Suspension innovation is probably the most important factor in a 4WD’s off-road superiority. Are trailers, caravans and campers trailer different in their design requirements to 4WDs? Yes, absolutely. The differences are so great, there are few vehicle suspension experts who understand the nature of a towed trailer.
At Kimberley, we have invested heavily in suspension design and realised that the generic air bags could be significantly improved. So we have designed, in Ballina, a premium air spring for Australian off road use and matched to our caravans and camper trailers.
A quick history in air-suspension in 4WD's: In 1970, at a time when almost all 4WDs used primitive leaf springs, Range Rover pioneered the use of soft, long travel coil springs on all four wheels. The benefit of extreme axle articulation became clear to everyone.
Then in 1993, Range Rover suspension design took a huge leap ahead into the next generation of innovation: electronically controlled, self leveling, variable height pneumatic suspension. (This is where we are today with off-road caravan air-suspension, some 20 years later). This gave the driver the ability to reduce ride height for high speed cruising, increase it for off-road clearance, maintain a level ride regardless of load, and maintain articulation. The staggering capabilities of this system, in combination with the simultaneously introduced electronic traction control, took the 4WD world by surprise.
It took several years for manufacturers and engineers to iron out some of the reliability issues but today, air suspension is a "must have" on any truck/ trailer travelling accross difficult roads.
At Kimberley, we started by studying the theory of suspension in commercial trailers which is a closer starting point than most people realize.
Our motto on innovation is: “Without understanding the theory, experience will not teach”. So it has taken years of experience layered on top of the theory of suspension in trailers to get to this point.
We believe you cant “add on” top of the line suspension. It has to be part of the design right from the beginning. It is for this reason that we tackled designing our own air springs (air bags) and doing it really professionally.
Firstly, Air-Springs have some negatives:
- They cost more than steel coil springs
- They are less reliable that steel coil springs
- They can have reduced roll stability
But some huge positives:
- Much less weight than springs, reducing the "unsprung" weight.
- A soft “magic carpet” ride on all trailers
- Using "anti-sway" bars eliminates roll stability issues in higher centre of gravity caravans
- Increased stability and safety on tandem caravans as the height can be leveled to match the vehicle regardless of the load.
- If your caravan has air-suspension, you can drive faster and more confidently on typical rough roads or corrugations, where higher sprung caravans may be reduced to a kangaroo-hopping crawl.
There are concerns of gibber and stone damage to air-bags, however, you can change out an airbag in less time than a coil spring. So you have the peace of mind of having a spare available.
In the photo on the right, we have the air-bag inflated to the mid-point so you can see the length of the travel in this Kimberley Karavan. (dont be concerned by the shape of the corrugated protection boot over the mono-tube shock absorber shaft - the shaft is straight and fine!)
Bump stops, that were originally externally mounted are now internal inside the air spring. This in a key design feature for these reasons:
- The bump stop is a triplex christmas tree design internally secured that can't be out of alignment.
- It adds to the spring rate to give protection for the big “thumps”.
- In the unlikely event, the airbag is punctured or fails, the unit can “bounce along” on the bumps stops at low speed.
To explain how an air-spring works, we will concentrate on how the Premium Air spring with it's variable diameter operates and what the difference is to standard air bags.
The air spring works when the lower "piston" pushes up into the "air bag" compressing the air. After it is compressed, the air pressure pushes back on the lower piston. This is the "spring" action.
For a given air pressure in the air bag, a larger diameter piston will carry a larger load.
For off road use, it is clever to vary the piston shape which changes the spring rate. Range Rover do this on all their models. So at Kimberley we have adopted this design feature.
How does this shape change work?
The air “bellows” rolls over the lower piston.
With a variable diameter piston design, the bellows folds over the larger diameter section of the piston at the bottom of its travel. However, it reduces to the longer narrower diameter for most of its length. See the circled larger diameter in the photo above. (in this photo, the air bag is "pulled" down over the bottom piston compared to the standard one)
The photo above shows the direct comparison of a Kimberley Premium Air-spring and the industry standard “Firestone” branded unit.
This Premium Air-Spring is a totally new design with only 3 air bag components instead or 4. But remember, it has an internal bump stop as well!
- The piston is widened at the bottom for increased spring-rate
- The “effective travel length” is longer as there are fewer components. (photo below shows the full travel at about 20mm longer)
- The upper and lower mounts are CNC machined from 6061 Aluminum and polished where the air-bag rolls over. (far superior for wear compared to polyurethane in standard air-bags)
- The bump stop is an internal triplex design
Finally, the resultant air-suspension has a longer effective active piston which provides 30-40mm more travel at a lower rate.
The result is a noticeably smoother ride when the suspension is set to “normal” with noticeably more control in highway travel at “low” mode, and more usable wheel travel.
The secret of the new Kimberley Premium air-springs lies in the custom-made lower piston which is CNC machined out of a solid aluminum billet, specially shaped to vary the spring rates according to conditions. The fatter bottom section gives a spring rate of 150% times normal.
Another feature built into the new Premium Air-spring is significantly longer travel -- with 170mm extension achieved via the longer piston and special bellows.
However, we have designed the new variable air-spring to be interchangeable with the standard air-spring so that spares and even third party “off the shelf” air-bags can be purchased and used if required.
In a nutshell the design features of an air-spring are:
- Long travel is still preferred like the coil spring
- Operate in a range of 3:1 on pressure only
- Standard air-springs use polyethylene pistons and heads with nut-serts for securing
- Premium air-springs will be precision CNC-machined from 6061-T6 aluminum.
- Premium air-springs will have at least a 1.3:1 spring rate increase from increased lower diameter
- The mounting holes are American UNC so they are interchangable with Industry standard Air-springs.