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Short Done Right!:
JB Conversions Super Short Kit
Slip Yoke, Heavy Duty Chain and SGI-5 Electronic Speedo Calibration

By Shawn Pagan

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JB ConversionsThe Problem Defined:
One of the often overlooked areas when a person first plans on lifting a short wheelbase vehicle is the impending problem they will eventually find with the angles of their drivelines.

In particular, Jeep TJ's and YJ's being lifted more then 3 or 3-1/2 inches generally need to install a slip yoke eliminator (SYE) kit, completely replace the rear drive shaft and/or lower the transfer case mount.  Dropping the mount works and it can generally address replacing the driveshaft and installing an SYE in many cases, but it also lowers the break-over angle of the vehicle and reduces ground clearance, which of course, negates many of the reasons for installing the lift for off-highway use.

After lifting the Jeep, a SYE kit is needed because the distance between the transfer case and the axle is now longer and the angle of the u-joints at the rear of the transfer case have been increased. Without doing a SYE kit, the result is usually felt thru the seat of the pants and termed "driveline vibration."

Some of this vibration can be solved by purchasing a new rear shaft (or modifying your existing shaft) and/or by "adjusting out" the rear end by installing adjustable control arms or degree shims or in some cases using eccentric bolts.  This, however, only partially addresses the angle issues and without addressing the entire issue, more than likely, during the cycling of the suspension it will put added stress and wear on not only the u-joints but the yokes and flanges that hold them in, as well.  Someone once told me that it paid to pay attention to your u-joints, their angles and their general health because if you don't pick and choose the time to maintain them, they will pick the time that you replace them.

There are plenty of "Slip Yoke Eliminator Kits" on the market from the basic "Hack and Tap" kits that require you to cut your rear output shaft and tap it to heavy duty kits that require you to replace the entire shaft.  The fact is that many of these kits are not there to just address removal of a slip yoke, but what they are really designed to do is put a yoke or a flange on the rear output of the transfer case and provide a driveshaft that is not only longer but one that incorporates a "CV" (Constant Velocity) joint or a "double cardan" joint in the t-case side of the shaft itself.  This double cardan joint (which is commonly referred to as a CV style drive shaft, even though it does not really have a constant velocity joint) allows the angle at the t-case and the rear axle to be on different plains.

Spicer recommends a constant working angle of 7 degrees, however, it's been written by many "experts" that a u-joint will handle more like 12 to 15 degrees with no side effects.  From what I have seen, the better the angle - the longer the joints will last.

Note: The working angle of the u-joint is the angle of the driveshaft minus the angle of the yoke.  So if you have a driveshaft at a 23 degree angle and your yoke is at an 18 degree angle you would have a 5 degree working angle on the u-joint in that end. For a double cardan Joint you can roughly estimate that the angle of the u-joints is going to be the angle of the driveshaft minus the angle of the yoke divided by two.

In common terms a standard, single u-joint on each end of the driveline (2 joint driveline) requires that the t-case end and the axle end maintain the same exact angle at the u-joint.  Picture in your mind that the t-case side must be a mirror image of the axle side (see picture).  This offsets the elliptical motion path that the u-joint travel thru (yep, you are right. U-joints don't spin in a true circle) and keep the vibrations to a minimum.

A double cardan style or similar driveline places two u-joints near the t-case which allows the upper end to maintain the proper u-joint angles by dividing the angle between two joints instead of one.  In a typical double cardan installation you want the rear axle yoke to point directly at the t-case output.  Well, in reality, most of the time with soft springs you really want it to point slightly under the t-case output in order to offset the torque load when you apply power to the rear end. This arrangement reduces the angles on all three u-joints which gives them a longer life and allows your suspension to travel up and down without binding the u-joints or the yokes.

Now having read thru all that you probably think all the needs are met and you're ready to move on to bigger and better things.  Well, let's just say we still have a lot to talk about - especially for the more extreme crowd.  When lifting a rig 5, 6 or even 10 inches as some kits and custom installs are doing these days, the process of getting the lengths and the angles right start all over again.  In addition, many people today are installing flat belly pans or skid plates that lift the transmission / transfer case up one to three inches higher into the frame rails, further compounding the driveline angle problem.

Not only do these type of installations now separate the axle and the transfer case in distance but they generally allow for more articulation of the axle and more movement of the suspension as a whole. This added force on the component on a short driveline causes many people to consider swapping transfer cases altogether in order to get adequate length for the rear shaft without giving up drivability and longevity of the u-joints.

