Updated 22 May 2016

Caravan suspension has requirements that are very different from tow vehicles. This is only too often misunderstood. Here’s why and what it should be

Caravan suspension basics

Were roads totally smooth, there is no need for sprung caravan suspension. Most roads, however, are far from that. Even trains on smooth rails need springing. Caravan wheels must traverse bumps, holes and sometimes corrugation. The must so without damaging the caravan and that carried in it.

Suspension is speed-related. Road shocks forces increase by the square of the speed. For instance, such shocks at 60 km/h are four times harder than at 30 km/h. At 100 km/h it’s over ten times as hard.

The effects are more severe than many suspect.

 

suspension - jayco independent

Jayco independent caravan suspension

How hard and how often

At 60 km/h, a bump one metre wide is crossed in a sixteenth of a second. It is not a gentle rise and fall. The wheel and axle is belted upward, compressing the associated spring/s. The heavier the wheels, tyres and moving bits of suspension (relative to that sprung) the greater the shock energy. 

Whatever the springing form, it acts as a strong bow. It momentarily stores energy. The instant the wheel has passed over the bump, that spring instantly releases its pent-up energy. This jackhammers the wheel, tyre and axle back down. Unless slowed it smashes wheel and tyre onto the road. The impact forces are taken by the rest of the trailer via its wheel studs.  Over corrugation, this happens about 1300 times per kilometre. And, likewise, per wheel.

Sprung suspension must dampen that downward release. This is typically done by friction (thus turning it into heat). 

Caravan suspension – energy damping the primary need

With leaf springs, as compressing spring leaves slide (slightly) between each other, friction is caused. This only happens, however on upward travel. As compression suddenly releases, there is no friction to dampen that movement, in other words the spring leaves are no longer pressed together.

Early carriage makers knew all this by 1800 or so. They understood that, if spring movement is damped by friction, spring energy is released as heat. By binding the leaves tightly with strong leather thongs they increased interleaf friction. (Vintage car owners still do). Today’s crude upper clamp only vaguely holds the leaves together.

 

carriage suspension old good

Early carriage suspension was surprisingly sophisticated.

Less crude was the Hartford friction damper invented in 1895. This had partially rotating clamped friction disks. A later version had hydraulic action. Such methods helped. That specifically needed was to dampen downward movement more strongly than upward movement.

Coil springs and torsion bars the made this essential. The solution, de Carbon’s telescopic hydraulic dampers, are used to this day.

Beam axle – or independent

Suspension needed for passenger cars is totally different from that for centre-axled trailers. The latter freely pivot about the tow ball. They readily absorb even major bumps by rocking. Extra-long travel suspension is not required. It has long since ceased to be (on all but fully off-road trucks).

Nor is there any benefit from using independent suspension. Beam axles have many advantages. For example, they provide a stable platform at all times. Leaf springs do not necessarily have to be used. Other forms of spring exist.  A major issue, however, is a lack of well engineered leaf spring systems for caravan use. That is for marketing/promotional reasons, but not engineering. Long, parabolic leaf springs are just fine.

Caravan suspension – little need for compromise 

Human physiology substantially dictates passenger car suspension. That needed to optimise road holding is too hard for comfort. Humans, on the other hand, do not travel (at least nowadays) in caravans. There is thus no need to compromise. But caravan makers pointlessly ape car suspension. Caravan suspension needs to be engineered to suit caravan needs!

(The effect on humans etc is covered in depth in my Caravan & Tow Vehicle Dynamics.)  

All this was thoroughly known by the mid 1930s. Despite that, most caravan makers ignore it. Or not aware of it.

As if their products are immune, some defy and/or deny the basic laws of physics. Many a Victorian horse-drawn cart, for example, had better springing than some caravans today. 

Caravan suspension – further information

I cover suspension in detail in the Caravan & Motorhome Book. Camper trailer suspension, likewise in the Camper Trailer Book. My other books are Caravan & Motorhome Electrics, Solar That Really Works (for cabins and RVs). Solar Success is for homes and properties. See also Wheels Falling off Trailers.

For information about the author please Click on Bio.

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