Unit Bearings Versus Rebuildable Hubs - The Unit-Bearing DebatePosted in How To on April 1, 2010 Comment (0)
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.
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.
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.
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.
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.
The Ford Super Duty unit bearing design is a bit different. First, the axle stub shaft doesn't hold the unit bearing together. You can drive with a broke stub axle or with the axleshaft removed. They other difference is that these trucks have selectable hubs. Thus, the Ford unit bearing overcomes two of the biggest shortcomings of the Dodge and Jeep units. The non-serviceable nature of the unit bearing still leaves many fleet owners and others who depend on their truck for work looking for another solution, though.
One thing is common to the Ford, Dodge and Jeep applications; a hub conversion kit offers stronger components - something that enthusiasts can universally appreciate.
So the next thought that comes to mind is, "Why aren't there kits to replace the unit bearing assemblies in late model GM trucks?" There are plenty of these trucks out there, and more people are building them with larger (heavier) tires than ever before.
There are multiple answers, but one key reason is that Dynatrac and SpynTec aren't receiving many calls for these applications. All Chevys (and all half-ton 4x4s) now have IFS, which have several other limiting factors. The Ford, Dodge and Jeep applications discussed earlier are all solid-axle vehicles. That means that people lift them high and stuff on heavy wheel and tire packages that the factory unit bearings were never meant to endure. An independent front suspension is limited by suspension geometry and the CV shafts. For those who do manage to stuff 38-inch-and-taller tires on their IFS GM trucks, the CV shafts or other components often fail before the unit bearing. It's a series of weak links.
As the industry continues to improve the other components available for these trucks, we may see the rest of the system improve enough that there becomes a demand for a hub conversion that replaces the unit bearing.
We hope we haven't simply denounced unit bearings as evil. There are millions of vehicles on the road with unit bearings. But, it seems clear that in some applications, there are better designs available.