Building a 4x4 Expedition Truck: Choosing the Right Front Axle
How to Build a 4x4 Expedition Truck: Choosing the Right Front Axle
When designing our expedition truck, opting for a 4x4 system was a nonnegotiable decision. We weren’t willing to compromise on the ability to tackle rugged terrains, and since there weren’t many off-the-shelf options for a new 4wd cab-over truck, we decided to build our own custom rig.
Why a 4x4 Expedition Truck?
The two key differences when comparing a simplified 2wd truck to a 4wd vehicle are the front axle and the transfer case. For our expedition truck build, we chose and installed the NP205 transfer case with a passenger-side drop. With that decision in place, the next step was finding the ideal front axle.
Choosing the Best Front Axle for Our 4x4 Expedition Truck
We needed a 4wd axle that would work seamlessly with our NP205 transfer case and handle the rugged demands of an expedition truck. After a lot of research, we listed our essential requirements:
Manual locking hubs for better control and reliability
Final drive gear options to suit different on and off-road needs
Locker options to ensure maximum traction
Ease of maintenance for roadside repairs
Simplicity in design
Additionally, we needed to maintain the leaf spring suspension and ABS functions. The brakes had to be robust enough to handle the weight of our expedition vehicle, and the axle itself had to be easily serviceable. Most importantly, it had to align with the NP205 transfer case's passenger-side drop.
Specific Requirements for the Front Axle
As we narrowed down our search, we became more specific in our axle requirements:
Wheel Mount Surface (WMS): The distance between wheels needed to match our tire and rim choice and ensure the wheels didn’t stick out too much.
Leaf Spring Width: The distance between the leaf springs is more crucial on the front axle. Since the differential is offset in the NP205 transfer case, modifying this width could require major custom work.
Load Rating: Rufus, our custom expedition truck, is heavy. We needed a front axle with a high load rating to handle the weight, especially because the cab sits over the front wheels. The axle had to be strong enough to handle the weight, and the brakes needed to be large enough to stop us.
These considerations were essential for making the right decision on the axle. We also had to consider that modifying any components—like axles, suspension, wheels, or tires—could void warranties and potentially require state or federal inspections. Always conduct thorough research before making decisions.
Steering Compatibility for Our 4x4 axle swap
Rufus originally came with a steering box that used a drag link to move the wheels forward and backward. The axles we were considering for our 4x4 build used either a similar forward/backward motion or a left-right setup. To make it work, we could adapt a bell crank, a common solution in the RV world, to convert the motion.
While we didn’t focus too much on the steering at this point, we knew we’d need to engineer a solution once the axle was in place.
Serviceability: A Key Factor for Roadside Repairs
When designing a 4x4 expedition truck, it’s essential to ensure the vehicle can be serviced in remote locations. We wanted an axle that didn’t require special tools and could be repaired with basic, commonly available parts. Ideally, parts should be easy to find throughout North and South America.
Finalizing the Axle Search: What We Chose
Thankfully, Rufus’s factory wheel mount surface (WMS) and leaf spring width were quite similar to those found in many pickup trucks sold in the U.S. However, modern trucks mostly use coil springs, and those few that still use leaf springs have much wider frames. This led us to consider axles from the 1970s and 1980s, which were better suited for our needs.
We weren’t interested in modifying an axle by cutting, lengthening, or shortening it. We didn’t want to deal with the complexity of removing radius arms or coil spring parts to fit a leaf spring perch. Furthermore, we preferred axles with kingpins and tapered roller bearings for their long-term reliability over ball joints and unit bearings.
The Best Axle for Our 4x4 Expedition Truck
After considering all these factors, we narrowed down our search to one specific axle that perfectly matched our needs; the GM-Dana 60. In the next post, we’ll dive deeper into the axle we chose and explain why it was the best fit for Rufus, our custom 4x4 expedition truck.
