Technical

Suspension Design

ONE TENTH OF THE IMPACT FORCE!!

There is a lot of myth and legend surrounding suspension design, and much talk about the relative benefits of independent suspension versus leaf springs. In the end it is not the type of system so much as the implementation that matters. A good leaf spring system will beat a bad independent suspension any day (and vice versa), but is much simpler to build and maintain.
 
There is a basic underlying principle behind suspension design that most builders forget, and it is the same principle that applies to air bags - slow deceleration protects the occupant. "It's not speed that kills - it's the sudden stop!"  If you were asked to punch a brick wall as hard as you can you would decline. However if someone placed a 150mm thick piece of foam rubber on the wall you would not have any issues.  So why is that?
 
Fundamentally we all understand about the sudden stop, and its effect on fists, or on heads hitting dash boards, or on the crockery in the camper trailer or caravan.  So why does almost every camper trailer and caravan maker insist on fitting bloody great big, enormously stiff springs, especially on their "off road" vehicles?  They might as well save the cost and weight and weld the axle directly to the body for all the good the springs do. Watch carefully as a camper trailer travels along the highway. Usually if you look closely you will notice that the vertical movement of the ?suspension? is in fact all in the tyres, not the springs.  OUR SPRINGS ARE APPROXIMATELY ONE TENTH OF THE  RATE OF THE 7 LEAF SPRING PACKS = ONE TENTH OF THE IMPACT FORCE.  This is what stops you breaking the eggs or the trailer axle.
 
And further, since shock absorbers require movement of the piston at speed in order to carry out their shock absorption duties there is absolutely no point fitting shocks to a stiff six or seven leaf spring on a camper trailer.  As Jeremy Clarkson (Top Gear) would say "they are as much use as a bright green spy".
 
So we at National Campers use very long, soft, automotive grade leaf springs on our extreme off road trailers.  We also fit automotive grade gas shock absorbers to absorb all the energy generated by the corrugations and bumps on the average outback road.  The axle is able to move up and down 180mm, which is a lot more travel than provided by many independent suspensions, enabling the suspension to do its work and cushion the trailer (and its contents) from all those road impacts, in exactly the same way that your car's suspension works.  Of course the springs come at a cost, but they will save your trailer and its contents from fatigue stress cracking and from shattering.
 
In placing the shock absorbers it is critical that the lower pivot of the shock moves in as near to a straight line as possible towards the upper pivot.  This means the shock absorbers need to be at right angles to the suspension arm or axle.  We see many systems in which the packaging of the shock absorber is dictated by the space available, and is lying at 45 degrees or worse.  This greatly diminishes the shock absorbing qualities of the shock absorber .  At National Campers we design the trailer so that it is shock absorber compatible, so the entire design is holistic, not patchwork.
 
The following is an absolutely true story.  Early in 2010 we went on a test trip from Melbourne to Cameron's Corner and back via Menindie, White Cliffs and Arkaroola.  That was over 3500 Kms in 7 days, on roads that were still wet in parts and damaged from the rains.
 
The journey from White Cliffs to Poole's Grave near Milparinka was initially on good surface that allowed us to travel at 100 km/h, but it deteriorated, with rough, rutted sections and endless creek crossings, but no water on the road. Maximum speed dropped to about 85, but was up and down endlessly as we negotiated the rough sections.

We had afternoon smoko about 90km from Milparinka.  On reaching Poole's Grave near Milparinka, our campsite for the night, we were astounded to find our smoko dishes still sitting where we left them, on top of the tool box on the trailer!  Even the bikkies were still in one piece.  There are not many trailer suspensions that would enable that to happen!


So if you care about your camper or its contents then you need a National Campers' trailer.
 
In future articles we will discuss the connection between suspension design and trailer weight, so please watch this space.
 
 

WHAT'S WEIGHT GOT TO DO WITH IT?

 

Answer - everything!

 

Firstly to state the obvious.  Heavy off road trailers won't go far off road, because they bog easily, or overload your car on steep inclines.  And of course they use more fuel, further limiting your traveling flexibility.

 

Extra weight also means increased cost of materials, and this compounds up.  For example a trailer that has a GVM of less than 750kg does not require brakes, but as soon as you exceed that weight the law states you must have brakes, and that automatically adds a further 40-60kg (and up to $900!!).  Rego costs more as well, since in many states the rego is linked to weight, and in NSW once you have brakes you have annual inspections.

 

But it is even more complicated than that.  A 750kg trailer only needs light springs, typically 3 or 4 leaf in standard trailer springs.  But go heavier and you also add the need for more leaves in the springs = more cost and more weight.

 

But to cap it off think back to the above article on suspension forces.  You add say 200kg to a trailer and you add the need to be able to absorb 600kg of additional impact force, because the impact force is caused by moving masses, which means about three times the actual weight being restrained as a rule of thumb.  So now on top of the 200kg you start with you have to add some more to protect against that 200, and then some more weight to protect against the weight you just added and then...................  Get the picture?

 

Car companies spend a fortune getting weight out of their cars through use of clever design and clever materials.  It should be no different with camper trailers.  A small investment in design actually pays off to the user in the long run.

 

So pay close attention to the tare weight on the trailer VIN plate when you buy.  And ask the maker whether that tare was measured before or after the furniture was added.  Many makers weigh their trailers before fitout so they can quote a lower figure.  It is illegal but no-one seems to be watching.