TJ Rear Shock Shifters
1. Overview and Evaluation
2. Fabrication and Installation
The shock shifter concept was developed by Nth Degree and then acquired by AEV when they took over Nth Degree Mobility's product line. The idea is to relocate the lower shock mount outboard on the axle and provide a new upper mount to work with the new shock orientation. In contrast to a fully outboarded rear shock set up for long travel shocks there is no need to cut the frame and these shock shifters work with any stock shock or one scaled up for a particular lift. For example, they work with a 2" longer shock designed for a 2" lift, or a 4" shock designed for a 4" lift. In other words, there is a 1:1 correlation between lift height and the shocks this mod accommodates.
The point to this mod is that it provides greater clearance between the shock body and axle housing, which is especially important if the housing has been rotated for a SYE / CV driveshaft install. It provides greater clearance between the shock and the exhaust. It has an improve shock motion ratio and the new splayed arrangement improves handling and chassis control for the rear of the TJ. Finally, by moving the lower shock mounts outboard and behind the LCA (lower control arm) mounts it frees up additional ground clearance between the axle tube and the ground, meaning, there is less there for the axle to get hung up on.
As advertised, this shock arrangement does improve the handling of the TJ. The back end feels more planted and doesn't have the "wiggle" when going over bumps at a skewed angled. It is a subtle yet noticeable difference. This is one of those mods that may have just as much beneficial impact on street performance as it does on trail performance. And while I have not had a chance to hit the trails with these yet there is ample ground clearance under the axle housing. However, the lower shock mounts do hang down about 1.5" lower than the LCA mounts, but they are tucked tight against the tires where they'll likely not get hung up on rocks.
Its an incredibly time consuming process to fabricate these since there is a lot of checking and double checking going on to make sure all the necessary clearances work out. Anyone thinking of making a set of these would be wise to simply buy the AEV shock shifters
as you'll be time ahead when the project is done.
2. FABRICATION AND INSTALLATION
Before doing anything it is important measure the susupension at ride height to get a base-line on the shock shifter setup. The TJ should be loaded up with the driver's weight plus a standard or typical loading which would be any particular tools, spares, or gear kept in the Jeep as well as about 2/3 of a tank of gas. Then measure from the top of the axle tube to the top of the frame above the upper spring perch.
Prepping the Axle:
Measure the ride height between axle and frame with with a typically loaded TJ.
Next support the rear chassis on jack stands and remove the wheels, springs, shocks, and disconnect the swaybar links. Loosen (but do not remove) the control arm and trackbar bolts. And remove the foam bump stops from their retaining cups. The idea is to get the suspension set up so you can easily flex it out through it's range of movement without fighting the springs and bushing binding. Finally, weld on some mini-skids to box in the rear LCA brackets. Not only does this strengthen the brackets but also provides a base for the lower shock mounts.
Cycle the Suspension:
Welding on mini-skids to box in the rear axle LCA bracket.
Then cycle the suspension and observe how it moves. As the axle moves closer to the frame (uptravel) it shifts toward the driver's side. As it moves down (droop) it shifts to the passenger side. The housing pinion angle will rotate as it moves up and down. Note also that it is the passenger side rear shock that will govern all the measurements
as this has the shortest up travel for the two rear shocks based on how the axle moves.
Relationship to LCA Mount and Upper Frame Hole:
Getting the shock shifters set up is a balance between the angle of the upper mount and how it relates to the lower mount. In general, the lower mount will line up with the outside LCA bracket plate. And the upper shock bushing will be almost centered over a hole just in front of the stock upper mounting location.
Upper Shock Mount:
View of the upper shock mount location on the TJ's rear crossmember.
The white dashed line represents the approximate location of the outside face
of the lower shock bushing and how it lines up the outer LCA bracket tab. The
actual vertical location will be dependent on the compress shock length. In
this photo, a secondary lower mounting plate was tack welded to the mini-skid
in a later step. I marked on this plate the alignment of the lower bushing at
ride height and at full bump stop compression.
