SUSPENSION

Tom Olthoff looks at the smooth rides or harsh bounces of suspension systems.

Words and photography by Tom Olthoff

Quite a few years ago some research was conducted to determine why people bought certain caravans. Not surprisingly, the outcome showed that about 75 percent of those who purchased a caravan did so because the female partner liked the decor and layout. While this is understandable, it is also a pity that more importance is not placed on decisions like the type of suspension used. Particularly as car manufacturers place great emphasis on getting the suspension right as they know how this contributes to overall safety and comfort.

Yes, I have heard it many times, no-one travels in caravans. But caravans and their contents still travel along the road, at times at speeds that is perhaps greater than desirable. And considering that the way a suspension absorbs road irregularities has a bearing on the stability of the vehicle, or in this case the caravan, it is a pity that caravan suspension has not kept pace with the improvements in other areas.

Recreational vehicle suspensions range from somewhat archaic leaf springs to state of the art air bags with many other variants in between. Many complaints about items moving around inside or even equipment breakages can be the result suspensions that are badly designed or not right for the application. In relation to safe towing, the design of the suspension is as important as the way the trailer is loaded or hitched to the tow car.

To absorb the road irregularities the suspension needs to be supple and have enough travel to smooth out the bumps. Springs should only be increased in strength if extra loads are carried. Another myth is that shock absorbers carry part of the load. This is only the case when the shocker is designed with an integral airbags as is sometimes the case on the rear of passenger cars. Otherwise the job of the shock absorber is to dampen the spring movement to prevent excessive bounce.

TYPES OF SPRINGS

On trailers a spring can consist of leafs, coils, rubber or air. Wheels on opposite sides can be joined by a common beam axle or be independently sprung. With trailers an allowance has to be made for the load that is likely to be carried. Some believe that it is better to allow too much than too little. That may be fine for a box trailer but not necessarily for a RV that has all the comforts of home. Erring on the too-strong side can cause the springs to be too stiff and create a rough ride, particularly on corrugations or where there are lots of bumps and potholes.

Rubber suspension like that produced by AL-KO International is becoming increasingly popular. AL-KO, from their factory in Melbourne, are able to supply rubber sprung suspension units in 100kg increments to suit almost any application. Shock absorbers can be added if required. If the load situation changes, the rubber inserts can be modified to suit different conditions.

Air bags have a lot to offer when it comes to ride quality. But they can be more expensive due to the additional equipment like the compressor, valves, regulator and height control mechanism. With air bags the pressure can be altered to suit the load or type of terrain. With the flick of a switch additional ground clearance can be obtained or the van lowered to fit through a low doorway.

INDEPENDENT OR NON-INDEPENDENT

Many are of the opinion that you can’t beat a beam axle and leaf springs. And if keeping it simple is the name of the game that’s probably right. As mentioned earlier, many leaf spring set-ups are too stiff, too short and don’t offer the flexibility to soak up the rough stuff. With a beam axle if one wheel moves up or down, it affects the position of the opposite one and can be detrimental to stability. This is where independent suspension can be an advantage over a beam axle.

But a smooth ride is not guaranteed just because a trailer has independent suspension. Even a coil sprung suspension can be too stiff. Matching the spring to the load is still very important. Many owners of off-road RVs complain about gear moving around or being damaged. While a rough road surface may contribute to this, it is equally likely that the springs are too rigid or there isn’t enough vertical suspension travel.

And independent doesn’t automatically mean that you’ll find coil springs. There are a couple of trailing arm independent units that successfully use leaf springs. The Austrak independent suspension made by Preston Chassis and Vehicle Components’ Cruisemaster suspensions are two examples.

