Camper Trailers Tech Tips

what makes a good off road trailer?

 

 

 

 

What makes a good off road trailer?

 

 

    It can be a little daunting buying your first camper trailer or perhaps upgrading to an off road model, not knowing what components make a good camper trailer or more importantly one that will handle the tough outback conditions & not let you down. Forums such as the Australian CamperTrailers Group make an excellent resource to gain a little knowledge before that big outlay. Although the group does not push one brand over another, we hope we can steer prospective owners in the right direction & help them make an educated decision.

    There are a few points to consider which makes a good off road trailer stand above an on road trailer design. Setting up a camper trailer to handle the harsh off road conditions is more involved than just throwing a set of heavy duty springs under it. Axle, suspension & bearing selection all play a vital role as well as the chassis they are connected to. On road trailers are commonly built from lighter RHS (Rolled Hollow Section) than that of a purpose built off road trailer and are generally considered too light for such work.

    The first question that you should ask yourself is WHAT WEIGHT LOAD WILL YOU BE CARRYING??? From there you can work out your suspension requirements, axle weight & bearing size.

  chassis. The chassis is the foundation of a good off road camper trailer. A full chassis is your best option for strength. The chassis rails could be built from say 60x60x4 RHS, the cross members 50x50x4 RHS and the A frame from 75x50x4 RHS. Spring hangers have been known to tear clean out of thin walled RHS in corrugated conditions.

  A frame. It is a good idea to position the A frame back to the front spring hangers for maximum strength and from RHS of a larger vertical dimension than that of the main chassis, as in the dimensions above. The National Code of Practice booklet ‘Building a Small Trailer’ which is available from motor registries, states drawbars must withstand certain forces applied at the centreline of the intended coupling without detachment or any distortion or failure which will affect the safe drawing of the towed trailer:

    Longitudinal tension and compression (N) .............................. 1.5 x 9.81 x ATM (kg)

    Transverse thrust (N) .............................................................. 0.5 x 9.81 x ATM (kg)

    Vertical tension and compression for rigid drawbar trailer (N) 0.5 x 9.81 x ATM (kg)

    The length of the A frame can be governed by what you want to carry on it, but keep in mind not to overload the ball weight. More on this later. In extreme off road conditions the shorter the A frame the better with a minimum ball weight of around 10% to 15% of the trailers overall weight.

   suspension Leaf, coil or rubber sprung??? Each have their good & bad points, but having any suspension too stiff is just as bad as having it too soft. There is no comprise, you must have it right. Either too soft or too heavy & the suspension over the endless corrugations will shake everything to pieces inside the trailer as well as the trailer itself & can cause components to fatigue & break. This either takes a lot of skill, knowledge & expertise to get right or a longer process of trial & error. Start by calculating your intended trailer weight & load & go from there.

   Slipper springs are not the right choice for off road work. The majority of off road camper trailers use eye to eye leaf or independent coil suspension.

  shockies Shock absorbers play a very important roll as part of the integral suspension of an off road trailer, especially when traveling on our very rough corrugated roads up north. Shockies on the suspension for kilometre after kilometre of corrugations are essential. Shock absorbers help in the rebound of the springs/coils. Without shockies the spring will rebound with equal force to that it was depressed resulting if fatigue of components & eventual failure.

    A type of leaf suspension with small reverse leafs positioned on the top side to reduce the rebound force of the main leafs are known as rebound leaf suspension and do a similar job to that of a shock absorber. 

  axle weight Under estimating the axle weight could see you with a bent axle. Around 1300kg is a good starting point for an off road trailer. You will need to reassess your axle weight to safely carry extra loads like a boat, outboard, fully loaded box on the A frame with food stuff and fridge etc.

    The axle chart at http://www.campertrailers.org/axles_bearings.htm sets out recommended axle size to weight carried. You will notice there is a maximum distance from the chassis to the centreline of the tyre before load capacity is reduced.  These charts have been reproduced with permission from Vehicle Components.

  bearing size The size of the bearings in relation to load weight is an important issue as well. The chart on the above page will also give you a recommended bearing size to load carried. Overloading your trailers wheel bearings may see them collapse at the most inopportune time leaving you stranded on the roadside miles from anywhere. It is always a good idea to carry a spare set of wheel bearings in your spares kit & know how to replace them.    

