4x4 Driving Tips
Hummer climbing out of a ditch.
A Hummer uses brake-throttle
modulation to climb out
of a ditch.

The Subject:

   Hummer Title
Thanks to my friend, San Francisco businessman Knut Akseth, who showed me what a Hummer can do. Special thanks also to Knut's son, Kristian Akseth, a mechanical engineering student at UCBerkeley, who helped me understand the complexities of differentials, both torque-biasing and otherwise.
TIP #1:
Why is a Hummer such a great off-road rig?
Its long wheel base and wide track make it very stable.
It has full-time 4WD, but the NP242 transfer case can be locked to give part-time 4WD for more traction in Hi range. In Lo range, it's automatically locked.
The standard 6.5L diesel V8 engine has a lot of torque at low rpm (290 lb-ft at 1700 rpm to be exact). With the turbo diesel option, the torque reaches an amazing 430 lb-ft at 1700 rpm.
The Zexel-Torsen differentials are torque-biasing, which means that a spinning wheel doesn't rob torque from the wheel with traction.
It has geared hubs, with the axle entering the hub above its center. This increases the ground clearance to a total of 16 inches, far more than any other stock 4x4 rig available today. The geared hubs nearly double the torque the drivetrain delivers to the wheels.
   TIP #2:
Electronic Traction Control. The new Hummers use a system called TorqTrac4 (TT4) to transfer torque from a spinning wheel to ones that still have traction. This uses the ABS system to brake a spinning wheel (the opposite of what ABS typically does when it's stopping a vehicle). This allows the torque to go to the wheels that aren't spinning and moves the rig out of the stuck situtation. Before the 1999 models, you moved a stuck Hummer with Brake-Throttle Modulation.
   A Hummer on the trail in Hollister Hills.
On the trail in Hollister Hills. This is
Knut Akseth's '92 wagon, with the
6.2L diesel engine. With a track of
72" and a center of gravity of 34",
the Hummer is the most stable
truck on the market today.
Brake-Throttle Modulation. This is way Hummers before 1999 models developed traction when one or more wheels are slipping. Let the spinning wheel slow down, then step on the brake hard with your left foot. Now gently apply the gas with your right foot and ease up gradually on the brake. Use the brake sort of like a clutch, which has the effect of directing torque to the wheel that still has traction. If you do this right, you can drive out of a stuck situation when only one wheel has effective traction.
   TIP #3:
The 4-wheel drive system. A Hummer's 4WD uses a heavy duty version of the NP242 T-case, which is a system similar to the one used for years in the Jeep Cherokee and available as one of the 4WD options in the Grand Cherokee. It's also available now as an option in the new Dodge Durango.

Military Hummer
The military version.
   The 2-wheel drive selection, available in the Cherokee and Durango, is locked out in the Hummer, leaving full-time 4WD Hi range, part-time 4WD Hi, and part-time 4WD Lo. The full-time 4WD selection activates the differential in the transfer case (T-case), and creates some play between the front and rear driveshafts. This means you can leave the system in 4WD all the time, even on dry roads. If the surface gets slippery, locking the T-case gives you so-called part-time 4WD with more traction, but you shouldn't use this setting on surfaces with good traction.
When you need to crank up the torque for hill climbing or rock crawling, shift into 4WD Lo range. This automatically locks the T-case for maximum ground-clawing capacity.

   TIP #4:
Crawl Ratio. Kristian Akseth helped figure out the final low range crawl ratio, which is a little tricky on a Hummer because of the torque converter and the geared hub assemblies. The numbers to use are:
    Transmission 1st gear - 2.48:1
    Torque converter - 2.2:1
    NP242 T-case low range - 2.72:1
    Zexel Torsen diffs - 2.73:1
    Geared hub assembly reduction ratio - 1.92:1
Multiply all these together to get the final crawl ratio:
                2.48 X 2.2 X 2.72 X 2.73 X 1.92 = 77.8

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   Hummer at Hollister Hills SVRA.
Hills aren't much of a challenge
in a Hummer.
The 12,000 Warn winch.TIP #5:
Accessories. AM General, the manufacturer of Hummers, offers several accessories. You can buy the rig with the famous central tire inflation system (CTIS). This means you can air down for rocks, sand, or mud while you're on the move, and then air back up again, all without leaving the driver's seat!
You can also get a Hummer with the optional 12,000 pound Warn winch mounted on the front. This cuts down the approach angle some, but is certainly an off-road accessory I wouldn't want to be without.
The run-flat system will get you back home, even with a deflated tire or two or three or four. With one tire flat, you can drive for 30 miles at 30 miles an hour.
   TIP #6:
Carry extra axles. A Hummer's axles are sort of like fuses to protect the rest of the drivetrain. It's relatively easy to break an axle, but it's also relatively easy to change one in the field. If you break an axle, that means you probably haven't broken anything else, like an expensive Zexel-Torsen differential, transfer case, or transmission. Protect the axles by letting a spinning wheel slow down before you apply the brake to perform brake-throttle modulation.
Knut Akseth and his '92 civilian Hummer.
Knut Akseth uses brake-throttle
modulation to keep the right
front wheel of his '92 civilian
Hummer hooked up. Each
one of those 37 inch
Goodyear tires and
wheels weighs
165 pounds!

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   4-Wheel Freedom: The Art of Off-Road Driving. By Dr. Brad DeLong.
The definitive book on 4x4s, off-road recreation, and all-weather driving.
ORDER NOW at 1-800-4X4ROAD (494-7623). Copyright Symbolcopyright Dr. Brad DeLong 2005