Front straight axles require an added complexity in that the axle has to allow the tires to turn as well as rotate. This is done through the use of a knuckle that pivots on the end of the axlehousing as well as a U-joint or constant-velocity (CV) joint that pivots along the axleshaft. In addition, the knuckle has both a spindle and hub similar to a full-floating rear axle or a newer style unit bearing and some sort of selectable or automatic hub or drive flange.
Open vs. Closed Knuckles The knuckle will be either an open or closed design. An open knuckle allows you to see the U-joint or in some rare cases a double-cardon constant-velocity joint. A closed knuckle looks like a giant steel ball at the end of the housing around which the knuckle rotates. Within the closed knuckle is either a U-joint or constant-velocity joint, depending on what make and model 4x4 you have. Most imported straight-axle 4x4s have a constant velocity joint within a closed knuckle whereas most U.S. built 4x4s have an open knuckle with a U-joint, though some older U.S. made rigs had closed knuckles with many different styles of joint. A closed knuckle that is properly sealed can be a benefit when wheeling in water or muddy terrain as it will protect the axle steering joint. However, most modern 4x4s have gone to an open-knuckle design as it is easier to have a seal within the axlehousing that seals against a smooth seal surface on the inner axleshafts to protect the differential. Plus we have found that an open-knuckle axle is much cleaner should you need to replace the steering joint in the axleshaft.
Ball joint vs. Kingpin With an open-knuckle axle you will have either two ball joints or an upper kingpin and a lower pivot pin around which the knuckle pivots, whereas a closed knuckle only uses a kingpin or pivot-pin design. A ball joint is a metal case that is crimped around a ball that has a threaded shaft protruding from it, while a kingpin is a solid pin that either a bushing or bearing rotates around. As for which is stronger, for the longest time 1-ton axles only came with kingpins, but now they are only available from the factory with ball joints, and though we should trust the OEMs with knowing what's best, they have been wrong before. We've run both, and seen both break, and though many will argue that kingpins are stronger, we have not seen proof. However, we do believe that a ball joint is more likely to come apart should a front axle break and the driver does not stop immediately and fix it.
U-joint vs. Constant-Velocity Joint A front axle uses four axleshafts-an inner long side, an inner short side, and two outer stub shafts. Between the inner and outer shafts is a U-joint or some sort of constant-velocity joint. A U-joint is a steel cross that connects the yoke of the inner shaft to the yoke of the outer stub, and has either roller needle bearings or bushings within caps that are pressed into the axle yokes. A CV joint (also known as a Rzeppa or Birfield) consists of a stub shaft that has a bell-shaped end with grooves for large ball bearings to ride around a central star-shaped piece. The center of the star has a splined hole into which the front inner axleshaft fits. As a U-joint rotates while turned it will actually slow down and speed up due to the fact that it can only pivot on two axes. A CV joint will stay at one speed no matter how much angle is put on it, and the design demands constant lubrication such that most CV joints are found in closed-knuckle axles. If your 4x4 has a closed knuckle with CV joints you can upgrade to either heavy-duty aftermarket CVs or some sort of CV eliminator kit that replaces the joint with a U-joint. If you have an open knuckle with U-joints there are many aftermarket alloy axles being made as well as extremely beefy U-joints of various designs and materials.
Unit Bearing vs. Hub In order for the front tire and wheel to rotate they need to be attached to something that is supported by bearings. The most common and age-old design is a spindle and hub setup that has a spindle which bolts to the knuckle and then a hub with races pressed in and bearings that ride between the outer races of the hub and the machined surface of the spindle. We like this design because all the parts are replaceable individually and if serviced and greased regularly they can last for ages. Then with some late-model vehicles the unit bearing was introduced. This was mainly done for ease of assembly, and it consists of a complete bearing, race, hub, and spindle of sorts that bolts on with just three or four bolts to the knuckle. It has some clear advantages as it can easily be removed and replaced, but it is not serviceable and as such it must be replaced as a complete unit, which can be costly. Plus these unit bearings have little to no warning signs before they fail, and when they fail they can do so very quickly especially at highway speeds and consequently can damage other parts such as brakes and stub shafts. Luckily there are some aftermarket offerings to swap in the tried-and-true spindle and hub design.
Auto hubs, Selectable Hubs, and Drive Flanges With the spindle or unit bearing supporting the weight on the front axle, we still need some way to transmit the power from the axle stub shafts to the wheels. Early on, all 4x4s had drive flanges so that the stub shaft was solidly splined or bolted to the hub via a metal slug. Though we like the burly, always-ready-to-wheel design, it can have a few on-road downfalls such that it is always turning and wearing on the axleshafts, joints, and differentials. Plus if there are any front driveshaft issues (not uncommon in a lifted 4x4) they will be compounded at high speeds with the front hubs locked up. And finally, with the added drag of spinning all these parts in the front axle-even when the transfer case is not in four-wheel drive-there is going to be a decrease in fuel economy which is hard to afford these days. Eventually selectable hubs were developed that require the driver to get out and turn a knob to lock the hubs before he or she is ready to wheel off-road, but the reduction in wear and increase in mileage made this upgrade near-mandatory for any wheeler with a street-driven trail rig. Nowadays a handful of companies are making selectable hubs for nearly every solid axle available. Along the way there were also some automatic locking hubs that engaged the front tires when torque was delivered from the transfer case, and then when you shifted back into two-wheel drive they would either unlock or require reversing the truck a short distance to unlock. Eventually a newer design was incorporated by companies such as Dodge and Jeep that was a cross between a unit bearing and a drive flange in that the stub shaft was locked into the unit-bearing hub at all times, and an inner axleshaft was composed of two pieces that could be locked together when four-wheel drive was activated. Though the system works OK, it seems like a conglomeration of parts to make a frail, inefficient, and cheap system whose only benefit is that lazy drivers do not need to get out and lock the hubs. As for us, we would look into the various aftermarket options for swapping in selectable hubs that give the driver some exercise by making him get out and lock the hubs (you're gonna be airing down anyways so lock them in at the same time), or if your 4x4 only sees dirt go with drive flanges, as they are cheap, bombproof, and always ready to rock.