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The Novak Guide to

Driveshafts

driveshaft_angle

This is typical of a stock Jeep® driveshaft. The angle - even from the factory - is a bit steep. A little lift is all it takes to put u-joints over the edge. Axle articulation and wrap during off-road maneuvers further worsen the situation.

What makes most Jeeps a "Jeep" is that they are compact. Spend a day in the woods wheeling a short-wheelbase-Jeep with the full-size truck crowd and you'll come away a believer that short can indeed be sweet. Short wheelbase Jeeps are immensely agile because of their short length.

One of the greatest challenges of most swaps is that of the rear driveshaft length and the resulting u-joint angles. The length of engine, gears, pinions and adapters must be considered. Yes, most of us would like a six-speed gear box with a 7:1 low and 0.7:1 high, along with a three-range transfer case. However, more gears means more length. Such a combination is not in the cards for short Jeeps with any sort of lift on them. But, with decent planning and good components, you can put together a Jeep gear train that may amaze you.

You'll find in our Transmission Knowledge Base that we list the length of each transmission and this is further detailed in our Transmission to Transfer Case Adapters Section. This information will be useful in planning your swap.

Driveshaft Length Changes

Gearing vs. Driveshaft Length and U-joint Angles

So, How steep can you run a drive shaft? Again, this is entirely up to you. However, most manufacturers recommend a maximum of 7 degrees. I personally believe they are conservative - they have to be. I also think they base their recommendations on the math for the largest semi-sized drive line and call it good for every thing else, which it would be. Doing the math for an automotive sized drive line, using a 4" swing diameter and assuming the transfer case output and pinion shafts are parallel, the actual cosine for an angle of 15 degrees. I calculate the result of the net difference in linear distance traveled through the arc of each of the "U" joints' path, to be roughly .0014" per occurrence. I believe there are enough clearances in the universal joint, the slip yoke & spline stub along with a torsional modulus of elasticity in the tubing and other components to accommodate this. Beyond this point the the power train components must themselves flex and distort to allow for this extra motion. This repetitive and continuos flexing will fatigue these components and cause premature failure.

Many conversions require propeller shaft length changes. This work is done in your locale by driveshaft specialty shops. One can have this done at a farm implement or tractor dealer, as well as RV service centers. Balancing must be done during the process.

It is important to note that it is usually preferable to change the length of the driveshafts than to relocate the engine to the wrong place in the engine bay. It is best to find the optimum position for the motor and then work backwards, measuring for the driveshafts last with the weight of the Jeep on the springs. Make sure to anticipate the flex of the suspension in the upper and lower most positions.

To restate; it is seldom a good idea to allow the driveshafts to make the decisions as to where the powertrain will be placed. Some of our customers, fearful at the perceived expense of new or modified driveshafts, attempt to let the existing driveshafts dictate engine, transmission and transfer case location, sometimes to the detriment of their project. New driveshafts are an option but seldom a strict requirement in regards to the actual successful conversion. Jeeps that require extensive travel or specialty-built driveshafts have this option available through several great fabricators across the nation.

Phasing & Offsets

mangled_u-joint_montage

Because of the strains induced by the steep angle of a short shaft, the u-joint bearing lubrication rapidly failed and the driveshaft forced the u-joint out of its axle yoke straps. When an installer does not heed good principles, an iminent disaster for many Jeeps and their occupants awaits. Think of a flying mace or nunchaku, with a couple hundred foot/lbs. of torque...

Nearly all front and rear axle yokes are offset downwards, expectedly. Most early Jeeps (featuring the Dana 18 or QuadraTrac transfer cases) had passenger-side offset rear axles. These offset axles brought the driveshaft inline with their transfer case. Individuals who install a rear axle behind an offset transfer case, or, install an offset transfer case in front of a centered axle, are in for serious vibration! So much so that it may be an immediate safety issue.

The general rules to follow are:

  • Use a conventional (non-Cardan, a.k.a. non-cv) u-joint driveshaft in your Jeep whenever possible
  • If using conventional u-joints, the joint angle at the transfer case must be complimentary with that of the axle, i.e., if the drivetrain tilts down about 5 degrees, the axle pinion should tilt up about five degrees
  • If using a Cardan / CV, the joint angle at the transfer case must be non-complimentary, i.e., the axle is tipped up some so it points towards the transfer case
  • Use a fixed-yoke (two-piece) driveshaft in lieu of a slip-yoke (one-piece) driveshaft when possible. OEM's use slip-yoke shafts for cost savings, not performance and flexibility
  • On a single / conventional u-joint setup, the transfer case yoke should be phased to the axle yoke, meaning that the yoke orientations are identical

Sources: