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Tech: 4Runner Air Shocks with In-Cab Pressure Control
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| Toyota 4Runner | Toyota Tech | Toyota Section |

By: Dan Eddleman - 12/21/02

Background

Spurred on by the recent article on Cross Linked Air Shocks, this article covers the installation of Gabriel HighJacker air shocks on a 4Runner and, adds an In-Cab air pressure control system. The twist of a knob permits optimum pressure settings for either on-road or off-road operation thereby controlling the cross linking effect and the rear ride height.

Summarizing from the above referenced article, supporting a small portion of the vehicle load on air shocks can result in a better off road distribution of weight in flex situations. With the springs less loaded, there is more flex for a given amount of load. The air shocks with their cross link connection, uses the compressed air to transfer load forces from one side to the opposite side for better wheel extension and results in a better distribution of weight between the two wheels. For a more detailed discussion of this feature, check out the above article. The in-cab controls covered by this article, allow easy adjustment of the air shock pressure to activate the positive off-road benefit of load transfer and increased rear ride height only when needed.



Air Shock Installation

In preparation for installing the new shocks, the spare tire was removed for easier access and some time spent examining the 4Runner shock absorber mounts. Both the upper and lower shocks mounts are made out of the same heavy gauge material as used for the spring mounts and suspension link mounts and, sufficient room is available to accommodate the installation of the Gabriel HighJacker air shocks.

Removing the rear Toyota shocks can be a little difficult, due to the shock turning as the top stem nut was being removed. A pipe wrench was used to hold the shock as shown, and the stem nut removed with a socket and rachet drive. Since the only place the shock outer "cover" cylinder has sufficient strength for using a pipe wrench in this manner is at the very top where it necks down to the shock stem, that's where the pipe wrench should be placed.


Spare Tire removed for Easier Access Position of the Wrenches to Remove Shocks Upper Shock Mount Lower Shock Mount

Gabriel HighJacker air shocks were chosen to be installed. In searching for a local source for the shocks, CarQuest Auto Parts carries the Gabriel HighJacker Shocks, repackaged as the CarQuest Air Adjustable XD. CarQuest lists part number SHK50897 as the correct shock for the 2000 Toyota 4Runner rear axle with 4 wheel drive. The Gabriel recommended part number for the 4Runner, #49387 is also printed on the box. The end of the box with the part numbers is shown below for easy identification. The Gabriel shocks are 1 1/4 inches longer at full extension compared to the Toyota original shocks. This should allow some extra droop of the rear axle in maximum articulation situations. For reference, the original equipment shock (Tokico #48531-3D011) measures 19 3/4 inches from the stem shoulder reference point shown below to the center line of the rubber bushing on the lower shock mount eye. The Gabriels measured the same way are 21 inches long. The minimum fully compressed length for both shocks is 12 1/2 inches.


CarQuest SHK# 50897, Gabriel #49387 Gabriel Shock Length versus Original Shocks View of the Lower Portion of the Shocks

Shown below first, are the rubber bushings and hardware that came with the Gabriel Shocks. The original Toyota metal bushing circled in blue was reused with the new shocks as this intermediate piece did not come with the new shocks. The second photo shows the upper portion of the installed shock. Note the shock air fitting is turned inward, as this orientation provides more protection and clearance. Lastly, shown is the lower view of the installed shock.


Shock Mounting Hardware Installed, Upper View Installed, Lower View

In-Cab Variable Pressure Control System

The next step was to proceed with installation of the control system to allow adjusting the air shock pressure from inside the cab. An air pressure regulator gives the most flexibility as the control mechanism. It can be adjusted to maintain the minimum required shock air pressure, or any desired pressure with a twist of a knob. For example while on road, the vehicle can ride at normal height with the cross linking effectively disabled by maintaining only the minimum air shock pressure stated by Gabriel to prevent air bladder damage. For off road, the air pressure can be set to the 100 pounds per square inch limit of my air compressor to get the maximum effect of the cross linking and, to raise the rear of the vehicle for additional clearance. I am also finding the feature handy when using my utility trailer or carrying heavy loads in the 4Runner to restore the vehicle back to normal ride height.

Control System Components

Regulator System Components View of the Installed Air Pressure Regulator System

The regulator components are shown here along with the Grainger part numbers. The part number information may be helpful either for using Grainger as a source for the components, or for further examination of the chosen parts specifications. The mini regulator and gauge operation are pretty much self explanatory, however the check valve and the air metering control valve need further explanation which follows below.

The check valve shown was installed in the air supply line between the compressor air storage and the regulator, so that if the compressor air storage is drained down, air pressure currently in the shocks will stay, and not drain back through the system.


Air Metering Control Valve

The air metering control valve is critical to proper operation of this system. The control valve is nothing more than an adjustable needle valve, but performs some important functions.

If you have ever been in an air bag or air shock equipped vehicle with an air pressure monitor gauge, you probably noticed that the pressure can momentarily come close to doubling when the vehicle hits a bump where back end tries to bottom out. In general, the air gauge pressure is bouncing all over the place as the shocks are performing their function.


