Introduction
For an outfit to be stable when towing it is imperative that the car and caravan are well matched and that the caravan is properly loaded. This means that the laden weight of the caravan should not exceed 85% of the weight of the tow car (100% for experienced drivers) and that any heavy items in the caravan should be carried low down and as near to the axle as possible. Of course the caravan should never be overloaded, it should tow level or slightly nose down, its tyres should always be correctly inflated and the nose weight should be about 7% of the caravan's laden weight.

Having taken care of all of that you may be forgiven for thinking that a stabiliser is unnecessary. Indeed I know a few people who never fit one. However I think the best way to regard a stabiliser is as a very important safety aid. It is not a cure for a poorly matched or loaded outfit, or for bad driving, but a good stabiliser will provide significantly improved margins of both safety and comfort. Don't be tempted to explore these new limits because if something goes wrong the higher energy levels involved could produce disastrous results. 

There is a view that twin axle caravans don't need stabilisers because of their inherently greater stability. Whilst there is some truth in this it is also true to say that twin axle caravans tend to be longer and heavier than their singe axle counterparts and that they are not immune from snaking. Indeed someone once told me that the worst snaking they had ever experienced was whilst towing a twin axle caravan. My advice would therefore always be to fit a stabiliser. 


Types and causes of instability
Towing instability can take three forms, which can occur independently or together. Vertical movements of the tow-hitch are known as pitching. These are normally not too serious (as long as no part of the outfit hits the ground) and not all stabilisers are designed to reduce them. Pitching instability is usually caused by a pothole, speed ramp, or bad dip in the road. The outfit will normally recover without the driver having to take corrective action. A variation on this is the porpoising that can occur when travelling fast over equally spaced bumps, such as those that can occur on the inside lane of heavily used motorways.

Horizontal movements of the tow-hitch are technically known as yaw but in caravan speak this translates to snaking. Violent snaking can cause complete loss of control of the outfit and so all stabilisers are designed to combat it. Snaking instability is often linked to speed. However speed is never the only factor. Something else is needed to act as a trigger. This can be a gust of wind, a bump in the road, or a sudden swerve. Beware long downhill stretches of road where you may be tempted to let your speed build up. In these conditions the load of the caravan hitch on the car's towball can reduce making snaking much more likely. 

Rotational movements of the tow-hitch (and caravan) about its axis are known as roll. In practice roll should not occur in normal driving since the presence of the wheels of the caravan on the road prevents it. However if a wheel lifts for any reason (possibly due to violent snaking) then the caravan may be about to roll over! Stabilisers are not specifically designed to reduce roll (the forces involved are too great) but some may help a little. Roll will normally only happen in extreme situations. The most common cause is severe snaking allied to heavy braking of the tow car. It can also happen in poorly loaded caravans which are then cornered much too fast.  


Main types of stabiliser
Over the years much thought has been given to the design of devices to combat towing instability and several distinct types have emerged. 


 Friction Blade Type - typical makes. Bulldog, SAS, Staymount, Scott

Bulldog 100Q and 200Q Stabilisers SAS Stabiliser Scott Stabiliser Bulldog twin stabiliser

These are the simplest and cheapest form of stabiliser. They normally comprise a single leaf spring where one end is placed in a saddle attached to the caravan's 'A' frame and the other end terminates in a pivoting friction damper attached to the car's tow bar underneath the towball. The damper consists of a couple of friction discs sandwiched between steel plates and clamped tight by a central bolt. The bolt is tightened until the force needed to turn the spring blade is correct. Typically this is 60-70lb (27-32kg) at the end of the blade. Beware that  new friction discs will bed down during the first thousand miles of driving. It is therefore important to check the friction setting at this time and readjust as necessary.

The friction damper controls the lateral movement of the hitch arrangement thereby reducing any tendency to snake. Pitching is reduced by the bridge formed by the curved leaf spring, which is designed to stay virtually parallel to the road at all times. This spring action also tends to reduce sinking at the hitch by transferring some of the weight to the towcar's front axle and some to the caravan's axle. The result is an effective reduction in the caravan's noseweight. The actual amount will vary from outfit to outfit but could be in the region of 10 kg so do bear this in mind. It could even save you having to fit rear spring assisters to your car.

Modern blade stabilisers feature a quick release cam to release pressure from the central bolt for easy removal and fitting of the blade to the saddle. 

For heavy-duty operations, such as large caravans or horseboxes, twin blade stabilisers are available. These have blade on each side the tow-hitch assembly.

One minor drawback to friction blade stabilisers is their effect, usually slight, on the overrun brake because the extra linkage between car and caravan tends to hold the two vehicles apart through friction. This can produce a small delay in the onset of overrun braking.

