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View Full Version : What is an SYE and When Do I Need One?


XJ_ranger
October 24th, 2006, 11:22
What is a SYE?

A Slip Yoke Eliminator (SYE) adapts the rear end of the factory transfer-case from a Slip Yoke style to a Fixed Yoke Style.

The origonal design for the XJ Cherokee rear driveshaft is set up with what is called a single cardan rear driveshaft with a fixed length, fixed at the pinion, and slipping at the Transfer Case to make up for droop and compression of the rear suspention.

An SYE eliminates the slip yoke on the t-case tailshaft by changing it to a fixed mounting point for the rear driveshaft, moving the slip yoke to the driveshaft. This also allows some u-joint setups with higher angle capability for a more vibration free ride down the road.

There are two main types of SYE's, based on the output shaft that is used. The "HD" SYE uses a larger 32 spline output shaft, and is made by various top name vendors. The standard SYE kit provides a shortened stock 28 spline output shaft. One company offers a standard SYE kit without providing another output shaft, so the existing shaft is cut, then drilled and tapped for the new flange bolt. This kit is referred to as a "Hack and Tap".

The main issue with the Factory Slip Yoke is the increased angles on the rear driveshaft after lifting a Cherokee. The more lift, the steeper the angle. The increased angles can cause issues with yoke clearance on the factory slip yoke and increased wear on u-joints. A common fix to cure the yoke bind issue is to use a Slip Yoke from a YJ wrangler.
The increased angle also causes a need to Shim the rear axle to re-set driveline geometry and reduce vibrations. The problem with re-setting the geometry is that this causes the angles to be even steeper because in most cases, the pinion needs to move down and be parallel with the t-case out put. This increases the angles on the rear driveshaft more and more, and (the biggest issue) causes the slip yoke to be pulled out of the tail-cone more than is comfortable, and can cause the slip yoke to pull clean off the t-case mainshaft.


The fix for this is to run a Slip yoke eliminator and a rear driveshaft with a slip joint in it.

There are two main driveshaft choices -
another single cardan (one u-joint at each end - two u-joints total) with a slip joint in it
or a "Double Cardan, Near Constant Velocity" drivesahft with 2 u-joints at one end conected by a uni-ball and one u-joint at the other end. These are often refered to (incorectly) as CV driveshafts, and MOST cherokee's have an example of one of these style shafts in the front end for a front driveshaft.

Most cherokee owners opt for the Double Cardan style shaft due to the increased angle it allows, and the corect pinion angle geometry allowing for a higher clearance under the pinon.
Most driveshaft makers (local or mail order) will have some system of measuring on the vehicle to determine the needed length of your new driveshaft once you have your SYE installed. Talking with them will insure you get your order correct.

If you choose to go with a Hack and Tap SYE, you might need to have a flange syle driveshaft, and in talking with your drivesahft maker, you should mention this, and make sure he has all the adaptors and will get you the parts you need.

I encourage you to search here on :NAXJA: and make your own decisions as to what SYE to run, proper driveline geometry, whch rear driveshaft will work best for you, and all of that.


When do I need one?

The general rule of thumb on lift height and need for an SYE goes as follows:
1984-1995 = SYE needed at 4.5" ACTUAL LIFT HEIGHT - ok at 3" lift
1996-2001 = SYE most deffinately needed at 4.5" lift, and most need one at anything over the 2" Budget boost.

These are general guidelines, and you will hear people needing SYE's at 2" and you will hear of people running no SYE and 6" of lift - it is all dependant on the vehicle and the ownwers description of "Fine".

If you put on a lift, and then notice driveline vibrations, this is most likely due to the rear driveshaft, and could possibly be that you need a SYE and driveshaft.

While you may be able to live with your driveshaft vibrations and how that affects you, it is certanly not good for the t-case bearings and seals.

If a Lift Kit is sold with a "transfer case drop", a "motor mount lift", or a "high clearance slip yoke" - thoes are bandaids for a real problem - the REAL solution is to use a Slip Yoke Eliminator and a new driveshaft.

I encourage you to do all your own research as to if you need an SYE and the cost effective ways to run them and how they work.

Good Luck!

-XJ_Ranger