Klune-V Wrangler install article

Klune-V Wrangler Install Article:

What got us most excited about installing the Klune-V Extreme Underdrive was, in a word, OPTIONS. We're gonna drive this jeep on the freeway all day long sometimes. We wanna play in the dunes. We drive the easy forest service trails near home on weekend picnics. We go run the Rubicon, Sledgehammer and Surprise Canyon trails and want some extreme low gears for this. In other words, we like to do it all. And the Jeep is a daily driver. Real low axle ratios won't work: Bad freeway operation. And we live in LA. No way. We also thought about low gears for the T-case. Gets you low enough most of the time in the rocks, but is worse on the easy forest service trails, and doesn't give enough tire speed in the sand.

Bolting the Extreme Underdrive to the T-Case


First we oozed on plenty of RTV onto the front of the T-case. Shown here is the supplied mounting foot placed over studs on the front of a transfer case in preparation to bolt the crawl box to the T-case.	RTV sealant on the rear of the Klune-V in preparation to bolt unit to T-case.


This small spacer was cut according to measurements made and directions given in the installation instruction. We drop it into the input shaft of the transfer case. It will control the amount of free play the crawl box main shaft will have when mounted to the transfer case.	Sliding Extreme Underdrive onto the front of the transfer case. RTV sealant was used liberally between the T-case and the mount, as well as between the mount and the crawl box, in order to ensure leak-free operation.

After tightening bolts between the transfer case and the crawler, we are wiping away excess RTV sealant.

Installing the Extreme Underdrive/T-case
Assembly Into the Vehicle


Installing the studs into the front of the Extreme Underdrive unit. Note that we are using the 4th out of each group of 5 bolt holes. This will cause the transfer case to be rotated about 9 degrees more horizontal than factory. The benefit of this is shown in later pictures.	RTV sealant on the front of the crawler in preparation to bolt crawler plus transfer-case combo up to the transmission.
	In the rear of the factory transmission, there is a small vent hole in the lower part of the bolt up flange. In the Klune-V installation, this hole must be blocked with RTV sealant. Otherwise, you will have a tranny fluid leak. Rather than rely on the factory rear transmission seal, the Klune-V uses a double seal in the front of the crawler.

The Klune-V Extreme underdrive offers the option of using stock high range for the highway, stock low for the sand and the easy trails. And then, leaving the stock T-case in high, can be shifted into a rock crawling low range needed for proper control on rock trails and waterfalls. And as a bonus, when you put the transfer case in low at the same time as the Klune box, it's like having an 11:1 T-case! With 4.56 axles, our jeep could be about 200 to one in this range. Yeah, options. Wow. I like it.

The table shows the final drive “or “crawl” ratio of the Jeep in all of the different gear combinations. 5th gear on the highway works out to about 2200 RPM at 70MPH. When you are at around 200 to 1, the vehicle keeps moving at idle up, down, over and around rocks at about 4 to 6 inches per second, or 20 feet per minute.

A 4.56 axle ratio was chosen so that the Jeep had proper highway gearing with 35" tires. With 33’s, we would have used 4.11. In this way, the engine RPM vs. speed comes about the way it did with the stock axles and tires. The speedo even comes out right.


Using a transmission jack to raise the crawl box/T-case combo in place. Here we are getting everything lined up to stab it onto the back of the transmission.	Bolting the Extreme Underdrive to the transmission.

Extreme Underdrive unit shown installed between transmission and transfer case in Wrangler YJ. Note the “Mounting Foot” hanging down in front of the transfer case. This will provide a geometrically correct rear mounting point for the longer drive line. A GM type transmission mount is used under the “mounting foot”. The stock jeep unit had studs which would have hung down below the skid plate. By using a round head bolt into the GM mount, there are no studs hanging down to catch on rocks.

Short Driveshaft

The one thing we were concerned about when we started this project was the rear drive line length. A Wrangler doesn't have much of a rear driveline to start with, and here we were supposed to insert a 6-½ inch gearbox in between the transmission and transfer case! The 6 cylinder Wrangler YJ with the AX15 transmission comes from the factory with about a 13" driveshaft. Really, now. Doesn't sound bloody likely!

There are two problems you run into with a short drive shaft:

The first is vibration. This may not be very important to those who trailer their rigs to the trail, but this Jeep will be driving on the interstate for hours at a time. Vibration is caused when the various angles the parts of the driveshaft operate at do not line up correctly or simply have too much angle. Minimize drive line angles, use a CV joint and make sure everything is set up with correct geometry, and there will be no vibration. Vibration measurements are made with the vehicle sitting at ride height, as that is where it will be when it is driving down the freeway.

The second problem comes about when you get full extension, or droop, of the rear suspension. This can cause the joints in the drive shaft to physically bind. If this happens while the vehicle is under power, the driveshaft will most likely break. You must make sure the drive line does NOT bind when the rear suspension is at full droop, as with the vehicle on a lift.

Well, you can see from the pictures, that these problems have been solved. There is no vibration on the highway at any speed, and the driveline is not even close to binding at maximum suspension travel.

