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Old 04-23-2012, 07:08 PM   #1
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Default Air Bags

To recap, I'm planning on getting a semi, removing the sleeper, putting a box on the back and pulling a trailer..... sort of like this:



I'm thinking about putting the box on the back of the truck on air. Bags are relatively cheap. I do need a way to keep the box located. Plates with slots? Panhard rods? And, ride height.

Does anyone have a good primer on air ride height control? Trying to figure out how to do that properly..... Also, can you put full weight on a deflated bag? Or do I need to have stops so the bags are not fully compressed when the system is not charged...?

Thanks.
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Old 04-23-2012, 09:46 PM   #2
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I would think you'd put stops on either side of the bag. Steel, maybe hardwood.

It sure looks purdy.
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Old 04-23-2012, 11:12 PM   #3
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I would think that hard rubber stops/mounts similar to what you would find on a leaf spring suspension to prevent bottoming out would be your best bet. You do not want the empty air bag to support the box weight. (air bags do not like to be over extended either) You will defiantly want some way to limit any side to side movement. Not thinking a plate with slots is going to work very efficiently. There will be a lot of motion going down the road, and you will need some type of bearing surface to prevent wear on the plates, and or lots of noise coming from steel on steel. some type of links with bushings or heim joints may work. Take a look at how some of the trucks are mounting their sleepers, especially the unitary cab/sleepers. Most of them are supported by air bags. Some of the older rigs had a hinge point for the cab way up front and the rear of the cab and sleeper was on bags, this gave a better ride but still had some side to side stability due to the hinges. Not very practical for a long box though. Also consider adding some shocks to the set up, an air bag is basically an adjustable spring, and will bounce like crazy without a shock to dampen it. The amount of air in the bag can be controlled with a ride height valve similar to what is on the rear suspension of an air ride truck, a lever is moved up and down as load is increased or decreased and the valve its connected to will regulate pressure in the bag to keep ride height the same. This can also be done manually with a regulator which can be adjusted up or down as needed depending on weight. This can work well if weight is not constantly changing. , and with an RV box might be simpler than an automatic system, unless you can pirate one from a junk yard truck suspension.

I have been contemplating some type of air bag setup for the conversion I am planning. As of now, I am still leaning toward the tried and true hard mount to frame rails. I am concerned with coordinating movement between cab and box, especially if both are on air. Not sure I would want to bolt/weld/Velcro the cab and box together into one long unit either, so much twist and torque going through the frame when going up driveways etc.

Keep us posted with what you end up figuring out. Always looking for a better mousetrap.

Good Luck

Dave
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Old 04-24-2012, 01:07 PM   #4
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With the plates I was thinking of a layer of UHMW or Teflon in between the steel. Very slippery. Yes, I agree the box and cap need not be rigidly attached to each other. The accordion seal is what I'm thinking right now. I am hoping to reuse whatever is there when I pull the sleeper off. My thinking is that both the cab and the box should be bouncing in the same direction. Yes, shocks for dampening did come to mind.....
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Old 04-25-2012, 09:19 PM   #5
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Quote:
Originally Posted by Oscarvan View Post
layer of UHMW
What is UHMW?
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Old 04-25-2012, 09:21 PM   #6
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Consider putting in a Watts Like, just like they use on race cars. The travel will be limited. Should be easy to do that will allow the box to move up and down with easy. Put one in front and one in back. Should be easy to fab up with big Heim joints and heavy steel tubing.

Watt's linkage - Wikipedia, the free encyclopedia
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Old 04-25-2012, 11:59 PM   #7
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Ultra High Molecular Weight, its a dense self lubricating plastic, very slick and tough. makes a great bearing surface, and comes in all kinks of thicknesses.
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Old 04-29-2012, 11:05 AM   #8
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I really don't think that you will like the box moving around on the truck. Probably best to get a truck with air ride suspension and mount the box solid to the frame.
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Old 04-29-2012, 06:35 PM   #9
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Quote:
Originally Posted by #90-GTSC View Post
Consider putting in a Watts Like, just like they use on race cars. The travel will be limited. Should be easy to do that will allow the box to move up and down with easy. Put one in front and one in back. Should be easy to fab up with big Heim joints and heavy steel tubing.

