If you own a late-model 4x4, chances are you have unit bearings in your frontend. If you do a quick Google search, you'll find that these pieces of engineering marvel are cursed by many, and have created an industry of conversion kits for several popular vehicles. But before we discard them wholesale, we thought we'd delve a bit deeper.
To get the dirt, we polled Jim McGean at Dynatrac and Matt Martin at SpynTec. Both of these companies make conversion kits to replace unit bearing assemblies on popular vehicles. You might call foul on our choice of sources for an objective article. Granted, they both think their solutions are better than what the factory offered, but each of them know more about axle assemblies than most of us know about 4x4s in whole. The info they shared on vehicles they don't make kits for was just as insightful as the info on their products.
Say "unit bearing" to most off-road enthusiasts and they'll probably cringe, even if they
To start with, unit bearings have a few names. The factory engineers refer to them as live spindles. Most 4x4 enthusiasts call them unit bearings or unitized bearings. They're used in most late-model trucks and Jeeps due to their relatively cheaper costs and longer service lives.
A front-axle drive flange is not necessarily a unit bearing. Full-time 4x4s have used drive flanges since before lockable hubs were invented. But once the new-truck industry moved away from selectable hubs some 20 years ago, it migrated to the unit-bearing design to lower costs, eliminate required service, and to make assembly easier. The main difference between a drive-flange assembly and a unit bearing is the serviceability. A drive flange still has bearings and seals that can be replaced. A unit bearing is a completely sealed, non-serviceable unit. When the bearings wear out, you have to replace the entire live spindle.
Not all unit bearings are equal, though. The Dodge and Jeep design (shown) are retained on
If we look just at the spindle and bearing assemblies, the advantage of a unit bearing is that you never need to pack the wheel-bearings with grease or replace seals. Also, the whole unit comes on and off as a complete unit without bearings falling out when you take the frontend apart. Plus, you'll get a few more miles between services with a unit bearing than with a rebuildable hub. The bad: Servicing a unit bearing means bolting a new one on and throwing away the old one.
So why is there a market for old technology like hubs and serviceable bearings for Ford Super Duty, Dodge heavy duty pickups and Jeep Wranglers? You might dismiss Jeep guys as clinging onto the old ways, but most of the Ford and Dodge truck owners are using their trucks as daily transportation and for work. They're not going to change out parts unless they need to, which leads us to the disadvantages of a unit bearing.
The Jeep unit bearing also has the dubious honor of using ball bearings instead of roller
Just because the unit bearings are designed to be non-serviceable, it doesn't mean that they can retain their grease and keep all contaminates out of the bearings for the life of the vehicle. Depending on the environment the truck is used in, they will wear out at some point. But unlike a traditional wheel bearing, they don't give owners much warning. And when they let go, they generally do it in spectacular form, making the vehicle impossible to drive until it is repaired. And the price of replacing the unit bearing is pretty impressive as well. Remember, you can't just replace the bearings and seals; you have to replace the entire assembly.
Not all unit bearings are designed the same, however. The Dodge and Jeep units are similar because the unit bearing is retained on the axle stub shaft by a nut. In this assembly, the entire unit bearing is held together by the axle stub shaft. If the stub shaft breaks, the wheel will leave the vehicle, still bolted to the unit bearing. This can cause additional damage to the brake system and the vehicle.
The Ford Super Duty unit bearing design is different in that is held together by a large n
For the Dodge and Jeep applications, converting to a hub design allows you to unlock all of the front axle drive components when they aren't needed. This improves fuel economy and reduces the wear on these components.