Illustration shows changes made to solve the drive line. problem: 1] The underdrive is added between transmission and transfer case moving T-case back 6.5” 2] The transfer case is clocked such that it does not hang down as far on the front-drive side 3] A slip yoke eliminator is installed. This is 4” shorter than the original slip-yoke tail housing. 4] The rear of the engine-trans-underdrive-T-case assembly is lowered about 1.5 inches. Because the T-case was clocked, this does not cost any ground clearance. The factory skid plate will mount in its stock location, no spacers are required. 5] We upgrade to a CV joint drive shaft. This arrangement is less prone to vibration, and is all-around a better way to do things. 6] since we moved the rear yoke of the T-case down in proportion to how far we moved it back, no change was required to the pinion angle from where we started with the project 7] The overall installation causes no loss of ground clearance 8] The finished vehicle has better drive line. angles, with less angle on each U-joint than when we started.

Making the Drive Shaft Happen

These are 1” spacers between the the belly pan and the frame. They cause the belly pan to drop 1” lower below the frame rails. This was part of a lift kit that was on the vehicle when we got it. Since we did not like the idea of losing 1” of clearance, we got rid of the spacers. We went through a bit of work getting things dialed just right. But once we are done, even though the spacers are gone and the crawl box is in, we have a better drive line geometry than we started with.

On the left is the stock slip yoke off of the back of the T-case. On the right is the replacement part: An Advance Adapter slip yoke eliminator. About 4 inches shorter, we can save some length. In addition, we will be able to use a CV joint in the drive shaft. A CV will allow better drive shaft geometry than the original Jeep design.

The left picture shows how the factory has the transfer case “Clocked” (the rotational angle) in the vehicle originally. The photo on the right shows the new, more level, T-case clocking. This allows us to mount the t-case closer to the belly pan, allowing the rear output yoke to sit almost 2” lower, without any clearance lost. By lowering the T-case yoke in this manner, we did not have to make any change in rear axle pinion angle.


Before the install: Here we see the original drive line geometry with the vehicle at ride height (weight supported normally on wheels). Note the long slip-joint tail shaft on the T-case.	This is the rear drive shaft in the vehicle as we received it. This picture was taken with the vehicle up on the rack and the rear suspension at full droop. Compare with the picture taken after the crawl box is installed with a new drive shaft. Note that even though the new drive shaft is 2 1/2 inches shorter, the angles on the U-joints are less.


We had a new high-angle CV joint drive shaft made for us. Shown here, the vehicle is at normal ride height. You can see that the drive shaft is almost straight. And we even got rid of the 1” spacers under the belly pan. This vehicle drives on the freeway without a hint of drive line vibration.	This view shows rear axle in full droop position. It is necessary that at full droop the CV joint does not bind AT ALL. in this application, there is plenty to spare: The high angle CV will flex 40 degrees before bind, we are only at 30 degrees in the YJ. Compare to the stock drive shaft pictures: The angles here are better.

The T-Case Shift Linkage

We kept the factory T-case shifter in the factory location. The shift rod was extended 6.5” to reach the new transfer case location. This was done by cutting the rod and welding in a section of tubing to lengthen it.

Lengthened shift linkage shown installed in vehicle.

Muffler Bracket

The original transmission mount bracket is trimmed to retain its function as an exhaust support. It is not used as a transmission mount in the installation.

Hook up the Cable Shifter to the Underdrive

Here we are using a universal multi-
purpose adjustment tool to get about 3/8” more clearance at the front of the skid pan.

The cable for the cable shifter is attached to the shift box on the Extreme Underdrive unit.
Cable shifter shown mounted on side of tunnel next to factory shifter boot.

Driving Impressions

On the highway: No, it is not completely invisible. You can hear the box making a slight whirring noise, a sound that wasn't there before. Otherwise, it is the same as it always was.

In standard low range, in the sand or on a trail, it is the same as it was without the box.

Shift the Extreme Underdrive into low, and the factory T-case into high: Now we are in a 4:1 low range. There is a noticeable gear whirring noise, but reasonably tame. And we have real rock crawling gears. Just like we were dreaming about.

Now, we weren't real sure what that "Extreme Underdrive" range would be good for, beyond winning turtle crawls and stuff. Seemed like it would be so slow that it would be impractical. Whoa, surprise, surprise. In Surprise Canyon it allowed us to creep up wet, slippery waterfalls without using the winch. With the 4:1 rock crawler gearing there was still too much of the vehicle lurching around and we couldn't stick to the rock. And in real tight, technical rock crawling, we just leave the clutch out, idle through the obstacle with feet off the pedals, and steer.

Yes. It does what it says it will do. It works, plain and simple. We got new capability without having to give up any existing capability to get it. No, the Klune-V Extreme Underdrive is not a beer-budget option. But if you want it all, this is definitely the way to get it all.

Here is the Jeep, ready for two days of hard wheelin' in California’s Panamint Valley. We played in Stone Canyon and Surprise Canyon. Everything worked fine with nary a mishap.

Special Application:

NP231 Replacement

Klune-V with

Left-Drop Dana20 Transfer Case