Caravan chassis and running gear - part 2

In this second instalment of chassis inventions that changed our world we look at auto reversing overrun brakes, semi-trailing arm axles, hitch head stabilisers and active stabilising devices

Auto reversing overrun brakes

L ong after the operation of the overrun brake was perfected its main drawback was that it would often apply the brakes when reversing, especially over rough ground or uphill. I well remember my first caravan in 1974. If you wanted to reverse you first had to make sure the hitch mechanism was not in compression, then you could move over a lever that prevented it going into compression so the brakes would not operate when you reversed. I seem to remember it automatical - ly disengaged when you went forwards again. However, if you were executing a multi-point turn because you’d taken the wrong road, it was a nightmare!

A better solution was needed and it came a few years later with the invention of the auto reversing overrun brake.

A German patent DE3203113A1 from 1981 describes ‘… two positively coupled, floating but limited tangential to the wheel axis movable brake shoes, thus preventing tensioning against the brake drum when reversing…’. The patent was assigned to Olbernhau Fahrzeug (Olbernhau Vehicle), although Knott GmbH claims to have invented it and manufactures such brakes to this day.

Like many of the best inventions the auto reverse overrun brake is simple but ingenious. AL-KO’s version is discussed here.

When reversing, the towing vehicle pushes in the draw shaft of the overrun device. The brake shoes (1,2) are pressed against the brake drum via brake linkage, Bowden cable and expander clutch (3). The brake drum turns backwards, taking shoe 2 with it. The transmission lever (4) swings back, over centre, allowing the brake shoes to collapse towards each other and so releasing the braking effect. The wheels can then revolve freely in reverse, despite the fact that the drawshaft is then fully to the rear.

When the vehicle moves forward again shoe 2 moves outward through friction with the drum and the reset

Semi-trailing arm axles

emi-trailing axles are an ingenious bit of thinking from axle designers that came to the fore in the 1970’s although they were introduced somewhat earlier by BMW. (One of the first cars to use the technology was the BMW 2002 which first appeared in 1968.)

On a standard trailing arm axle, the arm holding the wheel stubs pivots along an axis perpendicular to the direction of travel. The wheels are free to move up and down with the suspension but always remain perpendicular to the axle tube. Most of the time this works just fine but, if a vehicle starts to roll, the tyres tilt with the vehicle, losing grip.

spring (5) pulls the transmission lever back into its normal position. The potential problem of the handbrake not working in reverse is solved by applying it fully. Shoe 1 then pushes shoe 2 via the transmission lever and adjuster assembly such that it presses against stop 6 and thereby provides braking action.

Auto reversing brakes are now standard on virtually all European caravans. In other parts of the world, such as the USA and Australia, trailer brakes are often electrically operated, eliminating the need for an auto reversing mechanism.

With a semi-trailing axle, the arm holding the wheel stub pivots at an angle to the direction of travel. Then, if the vehicle rolls, the tyres go up and down in an arc at 90 degrees to the direction of travel. This allows them to stay in full contact with the road surface.

SWhen applied to a tall vehicle, such as a caravan, the other advantage of a semi-trailing axle is that the roll centre becomes higher and closer to its centre of gravity (C of G). There is therefore less ‘leverage’ to make the caravan roll as it might, for example, in sidewinds. The result is a more stable and comfortable ride.

AL-KO’s version of the semi-trailing axle, the Delta, appeared as early as 1975. AL-KO’s archive says “As a result of the development with Mr. Prof. Kirchgäßner, the first AL-KO Delta axle with upwardly shifted moment centre was presented at the 1975 Caravan Salon.”

In the meantime, rival BPW countered with their V-Tech design, which also employed semi-trailing arm geometry. Now, most caravans over 750 kg manufactured in mainland Europe come with semi-trailing axles as standard. The axle has yet to catch on with UK caravan manufacturers although the Explorer Group (now The Erwin Hymer Group UK) used the BPW V-Tech semi trailing axle on their caravans for many years.