When fitting a blade type stabiliser remember that it is not permissible to drill modern caravan chassis to attach the saddle. Instead make sure you have a saddle designed to be clamped on. All new blade stabilisers are so supplied but you may come across this if you buy second-hand. Clamp-on saddles are available as spares so there's no excuse!

Self Centring Blade Type - Dynametrics Straightliner

Kit for flange mounting Operation when towing Unit disengages for reversing

Finished installation

One recent development of the blade stabiliser replaces the friction pads with a gas-filled damper arrangement having a strong self-centring action. This is claimed to give significantly better overall performance, especially at higher swing rates where friction dampers can lose grip. The device is known as the Straightliner stabiliser and is manufactured by Trapezium Developments who are well known for their innovative trapezium stabiliser (see below). It is being marketed in the UK by Dynametric Ltd, Tadworth, Surrey. 

The stabiliser works by using a 100 kg force gas-filled strut to push a roller bearing into the neck of a 'V' plate (see above). When the caravan is affected by external forces, the 'V' plate keeps the bearing in place and resists any sway. If the caravan overcomes this force (set at 44kgs or 96 lbs) the bearing will climb up the 'V' plate.  The Straightliner will then, using stored energy in the gas strut, push the bearing back into the centre of the notch thereby moving  the outfit back into alignment to maintain towing stability.  When reversing the Straightliner uses a secondary cam system to automatically disengage the gas strut. For fitting purposes the gas strut can also be disengaged using the supplied detachable release bar.

Other advantages are that there are no friction pads to wear or need adjustment and a locking pin that prevents the blade from jumping out of the saddle during use.

Ball acting type - typical makes Alko AKS, Westfalia, Winterhoff

Alko AKS 1300 Alko AKS 2004 Westfalia SSK Winterhoff

These work by applying pressure via friction pads to the towball itself. For this reason the towball must be kept clean and free of grease at all times.

Not all ball-acting stabilisers provide damping of vertical movements. Until very recently all Alko stabilisers had just two friction pads acting on the sides of the towball. Whilst effective in damping out snaking they have very little effect on pitching. However the new AKS 2004 stabiliser contains front and rear friction pads to remedy this.

The increased size of the tow-hitch normally means that a long neck towball needs to be fitted. These are normally supplied with the stabiliser. Beware that the gripping action of these type of stabilisers makes them unsuitable for use with tow-balls having a single bolt fixing since the bolt could become loose.

The pressure applied by the friction pads is not adjustable but a wear indicators give a good guide as to when the pads need changing


 Trapezium stabilisers - typical makes, Alko Geomatic, Eurotech Trapezium, Trapezium Rollsafe

This is the Trapezium Rollsafe quick detach model

Trapezium stabilisers work by applying some clever geometry to move the actual point of articulation forwards, nearer to the car's rear axle. This arrangement creates a virtual pivot point close to the cars axle just as if the towball itself had been moved forwards. The effect is  to provide much improved lateral stability and in principle is similar to that of an articulated lorry. From the rear view mirror the caravan may seem to waggle a little more from side to side but the effects of this are little felt within the towcar.

Trapezium type stabilisers normally give excellent results as many satisfied owners will testify. However as they rely on geometry their performance can be affected if the geometry of the caravan is not correct. This is not a common occurrence so don't let it put you off.  Another minor disadvantage is that turning circles can be extended slightly because the effective length of the caravan has been increased. Most owners soon learn to compensate for this and rarely regard it as a problem.

Because trapezium stabilisers protrude some inches beyond the rear of the tow-bracket they introduce much more of a bending moment than conventional bolt-on towballs. Under EC Type Approval regulations this effectively means that the manufacturer would almost certainly have to seek type approval for use of the device with every type of towbar to which it was to be fitted. Clearly this is not a practical proposition and so has led to a situation where no new trapezium stabilisers are being marketed in the UK. You may however see used or reconditioned ones for sale. You can legally use them on towcars up to 'R' reg. - but not 'S' plate later where the type approval regulations apply. 

Trapezium type stabilisers are unsuitable for use with swan-neck tow-hitches.

 

Conventional Stabiliser Pros and Cons

snake Conventional Stabiliser Pros and Cons
Friction Blade Type Self-Centreing Blade Type Ball Type Trapezium Type
Pros Less expensive than others Excellent performance Ease of use Excellent performance in most circumstances
Most reduce pitching and snaking Nothing to adjust Nothing to store Most types are easy to use
Easy to service - Very difficult to forget to apply Very little to wear or go wrong
Friction pads have a long service life - Easy to install Impossible to forget to apply
Reduce nose weight - Ideal for most swan-neck tow hitches Easy to install
Cons Assembly needs to be stored when not in use Expensive Expensive Infringes type approval of modern tow bars
Initial installation more involved Heavy and bulky assembly to handle and store Needs special hitch lock Second hand only
'A' frame fairing may need to be modified to accept saddle Initial installation more involved Can be stolen Unsuitable for swan neck tow hitches
Special adapter needed for swan-neck brackets. May not fit all 'A' frame fairing may need to be modified to accept saddle May be no pitch damping Detachable types need to be stowed
Blade may come out of saddle on uneven ground, especially if alignment is not correct Swan neck model available but may not fit all Need to clean towball before each use Bulky and unsightly if not removed
Friction setting needs to be checked after first 1000 miles and periodically thereafter - Unsuitable for towballs with a single bolt fixing No pitch damping
Small delay to overrun braking - - Small increase in turning circle


