JK on a diet

How to build an expedition vehicle with good on and off road performance

Okay, I have been driving the JK for about a month.  The downside is I have yet to get it into 4-wheel drive.  After all, it is a Jeep and touted as being “Trail Rated”.  But, the intent of this Jeep is to be an “expedition" or "overland" vehicle.  And, that is going to require some careful planning to come up with the right combination of upgrades to maintain good on-road performance as well as good off-road performance.

First impressions... Well, it doesn’t have a Hemi like the RAM 1500 it replaced.  It will cruise at 65-70 mph and it has better gas mileage than the truck.  Wait, that is stock with no extra weight.

For clarification, the intent of this vehicle is to be an “expedition” or “overland” vehicle.  The primary consideration is to improve off-road performance and maintain on-road performance through a series of planned performance upgrades.

That brings up a significant point to consider with the upgrade path -- weight.  In order to keep a balance between on and off road performance, weight is a major factor.  For off-road performance, increased ground clearance is necessary.  The stock 225/75R16 tires will be replaced with a beefy 35x12.50 tire for ground clearance.

That brings into consideration two additional points -- fender clearance and gearing.

Easy item first -- gearing.  Stock, the 3.8 is not a high-power V-8 Hemi.  It does have respectable low-end torque when gearing and tire size are matched.  Conventional selection would say 4.56 gears.  But, that is based on experience with the venerable 4.0l and tire size.  There, 4.56 is good; however, 4.88 or 5.13 are worth consideration with the 3.8l JK and tire size.

After consultation with various RPM to tire size calculators, the choice is 4.88 or 5.13.  It appears a reasonable compromise for 35 inch tires is 4.88.  If the desire is for 37 inch tires, a better choice would be 5.13 gears. 

At this junction, the consideration weight comes into play.  That extra two inches of tire size can add 15 or more pounds of weight per tire.  That is an extra 75 or more pounds to the overall vehicle weight.  To keep optimum on and off-road performance, my desire is to keep overall vehicle weight gain to a minimum.  So, 35 inch tires with 4.88 gearing appears to be the optimum.

The gearing change will be complimented with a 4-1 transfer case compliments of another project.  In addition, traction assistance will be added.  Traction assistance, AKA lockers, are an important item necessary for improved off-road performance. 

There are a variety of choices available including electric, air, mechanical or physical.  Electric, air, and mechanical actuated offer big pro (locking differential on demand) verses a big con (always engaged locking differential).  That means there is no extra “drag” in the driveline system.  That translates into better on-road performance.  Downside is something extra to go wrong when away from ready repair sites.

As the intent of the “expedition vehicle” build is for optimum on and off-road performance.  A major part of the performance is dependability.  At this point, the obvious choice is venerable Detroit Locker in the rear. 

While the deliberation is on-going for the front, previous experience with a PowerTrak in the front has been very positive.  The PowerTrak is a little more forgiving in on road performance compared to a Detroit.  Together, when compared to the ARB or OX lockers, the Detroit and PowerTrak combination offer positive and dependable performance without extra switches, linkages, or air-lines to fail. 

In recap, the drivetrain combination will be 35x12.50 tires, 4.88 gearing, rear Detroit locker, front PowerTrak locker, and NP-241C 4:1 transfer case.

This combination will require driveshaft modifications.  Driveshaft modifications will also be required for suspension upgrades to accommodate the 35 inch tires.  At this point, it is clear that gearing and suspension upgrades need to be accomplished at the same time in order to accomplish driveshaft modifications at the same time.

The next installment will cover the various options in supplying articulation and clearance for the 35 inch tires.

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