Kinetic Energy Recovery Systems

Kinetic energy recovery systems are big news in the world of off roading, they are very efficient at recovering stuck vehicles by using the catapult principle of an elastic rope which stores energy. Such systems may be good, but are fraught with danger, and can kill people who do not know how to use them, or use them properly.

Before using such systems we should be aware of the dangers they pose, they are real, and can easily kill people who use them incorrectly; so read this section and understand it before attempting to use this recovery system.

Modern day kinetic energy recovery systems were developed by a British company called Marlow Ropes in conjunction with the British armed forces, originally they developed these as an easily used, lightweight system to recover stranded armoured vehicles. We can trace the origins back much earlier than this to several industries that used such systems in many other applications, but it was the developed principles which prompted the use of this system for vehicles. Earlier systems predominantly used steel laminated ropes of extraordinary lengths to attain the kinetic energy required to extract heavy objects. Records from WWII also show parachute shock cord woven into ropes of varying sizes for recovering aircraft parts from crashed aircraft on uneven or hostile terrain, particularly when the tiny cranes of the day could not get near enough to recover parts. We can see the kinetic system is not new.

Kinetic energy recovery ropes (KERR) work by using the stored energy of a stretched rope, a rope is connected between a stuck vehicle and one on firmer terrain, it is connected with some slack; the recovering vehicle drives off. As the rope stretches it stops the recovering vehicle which applies its footbrake and holds it on, the rope contracts and pulls the stranded vehicle from its terrain, it should be assisted with the stranded vehicle having its wheels turning. Basically it is a large catapult and unlike the average catapult, it stores immense forces which can do considerable damage, it is these immense forces which range into many tonnes, which cause the damage.

KERR’s should only be used with the appropriate vehicular attachments, these are not a tow bar or bull bar as these cannot withstand the forces, and are not rated to cope with such high forces, and certainly not the shock loads which will be imposed. KERR’s should not be used by soft roaders, only true off roaders with substantial chassis which should be in good condition, and using the correct chassis mounted rings or attachments designed for withstanding such high loadings. These can be purchased, or manufactured by suitably skilled fabricators with the appropriate equipment, and expertise in welding to high standards.

Many people connect the ropes to the vehicle, or rope to rope to extend them, using rated shackles, these should never be used, even if they are rated higher than the kinetic rope, very few are; they are not designed to cope with the shock loads imposed. Imagine a broken shackle hurtling towards you with a force of several tonnes, would your windscreen stop it? No, would your head stop it? No; these are the very real dangers which can easily kill you or a bystander. Shackles are rated for lifting only, they are not rated for shock loadings, the most common type is the 4.75 tonne shackle, with its factor of safety (legal requirement for lifting equipment) of 1.7 : 1 will work at a maximum load of 8 tonnes. When we factor in the shock loadings of a kinetic rope, the most basic kinetic ropes will potentially exert a shock load in excess of 30 or 40 tonnes for a very short period, it is this shock loading which causes the problems with shackles and tow bars.


How do I connect the kinetic rope then? Use a bridle from the vehicles mounting points to the kinetic rope, install a second bridle which is slack as a security measure to stop it from flying if it does snap. Even if the second slack bridle snaps it will absorb a considerable amount if the kinetic energy from the kinetic rope, this will significantly reduce the load on the rope, and anything attached to it, it may not now kill you but it can cause serious injuries just the same. Kinetic recovery ropes stretch by around 30% so the safety bridle should be longer than the connecting bridle to allow more contraction and shock absorption. Bridles are used because they are rated for shock loadings, and substantially higher shock loadings than the required kinetic rope can exert. Due to the complexity of calculating the shock loadings I would recommend seeking expert advice before buying a bridle as it needs to be a suitable type; the supplier of the kinetic rope should have this information on a data selection sheet. They should then select the correct bridle for your kinetic rope as they should carry a range to suit their kinetic ropes.

