descent by automatically limiting wheel lockup and
spin. Land Rover uses it on the Discovery Series II, and on the Freelander,
available in Europe.
Torque-biasing ratio (TBR): A term which refers to the manner in which a
differential transfers torque between the right and left wheels, or if the
differential is in a T-case, the manner in which torque is transferred between
the front and rear driveshafts. The Torsen differential usually comes to mind
when torque biasing is discussed, though the TrueTrac limited slip differential
from Tractech Incorporated also uses this technology. A 4 to 1 (4:1) torque bias
means that if a wheel starts to slip, as long as it still has at least
one-fourth the torque available to the other wheel that is not slipping, torque
will still be transferred to the wheel with traction. If it slips more than
that, torque won’t go to the non-slipping wheel, and brake-throttle modulation
or ETC has to be used to stop the wheel that’s spinning and transfer torque to
the wheel with traction. The TBR is not a fixed ratio – it varies somewhat
with the rpms, the input torque of the driveshaft, and other factors. A vehicle
with Torsen or TrueTrac differentials may use a TBR of approximately 3:1 to 4:1
in the rear and approximately 1.5:1 to 2:1 in the front. The Torsen I
differential uses a worm-gear arrangement to transfer the torque. The Torsen II
uses a system called parallel-axis gearing and relies on the internal friction
of the system to transfer torque.
Viscous coupling: A type of connection either inside the T-case or
attached outside it for transferring power from front to rear or vice versa when
the basic driving wheels start to slip. It uses a viscous fluid that becomes
stiff when it heats up. When a set of wheels start to slip, gears connected to
the driveshaft going to those wheels starts to spin inside the T-case, and
"stir" the viscous fluid. It gets hot, stiffens up, and turns
paddle-type devices connected to the other driveshaft, transferring power to the
set of wheels that still have traction.
Noise, vibration, harshness (NVH): One of the goals in vehicle design is
to minimize NVH.
Split mu: The Greek letter "mu" or "µ"
designates the coefficient of friction of a particular surface. High mu surfaces
(close to 1.0) give good traction. Low mu surfaces (toward 0.0) are slippery. A
rig is in a "split mu" situation when some of its wheels are on one
type of surface and the other wheels on another type of surface. An example
would be a vehicle trying to drive up a hill with the right wheels on an icy
shoulder and the left wheels on dry pavement. The goal of 4WD systems is to
provide traction in "split mu" conditions.
New Venture Gear T-case terminology: In a separate conversation with me,
Sankar K. Mohan of New Venture Gear defined what the numbers on their T-cases
mean:
The first number is the number of speeds:
"1" = one speed (high range). Example – the NV 136.
"2" = two speeds (high range and low range). Example – the
NV 241.
The second number is the strength:
The NV 241 is designated "4" in strength – tougher than
the NV 231, but not as heavy duty as their largest T-cases, which go up
to a "7" (eg, NV 273, for vehicles with a GVW of 17,500 lb).
The third number designates the type of T-case:
"1" = part-time 4WD
"2" = full-time 4WD with an open center differential plus
lockable part-time option – like the Selec-Trac NV 242 of the Jeep
Cherokee and Dodge Durango, or the Hummer’s NV 242HD AMG.
"3" = electrically shifted.
"4" = not currently used.
"5" = Torsen-type differential.
"6" = computer controlled multi-plate wet clutch, like GM’s
AutoTrac NV 246.
"7" = GeroDisc – like the Grand Cherokee’s Quadra-Trac II
NV 247.
"8" = not currently used.
"9" = viscous coupling.
The program, Day 1:
Scott Schmidt and Tony Sealey of Land Rover led off the discussion. Scott
discussed 4-wheel drive systems in general, and Tony discussed the Hill Descent
Control (HDC) system available now on the new Series II Land Rover Discovery and
on the Freelander. The HDC uses the existing ABS and ETC
to control descent on steep grades.
Richard Fanco and Robert Gula of AM General talked about the development
of the Hummer and described the 1999 civilian Hummer’s new traction control
system, called the TorqTrac4 (TT4), which uses the ABS for electronic traction
control. The ABS itself is also new to the civilian Hummer in 1999. The civilian
Hummer has switched from the Zexel-Torsen I differentials to the Torsen II.
Both are torque-biasing devices. The Torsen I relies on brake-throttle
modulation to direct torque from a wheel that’s spinning to one that still has
traction. The Torsen II offers less torque-biasing than the Torsen I, but
doesn’t need it, because it relies on the TT4 for traction control. Both
civilian and military models offer a central tire inflation system (CTIS), so
you can air up and air down from the driver’s seat as the vehicle’s moving.
