DRIVING/TOWING - RECOVERY TECHNIQUES
We’ve been concerned for some time at the trend towards stronger and stronger snatch straps, because we felt that elasticity, not ultimate breaking strain, was the key to successful snatch strap performance. We’ve also long suspected that the loads involved in snatch strap recovery aren’t as great as most people seem to think. But how to test these hypotheses?
The solution came in the form of a load cell test rig that Mitsubishi Motors Australia developed as part of an engineering program to prove that the monocoque Pajero could easily withstand snatch strap recovery without suffering body distortion. Mitsubishi’s engineers proved that the Pajero easily handled snatch strap recoveries – a video of the tests is available – and also recorded the loads involved.
This was precisely the test rig we needed to measure the loads experienced during a snatch strap operation. MMA kindly provided two Pajeros, the load cell and the necessary instrumentation so that we could perform our testing.
MMA’s engineering team was on hand to set up and monitor the test equipment.
We selected a sandy property in South Australia and chose two sites for our testing: one a flat, sandy stretch and the other a steep uphill section. The sand had about the same degree of softness as typical beach sand and was finer than the red desert stuff.
We had three straps available: an old one that had done many recovery operations; a brand new one and a US-made, red-coloured Staun SuperStrap (not the lighter-duty black one that’s available).
The designated recovery vehicle’s load cell was integrated into a linkage that pinned to a conventional towbar square socket and was fitted with a massive shackle. A short length of shock cord stopped the heavy assembly adopting a downward tilt. The cell was connected to instrumentation and a readout printer strapped to the passenger seat.
We did three test snatches to make sure the cell was correctly calibrated and then we were ready to go.
The test method
We performed three recovery operations with each strap on the flat section of sand and on the steep pull section. In each case the ‘victim’ vehicle was bogged first at the rear end, by selecting rear wheel drive, and then at the front by deliberately using too much right foot in four wheel drive. Fortunately, the victim vehicle was an NM Pajero, without traction control, because we know from experience that it’s very difficult to bog a traction-controlledNP Pajero.
The first snatch recovery was done in ‘copybook’ style, with a one metre ‘S’ of slack in the strap. The next recovery was done with double that amount of slack. In both these recovery operations the victim vehicle assisted recovery by driving in first gear, low range in the auto box.
The third recovery simulated a stranded vehicle, so the victim remained in neutral and was hauled out unassisted, using one-metre-S strap slack.
On the steep pull section we used a one metre ‘S’ of slack in each strap, but needed additional slack in the old strap to achieve the desired result. All steep-pull recoveries were assisted by having the victim vehicle use first gear, low range.
Because of the unconventional nature of the Staun strap we performed some additional tests with it: a flat section recovery with the strap doubled, halving its overall length; and a steep section recovery with the strap shortened by four metres, making it the same length as the conventional straps.
What we discovered
The heaviest load we recorded in any of the straps was 2.6 tonnes and that was during calibration, when we were making sure the cell was functioning accurately towards the top of its scale. During the normal recovery operations the loads were typically lower than that. Interestingly, when Mitsubishi performed its body integrity testing in 2004 the heaviest load recorded on the cell was a little over three tonnes – achieved with extreme provocation, apparently.
Both drivers of our test vehicles agreed that the smoothest recoveries were those performed with the regulation one-metre-S slack in each strap. These recoveries also produced the lowest loads, with readings between 1.3 tonnes and 1.6 tonnes.
Doubling the slack increased the loads to the range of 1.7 tonnes to two tonnes.
Unassisted recoveries, using regulation one-metre-S slack produced loads in the 1.6-2.1 tonnes range.
We added a test in the case of unassisted recoveries, halving the length of the Staun by connecting its free end to two recovery points on the bogged vehicle and running the mid section through the shackle on the recovery vehicle. This arrangement theoretically halved its elasticity and doubled its load capacity. Thus doubled, the Staun strap’s behaviour was closer to that of a conventional strap, with a higher peak load and a steeper rate of load rise and dissipation.
In the case of the steep pull tests, the old strap that had obviously lost much of its original elasticity failed to extricate the bogged vehicle when we used regulation slack in the strap, so we used 50 percent more slack to generate more load – around 2.3 tonnes
Snatch strap theory
Snatch straps are intended to be used on vehicle boggings that occur in a compressible, homogeneous substance, typically mud or sand. They can be used for some hard-ground recovery jobs, but only by very experienced operators.
An elastic recovery strap is the preferred de-bogging method in mud or sand, because it’s desirable to get the stranded vehicle back up on the surface, rather than to drag it through more soft material using a winch or a tow rope. If the stranded vehicle can be ‘bounced up’ out of its hole the surface of sand or mud acts like a membrane, supporting the vehicle weight, provided the tyres are deflated appropriately.
In operation, a snatch strap extends as the recovering vehicle moves forward, and as the strap stretches
it absorbs energy from the towing vehicle. At some point the energy absorbed is sufficient (hopefully) to pull the bogged vehicle out of its wheel
The more elastic the strap the more gradually it builds up and dissipates energy.
A flat tow in the same circumstances doesn’t work, because the towing vehicle is likely to become bogged itself long before it develops sufficient momentum to extract the victim. Using the bogged vehicle’s winch and a solid anchor can do the job, but winching tends to drag the vehicle through the sand or mud and it’s possible to damage under body components during the operation.
We discovered that elasticity is more significant than ultimate breaking strain in assessing snatch straps and that highly elastic straps impose lower loads on the vehicles involved than do stiffer ones. The next question we need to answer is the life of a snatch strap: how many recoveries can a strap make before it’s ‘dead’? We’re working on it.