"Ride Quality" is a great way to know if you have the best suspension on your off-road caravan or 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 provided by the suspension 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 caravan and camper trailers are the suspension and choice of tyres. We will leave tyres for now in this blog 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.
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.
So good suspension design requires:
- lowest spring resonant frequency which is achieved with air-springs
- lowest “unsprung weight” which is achieved with lightweight trailing arms, disc brakes, lightweight shock absorbers and of course air-springs. This combination is typically half the weight of the equivalent drum brake, coil spring, dual shock combination.
- The shock absorbers dampen the spring (coil or air) motion but also must be less stiff than would optimally dampen the wheel bounce.
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.
So how has Kimberley perfected such a low un-sprung mass on the “suspension/ wheel set” over the last 10 years?
The first item to focus on is the wheel hub and brake assembly.
The disc/brake hub we use and the PBR disc brakes are half the weight of the 12inch Electric drum alternative.
Then we use lower weight Mono-tube alloy shock absorbers, these are NOT steel.
Air-springs are half the weight or less of coils which are far less again then leaf springs.
The shock absorbers are mounted in the optimum perpendicular position and the mono-tube shocks use stainless steel bushes for resilience in corrugations. (If you see dual shock absorbers at an oblique angle; the units aren’t as effective in this position so 2 are used with the result of higher un-sprung mass).
Finally, the high tensile steel trailing arms have a unique curve pattern for the best weight/performance.
The benefit to customers is not only better vehicle control and better ride but a 5 year warranty on the chassis and the trailing arms. They are well looked after in a Kimberley!
The premium air springs that Kimberley uses have a variable spring rate. When they are up "high" the spring rate is less and you will see much more travel.
To demonstrate this, we have a simple video with the air springs running under a Kimberley Kamper Platinum model are 3 height settings of low, medium and high: