Installing TNT Customs Stiffeners for XJ
One of the problems with the Jeep Cherokee (XJ) platform that it is not very stiff torsionally. When Chrysler freshened the XJ platform for 1997 they revised the unibody to make it stiffer. I’m not sure what they did to stiffen the chassis, if anything. I do know the body is still not very stiff.
There’s a number or reasons why having a stiff unibody is important. From a road driving perspective it makes sense to have the chassis as stiff as possible so that the suspension does all the work and handling is predictable. Additionally, off-road it makes sense to have a stiff chassis again so that the suspension does the work. Off-road if the chassis flexes a lot then there will be metal fatigue in the chassis. This metal fatigue will eventually be visible as cracks in the unibody and the frame uni-rails that support the steering box.
There’s other benefits to a stiff chassis also. If you cross axle an XJ chassis Jeep and open the rear hatch you may not be able to close the hatch afterwards until you move the vehicle to flat ground. It doesn’t have to be very uneven ground either. When we first got our Cherokee we loaded up the Jeep and went for a picnic. I had parked on relatively even ground and unloaded the back of the Jeep. When we went to leave I couldn’t get the rear hatch closed. At first I thought there was a problem with the hatch mechanism but after looking at it closely I realized the latch was not lined up with the body side of the mechanism when the hatch was closed. Moving the Jeep to flatter ground solved the issue. Since that day I’ve been keen to figure out the best method of stiffening the chassis.
The stiffeners I used are made by TNT Customs. When I bought them they were the only pre-made product available on the market. Since that time there are a number of other manufacturers of stiffeners. They tend to be in two styles: the formed channel style like shown in this article and the tube stiffener rail style sold by companies like T&J Performance. I used the channel style because I wanted to protect the factory uni-rail from obstacle damage as much as possible. The channel style wraps under the factory rail to be welded at the inside corner. The tube style stiffener tends to be used by the JeepSpeed racing series builders.
Here is what the bare stiffener looks like. I drilled some 3/4″ holes along the faces of the stiffener in order to add some additional rosette welds.
Here is what the underside of the Jeep looks like. It may be a little cool outside; note the icicles. I wanted to do this work ouside as I was uncertain how much mess I would make cleaning the underside of the Jeep.
I used the floor jack and the block of wood to support the drivetrain while the usual skid plate that supports the drivetrain was removed to do the metal surface prep prior to welding.
I used a soft brush flapper wheel on a 4″ hand grinder to remove the underbody tar from the rails and prepare the metal. It is important to not use something that will remove lots of metal from the unirails. The residual tar was cleaned up by spraying brake cleaner onto the rails and wiping it down with a rag.
Once the unibody rails were prepared I moved the Jeep into the work shop in order to weld the rails onto the vehicle. I did it in a stitch fashion so as to not remove too much strength from the underlying material. The instructions that come with the stiffeners are quite good.
I am quite happy with the product from TNT. I wish my execution was better. I shouldn’t have rushed the job and waited for the new welding helmet that I can see out of properly before doing the welding. Some of the welds are ugly but strong enough. Once the body tar goes back on you won’t be able to see them any more.
The stiffeners do make a huge difference to the vehicle. I haven’t driven the Jeep since installing these stiffeners but you can tell there is something different about the vehicle by opening and closing the doors. The tinny sound is gone. When you open or close the door it almost sounds like a 1980s Mercedes sedan now. The sound is more of a solid “thunk” now. I expect that it will sound even more solid once the rock sliding rails are installed on top of this structure.