Centering the Front Diff on a Hummer H1

Dec. 03, 2007 By Manny MacMillan


[Craig Bernier scales a hill in his ‘96 H1 - probably the first to sport a centered front diff]

A few years back, I was on the trails in the Berkshires of western Massachusetts with fellow Hummer driver Craig Bernier, and we got to talking about the various spare parts and tools we typically carry on the trails.  The topic of half-shafts came up, and Craig explained that he only carried one or two spare shaft for the trails.  I crawled under his truck as he explained how he had relocated his front differential to the middle of his truck, eliminating the need for 3 separate sizes for each half-shaft.  Bernier wheels his truck very hard, and he has won several demanding off-road competitions (including “The Hummer Challenge” in Florida and the “WARN Winch Safari”) – so it didn’t take much more than a glance at the work he had done and a ride in his truck to convince me of two things: 1) it was a stable modification, and 2) I should do the same on my own rig. 

For those who don’t already know, the rear differential on a Hummer H1 lies right in the middle, but the front differential is skewed toward the driver’s side.  There have been a few different stories about why that happened when AM General designed the HMMWV.  One popular version is that the engineers were trying to eliminate a driveline vibration, and tried moving the front differential, but did not succeed, and that a carrier bearing was added mid-way up prop shaft afterwards to resolve the issue, but the project was already too far along to move the differential back to the center.  Whatever the reason was, the fact is there are now three different sized half-shafts required on a HMMWV or stock Hummer H1. 

By centering the front diff, a “rear” half-shaft can be used at all four corners instead, eliminating the need to carry 3 different spares.  By eliminating the extra-short half-shaft on the driver’s side front, that shaft no longer experiences the same kind of severe angles and stress – another advantage, particularly on a truck with modifications like a suspension lift and oversized tires.

hummer do it yourself

[ Centered Front Diff – DIY Style ]

The relocation on my truck involved a fair amount of work, but it was a relatively simple job – especially since Craig did the bulk of the work, and it was the second truck he had done it on.  We cut some existing brackets and re-welded them in a centered location, and then drilled a few new holes on the frame cross member.  I had to replace a water pump, fan and clutch anyways, so we pulled the whole front end of the truck apart, starting with the radiator stack, which gave us plenty of room to work in. 

diff hummerr

[Note the Splines Showing at the Slip Joint…]

[…And the Slightly Tweaked Carrier Bearing]

For those who may be considering this modification today there is now another, easier option available.  Travis DePew at Rubberduck4x4 ( recently came out with a complete kit for centering the front differential. His custom bracket eliminates the need to do cutting and welding, and the remainder of the kit addresses some other issues, making for a nice clean install.  As you can see in the photos above, on my truck the splines at the front slip joint of the propeller shaft are slightly exposed, and my carrier bearing is a little tweaked.  The Rubberduck4x4 kit includes a mounting plate that relocates the carrier bearing forward to eliminate both of those issues.  The kit also provides a new coolant pipe, which can eliminate clearance problems on some trucks, particularly for those who intend to switch over to 12” rotors and the heavier duty 12k half-shafts.  The parts in this kit are heavy duty and trail tested – as you would expect with a mod like this, it is all about function and no flash. 

hummer parts

[Complete Centering Kit Available from Rubberduck4x4]

Centering the front differential, whether you use the kit mentioned above or not, will change the angle of the front propeller shaft somewhat.  After miles of use in its stock position, the U-joints often times wear in a certain way.  While this has no effect on the shaft as it sits in its original location, this can reveal itself when you change the position of the shaft by doing something like centering the front diff.  For that reason, you may want to go ahead and replace the U-joints on your front prop shaft when you perform this modification, to ensure you don’t get any driveline vibration (which usually manifests itself as a recurring “wobble” while driving on the highway at certain speeds.) 

As with any modifications, this change isn’t for everyone.  And for those of you driving post-2005 trucks in the form of an H1 Alpha - forget about it.  There isn’t much room to work in that area on those trucks, so I wouldn’t hold your breath for the first person to make that mod on one of those rigs. Newsletter
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