   With the major structural components taken care of we will look at other factors which add to make a reliable bulletproof trailer which handles all conditions. 

  finish Painted or galvanized. Painting has its merits. You can weld additions at a later date and then camouflage it with a simple touch up. Gravel roads sandblast the trailers underside removing all traces of paint. You will need to maintain the steel underside & watch for rust.  

    Galvanizing stands up to the elements more, especially if you camp on the beach, however you need to be careful not to break this coating if adding items at a later date. It is better to bolt on attachments in these circumstances.

  wheel/tyre combination Having the same tyre/wheel combination on your trailer is another good idea along with a spare for the trailer if you are intending touring the remoter areas of our great outdoors. With two wheels interchangeable around the whole rig and a good tyre repair kit you should be able to get out of most situations.

  track If you are going into the desert or on the beach it is a good idea to set up your trailers track the same as the tow vehicle. This will reduce extra drag & reduce fuel consumption.

  ball weight A good rule of thumb is to have around 10% to 15% of the trailers weight on the towball. This will ensure it has a good forward tracking motion. Tow ball weight is not only an important aspect to the small & medium sized vehicle, but the larger vehicles as well. Make sure your vehicle manufacturers towing specs, as well as your towbar are rated for which camper you have in mind as this could void your warranty & insurance if overloaded.

  weight distribution Weight distribution is also of importance as this will also govern the tracking performance & trailing characteristics of the camper. Thought must also be given when packing the camper. We usually pack items one by one & the overall weight is not felt until the camper is hooked up & towed.

    An old mate of mine used to build box trailers for a living. The theory used to locate the axle was an old 'rule of thumb measurement' as a rough guide. The axle offset toward the rear of centre is an inch to the foot. That is if the trailer is six foot long, then the axle offset will be six inches off centre toward the rear of the trailer. This measurement is regardless of A frame length. He said this would allow the trailer to track well behind your vehicle. Most trailers he built were of the 'garden box variety' and used to get a load of sand or gravel & take stuff to the tip.

    As you know with camper trailers all sorts of gear of varying weights are loaded into the camper trailer so careful thought must be given when packing to give the camper the recommended 10 to 15% ball weight. Underslung waters tanks can be anything from 60 to 120 litres, jerrys for fuel & water on the front or sides, weighting around 1kg per litre. The above rule of thumb can be used as a starting point to located your axle taking into account loaded toolboxes, battery location, water tanks & fridges etc especially when forward of the axle.

    You can quickly add weight to the drawbar by adding things like batteries, front mounted fridge boxes & pantries with canned food, tool boxes, spare wheel & out boards, so a little thought is needed to loading this area up. A toolbox mounted to the A frame is a good idea for extra space, but don't forget that everything you put in the box & therefore onto the drawbar will affect the towball weight of the trailer & result in the trailer becoming 'unbalanced'. With an off-road camper trailer it is a good idea to have your hitch light enough to lift. A situation may arise where you might have to manhandle it in a recovery or even where you store it, like dragging it out onto the grass etc.

    Canned food is heavy & will quickly add to the weight. Water & fuel weigh about 1kg per litre & a 10Rx15 tyre & steel wheel combination weighs 30 kg. Just by adding the 3 jerrys + 60 litre water tank + spare tyre = 180 kg for starters. Then there's the bbq plate, camp ovens, spade, axe, recovery gear, batteries, tools & spares. Before you know it the trailer weighs a tonne plus. And don't forget the boat & outboard. Are your axle & bearings rated to carry such a load???

    It will only cost you a small fee to have your camper trailer & ball weight weighed on a public weight bridge. You can find these in your local area listed in the yellow pages under weighbridges.

links

axles & bearings http://www.campertrailers.org/axles_bearings.htm

weight distribution http://www.watchyourweight.com.au/weights.php

National Code of Practice ‘Building a Small Trailer’ http://www.infrastructure.gov.au/roads/vehicle_regulation/bulletin/pdf/vsb_01.pdf

All Trailer Spares tech information http://www.alltrailerspares.com.au/html/technical_info/technical.asp

 

 by Rob


 

december 2008