Outback roads and tracks are littered with trailers with broken springs, axles and trailer chassis. Many have wheel studs snapped. This is not helped by overloading. It is mostly due, however, to inadequate suspension. Road shocks are transmitted directly to the trailer superstructure and contents. The associated forces are surprisingly high. Chapter 5: Trailer chassis and suspension explains all.
Most trailer chassis are similar but differ considerably in how they are sprung. The MC2 shown (below) is used on TVans. It was designed initially for mobile radar units. Few are this good. Many cheaper trailers are like horse-drawn carts – yet towed at many times the speed.
The Tvan’s military-based (MC2) suspension provides long travel and soft well-damped springing. Pic: Track Trailer.
Chapter 5: Trailer Chassis and suspension springing basics
A trailer’s springs sandwich between a trailer’s wheels and axle (unsprung mass) and its body and contents (sprung mass). If unsprung mass is light relative to sprung mass, the springs absorb much of the blows. Springing thus works best with soft springs; plus light axle, wheels and tyres.
There is one other vital issue. A spring is compressed by a wheel encountering a bump. Once over it, released spring energy jack-hammers wheels and axle down. The wheels strike the road surface very hard. This imposes shock loads on their bearings and studs. Driving fast over corrugation is thus like smashing a tyre up and down with a sledgehammer. The forces at 80 km/h are 16 times higher than at 20 km/h, and applied about 1300 times/kilometre.
Ride harshness depends on the ratio of sprung weight (shown here in grey) to its unsprung weight (springs, axle and wheel assembly). The greater this ratio, the more effective the suspension. Pic: RV Books.
To dampen spring energy as it is suddenly releases, shock absorbers convert some of it into heat. They do so on both upward and downward strokes. Shock absorbers are an essential part of a sprung suspension system. They also assist braking: none can exist unless the tyres are firmly on the ground. Further, by lessening shock loads, shock absorbers enables trailers to be lighter built.
Sadly, a few trailer makers fit springs so stiff they barely deflect. They may insist that interleaf friction alone provides damping. It does, but only on the upward stroke. There is almost none on the downward stroke. This, they seemingly overlook.
Despite the outback littered by trailers with broken springs and axles, even that fails to change their views. Nor did a poll showing almost 50% had experienced failures. A trailer lacking shock absorbers has suspension so hard its springs do not deflect enough to break. Everything else breaks instead, yet makers seemingly cares little about it.
Basic hydraulic shock absorber. RVBooks.
Not all are like that. The rough looking trailer (pictured below) is an old Russian-made unit seen in Uzbeckistan in 2007. It had been carting heavy rocks over a 50 km dirt track every day for a then 35 years, and probably still is now. It has a tubular central backbone, swing axle coil sprung suspension supplemented by progressive rubber bump stops, and big shock absorbers.
The owner said, ‘its brakes stopped working in the 1980s but as we only need to stop when we get here, we’ve never bothered to fix them’.
Despite its looks, this 1960s Russian trailer is way ahead technically of many today. Copyright: RVBooks.
Chapter 5: Trailer chassis and suspension – beam axles
For reasons that are explained below, there is little if any point in using independent suspension on a camper trailer. The main reason why so many makers do is substantially marketing based.
This Finnish-made trailer has longleaf, supple and well-damped springs, plus disk brakes. Pic: RVBooks.
A well-engineered beam axle and damped supple springs are simple and very effective off-road – and axle/ground clearance remains constant. For self-building, consider a hollow beam axle and Toyota Hi-Lux rear springs with leaves removed so that it deflects 50 mm when fully laden. Use also the Hi-Lux’s shock absorbers.
Independent suspension for trailers
Independently sprung wheels are essential only for softly sprung steered wheels. Whilst marketed as ‘superior’ (for trailers), a well designed and engineered beam axle and leaf spring system is adequate. Sadly very few of the latter are. It is mainly that which may independent suspension a better choice.
Trailing arm independent suspension on a well crafted DIY trailer.
AL-KO rubber suspension
AL-KO rubber (independent suspension) is used on many travel trailers and camper trailers. It can be used without shock absorbers because rubber has is partially self-damping. Shock absorbers are optionally available.
This suspension is also worth considering by DIYers. It simplifies design and construction as the whole assembly bolts onto and strengthen the chassis.
Such suspension can be used off-road but if planning to do so contact AL-KO engineering directly as specialised advice is essential.
Trailer chassis and suspension: summary
Whether suspension is beam axle or independent matters less than how well it is designed and made. Sadly, apart from many well-made DIY camper trailers, only a few beam axles systems are.
The AL-KO axle assembly unit can be obtained in various forms – including those that bolt directly onto the chassis rails.
The AL-KO rubber suspension mechanism. The axle causes the inner section (shown in orange) in the ‘normal’ state to rotate (shown in red) and compress the rubber pads. Pics: AL-KO.
Except for carting rubbish to the local tip do not consider any camper trailer that lacks shock absorbers even for on-road use – let alone off-road. There are too many abandoned trailers with broken springs and stub axles, lacking even provision for shock absorbers, littering outback Australia to do otherwise. Moreover, think carefully before towing anything over 1300 kg (2870 lb) on sand for other than short distances. Many owners do, but such trailers are hard to retrieve if deeply bogged down.
Buying an off-road trailer then using it only on-road is not in itself a problem – but you may need a heavier and larger tow vehicle than otherwise required.