Once it is connected we need to refer back to the manufacturers instructions, kinetic ropes are generally rated by vehicle weight, maximum pulling speed, and minimum breaking loads of the kinetic rope. Vehicle weights from the largest manufacturer of kinetic ropes range from 0-3tonnes vehicle weight, 3-5.5 tonnes vehicle weight, and 5.5-7 tonnes vehicle weight; this should ensure selection of the appropriate type of rope.
Minimum breaking weights are something of a misnomer, this figure is for the purposes of testing and quality, and they will exceed this minimum breaking load by many times. For the purposes of testing they have to have a 10% deterioration of the rope itself, this is to allow for any damage to the rope which may occur during recovery work as well as the recovery conditions, and they have to hold this weight for 30 seconds without problems or failure. In reality the rope when used will not be holding weight for anything more than a fraction of a second before it contracts and disperses this load, but will potentially peak with a force of 30-40 tonnes or more for a fraction of a second. Always remember these testing conditions are for the worst case scenario, and to prevent legal actions against the manufacturer by giving a defence against incorrect use of kinetic ropes, something tried several times around the world, and all cases have failed.
Vehicle speeds are given in the manufacturer’s data sheet, this usually gives a maximum vehicle speed for the pulling vehicle, and this must not be exceeded and can range from 7 to 25 MPH depending upon the type of kinetic rope, application, and manufacturer.

Connection to the vehicle itself is crucial, as is the angle of pull, the kinetic rope should always be connected to one rail of the chassis, and it should never be connected across both rails as the potential forces will pull these rails together. Even with an additional bracing member installed, the shock loading will crush it and create stress fractures in the chassis, so never connect across both chassis rails. It is acceptable to fit the safety bridle to the opposite chassis rail as long as it is longer than the connecting bridle, with slack; as this is there purely for safety reasons to disperse any energy from a broken or defective component.
Always pull with the kinetic rope longtitudally inline with the chassis rail, never pull with the rope at an angle to the chassis rail, its strength is along its length.

Many manufacturers make kinetic recovery ropes, these mainly purchase their rope from one of the two large suppliers of such ropes, these being Marlow and Du Pont; they then splice their own ropes from this supplied rope. This means there may be many trade names from many manufacturers of such kinetic ropes, but they will almost certainly use the rope supplied in bulk from the above two rope manufacturers. One newer variant of the kinetic recovery rope is entering the marketplace, this is the kinetic tow rope, this is essentially a kinetic rope which can supposedly be used as a tow rope, and many recommend not using them for towing applications, only recoveries. This is because they can exert considerable forces in certain applications such as a hard accelerating vehicle on an uphill slope, this stretches the rope and as the towed vehicle crests a hill it has to disperse the stretch and kinetic energy. This literally fires the vehicle into the towing vehicle. Much the same happens when travelling downhill, if the towed vehicle suddenly breaks it builds up enough kinetic energy to stop the towing vehicle and fire it rearwards back up the hill.
Safety of towing is clearly an issue when a combined kinetic/tow rope is used, the best advice from the experts id use a kinetic rope for kinetic recoveries, and use an ordinary non-kinetic recovery or tow rope for towing.

Kinetic straps are a variation of the kinetic rope, in order to enter the market and not infringe patents or trademarks many companies opted for the kinetic recovery strap, these work in the same way as kinetic ropes, but have a few differences. Kinetic straps look like ratchet straps but do not have the same stretching capacity as ropes, generally they only stretch around 15-20% and build the same kinetic energy over a shorter “stretch”, and the fundamental rules apply. Never use these with shackles, rated or not; never attach to tow bars, bull bars, suspension points, or anything other than a properly designed and fitted recovery point. Always connect with a bridle which is correctly rated, and follow the manufacturer’s instructions for use.

Some current, and a number of earlier variants of the kinetic recovery strap had a finite number of pulls they were allowed perform, once these had been used the strap had to be rested, this meant not using it for a prescribed time period. This allowed the strap to normalise, or return to its normal working length by contracting, if you have one of these earlier types of kinetic strap then adhere to the manufacturer’s instructions. Many have a marker system for overloading, this is generally a coloured stranding woven into the fibres, as the strap works it stretches, this shows the indicator; once it shows it cannot be used and has to be rested, this should hide the warning strand. If the warning strand is visible after the prescribed resting period it is scrap and should be cut up and thrown away.

This article does not apply to ordinary tow ropes or straps, only kinetic straps and ropes; these should not be confused with kinetic recovery systems, and can be safely used with the normal rated shackles for off road recovery and towing.

On a safety note:

Many manufacturers of accessories and systems are placing disclaimers in their literature and warranty documentation, this is because people are attaching kinetic recovery systems to them when they are not strong enough. These are generally tow bar manufacturers, bull bar manufacturers, and accessory manufacturers manufacturing ordinary recovery points for a variety of vehicles.

Where possible I would recommend attending an approved course for kinetic recoveries, these are a mine of information and safe working practices, much like this article they dispel the myths and common misconceptions that are currently around.