Location of the Air Metering Valve and Cross Connection

The air pressure regulator used in this system will not only add air if shock pressure falls below the setting, but also will bleed air if the shock pressure is above the pressure setting. Without some form of protection, the regulator would be constantly over correcting. Therefore, the metering valve is a must to prevent the regulator from bleeding and adding air on the pressure transients. The air pressure regulator sees only the "average" pressure in the system when the needle valve is set to a very small flow rate. The metering valve was installed in the location shown to make it easy to reach under the vehicle and adjust the setting. The wire ties holding the valve in place were temporary; these were later replaced with a stainless steel hose clamp.


The correct setting for the metering valve was determined to be with the valve adjusted such that only very minor movements of the pressure gauge are visible during vehicle bounce. For example, while riding over a fairly significant dip in the roadway, and with the 100 pounds per square inch maximum air pressure set, the pressure gauges should only be moving no more than 8-10psi. By keeping transient pressure changes seen by the regulator this small, it does not attempt to correct. At this setting, the metering valve will still permit a free flow of air to the shocks when the regulator setting is changed. With the regulator not constantly overcorrecting by adding then releasing air, the desired air pressure will stay in the shocks without the need for the air compressor to run and make up for lost air.

The standard Gabriel "T" connector shown in the above photograph is already set up to tie the air lines together from each shock to make the "cross link" connection. Originally, I had considered replacing the very small diameter air lines with larger compressor type flexible air hose, thinking that some additional external air storage buffer between the shocks and the metering valve would be needed to prevent a "pogo stick" or bouncing effect. I have experienced this effect in a friend's full size pick up truck with air bags and assumed that air shocks might exhibit the same characteristic due to the small amount of compressible air space inside the shock air bladder. However, in ride testing with the shocks set at 100 pounds per square inch, I felt no "pogo stick" effect at all, and decided to leave the small diameter air lines.

Installing the Air Lines

A rubber grommet hole just forward of the gas tank was used to run the lines from the air shocks and compressor air storage to the in cab regulator. By placing the roll of soft copper pipe as shown and uncoiling, the bend on the uncoiled line was perfect for following the desired path underneath the floor carpet up to the center console. This part of the job could be done much easier with two people, one below to work in the dirty, greasy area near the drive shaft, and the other person inside the vehicle, helping work the line up to the desired position. Using plastic flexible line would be even easier to install, but my preference was to use the soft copper metal piping.

To facilitate routing the air lines, the driver's seat was unbolted and moved back out of the way. The console storage unit was also unbolted as shown. My console area was already pretty full with two #2 gauge welding cables running through to supply the 120v AC inverter and other cables and components for the mobile amateur radio system. This wiring and the air lines must be routed over to the right side of the console as shown to avoid interference when the console section with the cup holders is reinstalled.


Rubber Plug Hole Near Gas Tank Used to Enter the Body Lines Complete with Rubber Plug back in Place Driver's Seat and Console Unbolted and Shifted to Facilitate Running Lines Under Carpet Lines Routed Through Console to Clear Bottom of Cup Holder Line Ready to be Connected to the Regulator Assembly

Clearance Check with the Longer Shocks

Drive Shaft Clearance to gas tank skid plate Axle Travel Bumper Stops

When longer shocks are used on the rear axle of the 3rd gen 4Runner, the drive shaft to gas tank skid plate clearance can become a problem. Since the Gabriel shocks are 1 1/4 inches longer, the clearance was checked, along with the axle travel bumper stops as shown here. The photos are taken with the left rear tire stuffed and the right tire off the ground. There is approximately 3/8s of an inch clearance remaining between the drive shaft and the gas tank skid plate. The additional 1 1/4 inches maximum extended shock length with the Gabriels is pretty much the limit, considering a little bit of clearance needs to be left for further shifting due to the rubber bushings in the suspension. The upper limit on further axle travel looks good, with travel being limited by both the outer rubber axle stop and the inner spring rubber cone. This also shows that even with the rear sway bar in place, the rear axle is articulating to the maximum with limits being the travel stops on the left, and the maximum shock extension on the right.


Rear Axle Droop, Air Shocks at 100psi View of Axle in relation to Body Rear Axle in Relation of Front Wheels

The photos below show the maximum droop of the axle in relation to the body and photo underneath shows angle of the rear axle in relation to the front wheels, which are flexing very little with the front sway bar in place. With the fuel tank in its relatively low stock location, the tank shield hangs down below everything else when the vehicle is in this position, something to remember when on the trail.



Rear Lift Comparison at 100psi and 25psi

Rear Lift (left - 100psi) and (right - 25psi) Lift, measured at the Trailer Hitch

The difference in rear height is apparent especially when viewing the flare in relation to the tire. It would be interesting to see how much lift would be achievable (and what the ride characteristics would be) with the full 200psi limit of the shocks. I don't have a means of getting the air pressure up that high. Lift (as measured at the rear hitch), is about 1 3/8" at 100psi.



Conclusion

While there is no substitute for a locked rear axle on uphill twists, in preliminary testing on wet ground I did not get wheel spin where expected, until the shocks hit full extension. I plan to find some slick uphill twists where the tires are just at their limit of traction, and play with the in cab controls to see if bumping the air pressure up for more weight transfer to the unloaded side, does in fact make a noticeable change in tire traction.

With more production vehicles getting air suspension assist, there may be a larger selection of parts to try sometime in the future.


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