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July 2006 4x4 Tech Questions - Techline

Posted July 1, 2006

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Four Wheeler
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Question: I own a Jeep Liberty with the diesel engine. I tried to order it with the five-speed manual, but had to settle for the automatic. While it's not what I consider a real four-wheeler (I have a CJ-5 for that duty), it's a great commuter to work. I'm a ski instructor.

What I would like is a bit more power, as it just never seems to be in the right gear. Any thoughts?
Jimmy Long
Reno, NV

Answer: We will have to agree with you-it does seem to lack power right after a gear change. At one time we even considered a Liberty as a project vehicle.

During our initial investigation into increasing power output, we uncovered several companies who were in the process of developing "black box" computer modifiers to increase power. TS Performance (270/746-9999, www.tsperformance.com) has its plug-in Powerplay MP-8 now out, which the company says will make 50 more horsepower and 100 lb-ft of increased torque.

Question: You guys are my last resort. I have read articles on 4x4s in your magazine and others, called Ford directly, asked a local mechanic that works on a lot of 4x4s, and none could answer my questions. I have an '02 Ford F-150 FX4, with rear limited-slip. I put a 3-inch body lift on it to fit 35-inch Pro Comp XTerrains.

Will both front tires spin at the same rate even if one is completely off the ground? Second, in a stuck situation that I was in recently, I had my left front tire on the ground with the truck frame stuck on a 3-foot embankment. One of my back tires was on the ground but way up in the wheelwell, and the other rear was up in the air. The one in the well was not turning-I opened the window and looked-and the other on the passenger side sounded like it was just spinning in the air. I thought the limited-slip should have prevented this? You are my final hope at understanding.
Tomheerlyn
via fourwheeler.com

Answer: Let's see if I can clear this up for you. The differential, as its name states, is made up of a case, sometimes referred to as a carrier. To this carrier is mounted the ring gear, which is driven by the pinion gear, which is directly connected to the driveshaft. Within the carrier are two sets of gears. One set rides on each axleshaft, and the other set is mounted to the case. When the case turns, the differential gear turns the axle gear, which in turn drives the axle and, in turn, the wheel. The design is such so that when you go around a corner, one wheel can turn faster than the other to make up for the radius of the turn, or differentiate the two different wheel speeds.

The problem is the differential also will send power to the wheel with the least amount of traction, as it can't tell the difference between this and going around a corner. This means if one wheel is off the ground, as you experienced, that wheel will spin freely. Now, in the rear axle where you have a "limited-slip," the same thing happened. Surprise? No, not really. Limited-slips really don't work all that great. They still have the differential gears, but there also is a series of clutch packs that put pressure on the gears through various methods when a spinning tire is detected, and try in somewhat of a ho-hum manner to send power to both wheels. They are designed to limit tire slippage to some degree while not causing any undesirable driving characteristics. As you found with one wheel in the air and the other on solid footing, the tire with the traction got no power, and all of it went to the free wheel. The same thing will happen with one tire on ice and the other on dry pavement. One way you can cheat and fool the limited-slip to help out a bit is to slightly put some pressure on the brakes. You can do this with the foot pedal or with the emergency brake. Still, factory installed limited-slips just aren't worth the money you had to spend as an option.

Auburn makes a clutch-pack-type differential that is claimed to have the best of both worlds-that is, of a clutch-pack-type limited-slip and a true locking diff. Also on the market is the totally mechanical Detroit Truetrac differential. This uses some special-cut gears that allow normal differential action under turning, but will lock up so that both wheels get power sent to them when needed. It has been my experience that sometimes it's necessary to add a bit of brake pressure to get this to happen under certain situations. The Truetrac has none of the harsh driving characteristics that some people associate with its famous big brother, the Detroit Locker. The Detroit, and others of its type, lock both axles together until you make a turn, such as going around a corner, and then release but will apply equal power to both wheels when in such a situation as you encountered.

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