GMC quad bag install

We’ve been installing a quad bag system and thought you guys may be interested not only in how to install it but also why would you want to spend @ $2200 for it as your rear suspension system.  OK, lets do that, first let’s talk about why:

Though it was produced by the lowest bidder, Firestone did a good job at designing an inexpensive system that took into account the lateral movement of a tandem suspension by building in a flex rate to the bushing swing arms.  They also saved money by producing the first of it’s kind single air bag system for 2 wheels.  Much cheaper than what was done before and if it caught on they would have a real win for the trailer industry… a low slung RV… motorhome, trailer, utility trailer, truck whatever.  In short, it didn’t fly but they did continue production of the air bag all but exclusively…. there were some trailers made…. but they continued production for what 35 years…. thanks Firestone…. but the tool wore out… no money in retooling and besides Firestone moved their manufacturing plant tools and all to somewhere in South America… OK, they’re done with us, I ‘m ready to be done with a few problems the original rear air ride suspension had.

At the time the original bags were available, there was an exotic system using 4 Firestone air bags currently on Firestones price list.  The system was called the 4 air bag system and I for one bit on the idea of not only holding the butt of the coach up but also increased the stability of the coach having an air bag for each wheel also increasing the firmness of the rear end taking away what I call sort of a marshmellow feel.  Hey, nothing wrong with that but I would just like a little more stability, especially when I’m towing.  The 4 air bag system did all of that plus it was said you could take off a wheel and drive… saw it done at a convention… so it also made it where I didn’t have to get down in the ants and do something when a tire gave out or even if I blew out an air bag… sounded like a win/win to me so lets do that and over 10 years I put in a blu-million of the systems.   And I was proud to be an emissary for the system.  One day though I noticed that the vertical plates the bags were bolted to on my system there above were bent.  With only one pin at the middle of the plate the bags put lateral pressure on those plates and bent them to suit their load direction.  In short, the bracket was benting.    Crap, if I installed those without knowing there was a problem I can’t be held at fault but if I continued knowing that a key component of the rear suspension was flexing most propably under dynamic loads… well I need to open an offshore bank account…. and I told the manufacturer of it just that.  He said so what so from there I just could not be involved in a system I had been a promoter of for over a decade.    Bummer….. and other such thoughts.  Then a wonderful thing happened and I tell you it’s true… ” We all get by with a little help from our friends”,  the GMC venders were all pondering this problem… a pile of folks in their back yards were coming up with all sorts of bag systems, some using huge spacers and a woefully underrated air bag while others were clamping 2 together…. using big bags that present other lateral dynamics to the flex rated swing arms…. it was a real mess… heck I invested in a set of springs!  I have to give credit to all of us for trying to replace an old “mouse trap” with a new one but to be honest, this really important rear suspension system needed more of a solution than a pile of shade tree fixes or a system I knew was weak.

There was a design that a GMC owner had devised which was much stronger than the original 4 air bag structure that still offered the attributes of performance and ease of maintenance that I loved in the 4 bag system.  A couple of us dealers got together with that basic design, I was proud to add my 2 cents as far as installation of and features I loved in the old system and the engineering design of CAD and computer failure analysis …… the “Quad Bag” system was born.

Thanks to the manufacturing capabilities of Jim Kanomata at Applied GMC, the system was a water jet cut, zinc anodized, 2 piece construction using 3/8″ steel and a back to back vertical plate design bolted together there is no way THOSE plates will flex!  Having an average bag pressure of @ 70psi, it was under the rated bag max pressure which BTW was similar to the other 4 bagger system but with a centerline bolt pattern about any bag… Firestone, Goodyear, Continental or Air Lift bags would fit.  This was a truly “off shelf” repairable system!  The original nominal air pressure on the single Firestone bags was @ 105psi and the original 4 bagger needed @ 145psi… on a bag rated at 100?  True DOT requires a 3x’s burst pressure but designing any system beyond the rating of the component is bag JuJu!

So I’ve jumped upon the quad bag train and never looked back.  With the fact that you truly do not need to carry a spare tire if you would like to get away from the “continental” butt look….. you actually do have 2 spare tires on the ground, the quad bag system can be drive a short distance with only 2 tires on the ground in the rear.  Blow a front tire, take one off the back and keep driving!  Man, forget those ants on the side of the road… I’m still riding!  And forget needing a jack or “jack hook” to change a rear tire… I can use 2 wood blocks and no jack…

Being a universal fit, I know if one of my customers blows out a bag… unlikely but anyway…. most any air bag they find will work and Bubba will not have them by the throat if there is a problem….. that’s awesome.

Then there is the performance….. goes up and down just as far as the original single bag system… offers superior rear stability and man does it help towing….. plus running the system at a lower pressure I have less vibration from the road and less stress on the on board air system.   Lastly but to me a biggie….. if your on board system fails for whatever reason, you can shut down the system and because the air pressure is lower, easily find air to pump the shut off system up and get home…. superdooper!

The system was computer failure analyzed and passed with flying colors… see now THAT’S something I can really get behind, and I have.  The only negative if you call it that is it costs @ 2 times what all of these shade tree ideas do.  It offers soooo much more but as you know in this capitalistic society, the cheapest way will always be popular… and I get that I just want a true upgrade when I redo a system on a 40 year old machine.  I want better and the quad bag system IS much better.  The system cost is $1650.00, is available I know through us or from Jim K. at Applied GMC and shipping from his place usually runs @ $110.  No way around it unless you can pick the system up in California… those are the costs.  Now you need to add to that about 6 hours of labor.. of course depending on how well you catch on.  It’s a bolt in proposition.. a selection of 3/4″ wrenches for those 40 year old frame bolts, a few well placed cuss words and you can do it.

If this sounds like something you would like… and BTW this IS a system I included in the bringing to the road of James coach.  You just need this tool when you get out onto the open road.  Serviceability, countermeasures, emergency options and the bast handling you can get… why would you not want this on the road….. Oh yeah, there is that nagging budget but hey… no one forced you to buy this thing and one breakdown with Bubba fixing your rig may just cost you more!

OK, so here is a quad bag installation from the beginning.  Here is a normal, coach with an original single air bag system   Up on jacks in hover mode tires off.  You want it all off the ground for access and so you’re not looking at things Catywampus…. 

Step 1 is to remove the T skirt and original air bag.  Then remove the 4 bolts in the vertical part of the frame and install the new longer bolts along with the rear section of the quad bag structure.     The rear wheel air bag will be attached to this vertical member.  Next mount the rear bag to is and install the front structure with the front wheel air bag attached   Now install the riser plate that connect the air bags to the swing arms   So the structure is now in, you can see the air lines are run, next the air manifold and control valves will go in.  Stay tuned for that tomorrow…

OK, the guys are here & I gotta go.  Thanks to everyone who has stepped up to contact James to prevail upon his better mature anc contact me… I hope you efforts get something going… I’ll let you know….

So thanks for dropping by, we’ll see ya tomorrow… .   

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