Rear Coil Air Bag Installation

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Project BRYJ

By: Bryan Archambeault - 3/2001

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Air bag installed and plumbed
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A view of the plumbing
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The dual gauges
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The compressor mounted in the engine compartment
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A view of the bag at ride height

My Pro-Comp coil suspension works well on the trail. However, when I'm fully loaded down with trail gear for a long trip, the rear coils just don't seem up to the task. I have no idea if that is a deficiency in the design of the Pro-Comp coils, or just a problem with coil suspensions in general. Either way, I wanted a way to level out my ride when I was full of gear. So, I decided to put some air bags in my rear coils. The first place I had seen this idea was in an article by Ray Woo. The article is about 2 years old, so I contacted Ray to see if he still thought it was a good solution and he said that he was still very happy with it.

The air bags used are made by an outfit called Air Lift Company. Since my Jeep did not come with coils, they do not list any applications for the '87 - '95 Wrangler (YJ) on their site. Ray did his install on a TJ, which does come with coils, so they have applications for those models. Since the coils that come with the Pro-Comp coil kit are not the same size as TJ coils, I called Air Lift Company and had them fax me a list of available air bag sizes. The folks at Air Lift Company were very helpful both on the phone and via email. Here is what they sent me (may be out of date by now):

Diameter Height Diameter Height Diameter Height Diameter Height Diameter Height Diameter Height
5.38" 10.50" 4.90" 10.00" 4.35" 10.00" 4.00" 13.00" 3.44" 10.50" 3.00" 9.00"
  9.50"   9.00"   9.00"   11.00"   8.75"   8.00"
  8.50"   6.00"   8.00"   9.00"   7.00"   5.50"
  7.50"   4.25"   5.50"   8.00"   5.50"    
  6.50"           7.00"        
  4.50"           6.00"        

The inside diameter of my coils is 4", and with no load, there is about 9" of open space vertically. So, I went ahead and got the 4" x 9" bags. I also got the dual compressor setup, so I could control each air bag separately. The instructions say to mount the compressor on a frame rail, but I figured that it would get damaged or take on water or mud there, so I installed it under the hood. The kit comes with the air hoses already set up and loomed nicely, so there isn't any work to do there. The longest part of the install for me was figuring out where to route the air lines, and where to mount the gauges. Trying to get the air bags into the coils was also a lot of fun.

Once everything is in place, operating the air bags is very easy. There is a switch in between the gauges that turns on the compressor and fills the bags. Once you've got the bags to the desired pressure, there are buttons next to each gauge which allow air to be released from that bag. This way, if your load isn't level, you can have different pressures in each of the bags to correct it. There is also a valve stem (just like in a tire) near each air bag so that you can check the pressure externally, or quickly inflate or deflate the bags.

After getting everything installed, the system worked great. With no load, I could get the bags to raise the rear end of the Jeep with around 40 to 50 psi of pressure (the maximum recommended pressure is 35 psi). With some gear loaded, it didn't take nearly as much to level it back out. To keep the bags in good shape, the gauges have a pressure switch that will not allow the bags to deflate below 5psi. At this level, the bags don't expand enough to effect the ride quality in my Jeep.

On the trail, the bags seem compressible enough to not effect articulation. However, the problem I kept encountering was that as the coil expanded during droop, the air line would pull down into the coil a little, and then when the coil would compress back up, the line would get pinched, and therefore I could no longer add or release air from the bag. This is probably because the bags are meant to be mounted with the nipple and the air line at the bottom. But, the top mount for my coils already have a nice hole in the middle that appeared to be perfect for this. And, the bump stop and coil retainer are on the bottom in my kit, so there is no way to drill a hole through there for the air line. I'll have to figure something out...

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