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.
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.
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.
In general, a doubler conversion requires more vehicle modifications, but the end result is the increased versatility of a wider range of gearing options. In general, the differences can be summarized as follows:
• Two or three low-range transfer case ratios
• Improved front driveshaft angle
• Requires driveshaft length modifications
• Requires shifter/floorboard modifications
• Slight increase in driveline slop
• Usually requires crossmember modification
• May require speedometer cable/wire extension
• No other modifications required
• Preserves original drivetrain setup
•Cannot pick "stock" low-range
One word of caution when it comes to the use of low-low doublers mated to a manual transmission: If you have a 10:1 low-low-range, you're geared four to five times lower than a stock T-case. Should you be coming down a hill in deep low-range and gain too much speed with the clutch pushed in, it's possible to quickly exceed the maximum rpm rating of the clutch disc. In such a case, you can literally spin one apart and destroy it under these circumstances.
We've also seen some clever homebrew adaptations where fabricators have found ways to create doublers for t-case models not addressed by the aftermarket. These undertakings often involve the ability to do precision machine work and quality welding. A successful assembly involves building a true and strong shaft coupler, finding a way to reliably mate two housings together for leak-free operation and good mechanical strength, and ensuring that the input/output shafts all align true on axis.
With the various gear choices available today, you can optimize your rig for the engine, tires, and terrain you're dealing with. You can retain good road gearing and still gear down for pursuits in dirt, mud, snow, sand, or rocks.
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