Chassis and suspension are the key to a smooth and safe ride off road...
You can travel on virtually any road with nearly any suspension. People do the Cape towing a low cost box trailer and tent. However, the poorer the chassis and suspension, the slower you will need to travel, take less load and travel with a degree of stress watching for every serious road crevice.
Are trailers, caravans and camper trailers different in their design requirements to 4WDs? Yes, absolutely. Vehicles move around on 4 sprung wheels whereas trailers are fixed to a bouncing vehicle with a large mass over an axle to the rear. It is like placing a weight on a 12 inch ruler at one end and swinging it from the other end. The forces on trailer suspension can be 3 times harsher in some circumstances compared to most vehicles!
Kimberley uses engineering design tools and has 3 qualified engineers including one admitted to the Institute of Engineers to complete their designs. These engineers have depth of experience in trucks, mining vehicles and other off-road applications.
Discover how air suspension really works for offroad caravans and camper trailers. Using a simple model, Bruce Loxton unravels the mystery surrounding the suspension needs for offroad travel with single & dual axle trailers and caravans.
The chassis on a Kimberley Kamper is virtually the same for all models.
- High Mpa Steel
- Laser cut with locking tangs for external butt welding
- Flexible "walking" A frame design
- A Frame connect DIRECTLY to independent suspension arm "hangers". So the linkage from the vehicles hitch to the trailer wheels is absolutely direct. It is NOT through the rest of the chassis
- The chassis main section and sub-frame sits ABOVE the "line of pull"
- Hot dipped galvanized
- Adequate drain points and flow for galvanising the inside of the RHS
- 5 Year transferrable warranty is available on the chassis
"Ride Quality" is a great way to know if you have the best suspension on your off-road camper trailer.
It really is this simple. Lets see if I can do a simple job explaining the linkages between suspension design and ride quality.
"Ride quality" measures the amount of isolation the suspension provides from road inputs, without compromising vehicle control. If the "ride quality" is high then the suspension is minimizing the road forces and vibration. Keeping these to a minimum results in less discomfort and lower vehicle and trailer maintenance cost. These are three big benefits of having the best suspension.
However, there is a 4th benefit: if you have better ride quality, you can travel over the same road at a better speed, providing it is legal and safe, based on the conditions. We are not saying "go faster" but everyone enjoys shorter travel times providing the quality of travel is good. Speed is a huge factor in the resultant forces from road inputs because doubling your speed generates four times the kinetic energy. If your suspension isn't good, you will be kangarooing and jumping all over the road!
So what are the suspension design factors that provide the best "ride quality"?
The major influences for off-road camper trailers are the suspension and choice of tyres. We will leave tyres for now and focus on the suspension. Of course, the chassis has to be the right design and flex in the right way.
The best ride outcome is determined by the suspension’s "natural frequency" and degree of travel. "Natural frequency" is a measure of the suspensions response to regular road forces. In general, suspensions with lower natural frequencies reduce the forces transmitted to the caravan or trailer and improve ride.
There are 2 ways to change this "natural frequency". Firstly, it is to change the "spring rate" of the suspension. Secondly, it is to change the amount of un-sprung weight the suspension is supporting.
Let's look at "spring rate": Coil springs generally can have lower spring rates for their load carrying capacity. Air suspensions are capable of achieving even lower very low natural frequencies and providing very high levels of protection. Typically, the natural frequency of air-suspensions are below 1.5 Hz.
Typically, coil and/or leaf spring suspensions have natural frequencies that range from 2 to 5 Hz, depending on payload. This higher amount can give poor ride quality.
The second factor is what sits below the springs and is called the "un-sprung weight". Lets explain this in more detail:
The "trailing arm/wheel/hub/brake/tyre set" comprise the "unsprung weight". This mass "hangs below" the spring system and so is not cushioned by the spring system. It bounces around on the tyre only.
Bumps in the road cause tire compression; and induce a force on this "un-sprung weight". The "un-sprung weight" then responds to this force with movement of its own. The amount of movement, for short bumps, is inversely proportional to the weight. A lighter “trailing arm/wheel/hub/brake/tyre set” which readily moves in response to road bumps will have more constant grip when tracking over an imperfect road.
For this reason, lighter “trailing arm/wheel/hub/brake/tyre sets” have a lower resonant frequency and give better road holding. This is why alloy wheel sets with the absolute lightest weight braking systems are chosen for performance vehicles which want the maximum rubber on the road. It isn't done for "looks!"
In contrast, a heavier “trailing arm/wheel/hub/brake/tyre set” will move less but with more force. It will not absorb as much vibration. The irregularities of the road surface will then transfer to the caravan through the geometry of the suspension and hence ride quality is deteriorated. For longer bumps that the wheels actually follow, the greater un-sprung mass causes more energy to be absorbed by the "trailing arm/wheel/hub/brake/tyre set" and makes the ride worse.
