Driveshaft and Gearbox Upgrades - Big-Truck Tech

It’s like a good recipe: Take one big Super Duty, add equal parts big lift and big tires, stir in some shop time, and let cool. Then go ’wheeling. If you follow directions and cook it right, you have a killer rig that can perform. But if you don’t pay attention to the whole recipe, you’ve just cooked up disaster.

As it turns out, big lifts and tires on big trucks aren’t just bolt-on propositions. Sure, the lift kit fits and those 44-inch tires look way cool, but even after you’ve regeared the axle with the lowest cogs, you still don’t have enough grunt to get out of your own way, even in low range. Worse yet, the front driveshaft keeps spitting out CV joints because of the poor operating angles, and you just blew out your third axleshaft from banzai maneuvers in the mud. What’s a guy to do?

In this type of scenario the first thing to do is to keep in mind what the truck is for. Is it a trailer queen (not likely), a rock buggy (hardly), or a real mud whumper? For our case this truck may look spiffy, but it also does weekend duty at the local mud hole, where Dana 60 front stub shafts are eaten for lunch. However, its biggest problem was keeping the front driveshaft alive. Oh yeah, more gearing sure wouldn’t hurt either.

After scratching some heads and throwing some ideas around, Jason Bunch at Tri-County Gear in Pomona, California, came up with the solution long favored in rockcrawling circles. Rockcrawling? How could that radical technology help a mud truck? The answer was surprisingly simple as soon as the “oh yeah, more gearing sure wouldn’t hurt” was bandied about.

Deeper gears for the axles are fine, and this truck already sports low-geared Dana 60s front and rear. But that means more work on both ends of the truck, which results in sacrificing the highway top speed, and is why this solution didn’t come up first. Besides, it was the driveshaft that caused problems, and changing axle gearing wouldn’t help that at all. The simple and easy way to eliminate these concerns was to install a Klune-V Extreme Underdrive gearbox, a longtime standard of rockcrawling and trail-riding aficionados. Since this trick unit mounts between the transmission and transfer case, the high range of the rig is not affected unless the box is engaged. It’s similar to a two-speed rear axle, but it’s mounted in front of the transfer case, so you affect whatever position the transfer case is in. The extra length of the unit also moves the transfer case rearward, helping to alleviate some of the front driveshaft angle problems. But the best reward is the fact that the Klune-V has multiple clocking positions on each end of the unit, which allows the transfer case to be rotated to virtually any angle imaginable. This means that, unlike the stock position, the front output of the transfer case can be lowered for optimum driveshaft angle.

While simply bolting a Klune-V unit in place and driving away is entirely possible on some rigs, the added length requires driveshaft lengthening in the front and shortening in the rear for most vehicles. In addition, crossmember modifications are usually needed, and the shifter needs to be mounted inside the cab. For this install we chose the Goliath model, which features a 2.7:1 low ratio, more than enough for this truck, and super strong to boot. Check out how this Super Duty was saved by the addition of a Klune-V and now romps freely in the mud, destined to break more front axle parts.