by Collyn Rivers
Mechanical sway-control
The overhung hitch of a caravan tow vehicle can cause (not just permit) a caravan to sway. This sway annoys at low speed and is dangerous at high speed. Until recently attempts to limit such sway rely on friction to dampen and ideally prevent it. Unfortunately, mechanical sway-control has little value at speed.
Mechanical sway-control has little value at speed – fundamental limitations
The reason why mechanical sway-control has little value at speed is its fundamental limitation. This is that frictional damping forces remain constant. Sway forces, however, increase with the square of the caravan‘s speed. Few caravanners are aware of this issue. Makers of these devices also seem unaware of mechanical sway-control’s limits.
Controlled tests show that, at 100 km/h, a typical friction control damper has only 1% or so of its low-speed effect. At speed, it is virtually useless. That mechanical sway-control has little value at speed has long been accepted by UK/EU caravan makers. Most rely on the AL-KO friction tow hitch to control ‘nuisance swaying’. They use electronic caravan braking to control high-speed swaying.
Friction between the tow ball and receiver assists low-speed sway reduction. Never lubricate a tow ball. Chromium-plated tow balls are prone to surface breaking-up. Replace any such tow ball by the AL-KO friction product. While effective at up to 50 or 60 km/h mechanical sway-control has little value at speed.
Minor swaying at low speed is virtually inherent and fixable. Any more than that is a sign that something is seriously wrong. There is usually more than one cause. These include inadequate tow ball mass, weight at the rear of the caravan or too low (tow vehicle) rear tyre pressure.
Address and fix these issues first. Only use friction devices to limit low speed ‘nuisance swaying’. They work well for that but mechanical sway-control has little value at speed. Instead, add one or other of the electronic stability systems (described briefly below). See them, however, as ‘parachutes’ for exceptional events.
Electronic stability systems
Electronic stability systems detect excess sway and brake the caravan wheels accordingly to reduce speed. One type (Dexter) does so by braking one or other side wheels. The AL-KO system brakes both sides. These systems rely ultimately on the friction between rubber tyre contact patches and the road surface. That is partly molecular and not lessened by speed.
For a full explanation (plus technical explanations) of why mechanical sway-control has little value at speed see our recently published ‘Why Caravans Roll Over – and how to prevent it’. Full details are on our Bookshop page.