SHARING THE LOAD

While a good two-wheel independent suspension is generally superior to a beam axle and leaf springs, it can’t be assumed that the same applies to a four-wheel or tandem set up. When a tandem or 4-wheel suspension is used, load sharing becomes highly desirable, particularly on uneven surfaces. Load sharing refers to the suspension’s ability to share the load of the trailer between the wheels on the same side when road surfaces are uneven. Without load sharing, the wheel that is in the highest position will carry a greater amount of the load. This can overload the wheel, tyre or bearings.

Pair of leaf springs joined by a ‘rocker’ offers a degree of load sharing and are certainly preferable to two leaf springs that are located one behind the other without any interaction. However if ride quality, suspension movement and overload prevention is the name of the game then they don’t come much better than the Simplicity suspension. The Simplicity system always has the same load on all wheels, irrespective of the unevenness of the terrain.   Others that do a similar job include the suspension fitted to the larger Golf caravans and a few rugged off-road trailers that use inter-connected air bags.

GOING OFF ROAD

When it comes to ‘off-road’ trailers some manufacturers believe that the suspension has to be more rigid than for a similar on-road version. While individual components may need to be stronger, spring rates don’t necessarily have to be higher. Good performance is all about matching the springs to the load and offering enough suspension and wheel travel to cope with the terrain.

 Although many trailers designed for off-road use are fitted with leaf springs, these are not necessarily the best option. Most leaf spring suspensions are very rigid and to make matters worse, often have extra rebound springs attached to further reduce flexibility. And with a leaf spring set up it is the axle that is usually the lowest part of the trailer thereby reducing overall ground clearance.

Of course, leaf springs and beam axles have done a good job for many years. One advantage sometimes given is that if a leaf spring breaks it is possible to do a temporary repair to get you to the next workshop. In reality, if a leaf spring breaks, you could be in big trouble as the axle may then no longer be held in its proper position. And should a U-bolt break it could be even worse.

With a coil sprung independent suspension, you only lose some clearance if the spring broke. But then it may be possible to jam a piece of tree branch in between the coils to keep the trailer up.

TYRES AND SUSPENSION

In a general sense, tyres also form part of the suspension as the tyres themselves can assist the springs in absorbing road irregularities. With motorcars, suspension designers consider the type and size of tyre to be used in conjunction with the rest of the suspension. As far as I’m aware, when it comes to caravans or trailers in general that is certainly not the case. In fact, it is not unusual for caravan and camper trailer tyres to smooth out most of the bumps as often the springs are too stiff to play a meaningful part in levelling out the ups and downs.

If you observe a car with a good suspension negotiate a rough road you’ll notice the wheels moving up and down as should be the case. Watch a caravan or trailer under similar conditions and it is often quite obvious that the complete trailer moves up and down rather than just the wheels. And at the same time the tyres are flexing excessively.

Caravan tyres are often criticised for failing prematurely or wearing out too quickly. Considering what a caravan tyre has to contend with, particularly when it comes to smoothing out the ride, it is a wonder that they last as long as they do. A caravan with an efficient load sharing suspension is usually much kinder on the tyres. Due to less flexing of the tyre sidewalls and no overloading (assuming the tyre pressures are correct), tyre life can be noticeably increased when a well designed suspension is used.

SHOCK ABSORBERS

We all know that shock absorbers can make a big difference to the way that a vehicle handles and rides. So it stands to reason that fitting shock absorbers to a caravan or trailer suspension can improve the stability and ride quality. Not necessarily!

As mentioned earlier, shock absorbers do not carry any weight. Their function is purely to prevent excessive spring oscillation. Hence, they are often referred to as dampers. Simply put, shockers prevent the suspension from moving up and down too much. And when coil springs are fitted, shock absorbers are a must. But as many leaf springs are too short and stiff to start with, fitting shock absorbers can actually make the situation worse as it inhibits spring movement even further.

SELECTION

Before deciding on a particular suspension system, consider your needs and type of terrain you intend to travel along. If most of the time the surface is hard, black and smooth, just about any suspension will do as long as the strength of the springs match the load and it offers a smooth ride.