Randall Yost from GM described the AutoTrac system, available on GM
pickups and SUVs. These vehicles are basically rear wheel drive rigs. When the
rear wheels slip, the AutoTrac activates an electronic control module, which
uses a wet clutch pack, activated by an electric stepper motor, to send torque
to the front wheels that still have traction. It is a "4WD on demand"
system, activating 4WD when needed. It has part-time 4WD and 2WD options, as
well as a low range in the T-case. There is some advantage to having a variable
torque split between the front and rear. As the surface mu
increases, higher peak acceleration occurs when the torque split is weighted
toward the rear wheels.
Edmund Browalswki and Ron Paul of GM discussed vehicle stability
enhancement systems (VSES) and their interaction with all-wheel drive systems.
They pointed out the advantages of full-time all-wheel drive with an on demand
supplement and stressed the importance of integrating AWD systems with VSES.
Murat Okcuoglu of ASHA Corporation discussed the use of the Gerodisc in
AWD applications, both as a single disc and in dual TwinDisc applications. The
Gerodisc is speed and torque sensitive, operates full-time, and offers
proportional torque transfer with progressive action. It uses the transmission
or differential oil to operate the Gerotor. These devices can be integrated with
electronic stability control systems. TwinDiscs provide one coupling per wheel
and can be used in place of standard differentials for traction control from
side to side. A Gerodisc can be used in the T-case to provide on demand 4WD. He
feels that using TwinDiscs as limited slip differentials offers advantages over
electronic traction control, since the TwinDiscs are proactive and the ETC is
reactive.
Heribert
Lanzer of Steyr-Daimler-Puch Fahrzeugtechnik discussed the Geromatic
traction system, which uses a T-case with a Gerotor pump to activate a wet
clutch pack. He compared this technology to viscous couplings. It can also be
used as a limited slip differential.
John Barlage of GKN Automotive discussed Viscodrive technology and the
Visco-Lok limited slip differential. The Visco-Lok uses a shear pump, which
operates with viscous fluid in a closed loop to activate a wet clutch pack and
transfer torque from side to side when the device is installed in place of a
differential.
Jun Yoshioka of the Spicer Axle Division of the Dana Corporation talked
about principles of vehicle mobility and discussed the Hydra-Lok limited slip
differential, an application of Gerotor technology. The Hydra-Lok differential
is the one used on the front and rear axles by the 1999 Jeep Grand Cherokee in
the Quadra-Drive 4WD system. Chrysler calls it the Vari-Lok.
The program, Day 2:
Klaus Lippitsch of Steyr-Daimler-Puch Fahrzeugtechnik discussed the
integrated transfer case (ITC) developed by Steyr-Daimler-Puch. It uses a Hi/Lo
planetary gear set with a Torsen II parallel axis differential in the same unit
to provide full-time, two-speed (high and low range) 4WD. It is one of the few
systems that provides full-time 4WD in low range, an advantage in off-road
situations requiring turning in mud or heavy snow, and an advantage in towing a
heavy load uphill, on-highway or off-road.
Joel Jermakian of the New Generation Motors Corporation discussed
electric vehicles with AWD with emphasis on the variable air gap axial flux
motor developed by New Generation Motors.
Dan Showalter of BorgWarner Automotive discussed active on demand
electro-mechanical AWD systems, based on front-wheel drive vehicles, integrated
with electronic stability systems.
John
Zalewski of New Venture Gear described the new NV 247 T-case, which uses
Gerotor technology for progressive coupling and smooth passive torque transfer
from rear to front axles. This is the T-case used in the 1999 Jeep Grand
Cherokee. By itself, it’s called the Quadra-Trac II system. When combined with
the Dana Hydra-Lok (Vari-Lok) differentials front and rear, Jeep calls it the
Quadra-Drive system.
Larry McAuliffe of the Eaton Corporation described the Eaton locking and
limited slip differentials. These are primarily OEM products and are not
generally available for after-market applications. Eaton is developing an
electrical positively locking differential, which will function similarly to the
ARB locker.
Robert Atkinson of Daimler-Chrysler described the AWD system used on
Chrysler mini-vans, which is based on front-wheel drive, with a viscous coupling
transferring torque to the rear axle on demand when the front wheels start to
slip.
Paolo Sacchettini of Zexel-Torsen described applications of the Torsen
torque biasing differentials, both type I and type II. Type I is the model used
on the military HumVees and on civilian Hummers before 1999. Type II is the
model in the 1999 civilian Hummer.
The program, Day 3 – Ride and Drive:
The participants in the ride and drive session broke up into small groups and
rotated through five separate activities.
