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

Words Terry Owen

Auto reversing overrun brakes

Long 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 automatically 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 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.

Semi-trailing arm axles
Semi-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.
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.
When 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.

Hitch head stabilisers
Hitch-head stabilisers have so transformed towing today, it’s hard to remember what went before. Nevertheless, it’s worth taking a look into the past so we can see these devices in their true perspective.
Most people who’ve towed a caravan will have experienced that uncomfortable feeling when the outfit becomes unstable as the ‘tail tries to wag the dog’. Instability can be a function of several variables such as loading, speed, weight ratio, side winds and so on. Although it’s true to say that a well matched, sensibly loaded outfit, driven well, should rarely experience any problems; being hit by the bow wave of a large overtaking vehicle can be very uncomfortable. Also, a sudden avoidance manoeuvre or tyre blow-out can bring about instability.
For these reasons aftermarket stabilising devices became very popular many years ago but most had one thing in common – weight and inconvenience.
Between 1968 and 1985 AL-KO produced a device known as the Exact Drive, or Bügel Stoßdämpfer (Bugle Shock-absorber) as it became known. It employed a yoke and pair of oil-filled shock absorbers to dampen the yaw movements of a trailer. Whilst undoubtedly effective, it was a cumbersome and heavy device, and this limited its success.
In 1985 the bugle device was superseded by the AL-KO’s APS 4, which worked in a similar way to other blade stabilisers that had become common about this time.
Shown here is a typical blade type stabiliser from British company, Bulldog Security Products. It works by using friction discs to control the movement of a blade attached to the ‘A’ frame via a special sliding shoe. I had a blade stabiliser for many years and it worked well enough but it was a bit cumbersome, and heavy to carry around at 10 kg. The blade stabiliser was not the only yaw damping device available at the time, although it was perhaps the most popular for its combination of weight, simplicity and cost.

Below: AL-KO AKS; The first hitch head stabiliser AL-KO’s AKS 2000

In the late 1980’s AL-KO and Winterhoff triggered a paradigm shift in stabiliser technology by designing friction stabilisers with the friction built into the hitch head. The idea was to grab the towball very tightly and damp out oscillations that way. It was great idea, not only for saving weight, but for the sheer convenience it offered.
In each of the two designs (AL-KO and Winterhoff) the hitch head coupling contains friction pads that grip the towball very tightly when the operating lever is deployed. Needless to say, the towball has to be perfectly dry for this to work – a significant departure from the previous situation where the towball was normally greased.

Below: AL-KO Exact Drive Bügel Stossdämpfer 1968-1985; Blade type stabiliser

The very first such stabiliser, AL-KO’s AKS 2000, was produced as long ago as 1989, with the smaller AKS 1300 appearing in 1995. The numbers denote the MTPLM of the trailers to which they are suited. A friction pad on each side of the hitch gripped the towball and dampened out any tendency for the trailer to yaw (sway from side to side) or roll, although pitch damping was minimal.
At the same time Winterhoff was working on its own hitch head stabiliser, the WS 2000/3000. This also uses just two friction pads but, being positioned front and aft within the hitch head, they also damp pitch as well as roll and yaw. A nice feature of the Winterhoff stabiliser is that you can’t deploy the operating handle until the towball is safely inside the hitch head. In this way the chances of mis-hitching are greatly reduced. The set-back hinging arrangement also gives more room for those with tailgate-mounted spare wheels.

Below: Winterhoff stabiliser; Winterhoff friction pads

By 2002, end user demand was such that OEMs had started to fit hitch head stabilisers as original equipment and, in 2003 AL-KO launched the AKS 2004 stabiliser which had no less than four friction pads. These enabled it to damp pitching movements as well as rolling and yaw.
According to AL-KO at the time ‘On the AKS 2004 Comfort, four long-term friction linings enclose the towball on the right, left, rear and front with around 1.5 tons of force. There is no metallic contact whatsoever. The friction linings can be easily replaced if they are too worn.’
In 2008 AL-KO claimed their AKS 3004 hitch ‘increases the critical driving speed by around 20% (ADAC test winner).’
In 2014 BPW launched its own brand stabiliser, the iSC (Type ZKAS), based closely on Winterhoff’s device. Sadly, both these devices have recently been discontinued. (The Winterhoff Group was acquired by AL-KO’s parent DexKo in 2016, and merged with the AL-KO business unit.)

