Left: Beam axles.
As the caravan rolls the axle remains parallel to the ground, so only its body can roll – and at about spring height at wheel/s location. The Moment arm (leverage) is thus desirably short.
Centre: Swing arms.
Used on Allard sports cars, the independent half-axles raise the roll centre yet higher. A minor downside is that the wheels tilt in slightly as it rolls, thus requiring some positive camber whilst level. (Alan Mawson
, who designed the Tvan’s brilliant suspension
, substantially overcame that by having a full length beam axle for each wheel).
Right: this is typical of most independent suspension
systems (the typical pivoted trailing arms that locate the wheels are omitted as they cannot be shown in this plane). The Moment arm (shown here at wheel location) progressively lengthens. At the far end roll centre is way below ground level thus the Moment arm at that location is substantial. This is the reason why adding weight (such as twin spare wheels) high up at the rear of caravans with independent suspension
is, to put it mildly, not a good idea.
Most independent suspension
systems for caravans however, have a roll centre close to, or at ground level. A trailer with independent suspension
is nevertheless still forced, by the tow bar, to roll (at the hitch) at tow ball level. Most, but not all, independent suspension
systems thus, by virtue of a roll centre at about ground level, introduce a much longer (vertical) moment arm at wheel level.
For the caravan to be restricted to only roll around the hitch at the front, and only around a close to ground level roll centre at its wheels, creates a roll axis. In lay terms, that term refers to any point along that axis where, if one were to push it horizontally whilst the caravan was moving, the caravan would move without inducing any roll. For example, pushing at any point along that line, would cause all of the ‘van above the line to roll away from you. If pushed at any point below that line, the ‘van above that line will roll toward you. (To clarify my point for technical thinkers, the roll axis can be defined as ‘a situation where, in the median plane of the caravan, a transverse plane in which horizontal lateral forces applied to the rolling mass of the caravan will cause that caravan to move or to turn sideways without causing it to roll.’)
As the roll centre and roll axis tends to move slightly as the caravan traverses bumps etc, roll centre needs to be seen as only a concept (that may aid your visualisation of what's going on). There is nothing absolute about roll centre position at any time whilst a vehicle is in motion. It can and will move – but not by any major amount.
As can be seen, the effective centre of mass (centre of gravity) of a caravan with independent suspension
will be way above its roll centre, and extremely so at the rear. The result is a considerable lever action as the caravan’s mass will now roll around a centre close to or at ground level, and even more so at its extreme rear. This is not as serious an issue with beam or swing axles as the roll centre line drops far less. It is rarely understood that the soft ride provided by car suspension
is tailored to human physiology but what is comfortable for people is less necessary in a well-made caravan (one would not want to ride in an early Phoenix, that nevertheless are known for their longevity). Independent suspension
is far from required. A well sprung beam axle caravan with long leaf springs (to limit bump steer) overcomes all of the above.
Do note this article is primarily relates on-road and dirt-road going. Suspension
systems for serious rock-hopping, mostly confined to specialised off-road camper trailers, have very different criteria.
The issue is not independent suspension
per se, but rather the forms commonly used on caravans that introduce a low centre. It is perfectly feasible to have an independent suspension
that provides a high roll centre. A swing axle system for example has a roll centre much at the level as a beam axle. The roll centre issue is brilliantly explained in Maurice Olley’s superb book Chassis Design – Principles and Analysis. It does use full-on engineering terminology and mathematics throughout and is unlikely to be understandable without such a background.
Diagrams below show the Line of roll centre between two differently designed caravans.