The
Hummer ride took us through AM General’s wooded (300 acres) off-road course,
which included sandy and muddy V-ditches, 6 to 12 inch logs, deep ruts, sharp
bends, steep hills, and hood-deep water. We each had the chance to drive one of
the vehicles, which were all 1999 models with the TorqTrac4 (TT4) electronic
traction control system. I was impressed with the Hummer’s performance. My
friend, San Francisco businessman Knut Akseth, attended this TopTec because of
his interest in the engineering aspects of 4WD. Knut bought a 1993 civilian
Hummer when they were first released in 1992, and has driven it in a variety of
extremely tough off-road situations. He liked the new TT4 system, and felt that
there was less strain on the drivetrain of the 1999 Hummers he drove over the
rockpiles at the TopTec, compared to his own, which uses brake-throttle
modulation for traction control.
There were two stations called "Off-Road II," which used the
obstacle course on the AM General grounds. We had another chance to drive the
Hummer over challenging obstacles, including a 16" wall, 10" to
12" logs, and a rockpile. These obstacles require using enough throttle to
get the vehicle up and over the obstruction, then getting on the brakes quickly
and feathering them to ease the descent of the rig. This was tricky, because if
you get on the brakes too soon, you turn off the TT4 because the ABS has
priority.
The
Off-Road II station also gave us a chance to drive other vehicles, including a
Range Rover with open differentials, relying on ETC for traction control. There
was a Toyota pickup equipped with a Detroit locker in the rear and a TrueTrac
limited slip differential (LSD) in the front. The pickup performed particularly
well on the three-wheel roller ramp. This device puts rollers under the left
front wheel and the two rear wheels. The 4x4 system has to provide enough torque
to the remaining right front wheel to pull the vehicle the rest of the way up
and over the ramp. We also rode, but didn’t drive, a Pinzgauer, a 6X6 vehicle
by Steyr-Daimler-Puch. It was amazing, with its three locking differentials
between the right and left wheels, and a locking T-case.
The
Off-road I station demonstrated several vehicles, which had to climb a steep,
sandy hill, then traverse a deep, sandy ditch with fairly steep sides. The
vehicles included: A Tahoe with TwinDiscs on the front and a single disc on the
rear, a Grand Cherokee with the ITC Hi/Lo open T-Case and ETC, and a Grand
Cherokee with TrueTrac differentials front and rear and the Quadra-Trac II
T-case. Others were available, but we didn’t have time to ride in them all.
At the On-Road I station, we rode as passengers and drivers in a variety of
vehicles to demonstrate their traction capability on slippery surfaces. Tony
Sealey drove us in a Land Rover Discovery Series II, using a tight circle to
demonstrate the Discovery’s vehicle stabilization system, an option this year.
It uses something like lever-arm shocks with an accelerometer sensing lateral
acceleration to keep the vehicle relatively level even in sharp turns. A GM
pickup performed well starting from a standstill with the rear wheels on a wet,
soapy patch of vinyl, showing that the AutoTrac transferred torque to the front
wheels without hesitation. A RAV4 used an Eaton Dynatrax to link front and rear
driveshafts, and maintained traction with the front wheels on the skid pad,
transferring torque to the rear. A minivan used TwinDiscs on the rear axle, with
front-wheel drive and an open differential in the front. The skid pad was under
the right wheels, and the vehicle moved right out with torque all being
transferred to the left rear wheel.
Summary
The automotive manufacturers and their various OEM and after-market suppliers
are trying to meet the challenge of providing consumers with 4x4 vehicles which
offer superior handling, comfort, and stability on the highway while still
providing mobility and safety off-road. The gold standard for off-road mobility
remains a lifted vehicle with big tires and lockable differentials front, rear,
and center. But the technological advances available now may provide vehicles
superior to those which sport nothing more than locked diffs. The Hummer is an
example, which uses portal hubs to bring the axles into the hubs several inches
above the center line, increasing ground clearance. It uses a beefed-up version
of the NV 242 T-Case used in the Jeep Cherokee, giving it both full-time and
part-time 4WD in high range. The version used in the Hummer is designated the NV
242 HD AMG, and has a maximum torque output of 2340 lb-ft, compared to the 1486
lb-ft of the standard NV 242. The addition of ETC this year on top of the Torsen
II torque-biasing diffs makes ground-grabbing somewhat easier than it was
previously, when brake-throttle modulation was required to stop a spinning
wheel. The Hummer continues to offer central tire inflation (CTIS) as an option.
Another example of high-tech at work is the current Jeep Grand Cherokee. The
Limited Edition can be ordered with the Quadra-Drive 4WD system, consisting of
the Quadra-Trac II NV 247 Gerotor T-case and the GeroDisc Hydra-Lok (Vari-Lok)
diffs front and rear.
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