Updated February 2016

Poor caravan suspension wrecks wheel bearings, breaks axles, sheers wheel studs, causes wheels to fall off. This article shows what’s wrong.

Caravan suspension basics

Were roads totally smooth, there’d be no need for sprung caravan suspension. But most roads are far from smooth. (Even trains on smooth rails need springing). Caravan wheels must traverse bumps, holes and sometimes corrugation, without damaging the caravan and that in it.

Road shocks forces increase by the square of how fast the vehicle is moving, e.g., caravan suspension at 60 km/h gets clobbered four times as hard as caravan suspension 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. The effect is not a gentle rise and fall. The wheel and axle is belted upward, compressing the associated spring/s. The lighter the wheels, tyres and associated moving bits of suspension, and the heavier the rest of the caravan, the greater the amount of road shock energy absorbed by the spring. The rest of the caravan is subject to road shocks, but less so.

The main problem is not wheels being thrust upward. It is that a leaf spring acts as a very strong bow. When compressed it holds much of the upward energy. The instant the wheel has passed over the bump, that spring instantly releases the energy. It jackhammers the wheel and axle downward. Unless that force is restrained, it smashes the wheel and tyre onto the road surface. Over corrugation it does that about 1300 times each and every kilometre. Per wheel.

That’s what’s happening underneath many a low-priced trailer and caravan travelling over anything other than totally smooth roads unless it has adequate shock absorbers (energy dampers).

The need for energy damping

With leaf springs, some friction results as the spring leaves slide slightly over each other when compressed when passing over bumps. But this happens only the upward travel. There is next to no friction on the vital downward movement. The spring leaves are no longer pressed together.

Much of the above was known by 1800. So too was the understanding that, if that downthrust could be dampened by friction, part of the spring’s energy could be released as heat. Most vehicles back then had long, supple multiple-leaf springs. To assist damping, some carriage builders bound the spring leaves together with strong leather thongs. (Vintage sports car owners still do). Today’s crude attempt employs an upper clamp that only vaguely holds the spring 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 used hydraulic action. Such methods helped, but that needed was for the downward movement to be damped more heavily than the upward movement. Coil springs and torsion bars made that totally essential. The solution was de Carbon’s telescopic hydraulic dampers. They are used in various forms to this day.

Beam axle – or independent

A great deal of nonsense is written about this. The suspension needed for passenger cars is totally different from that needed for trailers that are free to pivot about the tow ball and readily absorb major bumps by rocking. Car suspension is also a compromise dictated by human physiology. This issue is thoroughly covered in the more technical articles Linked below. What is far more important is that caravan suspension be properly engineered.

Despite all this being thoroughly known for so long, it is totally ignored by some trailer makers. They ignore or deny basic physical laws, arguing that their products are somehow immune. The underpinnings of cheap trailers (including some fifth-wheelers and caravans) is now at the level of low-end horse-drawn carts.

Caravan suspension is too complex to be covered in brief article form. That fuller is in my article Caravan Dynamics. A longer and more comprehensive version is at http://caravanandmotorhomebooks.com/caravan-and-tow-vehicle-dynamics/

Caravan suspension is also covered in considerable detail in the author’s all-new Caravan & Motorhome Book. Camper trailer suspension is covered in depth in the Camper Trailer Book. My other books are Caravan & Motorhome Electrics, Solar That Really Works (for cabins, caravans and motor homes). Solar Success relates to home and property systems. See also the related article Wheels Falling off Trailers.

For information about the author please Click on Bio.

This topic often arises on forums. If you found this of interest please consider posting this Link in the thread concerned.