Suspension

SUSPENSION DESIGN.

Adventure Trailers wanted to share with you some of the history of the suspension systems AT has used throughout the development of the company. At the end of a seven year journey we have developed a highly sophisticated, responsive, suspension on a par with current vehicle suspension.

Chapter 1  LEAF SPRINGS

Chapter 2  SHOCK, ABSORPTION, and REBOUND.  MAXIMUM LOAD vs. SMOOTH RIDE

Chapter 3 INDDEPENDENT RUBBER TORSION AXLE

Chapter 4 AIR BAG SUSPENSION

Chapter 5  TAAS. TRAILING ARM AIR SUSPENSION
TAAS Generation 1
TAAS Generation 2
TAAS Generation3

TAAS Generation 4


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SLIPPER SPRINGS: The first trailers we used a variety of leaf springs called flat end slipper springs. The springs had a front eye, and the flat end sat in a pocket       welded to the chassis. Using this set up we were able to squeeze a 33 inch tire under the trailer, which in 2001 was unheard of.

Fitting a larger tire with a slipper spring is limited as it only has one eye. The lack of a second eye makes lifting the suspension with shackles impossible.

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MILITARY WRAPPED LEAF SPRINGS: The next evolution of AT’s suspension was to a double eyed, military wrapped, leaf spring with shackles. The shackle had two settings high and low, high gave us the lift we needed to fit up to a 35 inch tire.

We found the leaf spring suspension to be extremely heavy, and the ride it provided was only adequate when the trailer was loaded down. With the trailer empty or only lightly to moderately loaded Unloaded or there was lots of bouncing and feedback to the tow vehicle.

We started our research at a very simple level, trying to understand what a suspension system did, and came to the conclusion that a suspension system should isolate a trailer body from the shock caused by the tires hitting irregularities in the road or trail.

When we looked at what we had been using, a dead axle (to be a live axle it also has to be a drive axle), or solid beam axle attached to the trailer via leaf springs, we saw issues isolating the trailer body from road shock.

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SHOCK, ABSORPTION, AND REBOUND: A trailer equipped with leaf springs shock absorbers, and a solid beam axle will tip sideways, shifting the trailer contents, when one tire goes over an obstacle. The tipping is a result of both the left and right sides of the trailer being connected by the axle, the spring’s lack of articulation, and stiffness.

After the tire cleared the obstacle the shock resulted in rebound as the leaf springs tried to dissipate the energy and the shock absorber tried to dampen it.

We asked ourselves “If the leaf spring suspension is prone to the tipping movements and rebound why was it used on 90% of all the trailers on the road?”  The answer seems to be that it’s an off the shelf system, it’s easy to get the parts and install them. It’s also an inexpensive system, so there is cost savings involved, and it’s well suited to heavy loads, in that the load is spread more widely over the chassis.

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MAXIMUM LOAD vs. SMOOTH RIDE: The challenge is that the leaf springs must be rated for the maximum load that the trailer might carry. We wanted a smooth ride no matter if the trailer was full or empty. We also wanted to maintain travel regardless of load. A trailer with leaf spring compresses the springs under load effectively reducing the travel, or reducing the amount the springs could compress to absorb shock. Our ideal system would have to have some method of maintaining travel no matter what the load.

LOAD: A typical SUV ’s (Jeep Grande Cherokee) curb weight is 3669 lbs, the gross Vehicle Weight Rating (GVWR) is 5700 lbs. a payload of 2031 lbs. The payload is 55% over the curb weight.

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When we look at our trailers we realized that the curb weight was 700 lbs, and the GVWR was    2700 lbs. a payload of 2000 lbs. The payload was 285% over the curb weight !!! Extrapolating that out, to get some idea of the GVWR of an SUV if it’s payload was 285% of the curb weight, we found that the vehicle would have weighed 10,457 lbs.

That thought sent us into a tailspin, it was at that point we discovered the Independent Rubber Torsion Axle.

 

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INDEPENDENT RUBBER TORSION AXLE: was becoming very popular and had a reputation for being very reliable and maintenance free. The suspension system was all inside the main beam and it functioned by compressing the tubes of rubber.

We chose a 3500 lb axle and had it de-rated to 2500 lbs, this way we could get the larger bearings but have it weight rated for our light weight trailers. Although we were able to soften the suspension it was still rated at the maximum GVWR for the trailer, which, as in the case of the leaf springs worked well when the trailer was fully loaded, but produced bounce at lower weights.

Installation was simple just four bolts held the axle to the plates on the chassis. Everything was tucked up under the body of the trailer, and due to the drop arm design, there was good clearance beneath the trailer. We had a selection of angles we could have the drop arms set at and so we could fit the 35” and 37” tires, that were now becoming common of off road rigs, under the trailer. This design looked like an answer to our problems.

The torsion flex axle performed very well on road and even in severe off road conditions where the pace was slow and there were large drop off’s. Where the torsion flex design had problems was dealing with wash board. The design was too slow to compensate for the changes in terrain and instead of skimming over the bumps or absorbing them the suspension started to bounce.

The biggest challenge for this suspension was Baja, Mexico where wash board is the norm, and it can last all day. Our experiences in Baja were that the rubber inserts started to work their way out of the main beam, and ultimately result in a failure of the drop arm. The arm would work it’s way from a downward position flexing to horizontal, and finally flexing past horizontal to an upward position.