As you can see here on my Jeep, I have a typical slip yoke kit installed.  It was one of the original standard duty kits produced by Currie Enterprises and uses a flange on the t-case end with a double cardan joint and gives me about 4" of length when compared to the stock transfer case. Because of the double cardan joint, though, it greatly improved my driveline angles and eliminated the slip yoke on the transfer case side.  When I had a basic four inch lift and the stock t-case skid this was fine and it served me well.

Measuring the length and angle

 View of the Currie Double Cardan joint

Over the last "few" years my suspension and other components have been changed and modified to the point where I have almost 6" of suspension lift and run a modified Skyjack Transfer Case Skid Plate.  With a 1" motor mount lift the Skyjacker pushes the transfer case up approximately 2" higher between the frame rails than it did in stock configuration.  Since it doesn't lift the transfer case at the rear end, it creates a situation where the rear end angle is actually made slightly more severe because of the arc that's created by the lifting point.

Driveshaft angle approx. 26 degrees

Length to first joint in the cardan 16"

While I have not had many problem with driveline vibration, I was occasionally popping a u-joint because with only a 16" long driveline (about 18" to the middle of the second u-joint in the cardan joint), at droop I was putting too much pressure on the u-joints in the shaft.  Like many people, I assumed I would have to live with it until I could afford to put in an Atlas T-case or build up a Dana 300.  Prior to installing the original SYE, my original driveshaft was only about 13" in length.

The solution:
The last time I replaced my u-joints, I wondered what I could do and began the search for a replacement transfer case.  What I ended up finding was a new Slip Yoke Kit being made by JB Conversions that would allow me to add some serious length to my driveshaft, while keeping my existing transfer case, my skid plates, my front driveshaft and much more of the investments I had already made.

With anticipation I called JB on the phone and discussed my needs with them.  What I came away with were three distinct kits (well four if you count the new rear driveline separately) that I will now be installing on my TJ.

JB Conversions Super Short SYE Kit, Dakota Digital SGI-5 & 231 Wide Chain Kit

NP231J HD Super Short SYE Kit with new JB Double Cardan Driveline:

  • Replaces the existing 231 main shaft with a heavy duty 32 spline shaft
  • Creates a NP231 Transfer Case that is equivalent in length to an Atlas transfer case
  • Uses a removable, replaceable yoke on the rear output shaft.
  • Allows the use of different size u-joints (1310 up to 1410) by replacing or installing proper yokes.
  • Replaces the plastic gear speedometer assembly with an electronic sensor and tone ring.
  • Includes a new rear bearing housing assembly complete with seals.

JB Conversion Wide Chain Kit which:

  • Replaces the existing chain with a wider version which helps to eliminate stretching
  • Replaces the front output shaft with one using a wider cog for the wider chain
  • Replaces the main sprocket on the rear output shaft with one using a wider cog

Dakota Digital SGI-5 Universal Signal Interface which:

  • Allows adjustment of the electronic signal from the new speedometer sensor to accurately calibrate the speedometer.
  • Allows use of different-sized tires or gear ratios down the road with no additional investment

For many people, the Dakota Digital device may not be needed as JB Conversion has setup their electronic control in a range that emulates the "factory" and will provide an accurate reading to your speedometer if you are using a tire and gear combination that is close to a 4.10 and 31" such as the stock configuration of a Jeep Rubicon.  For example, if you are running a true 35" tire and 4.56 axle gears which are pretty common on YJ's and TJ's these days, you probably won't need the Dakota Digital SGI-5.  I opted for the SGI-5 because I run two sets of tires and would really like the ability to flip a couple of switches when I change tires and know that my speedometer is still accurate.

I got the wide chain kit and almost ordered a complete rebuild kit (which I was talked out of) since I was going to have to open the t-case up anyway to install the Super Short SE Kit.  JB Conversions said that since my transfer case only had 60,000 miles I probably didn't really need to rebuild it. I was interested in the wide chain kit because while I believe that the NP231 generally gets a bad rap for being a "weak" transfer case. The weakness seems to be the ability of the chain to stretch.  Usually, this occurs because of poor maintenance and the transfer case overheating. Basically, not keeping good, clean fluid at the appropriate level in the transfer case. While I am pretty religious about maintenance, I figured the wider chain would absorb more shock and make up a little for those times when I don't get the ATF replaced quite like I should.

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