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JB Conversions Slip Yoke Eliminator Kit
& Tom Wood Drive Shaft

By Cole Ford

Discuss This Article

JB Conversions/Tom Wood Slip Yoke Eliminator Kit
The stock drive shaft angle just doesn't cut it anymore once you lift your Wrangler.
JB Conversions/Tom Wood Slip Yoke Eliminator Kit
You really can do the SYE kit in your own garage and save some bucks. As far as mechanical modifcations go on a Jeep, it's actually one of the easier ones to do. You may need a buddy to help you with removal and moving the transfer case.
JB Conversions/Tom Wood Slip Yoke Eliminator Kit
The JB Conversions NP231 output shaft (top) and stock shaft (bottom) are quite different on the outside end (on right). The new shaft is much fatter, has a higher spline count, and is several inches shorter than the stock one. Which would you rather use?
I Hate Worms
If you read the Superlift Rock Runner suspension article you know exactly what I am talking about. Any time you make a modification to your Jeep you open a can of worms. A suspension install can bring about a lot of worms. Two of the biggest worms you will encounter are a change in gear ratio and driveline vibrations. The one we are going to address here is the driveline vibration problem. Since not all of us are mechanics or engineers, I think the first thing we should do is define the problem. Once we have done that we can talk about the solutions and, of course, how JB Conversions and Tom Wood's Custom Drive Shafts helped us solve them.

One of the greatest problems with Jeep Wranglers is a short rear drive shaft. The combination of a short wheelbase and a long driveline (engine, transmission, transfer case) makes for a very short rear drive shaft. This is not much of a problem when the Jeep is at stock height. The back of the transfer case and the front of the rear differential are pretty close to the same height. This means that the drive shaft itself does not need to have much of a slope. This lack of slope allows for the u-joints at each end of the drive shaft to move freely and without causing any vibrations.

When you lift the Jeep to gain ground clearance, you move the tail end of the transfer case up and away from the rear axle. This causes the operating angle to be steeper, which in turn causes a binding effect on the u-joints at the ends of the drive shaft, leading to vibration. The extent of this effect can vary from Jeep to Jeep. Some may only have a small annoying vibe, but even a little vibration can wear out u-joints in a hurry. The effect gets much worse off-road as your suspension flexes even farther away from the transfer case. Broken u-joints, drive shafts and transfer cases could result in short order.

Vibration Robbery
Unwanted vibrations in a Jeep are like little gremlins. It is easy to dismiss the importance of a smooth-running rig. Since most of us Jeep owners fancy ourselves as rugged individuals with Jeeps, we also tend to come up with excuses for the vibes. We say things like "It's a Jeep, its supposed to do that!" The truth of the situation is that vibrations can rob you of some very important things.

The first thing you lose is the comfort and smooth operation of your expensive Jeep. The second thing you loose is reliability on and off-road. The constant vibration and resonance of the driveline shakes bolts and u-joints to pieces.

The last thing is horsepower. Yes, horsepower! What do you think actually powers the vibration? The power from the engine makes it through the transmission to the transfer case and then to the rear drive shaft. If all of that power makes it to the rear axle, you feel it as forward motion of the Jeep. If there is something binding in the driveline, that energy is being transferred into a vibration and not forward momentum. The power loss may be minor in most cases but think about how many horsepower it takes to shake your whole Jeep with a vibration. I just installed a K&N air filter in hopes to gain 5-10 hp. What if I am losing the same amount in my driveline due to a poor drive shaft angle?



Worm Farm?
Since we opened the can we have to deal with it right? Right! There are several ways to deal with this vibration problem.

JB Conversions/Tom Wood Slip Yoke Eliminator Kit
You can typically remove those transfer case drop kit spacers once you do a SYE kit and tuck your t-case back up where it belongs.
JB Conversions/Tom Wood Slip Yoke Eliminator Kit
The JB Conversions SYE kit and the Tom Wood drive shaft combine to give you everything you need for a better driveline angle, including a new output shaft housing for the transfer case.
JB Conversions/Tom Wood Slip Yoke Eliminator Kit
The stock (left) output shaft housing will be replaced with a brand new housing (right) from JB Conversions, which will help lengthen the driveline.
JB Conversions/Tom Wood Slip Yoke Eliminator Kit
Big, fat, beefy, stout. These are all perfect adjectives you can use to describe the JB Conversions output shaft (top). Looking at the realatively dinky stock shaft (bottom), you can see why Option B+ is not the best way to go.