Once this had occurred there was no way to fix the axle in the field. The only options were to get a replacement axle, or to weld the drop arm in one set position and limp home. We were able to mitigate total axle failure by placing bump stops preventing over flexing of the drop arms, but the ride quality remained low.

Ultimately the torsion flex
design wasn’t suited to off road conditions. The consensus of opinion reached by ourselves and other Overland companies who had used the Independent Rubber Torsion axle was that;
• The end of the main beam wasn’t’t sealed against dirt.
• Once dirt began entering the beam the flexing movements would cause friction and heat.
• The heat and movement reached a tipping point where the rubber inserts broke loose and worked their way out of the main beam.

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AIR BAG SUSPENSION SYSTEM: By 2004 we were back to the point where we had a lot of questions but not too many answers. Fortunately this was also the time we started to see parallels between the problems we were trying to mitigate and those the trucking and low rider industries were experiencing; tipping, rebound, maintaining travel, and dealing with varying payload.

Tractor trailer rigs were dealing with payload issues similar to ours, an empty rig rolling versus a fully loaded rig, and the Low riders had the ability to lift or lower their suspension at the flick of a switch.

What they had in common was airbags.

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Fortunately for us the cost of airbags had come down and you were no longer just seeing them on big rig trucks or high end vehicles. A small number of companies had started to use air bags as helper springs and retro fit kits for vehicles.

The airbag would allow us to maintain the travel in the suspension. As the load increased compressing the airbag we added air to maintain the height.

Ride quality was superb with air bags, they were extremely fast in reacting to changes in terrain, and an airbag transmits much less vibration and shock from the road to the trailer. With this knowledge we could then design a suspension system around the air bag by using it as a spring.

Many factors influenced the design, rear suspension on motorcycles, swing arm suspension on VW buses, and the need for simplicity. With all of these factors in mind Adventure Trailers designed an A arm trailing arm suspension with airbag and shock absorber.

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TAAS:  We called the system TAAS.       TRAILING ARM AIR SUSPENSION. The independent arms allow the axle to pass over an obstacle without the side to side tilting effect on the trailer, and without the compression of the    springs on one side transmitting over to the other side.   The airbag spring absorbs changes in terrain by quickly compressing the air in the bag, once the object is cleared the air wants to rapidly expand to it’s original volume. Left unchecked this would result in the trailer bouncing. To slow down the rebound we installed a shock absorber from the back of the trailing arm to the chassis.

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TAAS GENERATION 1: In the first generation of the TAAS system we used an Old Man Emu shock valved for our application. Off Road capability was impressive, the prototype  ran 800 miles of Baja wash board with the trailer handling smooth, level, and transmitting very little feedback to the tow vehicle.

In difficult rocky terrain the fact that the leading edge of the arms are tucked up close to the chassis prevented them from being damaged. The arms themselves are tucked in behind the tires, in this location they are well protected from obstacles. In turn the trailing arms provide good protection to the air bags.

TAAS GENERATION 2: The second generation of TAAS involved using a Rancho 9000 shock absorber. This shock has 9 adjustment point to dampen the vertical movement. The adjustability allowed the suspension to absorb large changes in terrain without transmitting it to the trailer, it also kept the wheels firmly planted on the ground.

The Rancho 9000 adjustability is achieved by changing the size of the internal valve, or hole that the oil within the shock has to pass through when compressed. If the valve is left open it produced a soft ride, as the valve is made smaller the suspension becomes stiffer. By adjusting the setting the owner can control trailer roll on the highway and bounce when off road.

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TAAS GENERATION 3: In 2007 we found a promising product that would eventually become the third generation of TAAS. The design involved refining the way that the airbag and shock absorber worked together.

The Airshock consists of two parts, a short adjustable shock absorber, and an air bag. Both the components are adjustable, the airbag by inflating or deflating with air, and the shock absorber by changing the valving with an external dial.

The beauty of the system is in combining the two by placing the shock up through the center of the bag. In this combination the shock absorber is closer to vertical, allowing it to work more efficiently, while the airbag works efficiently responding to variations in the trail as well as maintaining ride height.

The Airshock has 16 adjustments for compression and extension that improve the ride quality.

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So it would appear that after seven years of working toward the “perfect” lightweight trailer suspension Adventure Trailers is getting close. By incorporating the air-shock as part of our trailing arm suspension system we have developed a suspension system that is light weight, extremely quick in responding to changes in terrain, transmits little of the road noise to the trailer, does not have the side to side rocking of a solid axle, maintains travel, is fully adjustable for load, and gives little or no feedback to the tow vehicle.

TAAS GENERATION 4: By 2010 you would think that once you have a good thing you would just stick with it from then on. Racing technology proves if you depend on sticking to the same thing you never get better. Adventure Trailers is always looking to the future and better ways of doing things hence our latest suspension system. We are now using a mix of things we have tried and made changes to get it dialed in for an even better ride both on and off road. We are using the Rancho 9000 shock absorber to be able to adjust to all types of terrain with just a quick turn of the adjustment knob.  On top of that we use a high quality air bag to be able to adjust ride height and other great features of the air bag as in less vibration transmitted to the trailer and the fast reaction for all terrains. And then as a final touch we add a limit strap so that we are not depending on the shock as a limiter. We could go on for pages about the angle changes we made on Gen 4 but to keep it short… IT WORKS.