Driveshaft 101

There also may be a grease fitting on the slip yoke (the female sliding component) of the driveshaft. The slip yoke and spline stub are a couple of the more expensive components in a driveshaft. I don't believe that you can ever grease it too much (the EPA may disagree), and grease is a lot cheaper than finish-machined steel parts.

Proper greasing of the slip yoke will depend on the location of the grease fitting. On most older applications, the grease fitting is in the body of the slip yoke near the area that accepts the U-joint. On many newer applications or reworked driveshafts, the grease fitting is in the dust cap at the end opposite the U-joint (on either type of slip yoke, you'll find a relief hole in a plug that's in the end of the yoke under the U-joint). With the first type, you need to put your finger over this hole and pump grease until you see clean grease coming out past the dust cap on the opposite end. Unless you do this, grease will simply fill the cavity in the slip yoke that's not filled with the spline stub, and any excess will come out of the relief hole. With the second type, pump grease until you see clean grease coming out this relief hole. The object is to make sure that clean grease will completely coat the wearing areas of the slip yoke and spline stub.

Performing this procedure regularly is especially important on the front driveshaft. That's because every time you hit a bump in the road, your driveshaft will compress and extend. This is a lot like a sawing motion. On your rear shaft, the driveshaft is always turning and circulating the grease around. But on the front shaft, because it's seldom used at high speeds, this sawing motion will wipe out the grease film, allowing for metal to metal contact and will accelerate wear, as compared to the rear shaft. Many people complain to driveline shops of the short life of their front driveshaft even though they "hardly ever use four-wheel drive."

Hardly ever using four-wheel drive is, in fact, a big part of the problem. We suggest that you periodically engage the front hubs or put the transfer case into four-wheel drive mode without the hubs engaged and drive for about 30 minutes at a relatively high speed. By doing this on a regular basis, you should also be able to notice any problems in their infancy on the front shaft.

If you have a double cardan or constant velocity (CV) type of driveshaft, grease fittings are usually present, although short of removing the driveshaft, they may be difficult to see and impossible to get to. On a Spicer type of CV driveshaft, there will usually be a flush-type grease fitting for lubrication of the center pivot point. The problem is when the CV is opened where it would be accessible, the fitting is at the top of the shaft, where you can't see it. If you turn the shaft until the fitting is at the bottom, the knuckle closes up and you can't get to it. The only viable solution is to disconnect the driveshaft at the transfer case end, drop it down, and then grease it. This should be done at least twice a year.

Of course, most of us use our Jeeps for much more than the occasional trip to the ski lodge. We continually build problems into our vehicle while trying to improve its performance in other areas. Taller tires, differential changes, higher-horsepower engines, suspension lifts, and transmission swaps will all affect the life of the U-joints and driveshaft. With many of these modifications, there's no factory-approved solution.

Suspension lifts are the single biggest factor in unacceptable driveshaft and U-joint life or performance. Most lift kits only address the issue of elevation. The truth is most suspension lifts adversely affect proper driveline geometry--especially on short-wheelbase vehicles. We need that lift, though, to upgrade our suspension and to get the clearance required to accommodate taller tires.

With conventional two-joint driveshafts, it's very important to keep the output of the transfer case and the pinion parallel within one degree and in relationship to either of these two shafts. The driveshaft itself should be running at an angle no greater than 15 degrees. This 15-degree limit is a pushed limit, well beyond what most factory engineers would allow. Don't think, "Hmm, if they say15, I can go to 20. If you can't fall within this parameter or are close and want to optimize the life and smoothness of the driveshaft, you'll want to pitch up the differential so that the pinion points directly at the output of the transfer case and use a double cardan (CV) type of driveshaft. The net effect of this will be minimal joint angles at the differential end (ideally less than 3 degrees). This will also lower the joint angle at the transfer case end of the driveshaft.

Unless you do one or the other of those two things, you'll have torsional vibration. This torsional vibration is a result of the nonuniform rates of acceleration and deceleration through the elliptical path of the universal joint (that's easy for us to say). In some cases, you may not be able to feel it, but we can assure you, it's there. You just can't argue with physics. You might think that this vibration is something you can live with, but you are building and releasing torque two times per revolution on the driveshaft and other powertrain components. This pressure has to go somewhere, so it causes the drivetrain components to flex and distort two times per revolution. Eventually, that will cause fatigue failure.

Many times people will install very beefy driveshafts and do the most extreme trails with no problems. Then, while pulling out of their local supermarket, they'll suffer a broken driveshaft that's only under a modest load at the time. That's a classic example of fatigue failure.

Excessive angles are also a big problem. Often it's necessary to relief-grind portions of the driveshaft or the attaching yokes to allow for extra momentary articulation through the driveshaft. This should only be done to allow for things like axle droop, spring wrap, and frame flexing and not as a cure-all.