Back in the bad old days, going down a steep hill in a horse-drawn wagon or stagecoach called for desperate measures. The horses or mules couldn’t hold the heavy coach back, and it could even run them down. The drivers or muleskinners would tie a log or spare single-tree to the rear wheels to lock them up. The drivers would then have the horses drag the coach down the hill. This kept the horses and passengers alive, but the coach’s steering control was all but lost. The steering would have been vastly improved if the drivers could have figured out a way to hold back the coach and still allow its wheels to rotate.
Sliding down a steep, loose trail with both feet on the brakes can give a Jeep driver the same butterflies in the belly as the stagecoach driver. With the brakes locked up, steering becomes very iffy—the tires are sliding rather than gripping—so you’ll want the tires rolling. If they’re rolling, you’re under control; if they’re sliding, you’re on the edge of control.
Low range axle gears can provide the hill holding solution, but that also uses more fuel while on the highway. However, TeraFlex has a better solution to the butterfly problem—and it won’t use any more gas on the way to work. It’s a 4:1 low range kit that is installed in the transfer case. With our ’93 Jeep Wrangler YJ the transfer case is an NP231, and TeraFlex offers 4:1 kits for other models too. Only the low range is affected, therefore highway gearing is not affected and neither is highway mileage.
We also added another product to the NP231 transfer case for safety’s sake and longevity, a slip yoke eliminator kit. This also necessitated a different rear driveshaft. (If the slip yoke is eliminated, a driveshaft with the older style of U-joint is required.) A slip yoke eliminator kit is highly recommended. The nice part is we did it all ourselves at home—two people and a weekend can get things rolling.
1. The two TeraFlex kits from Summit Racing Equipment comprise these major components. From the top: rear U-joint yoke, Tera rear housing, Tera heavy-duty mainshaft, and Tera heavy-duty low 231 case.
2. Kevin Lake cleaned off the NP231 transfer case after it had been removed from the Jeep. Lake’s expertise shortened the Jeep’s downtime significantly.
3. The owner, Dash Dornbush, a newbie to Jeeping, assisted Lake in the rebuild.
4. This entire project is only a two-person job, and it’ll take an impact wrench and a second pair of hands to remove the front U-joint yoke and slip yoke.
5. The heavier-duty mainshaft from TeraFlex is on the left. It is not only beefier but also ready to accept the new U-joint yoke.
6. If your YJ is a ’96 model or earlier, you’ll need to remove the needle bearings from the drive sprocket. We used a large socket to hammer them out.
7. Make certain that all the mating surfaces are clean and free of old sealant and debris. For reassembly we used black silicone between mating surfaces of the cases. No gaskets are used in the NP231 transfer case.
8. Sliding the shafts and chain into place while keeping everything level and even takes two pairs of hands. We used thick sticky grease to hold parts in place while assembling the new parts.
9. Make sure you install the new blue plastic gear for the speedometer sensor. The sensor can be installed in its slot in four different “clock” positions. Before removing the sensor, mark both it and the case so that it goes back into the proper clock position or you’ll not have a speedometer when you complete the project.
10. With these two kits, only the back half of the transfer case is reused. The TeraFlex low range kit provides a new half-case, and the TeraFlex slip yoke eliminator kit provides a new rear output housing.
11. Rock Lizard 4X4 built the replacement rear driveshaft, using a salvaged Cherokee front driveshaft in an effort to save time and money. By using an existing driveshaft, the shop didn’t have to spin-balance the driveshaft nor weld up both ends. Rock Lizard just replaced the two U-joints with new ones and shortened the shaft.
12. Dornbush’s YJ and Trailblazer tent camper are ready to tackle the high country around Silverton, Colorado.
Why Lower Ratios?
An OEM NP231 comes from Toledo with 2.72:1 low range and 1.00:1 high range. While this works great in most situations, when a Jeeper wants to become more adventurous with higher suspensions, taller tires, and more technical trails, lower off-road gears are needed. Lower gears allow the Jeep’s engine compression to slow the wheels down so that they rotate more slowly without using the brakes, holding the Jeep back while still rotating and keeping the Jeep’s steering under control. The lowered gearing also aids in traction control while climbing steep hills. Through gear multiplication, the Jeep’s torque can be multiplied for easier climbing. The ’93 Wrangler’s three-speed TorqueFlite automatic’s gear ratios are 2.74:1 in First, 1.54:1 in Second, and 1.00:1 (straight through) in Third. Since the YJ’s differentials are equipped with 3.07:1 gears.
Running the Numbers
Gear multiplication can work even better with a manual transmission because an automatic goes to First gear automatically, whereas a manual tranny can be shifted to Second or even Third to start off with slightly higher gears, lessening the chance of spinning the wheels during takeoff.
Differential gear: 3.07
Transfer case OEM low range: 2.72
TorqueFlite First gear: 2.74
These numbers mean that to understand by what number the engine’s torque is multiplied, you have an equation such as this:
3.07 x 2.72 x 2.74 = 22.88
With the TeraFlex 4:1 low range in place, the equation would read:
3.07 x 4.00 x 2.74 = 33.65
That it is an increase of 33 percent to the engine’s power and compression holdback. And that’s without changing the differentials’ gearing!