Option A
The suspension manufacturers are well aware of the driveline issues in Jeeps. Most of the kits you can buy will include some sort of transfer case drop kit. What this does is place spacers between your frame and transfer case skid plate. This accomplishes two things. First, it lowers the transfer case closer to the rear axle height in order to decrease the angle of the rear drive shaft. Secondly, it tilts the case slightly to alleviate the strain on the upper u-joint.

Drop kits also have several negative effects, though. You lose some of your precious ground clearance you were trying to gain by installing the lift. Your shift levers are now shorter in the cab of the Jeep and may even require floor trimming in some models to make them work right in second, fourth, and reverse. This is more common in YJ's than TJ's. It also causes an extra bind on the motor mounts since the engine is now tilted rearward.

Option B
The model 231 transfer case found in the Jeep Wrangler since 1987 is too long and uses a slip yoke design at the back of it. The drive shaft slips into the back of the case making the rear seal of the transfer case. If, for some reason, you have to remove the rear drive shaft you will lose all the fluid inside the case. This is because there is no longer any type of oil seal. This means that if you break that rear drive shaft or u-joint on the trail you CAN'T limp it home in front wheel drive. Editor's note: We have seen people plug the hole with everything from rags, to duct-taped in plastic soda bottles.

A slip yoke eliminator kit (SYE) uses a C/V (constant velocity) joint, which bolts on to the back of the transfer case. The kits eliminate the slip yoke (thus the name) and use an oil seal to keep the fluids inside. By eliminating the slip yoke design on the back of the transfer case, you benefit in several ways. First, you can shorten the overall length of the transfer case output shaft by 4-6 inches. This translates to 4-6 inches that you can use for a longer drive shaft. This alone makes for a better drive line angle without lowering the transfer case. It also gives you the ability to remove the rear drive shaft should you break it.This means that you CAN limp it back home in front wheel drive.

It also means that your shift levers etc. stay where they were designed to be, because in most cases, you can get rid of that transfer case drop kit and tuck the case back up where it belongs. This actually makes the Jeep run and drive like it was designed to do. The SYE kits and drive shafts integrate into a Jeep seamlessly. You will have to keep in mind, though, that the rear pinion angle needs to be adjusted, no matter which method you use.

Option B+
There are several SYE kits out there on the market. Some of the options include cutting, drilling and tapping your current output shaft to shorten the overall length. While this is the cheapest option and can certainly be effective, it is in my opinion a cut and paste approach that does not seem to offer much consistency from the installer's stand point. What if I cut wrong, drill crooked or tap it wrong?

The best approach is to buy a kit that includes a whole new shaft to put inside the transfer case. The kit from JB Conversions is exactly that. It is a heavy-duty shaft made to the exact transfer case manufacturer's specifications. This means that not only do I get to put NEW parts in the Jeep, but I have also now increased the strength of my transfer case.

The JB Conversions shaft has an 18% larger shaft diameter than the stock NP231 shaft. The shaft also has 32 splines compared to the original 27 splines. That gives the shaft a 54% increase in torque capacity. The JB conversions kit comes with everything you need for the transfer case, including a NEW rear tail housing.

The JB Conversions' set up places the output at the shortest possible point without sacrificing the manufacturer's design specifications. That is why it comes with the new tail housing. This makes it the strongest and most reliable SYE kit on the market. I don't want to have any worries about the integrity of my transfer case on or off-road. Doing it right the first time means that I won't have to do it again. And as long as we are upgrading to a heavy duty transfer case, we may as well upgrade to one of Tom Wood's heavy-duty rear drive shafts.

JB Conversions vs. Stock
JB Conversions/Tom Wood Slip Yoke Eliminator Kit
This is the stock setup. Notice how long the output shaft is and the corresponding housing.
JB Conversions/Tom Wood Slip Yoke Eliminator Kit
This is the new, JB Conversions output setup. It's tucked in further and the shaft is a lot shorter.




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