Start work on the upper mount by cutting and drilling mounting tabs. The rear tab will span between the to stock shock bolt holes. The forward tab will be bolted to the front of the rear cross member and require a new hole to be drilled. I opted to match the new hole and fastener to the metric hardware used to secure the stock upper shock mount. Bolt the tabs into place.
Next the mounting plate needs to be fabricated. Use a piece of cardboard as a template to figure out the size of the rectangular open for the upper shock bushing eye and the location of the mounting hardware flanking this opening. The orientation will be perpendicular to the stock shock mount. The plate can be cut and drilled out of a piece of 3" x 1/4" plate steel, length will vary. It shouldn't be overly long but will need to be long enough to provide welding clearance for the various hardware. For the upper hardware for the new shock mount I used some carriage bolts. This means the holes for the carriage bolts need to be square. Drill the hole the same size as the squared shoulder of the carriage bolt and use the jig saw to cut out the corners. To cut out the square for the shock eye hold drill holes in the four corners and then cut with a jig saw or plasma cutter to connect the holes. Leaving a triangular point at one end of the plate will allow you to do a half-cut and bend to make placement on the Jeep easier.
Next, hold the shock in place and / or bolt it to the new mounting plate and hold the upper end in place and see where the lower end ends up. Keep in mind the relationship to the outer LCA bracket tab and lower shock bushing. Once you have it somewhat located tack weld the upper plate to the forward mounting tab. The tack weld should be strong enough to hold while allowing some tweaking of the plate.
Shock Length and Lower Shock Mount:
Some pieces of steel were cut, drilled, and mounted as the start of the upper shock bracket.
A cardboard template was created and roughly test fit for the upper shock mount before
fabricating the piece out of steel. Note, to create the square upper shock bushing cut out
drill holes in the four corers and then cut between them with a jig saw and a metal cutting
blade. Clean up with a file.
The pointy end of the plate was partially cut through with an angle grinder, bent, and
then tack welded to the forward mounting tab. This provides enough strength and
yet adjustability to mount the shock and further bend the plate to the propper angle
in relation to the lower shock mount location (as yet To-Be-Determined).
Next, determine the extended, compressed, and ride height lengths for the shocks you are using. Ideally, the ride height of the shock should be mid-way through the suspension travel. However, it is important that the shock does not bottom out before the axle reaches the bumpstops. To determine this length measure from the empty bumpstop cups to the axle spring perches. This is the full amount of up travel available and should approximately equal the amount of travel from the ride height length to the fully compressed length. Wire the shocks to set their respective lengths, fit them to the axle, and cycle the axle to see where everything ends up. You should get a good idea of where the lower mount should end up.
Now cut a 3" x 1/4" plate and weld it straight down and tangent to the axle housing and LCA mini-skid (see one of the earlier images). It should be oriented such that the lower shock tabs should land in the middle of the plate somewhere. Leave the plate oversized for now and trim it down later.
Fully compress the suspension to the bumpstops. Take the compressed shock and hold it to the upper shock mount plate. Mark on the lower shock plate where the centerline of the bushing eye is. This is where the lower shock mount tabs will go. However, the shock mount tabs will be oriented based on the ride height length of the shock and the ride height of the axle. So reset the axle to ride height and now hold up one of the ride height shock and mark that centerline angle on the shock plate.
Do check clearances between the shock body and the various suspension an brake components by cycling the suspension once everything is tack welded and the shocks are mounted. On my first tack weld I got the lower tabs too far outboard and had the rear disk caliper brake line bolt just barely kissing the shock body when one side was fully stuffed and the other was at full droop . . . too close for comfort. So I cut and moved the tabs in about 1/2"
The shocks need to be measured to determine total available travel. But it is
the compressed and ride height measurements that will be most critical.
Because these shocks are pressurized, some wire was wrapped around
the ends to keep them from expanding.
The compressed shock mounted on the upper bracket with the suspension
on the bumpstops to see where the lower bushing will rest when the shock
and suspension are fully compressed. Mark this location.