Driveshaft dilemma — Why choose a new joint over a beefier shaft?

I recently shared discovering my front driveshaft was leaking grease from its constant-velocity joint boot. I’d read enough about JK Wrangler driveshafts to know this and the vibration I’d been feeling each time Smokey approached 60 miles per hour likely spelled an early death for my driveshaft.

My immediate course of action seemed obvious: replace the driveshaft with a suitable double-cardan design. Ultimately, though, I decided to instead replace just the constant-velocity joint with a Teraflex Rzeppa High-Angle Factory Replacement CV Kit (try saying that three times fast), keeping the factory driveshaft for as long as it will last.

Here’s why:

Cost

As I explored options like Tom Woods, Adams Driveshafts or JE Reel, one fact was inescapable — this could get expensive. Replacing my factory shaft stood to cost as little as $400, but potentially as much as $900 for a single shaft and the yokes by which to mount it. My wife and I are diligently working to eliminate a good deal of medical and consumer debt this year, with the Jeep falling into the latter category. I could’ve redirected some of our aggressive Jeep payoff funds to this project, but I wasn’t looking forward to doing so. I could also find another Jeeper’s factory take-off shaft, but what kind of abuse might it have suffered before being cleaned up for posting on Craigslist or eBay? The Teraflex kit, however, comes from a respected, reliable company and reduced my cost to a scant $143. Even if I were to for some reason get only two or three years’ use out of the new joint, it will have served its purpose.

Practicality

Let’s face it. I don’t put in nearly the trail time I’d like to and, when I do, my wheeling style isn’t exactly what you’d call kamikaze. I’m not scared of the thin-walled factory shaft failing me, nor of gashing it open on a rock. At the same time, though, I do wheel when I’m able, and I don’t want my equipment to be the limiting factor in determining where I can or can’t go. The factory joint is designed to provide the best on-road ride, but the primary weakness of the Rzeppa joint, invented in the 1920s by Ford Motor Company engineer Alfred Rzeppa, is its distaste for extreme operating angles. To that end, the Teraflex joint has been specifically designed with a flared outer flange to operate at angles more than 30 percent higher than the factory unit. With that added flexibility, I should have the freedom to stretch out my suspension on the trail, even after lifting, without worrying about cratering my factory shaft. (Presumably, the rear driveshaft will need a similar modification, but only after I install my final coil lift and, even then, only if my actual lift height overextends the factory rear joint.)

Strength

Yes, it’s true these factory JK driveshafts are as thin and cheap as a tin can, and a hearty steel replacement offers significantly greater strength. But, as noted above, it’s unlikely I’ll be dropping my rig’s two tons down onto the shaft, making the more relevant strength to consider that of the joint itself. As designed for the JK, the factory Rzeppa joints offer higher breaking strength than a comparable double-cardan CV joint. It’s that joint, after all, that will absorb the bulk of the rotational force being applied by the transmission and transfer case. Based on my research, the tensile strength of a typical 1310 U-joint is about 1,800 ft.-lbs., while the Rzeppa will withstand some 2,100 ft.-lbs.

NVH

img_0177In automotive parlance, NVH stands for noise, vibration and harshness. Now, I make no claim to be an expert on drivelines or joints, but, I came to an interesting realization as I researched my options, one which finally helped me see why Jeep chose to use a Rzeppa joint (upper animation at right) in its driveshafts in the first place. This factory joint consists of eight steel ball bearings that move along grooved tracks within a rotating assembly. The rotating u-joint-dsmotion of the joint keeps the balls moving at a constant rate in relation to the force being applied through the transmission, making the Rzeppa joint far more smooth than a more traditional single- or double-cardan U-joint (lower animation at right), in which one side of the joint is speeding up as the other is slowing down during each rotation. Why does this matter? Because it means a Rzeppa joint will transfer less NVH up into the body of the Jeep than other joints. Additionally, my research showed how painfully common it is — even among top-of-the-line driveshaft manufacturers — to have shafts that aren’t perfectly balanced. “My shaft only vibrates at highway speeds” was a very common statement from end-users on web forums and other sites across the internet. Now, call me crazy, but vibration is one of the things I’m trying to get rid of. So, why would I pay hundreds of dollars to simply move my vibration from 60 mph to 80 mph?

 

Conclusions

Hopefully, this helps you see why I believe the Teraflex kit best suits my needs at this time. It’s no magic bullet, and I’m not suggesting it’s the right solution for all JK drivers. Both the Teraflex Rzeppa and traditional double-cardan designs offer both benefits and drawbacks, which must be considered before tackling this kind of project. As with any vehicle modification, the best advice I could share with my fellow Jeepers is to take your time and diligently research your options. Internet enthusiast forums are a trove of valuable information, but beware not all the information there is well-informed; some users will push a specific option only to justify their own past purchases. Examine all choices and choose whatever makes the most sense for your situation.

 

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