Watt's linkage - Wikipedia, the free encyclopedia
A simple panhard bar might work too. All depends on how much up and down travel you want for the box.
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Started looking for 379 Peterbilt TC, 24' to 30' box, bumper pull--but ended up w/1999 Liberty Coach conversion of 45' Prevost XLV bus. 1,000sf heated/AC'd race shop w/dump station, 50amp shore pwr where bus parks, 3 NASCAR/ARCA race cars & 26' Bravo trailer.
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Old 04-29-2012, 07:39 PM   #10
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i think a guy put his box on cab air ride bags. travel was limited to about 3". he just made steel plates to mount. i think he had 4 or 5 per side.
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Old 04-29-2012, 08:02 PM   #11
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Quote:
Originally Posted by volvoman View Post
i think a guy put his box on cab air ride bags. travel was limited to about 3". he just made steel plates to mount. i think he had 4 or 5 per side.
Was he happy with the results?
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Old 04-30-2012, 09:37 AM   #12
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Oscarvan -

Can I ask why you're not letting the suspension do this work?

My current conversion rides pretty much like a Cadillac. I don't have front air-ride, my cab is not air-ride and I run my seat without air (just like the lower seat position). Out of the 3 conversions I have owned this one rides the nicest, the first one is close with all air-ride front and back and the second was just never setup right and had a rough ride. This one has the quiet ride rear end (I have yet to determine if this is something different than just normal truck air-ride, it was something FL mentions in their truck spec ) my wife and kids all say the ride is amazing in the back. Better than our previous all air-ride bus style RV. If I really wanted to go crazy and put the ride over the top I could add air-ride to the front axle. But I cannot even justify doing so because it's just that good. Current setup is 16000# Front Axle, 23000# Quiet Ride Single Rear Axle, 325Ē Wheelbase Current weight is approx 32,500 lb which puts me way under max

I would guess you either have driven, ridden or owned a conversion that must have had a horrendous ride. And when these things are not properly configured than they can definitely ride like a truck. But get the geometry right and I think you can save yourself alot of grief and potential issues by utilizing the suspension.

Your challenge is going to be running all of you're wiring & plumbing without being able to tie it to the truck frame. So I am assuming your lower bays will hang from the upper portion of the box and as you add weight to these you will need to insure leveling capabilities. Also as your box is coming down during a bump and your tires are coming up you will need to have added clearance here as well. A few other things to consider, gone are the days of oak planks and U bolts if you did not do air-ride on the box. I see this thrown around alot by guys building their own and usually justify it by the fact that box truck mfg'ers are doing this so it must be o.k. The product in a box truck can be replaced during separation in an accident, family cannot. Many conversion companies used this method early on until issues started to arise. I know Showhauler brought in structural engineers to determine the strongest most effective way to tie the box & frame together. These boxes are getting welded directly to the frame now, making one solid unit. Far stronger and safer than ever before. In my 2012 I notice a much more solid feel, gone are the sounds of popping when the jacks are deployed and the frame is twisting. Also keeping cab air-ride means you will have a much larger gap between the box & cab to accommodate movement. If a seal goes bad on a truck and sleeper it is pretty easy to remove a sleeper and replace the seal, not so easy to remove your conversion box to do that repair. This was an issue with conversion mfg'ers early on as well and a big reason the practice is no longer used amongst limiting the cab opening.

Which ever way you go, just be sure to document everything you do with photos and drawings. If you ever plan to sell this, it will make it so much easier to convince a buyer why and how things were done. It will also help him justify the price you maybe asking.

As my wife and I were driving home yesterday from a weekend trip we were discussing how much we loved this rig. We finally got it right, we took the time to list out what was important and the pros & cons to each item. After owning 2 other conversion we realized that there is a trade-off with everything you do and add to these things and I mean everything. A perfect example is that our current conversion does not have a skylight above the shower which is so commonly found in most and was in our other 2. That skylight is great when you shower, letting in all that great light, but your only in the shower for say 5-10 minutes. Now if it's raining all night and that rain is pounding on that skylight you potentially are going to ruin 8 hours of sleep due to the noise that makes if that is something that annoys the heck out of you.

Keep us posted on your progress.
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Old 04-30-2012, 10:33 AM   #13
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Thanks for your reply. All this is still in the design phase. Your point about wheel clearance is VERY valid and one I had not considered yet....thanks!
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Old 04-30-2012, 12:34 PM   #14
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Great comments Warpath, the only down side I can really think of at this point to welding the box down to the frame is if the truck gets some significant damage from some sort of accident, frame rails go bad, or chassis just plain wears out, then there is no real way of swapping the conversion to a new chassis. For many this may seem far fetched, but for those of us on a budget that can not afford a new chassis to convert, it is conceivable that one may want the option to replace later rather than buy a whole new rig. I do not like the idea of U bolts and a spacer for many reasons, perhaps a series of stout angle brackets bolted or welded to the frame rails, with a bolted connection up to the floor system.
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