by Collyn Rivers
Caravan wheel placement
Caravan wheel placement is a vital issue affecting on-road stability. This article explains why, and the position along the chassis with which they should best be located.
A conventional caravan is always a compromise. This is because it is towed via hitch at some distance behind the tow vehicle’s rear wheels. If that vehicle sways clockwise, that hitch overhang causes (not just permits) the caravan to sway anti-clockwise. If the caravan sways clockwise, it causes the tow vehicle to sway anticlockwise. It is essentially an unstable concept, but safe within limits.
The major constraint of caravan wheel placement is the basic laws of physics. These include two major distances. One is the tow vehicle’s wheelbase (distance between the front and rear axle). The other is the caravan‘s so-called radius of gyration.
Caravan wheel placement – the radius of gyration
The radius of gyration is a distance. With a trailer, it is that distance from its tow hitch to that point along it was all its (laden) mass concentrated in one place. The greater distance the better. For optimal caravan wheel placement, the axle/s should be just behind that location.
As, like an arrow, to be stable when moving, a trailer must be nose heavy (here by about 8-10%). For ideal caravan wheel placement, they should be as far back as feasible. This is readily possible by centralising weight either side of those axles. Moreover, by minimising all other weight, particularly at its extreme rear.
Locating heavy spare wheels on a caravan‘s extreme rear indicates one of two things. Whoever designed does not under basic physics. Or does – but ignores it. And why two spare wheels – when the tow vehicle has one (and with some- just a repair kit and inflator).
Measuring the radius of gyration
This is readily done, but as far as is known, no caravan maker in Australia does so. That required is a frictionless turntable, the rotation of which is constrained by springs. The caravan is then centralised on that turntable and twisted by (say) 30 degrees and released. The time it takes to return to the starting position is a measure of that radius of gyration. The longer it takes, the greater that radius.
You can readily simulate caravan wheel placement by holding a bottle of wine in each fully extended hand – and twisting at varying speeds. (Either red or white is just fine).
Correct caravan wheel placement can also be done arithmetically. Like a loaf of sliced bread, the caravan is (theoretically) ‘sliced’ along its length. The ‘weight’ of each slice is then measured.
The tow vehicle’s radius of gyration is limited by the lateral distortion of its front and rear tyres. Moreover, also by how far they are apart. Here, the greater the better. Most vehicles used for towing (in Australia) are about three metres. In this respect, one of the best tow cars ever made is that least probable. It was the DS Citroen. It had virtually a wheel at each corner.
DS Citroen towing an Airstream Caravan – here again notice the ideal caravan wheel placement. Pic. source unknown
Only one local caravan maker appears to have realised the need for correct caravan wheel placement. He was Barry Davidson. He did so for his 1990s Phoenix range. They have a legendary reputation for excellent towing stability.