Other stability aids

Active caravan braking systems - IVRA, AL-KO, BPW, LEAS

These systems work by applying the caravan's brakes to stop snaking. All carry a weight penalty of around 6kg.They are not designed to replace conventional stabilsers but to work with them thereby improving safety margins.

First on the market in 2003 was the Brake Control system from Dutch company IVRA. This applied the caravan brakes when the towcar's brake pedal was depressed. Early models took their power from the car but needed a really good supply to work properly. Later models hooked into the caravan battery and used the fact that it is automatically charged when towing.In addition to the caravan being braked at the same time as the car, motion sensors detected when the caravan was began to snake then applied the caravan brakes automatically without input from the driver. The main problem with the Reich system was the excessive wear induced on the caravan's brakes. The complete system was not sutable for all caravans especially where undeslung water tanks were fitted ahead of the axle. Aas far as I know, it is no longer marketed.

In 2007 AL-KO introduced their ATC trailer control. It too uses motion sensors to detect the onset of snaking, The caravan's brakes are then gently applied to stop it. The system connects to the car via the normal 12S or 13 pin connection cables.  When it is ‘plugged in’ the AL-KO ATC automatically starts a ‘Pre-Flight’ test that engages the plunger, applying the caravan brakes.  Once this test is complete, the LED warning light on the front fairing changes from red to green to indicate the system is monitoring the caravan and in a ‘ready for travel’ state. It is offered as a factory fit option on some new caravans and can be retrofitted to most caravans with an AL-KO chassis. It costs around £375 fitted.This can be done at AL-KO's UK service centre in Southam, Warks. or by an approved dealer.

Shortly after AL-KO introduced their ATC system BPW introduced a similar sytem called iDC, short for Intelligent Drive Control. It is designed for use with BPW chassis (Elddis, Explore, Compass, Buccaneer) and is retrofittable to all BPW chassis with ZAF-2 overrun hitch. The main differnce in function between the ATC and iDC sytems is that in the event of a power failure (blown fuse, bad connection etc) the iDC sytem will return to its starting position therby releasing the caravan brakes for normal functioning. Like the ATC the iDC is activated by means of a function test, which is starts as soon as the electrical connection is established between the towing vehicle and the trailer. A status display on the drawbar indicates that the system is fully functional. The fitted cost is around £500.

The LEAS is another German system. It has been around for some years but is now being marketed in the UK by Conrad Anderson.It works using a large spring charged by a cam on an electric motor, when a snake is detected the motor turns the cam and the spring is released. This activates 20% of the vans braking capability. Once the lateral movement of the snake is no longer detected the motor turns the cam and the spring is recharged ready for the next release. When the system activates it sounds a buzzer in the tow car to alert the driver. Power for the system is taken directly from the towing vehicle engine battery and fed through an additional socket at the rear of the tow car. This is because the required current is too great to be taken through a normal tow hitch connector. Conrad Anderson claims it is suitable for all single and double axle caravans. They do not quote a unit price but say they will will fit it for £100.


Stabiliser service issues 
If looked after well stabilisers are reliable devices but wear and tear can eventually take its toll. All stabilisers need periodic adjustment to prevent wear. One service I can recommend is that offered by the Stabiliser Clinic operated by John Ward.  He attends many major rallies (often including our AGM) and also operates a mail order service.

If you must have a go yourself then the two most common problems you are likely to have to fix with a friction-based stabiliser are loss of effectiveness due to worn or badly adjusted friction pads and noise as the pads move around in use. 

With friction blade types a simple re tightening of the clamp bolt should restore the blade setting and you can check it using a set of bathroom scales against the end of the blade. 

Ball acting stabilisers rely on spring pressure and cannot be adjusted. Their effectiveness is checked using a bolt mounting towball on the end of a torque wrench. I came across one example where the innards had seized following winter use so beware of this if all does not seem well. 

Noisy operation is a fairly common problem, especially with blade stabilisers. This can be due to the disc surfaces becoming 'glazed' whereby a light sanding with fine grade (600+) abrasive paper can sometimes do the trick. Failing this you will need to fit new friction discs.

Links

Changing the pads on an Alko AKS stabilliser


© Terry Owen 1999 -