This is all about the best design of a wheel’s “bump-following” ability and its vibration isolation.
The wheels vibrate after each bump before coming to rest. These motions form the “road corrugations” which we hate. It is the sustained wheel bounce in subsequent vehicles that enlarges the corrugations and deteriorates the road!
High un-sprung weight also exacerbates wheel control issues under hard acceleration or braking. Vertical forces exerted by acceleration or hard braking combined with high un-sprung mass can lead to severe wheel hop, compromising traction and steering control.
Overall: the suspension has to be light, strong and agile. This takes very good engineering and is easily visible when you do an inspection.
Are their other benefits of good suspension?
- Safer towing control overall - so less stress for the driver
- Travel smoothly over corrugations
- More control if partially air-borne after a tricky grid or unforseen bump
- Better braking on difficult corners
- Higher safer travel speed (within speed limits of course)
- Safer braking in wet conditions
- Adjustable height for different trailer loads (like boats or second water tanks)
- Easier travel with swing away items on the rear like outboards and second spare wheels
What are simple rules for good suspension design?
- lowest "unsprung weight" is the key. This is the weight of every item hanging from and including the spring system.
This includes the springs, the independent trailing arms, the hubs, the brake system, the wheels and tyres.
- long travel in the springs is a big advantage: this requires long travel by the suspension arms
- the shock absorbers must be perpendicular to the suspension arm travel to be effective
- Bump stops for the final protection to large "thumps" are essential
- If using air bags and air suspension on a high centre of gravity caravan, then anti-sway bars are essential (only needed on a camper trailer with a high boat)
Our downloadable book on suspension design has a lot more technical information. If you want to research more details on engineering in suspension design, click on this eBook below to download a copy:
Do you use 60 tonne tool steel and 4 tonne stub axles?
- No we don't. The stub axle design of our independent trailing arms is to be as light as possible with a 2 tonne per side (4 tonne overall) rating. A well engineered suspension is as light as possible to lessen the unsprung weight. We use a duty factor of 1.8 on top of the finite element analysist of dynamic yield or fatigue failure.
- If you see large clunky oversized axles and suspension pieces it is most likely designed in China.
- Alloy wheels and most tyres have a 1250-1400kg rating so these are vulnerable to failure if you had a load as high as that in any case.
Kimberley offer 2 classes of suspension design:
The suspension on the Classic and Special Edition is the heavier and less expensive:
- Independent trailing arms (same as all models)
- Coil Springs
- Drum brake hubs (we use 10 inch; 12inch are absolute killers on weight)
- electric brakes incorporated into the drum hubs
- bump stops external
- Twin-tube shock absorbers
The suspension on the Limited Edition and the Platinum is the lightest and best available:
- Independent trailing arms (same as all models)
- Air bag Springs
- Hub Discs
- Hydraulic disc brakes
- bump stops internal xmas tree design
- Mono-tube shock absorber
Questions and answers?
Why don't you use twin shocks like many other do?
- Twin shock absorbers are twin tube with less than half the heat dissipation area of a single (2.1inch) Monto tube shock. So we prefer a single shock of double the capacity than 2. The monotube has several other advantages. Read our blog on the "Myths of Twin Shock Absorbers"
Why don't you use "coil over shock" designs as cars do?
- The shock absorbers need maximum cooling when travelling over corrugations. Coils around the shock absorbers impair the cooling rate by up to 30%. This reduces the performance of the shock absorbers.
- Whilst intuitively it would appear that the shock absorber is better protected inside the coil, in fact if debris is thrown into the coil, it will build up and we believe is a higher risk than the exposed shock absorber following the coil suspension.
Are you worried about damaging an air-bag out in the middle of no-where??
- We were! It is one of the reasons we have taken time to introduce air suspension.
- The success of the air suspension in the Kruiser coupled with increasing number of 4WD's using air suspension boosted our confidence.
- Even so, it is a harsh world off-road.Taking a spare air-bag is easy, it would only take less than 30 minutes to replace!
You use anti-sway bars with the off-road caravan, do you fit these to Kampers?
- Off-road caravans have a much higher centre of gravity and are more succeptible to sway if the air bag pressure is low or suddenly drops. The only time we will fit to a Kamper is if there is a boat with high mount points above the airconditioner and as a result, there is a higher centre of gravity.
Can Wheels be matched to suit most vehicles?
Yes they can. There are 3 important match points.
- The tracking width should be matched as closely as possible to the vehicle. This is important in travelling over a lot of sand, but don’t worry if the track width is out by 20mm or so for some vehicles. We match very closely with the major brands. Other vehicles are close.
- The hub pattern and the wheel offset need to match. This can be done for most vehicles.
- The tyres are matched. This is the most important matching as tyres can always be swapped from rim to rim.