Below: BPW iSC stabiliser; Knott KS30 stabiliser

Those looking for a similar device to the Winterhoff can opt for the KS30 from brake and trailer technology specialist Knott. Launched around 2020, it too boasts single lever activation, and a set-back hinging arrangement.
The major virtues of hitch head stabilisers are their simplicity in operation with instant hitching up and no need for additional, often heavy components. A dry towball is a bonus too as there’s no grease to get on your clothes. Correctly set, the friction pads have a long service life; in the case of the AL-KO models, a lifespan of 50,000 km is claimed.
Such has been the impact of the hitch head stabiliser that today virtually all new European caravans are fitted with one as original equipment.

Active stabilising devices

LEAS universal stabiliser

Active stabilising devices use sensors to detect when a trailer is becoming unstable and then apply its brakes to pull the trailer back into line. In this way any tendency for oscillations to set in is quickly damped out. Active stabilisers are not intended to allow you to tow at faster speeds but will give you an extra margin of safety if, for example, you have to make a quick avoidance manoeuvre. Operating power is normally taken from the towing vehicle so it doesn’t matter if the trailer has its own power or not.
Active stabilising devices are not to be confused with add-on systems that apply the trailer’s brakes every time the tow vehicle’s brakes are operated, such as the one introduced by Dutch company IVRA in the 1990’s. Instead, they only apply the brakes when instability is detected, and have no effect at other times.
The first such device is claimed by German inventor Wolfgang Lubs, as long ago as 1986. European patent DE3600708A1 from that year describes an anti-roll device ‘…which, connected to a trailer, is intended to prevent a dangerous building up of oscillations (rolling) beyond a certain movement, in which the trailer is automatically braked independently of the tractor vehicle.’
A box clamped to the trailer’s ‘A’ frame contains a spring and tensioning motor. When swaying is detected, the spring is released, gently applying the brakes. A warning buzzer alerts the driver to the actuation. Once stability is returned the motor re-tensions the spring for further use.
The invention appeared as the LEAS universal stabiliser around 1995. Until this point the only stabilising devices for trailers were friction-based ones which worked as constant friction dampers on every movement. None of these actually applied the trailer brakes. The LEAS stabiliser was the first one to do so, but only when lateral movement was detected. Further patents followed as the device was improved. It remains on sale to this day.

Below: AL-KO’s first ATC unit; BPW’s iDC stabiliser; AL-KO’s ATC 2 device

In the meantime, AL-KO and BPW set about producing their own active stabilisers. These appeared as the AL-KO ATC and BPW iDC around 2008. Although similar in principle to the LEAS device these new safety devices work in a slightly different way. As soon as any instability is detected, the ATC and iDC devices use an electric motor to gently apply the trailer brakes before snaking becomes a problem. There is no sudden release of stored energy from a spring.
Also, unlike the LEAS, which uses a sensor at the rear of the trailer, the AL-KO and BPW devices have their sensors mounted on the axle. This is logical as these companies also manufacture the axles and chassis so the exact performance of the systems can be known in advance.
In 2013 Knott introduced their ETS (Elektronische Trailer Stabilisierung) system for stability control. It was unique in that it used a hydraulic valve block with actuating cylinder to activate the trailer brakes ‘in milliseconds’. Meanwhile a two-colour LED display showed the status of the device. For the 2017 season the ETS202, as it became known, was replaced with the ETS Plus. This is bolted to the axle tube and acts on the brake rod mechanism in a similar way to AL-KO’s ATC. Indeed, it can even be fitted to an AL-KO chassis. The ETS Plus was the first electronic stabiliser to come with a remote display and smartphone app.
Following the acquisition of BPW Fahrzeugtechnik by AL-KO’s parent, DexKo Global, in 2017 the iDC device is no longer being made. In 2019 AL-KO updated its ATC device to be ‘even faster and more sensitive’. The new device is claimed to be the lightest on the market, weighing in at just 4.5 kg.

Below: 2LINK app; Knott ETS Plus stabiliser

The ATC-2 device is distinguished from the previous one by being horizontally, rather than vertically, mounted. It can also communicate with AL-KO’s 2LINK system to display its status via a smartphone app.
AL-KO says ‘The system is adapted to the respective trailer weight, recognises critical swaying movements earlier and at the point of origin, and is thus more likely to intervene in time. The car stabilisation system with trailer recognition in the towing vehicle is generally no longer needed.’
Whilst we have Wolfgang Lubs to thank for the original invention, we have to thank AL-KO, BPW and Knott for engineering these stabilisers to a high level of overall performance. Active stabilisers have now become so popular with end users that they are either standard equipment, or an option, on virtually all new caravans in Europe.