Lower Transfer Case Gearing - Gears - Dual Case

Variety is a spice of life and so it can be with off-road pursuits. Most of us don't have the dollars, time and garage space to maintain a fleet of specialized off-road vehicles, so we often try to make our one 4WD as reasonably capable as possible for a wide variety of terrain. You may be commuting back and forth to work during the week, then meandering down a tight rocky trail one weekend, followed by a day of mud slopping back to your favorite fishing hole. Trying to satisfy all the gearing needs for those activities can be tricky, but with some aftermarket transfer case "doubling" solutions, you can have a much more versatile rig.

Over the years we've used our vehicles to take us to bigger and badder places, and we've increased tire sizes to get us there. Swapping to lower ratio (higher numerical) axle gear ratios can help keep the engine turning in its useable torque powerband on the road, and sometimes off-road. However, there are times when we simply need more low gearing to turn big meats or traverse tough terrain.

Here is a good situation where low-low T-case gearing can be a plus. The deep gearing provides great control over tricky terrain in cases where excessive throttle with too high gearing might break traction or cause the vehicle to bounce more, risking body damage or bouncing the tires off the intended line.
Here is a good situation where low-low T-case gearing can be a plus. The deep gearing prov
Axle gears can be swapped but can only get you so far. With tires that are over twice as tall as stock, driving in low-range on the street gets you in the ball park of the original final drive ratio. To get some torque in the dirt, lower gearing is needed and a doubler setup could offer that, and possibly improve driveshaft angles when dealing with extreme heights.
Axle gears can be swapped but can only get you so far. With tires that are over twice as t

There are several choices that can get you to lower transfer case gearing. One is to simply swap in aftermarket gears that provide a lower ratio low-range while leaving the 1:1 high-range intact. These gears have become more available in recent years for a good variety of popular older transfer cases, but not commonly for late model versions.

The second method to getting lower T-case gearing is by adding a "doubler" (or dual cases as it may also be called). Simply put, this means adding another high-low gear reduction unit between your transmission and some existing transfer case. Depending on vehicle, there are several ways with which this can be done. In some cases, a second transfer case (gear reduction portion) is added to the drivetrain. Another option involves the addition of a planetary gear set behind the transmission case to get a second reduction. With these setups you end up with four possible transfer case gear ratios instead of just the typical high (1:1) and low-range.

Offroad Design (ORD) has created a dual case solution suitable for a wide range of full-size applications. They offer a conversion kit that mates the reduction portion of a NP203 to the venerable NP205 case. You can use their kit along with a couple of salvaged T-cases to build a doubler setup. They can also provide a two or three stick shifter mechanism to control both cases.
Offroad Design (ORD) has created a dual case solution suitable for a wide range of full-si
NorthWest FabWorks offers add-on planetary gearboxes as well. Their latest incarnation is their Black Box-i (integrated) assembly. It can be used to turn a 2-speed Atlas into a four-speed transfer case, or can provide an additional 2.72:1 choice to a Dana 20, Dana 300 or a STaK T-case.
NorthWest FabWorks offers add-on planetary gearboxes as well. Their latest incarnation is
The 4:1 low-range ratio Rock-Trac transfer case found in late model Wranglers is a great unit. John White at JB Conversions shared with us how it was originally designed to be used in the Jeep and in Hummer H2 models, so it is a beefy unit. The deep low-range works great, but sometimes you want gearing that's in between the high and low ranges. Advance Adapters came up with the Rubicrawler Kit that adds a 2.72:1 planetary set to this unit to provide two more gear range options. The slick thing is that Advance found a way to stuff the six-pinion planetary in the area taken by a dead-space transmission housing on the tail of the 42RLE automatic transmission in '03-to-'10 Wranglers. This allows installation of a doubler kit without the need for driveshaft or crossmember modifications.
The 4:1 low-range ratio Rock-Trac transfer case found in late model Wranglers is a great u

It is often possible to combine lower aftermarket gears in one transfer case with the doubler setup giving you an even wider range of ratio choices. The versatility of such a setup can be tremendous. High-range (1:1) is still used for road driving or any high speed off-road driving. The middle ratios (typically ~2:1 to ~4:1) are often useful for running rough dirt roads, powering through sand or mud, or doing milder crawling. Finally, a low-low ratio (~4:1 to ~12:1) can be used for the nastiest boulder crawling. This may seem uber low and it is, but the slow control it offers is incredible.

Since adding a doubler typically moves the transfer case outputs rearward on the vehicle, the front driveshaft length grows while the rear driveshaft length shrinks. Whether or not this is beneficial on any particular vehicle will depend on wheelbase and other drivetrain factors. Sometimes this change improves driveline lengths and angles, and sometimes it can hurt them. In cases where a front driveshaft comes very close to an auto transmission pan, adding a doubler can often increase the clearance, allow more uptravel for the front axle, or use of a high pinion front axle.