Take a '94 Jeep Grand Cherokee Limited -- 5.2L V-8, O/D auto, leather, A/C, sunroof, factory Infinity stereo, power everything -- mix it with a major urge for backcountry adventure and a taste for extreme rockcrawling, and you have a recipe for vehicular disaster, or so we're told. "That Grand Cherokee is nothing but a yuppie car. It wasn't made for that." Of course, making vehicles perform in ways they weren't necessarily designed for is a major part of what we like to do. We wanted our Grand to be capable of real adventures through forests and deserts, mud and sand, and rock and water, but we still wanted to arrive in relative luxury with a quiet running engine and drivetrain, no vibrations, plenty of overdrive, and low rpm when cruising at 80 mph. Seem like a lot to ask for?
Running backcountry roads with deep ruts and occasional mud holes is what Jeep had in mind when it came out with the "normal" Low range in its T-cases. And a normal Low range between 2:1 and 2.7:1 really works quite well for that kind of four-wheeling. But when trails turn really rocky, four-wheelers need gears that are much deeper than what you normally find in a stock Jeep. It's like a mathematical formula: As the gearing becomes lower, the control and capability of the vehicle goes up.
Eventually, Jeep started installing super-deep axle ratios and granny gear manual transmissions in its CJs and flatfenders. Not surprisingly, as the rigs received lower gearing, their off-road prowess became more capable. They also became less useable on the highway, but that was the compromise: As vehicles become better trail rigs, they become worse daily drivers. Those with deep pockets bought trailers and tow vehicles to haul their Jeeps to the trailheads, while the regular folks, as usual, just had to deal.
Before long, some bright people had the idea that if you took a second transfer case and stacked it in front of the first so the gear reduction would multiply, you could have super-low gear ratios. And, as a bonus, if you rigged the shifters right, you could have three different ranges: normal High range, standard Low range (only one T-case in Low), and double Low range (both T-cases in Low, multiplying the Low gearing).
About the same time, others began to realize that if you simply put a deeper Low range in the transfer case, you could have proper gears for the highway and an extra-deep Low range for the rocks. Leave the differential and the tranny stock and put the gears somewhere around 4:1 in the T-case. This was a much easier install, and it allowed you to receive most of the benefits.
Somewhere along the way someone realized that if you used a twin stick system to control the transfer case, you could have two-wheel Low and even a front-wheel Low. When you are in loose, steep terrain and the vehicle is in four-wheel drive, the truck fights you when you try to turn. The rig keeps sliding straight when you want to go left or right. Shifting into 2WD or FWD allows a new set of driving tricks and techniques to get you where you're trying to go.
The Atlas II
The Atlas II is the top of the heap when it comes to Low-geared transfer cases. The aluminum case is thick and durable, unlike many modern aluminum T-cases, and it is stronger than stock Jeep transfer cases, without the brutal weight of the older cast-iron boxes. The Atlas is all-gear drive; no chains, and its gears are helically cut, not straight. It comes with a twin stick shifter, and it is fully synchronized, meaning you can shift it on the fly (although most manufacturers recommend that you stop the vehicle before shifting into or out of Low range.) The final feature is a big one: The case was designed to fit almost any vehicle. Left- or right-drop versions are available, and different input shafts are available to match all popular transmissions.
This box is one of the best in the dual transfer case group; however, it is not a transfer case at all. It is a purpose-built gear reduction unit designed to bolt in between the transmission and transfer case of a four-wheel-drive vehicle. It comes in two versions: the David and the Goliath. The David uses a double-compound planetary gear set to achieve a 4:1 reduction, and the Goliath offers a 2.7:1 ratio planetary gear reduction. Both models have an excellent reputation for durability. Because the Klune is used in front of the transfer case, it can be shifted into Low, leaving the T-case in High, or the T-case can be shifted into Low, leaving the Klune in High. They can also both be shifted into Low, with the Klune multiplying the gearing of the transfer case for extreme underdrive.
Klune-V + Atlas
For our Grand, we decided we needed it all, or, at least, we wanted it all, so we elected to shoot for the ultimate by mating an Atlas to a Klune. In order to have the maximum gearing options possible, we selected an Atlas II 4.3:1 ratio transfer case and coupled that to a Klune-V Goliath 2.7:1 ratio box. When one unit has a much deeper reduction than the other, four different gear ranges can be achieved:
Atlas High, Klune High: Leaving both boxes in High range provides optimal highway driving.
Klune Low, Atlas High: This results in a normal 2.7:1 ratio Low range and is best for forest trails, mud, and sand.
Atlas Low, Klune High: This provides a 4.3:1 ratio suitable for good rock trails such as the Rubicon.
Klune Low, Atlas Low: Now you're talking extreme underdrive with a ratio of almost 12:1! This is far too slow for most trail riding, but it's awesome for super-technical boulder groupings and waterfalls.
From the ability to slow things way down, we can traverse some pretty tough stuff with a healthy degree of confidence that we can make it through without breaking and maybe even keeping the sheetmetal off the rocks. And in the event that we do have to use a winch, the tires will turn at winch speed, allowing the winch and engine to work smoothly together.
Effective Klune-V / Atlas II Combined Gear Ratios
There are four possible T-case ranges made available by multiplying different combinations of the Klune and Atlas shift positions:
Gear Ratio: A gear ratio is the number of times the input shaft must turn compared with the number of times the output shaft turns. With 4.56 gears, the pinion shaft (input) must turn 4.56 times in order to turn the axles (output) one time.
Final Drive Ratio: This is obtained by multiplying all of the different gear ratios in the chain. These include the ratio of the transmission gear selected, the transfer case ratio selected, the ratio of the axles, and, in our case, the ratio of the Klune underdrive as well. Hence, transmission multiplied by Klune multiplied by T-case multiplied by axle equals the final drive ratio.
Crawl Ratio: In terms of four-wheeling, this indicates the lowest, deepest (numerically largest) ratio that a vehicle's gearing can be shifted into. In the case of our Grand Cherokee, the crawl ratio is 129.71. That means for the axles to rotate one time, the engine must turn 129.71 times. Take the rpm of the engine at idle (500 in the case of our Grand) and divide that by the crawl ratio (129.71 on the Grand) and you get the approximate number of revolutions per minute -- 3.85. In reality, they turn even slower than that because our Grand has an automatic with a torque converter.
|APPLICATION||KLUNE-V GOLIATH GEAR RATIO||ATLAS II 4.3 GEAR RATIO||COMBINED|
|Highway||High 1.0||High 1.0||1.0|
|Trail||Low 2.7||High 1.0||2.7|
|Rock||High 1.0||Low 4.3||4.3|
|Extreme Underdrive||Low 2.7||Low 4.3||11.6|
Grand Cherokee Project: Final Drive Ratios
|4.56 AXLE RATIO|
|Combined T-case Ratio|
|Transmission gear selected|