In the final analysis a good two-wheel independent suspension or a four-wheel load sharing one is not that much more expensive than your standard beam axles and leaf springs. Considering the cost of a new caravan, pop-top or camper trailer, a few hundred dollars extra for a good suspension shouldn’t be hard to justify. After all, these days you wouldn’t drive a car that had leaf springs and beam axles.

AIRBAG SUSPENSION

While air suspension is a relatively recent innovation on caravans, the idea was born about 160 years ago. In his book Riding on Air – A History of Air Suspension, author Jack Gieck writes; “In 1847, the year Thomas Edison was born, only three years after Charles Goodyear’s rubber vulcanisation patent was issued, inventor John Lewis was granted U.S. Patent No. 4,965 for ‘Pneumatic Springs for Railroad cars &c’”.

To allow the air suspension to do its job efficiently it is not enough to just place an airbag between the axle and the chassis. The air bag chosen has to suit the application in regard to the load carried as well as the amount of suspension travel needed. Then there needs to be a compressor to inflate the bags as and when required. And finally a valve that can control the amount of air in the springs and maintain the chosen ride height.
 
Although air suspension has many advantages, the biggest is probably the ability to adjust the spring rate to suit the load being carried. But being able to add or remove air means that also the height can be altered. Recently we visited the premises of Vehicle Components in Brisbane. Not only was I impressed with the efficient operation of the business, I came away wondering why more caravans didn’t use air bags as a springing medium.

THE FINAL BOUNCE

While overall build quality and the comfort features of a caravan or camper trailer may be high on the list of purchase criteria, the suspension fitted should not be far below when considering the priorities.

Opting for something better than a leaf-spring/beam-axle suspension shouldn’t mean that you have to find a lot of extra money. But a word or warning. If you raise the question of suspension options when looking at a particular van, you may well be quoted the total cost of that suspension. Often the cost of the system fitted is not deducted.

For example, if a particular load sharing suspension system costs $2000, the real costs may only be $1000 if the cost of the springs and axles that are not being used are taken away. So ask the right questions regarding costs. Considering the total cost of the rig, the cost of using a decent suspension shouldn’t be hard to justify. This is one time where “out of sight, out of mind” should not apply.

Those who have towed caravans or trailers with a rudimentary suspension and then upgraded to one with a well designed system will often remark on the difference in ride, stability and comfort. As our roads are not known to be the best in the world, it is worth spending a little extra to get the best equipment at the start.

For more information on suspension systems check out:

http://www.alko.com.au/

http://www.vehiclecomponents.com.au/content/contentdetails.asp

http://www.simplicityaxles.com.au/

The air compressor is the heart of the air bag suspension system.

The AL-KO independent suspension offers maximum ground clearance as well as an excellent ride.

Preston Chassis’s Austrak Independent Suspension has a ‘roller’ leaf spring for smooth action as well as a shock absorber.

A well designed single axle independent suspension with coil spring and integral shock absorber.

This 4-wheel swinging arm independent suspension has no load sharing and limited travel resulting in the tyres doing much of the work of coping with road irregularities.

Cruisemaster independent suspension uses a leaf spring and leading/trailing arms. Note the swinging shackle at each end of the spring.

European caravans like the Geist use high-tech independent suspension that uses a rubber spring plus shock absorber.

This rugged independent suspension is ideal for off-road camper trailers.

Leaf springs can be combined with wishbones to provide independent suspension while having lots of ground clearance.

Most vans are bought because of the internal layout, features and décor rather than the type of suspension fitted.

A 4-wheel independent airbag suspension offers the best of all worlds.

An example of a poorly designed tandem rocker suspension. Normally the ‘rocker-beam’ between the wheels would be more horizontal.

A fully load sharing Simplicity suspension ensures that the weight carried by each tyre is always the same.

With the right suspension tyre life can increase significantly.

Some of the parts that make up an airbag suspension.