Different camper trailer suspensions on test
CAMPING – BUYING ADVICE
In the fleet of six camper trailers we evaluated during a Camper Trailer Torture Test, we had a selection of independent and leaf sprung designs that gave us an excellent opportunity for comparison under all operating conditions.
There’s no substitute for a long-distance, varying-conditions bush trip when evaluating camper trailer suspensions. We took six different trailers on a 10,000km Torture Test and learnt much. Although the test was done a few years ago, the suspension basics haven’t changed much in the interim.
Although the tried and proved beam axle with leaf-spring suspension is obviously on the way out of the 4WD wagon scene it’s still very much alive in the 4WD ute and camper trailer market.
The beam/leaf combination offers inbuilt suspension rate variability, to compensate for different loads, by the simple expedient of having leaves that come into play only when the load increases.
Another plus for the beam/leaf is that the internal friction in the tightly packed leaves ‘damps’ spring action to some extent. If rebound leaves are fitted to the springs as well, to control the natural spring-back action of the leaf pack, additional damping may not be necessary on a very lightweight trailer.
Where the beam/leaf arrangement meets its comparative Waterloo is when its handling and response to braking and acceleration torque are taken into account.
A driving beam (‘live’) axle, has much more unsprung weight (weight that’s between the tyres and the suspension) than an independent system and places sometimes impossible dynamic loads on the shock absorbers that are attempting to control the bump and rebound actions of the axle.
In addition, the axle tries to rotate around its point of attachment to the spring pack when braking and acceleration forces are applied. In heavy braking and acceleration conditions this ‘wind up and release’ effect can cause the tyres to ‘hop’ along the road surface, reducing braking and driving power.
However, the beam trailer axle fitted to a camper is far lighter than a 4WD drive axle and there are no drive forces involved at all, so it can be made to behave quite well. Heavy braking can still cause torque reaction, but most trailers have far less braking power than a 4×4’s set of front wheel ventilated discs, for example.
It needs to be pointed out that leaf springs change length as they extend and compress, which is why the trailing end of a leaf spring is fitted with a swinging shackle or the main leaf slides fore and aft in a greased hanger. Some designs have slide-hangers at both ends, with axle location by radius and Panhard rods, and such suspensions have no self-steering geometry, but higher maintenance needs.
This means the axle moves laterally as the suspension rises and falls, imparting some steering action to the axle. However, this is more likely to be an issue in the case of heavy camper trailers that have highly ‘cambered’ (curved) springs. Highly cambered springs are a ‘cheap’ way of improving ground clearance.
The ideal shape for a leaf spring isn’t the heavily cambered type, but a shallow-curve spring that actually inverts slightly under full cornering load. This design has minimal self-steering geometry.
When it comes to a vehicle’s roll-over resistance the beam/leaf combination doesn’t compare with the best independent designs, but, unfortunately, the best of independent designs is not available on any camper trailer.
A car-type strut or transverse-wishbone independent suspension has a much lower roll centre (the point about which the vehicle will tip over) than a beam/leaf. However, trailer independent suspensions are trailing-arm types that have much less roll resistance than transverse-wishbone types. Trailing arm suspensions also suffer from camber change when the trailer sways, reducing tyre grip on both tyres.
Theoretical roll-over resistance in a camper trailer doesn’t have the same degree of significance as it does with 4WD vehicle design, because low centres of gravity are the norm in the camper trailer world. Most trailers will slide rather than roll.
Caravans are an entirely different story that we’ve covered in the Towing section.
Another roll-stability feature of a camper trailer that cannot be emulated in a 4WD is the ability of the trailer designer to concentrate load around the axle position. Good camper designs have their water tanks, jerry cans, gas bottle holders and kitchen modules as close to the axle position as possible.
In our Driving/Towing section we’ve referred to a research paper entitled “An experimental investigation of car-trailer high-speed stability” that was published by the Department of Mechanical Engineering at Bath University in the UK. The University research engineers discovered that central weight location was one of the most critical factors in trailer stability.
In the case of our six different test camper trailers there was very little on-road stability variation among the suspension types.
All tracked well, but those with longer drawbars stayed more ‘in shape’ when we were snaking through the Diamantina Channel mud bath. However, when that drawbar length came at the cost of a heavy ball weight, in excess of 10 percent, the towing vehicle’s rear suspension was overly compressed, causing bottoming on transverse road ruts. Off road, all the trailers traversed rocky terrain without snagging anything more than their couplings.
Even the beam-axled trailers had sufficient ground clearance to handle anything the towing vehicles could climb over. However, the independent suspension trailers bounced less when driven over sharp undulations.
All the trailer suspensions coped well with corrugations, but some of the shock absorber bushes looked tired at the end of our 10,000km trek.
Proponents of the beam/leaf arrangement cite reduced maintenance as a big plus, but a leaf-spring suspension will remain reliable only if it’s cleaned, lubed and fitted with new bushes and reassembled.
Independent units also need their bushes replaced after severe use, new or rebuilt shock absorbers and springs inspected for wear.
Vive la difference, but if it were our money, we’d opt for a beam/leaf suspension arrangement.
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