Why use a trailer?
1. Equipment Storage:
Most SUV's designed for technical trail environments have limited storage volume. In the case of a Jeep Wrangler or Toyota FJ40, a trailer can nearly double the available space. Families traveling in a compact SUV deal with the same constraints, and must resort to a roof rack with heavy tents and other equipment to allow for passenger room. A trailer is nearly always a better solution than a roof rack, unless very lightly loaded.
2. GVW Constraints:
SUV's also suffer from low payloads. In many models, three adults and a few bags can exceed the Gross Vehicle Weight (GVW) ratings for the vehicle. Adding aftermarket bumpers, winch, auxiliary fuel and water, etc. results in an overloaded vehicle. Overloaded vehicles will be more likely to break parts, etc. A trailer only adds tongue weight to the payload, which on the trailer is typically less than 200#. A well designed trailer can carry 1,000 lbs. or more over another axle.
Remember, never exceed the Gross Combined Vehicle Weight Rating (GCVWR) rating for the vehicle
3. Base Camp:
With payload and storage space freed up, you can keep the tow vehicle light and add trail modifications. Once in camp, you can drop the trailer and go explore more challenging terrain without all of the weight. A light vehicle always performs better.
4. The Integrated Camp:
Typically, a well designed expedition trailer will tightly integrate camping, cooking, sleeping and storage requirements. This can be difficult to accomplished in an SUV. The trailers primary function is to store gear and deploy into a tent, where the vehicles goal is to carry passengers and mount the drivetrain. That limits or complicates integrating camping features.
The trailer can be set-up in less than the time to deploy a typical family tent, and will have water, a kitchen, power, and sleeping accommodations all together.
5. Ready to go:
Most use their exploration vehicle as primary transportation during the week, which means lots of packing to get ready for a trip. With a trailer, everything can be packed and ready to go. That means water, extra fuel, all of the camping gear etc. That can make trip planning more enjoyable and the day of departure less stressful.
A trailer can also be an emergency life support. With the frequent natural disasters, just hook up the trailer and you will have food, water and shelter.
Critical Design Elements:
Suspension design, travel and dampening cannot be overlooked in a trailer. Having towed several torsion style suspension trailers over the years, they should be limited to either smooth highway or slow speed technical trails. Anything over a crawl off-road will cause a torsion axle to destroy the contents of the trailer and transmit significant force to the tow vehicle. Torsion axles are typically limited to 1-2" of wheel travel, which compared to the 7+" in the tow vehicle makes the problem obvious. I have found that the trailer suspension must equal at least 80% of the wheel travel on the tow vehicle. The best way to accomplish this is with a trailing arm suspension, supported by coils or airbags and dampened by shocks.
It is not common to see off-road trailers with brakes, but even one weighing less than a 1000lbs. should have them. It is not so important while on the road, but on loose, icy or muddy trails, the brakes are essential. Imagine how the rear of the tow vehicle would behave on a loose, cambered decent. The trailer will want to push the rear of the tow vehicle downhill.
3. When it rolls
The trailer should have a wide range coupler, which also allows the trailer to roll over completely. On a side hill, if the trailer does start to roll, it is better to let is go than stop and pull the tow vehicle over too. Most off-road trailer are tough and will see little damage in a light roll.
4. Draw bar length and weight distribution
Generally, you want the distance from the trailers wheel hub to the coupler to equal the wheelbase of the tow vehicle. A little longer is ok, and will help with stability and backing, but too long will cause the trailer to track outside of the vehicles line around corners.
In all cases, you want greater than 50% of the trailers weight to be forward of the axle, but not so much that tongue weight becomes excessive. A tongue heavy trailer will track properly, and handle as designed with little sway on the highway.