THE POWER OF FORCE
SO, HOW much force and load is involved in a vehicle recovery? Well, that’s a very difficult question to answer due to the nature of recovery situations. Each recovery situation is unique and will contain a number of variables: the weight of both the bogged/stuck vehicle and the recovery vehicle; the substance the vehicle is stuck in (mud will add much more suction force to the recovery compared to sand); whether either vehicle is on an incline/ decline; and whether the tyres on the bogged vehicle are inflated and/or are at any particular angle. Adam Craze added: “As Newton states, the force is equal to the weight, multiplied by acceleration”, with all of these affected by the above.
Having said that, controlled tests have been conducted that measured force in a particular recovery situation. One such test was undertaken by the late, highly regarded 4x4 journalist Ian Glover, in conjunction with Mitsubishi Motors Australia Limited (MMAL), to test/check for any potential distortion in the then-current Pajero’s monocoque chassis during a recovery. This test involved using a stock Pajero stuck in relatively soft, shallow sand (just under the sill). The 4x4 was subjected to a straight-line pull snatch recovery on level ground and then a recovery on an incline.
The MMAL engineers used a load meter to record the load in kilonewtons (kn) through the snatch strap via a ‘strain gauge’ (attached to a plate metal block on the meter that stretched under load) to a laptop on-board the vehicle.
The load recorded for a straight-on (no angle) recovery on a flat surface was 2.06-tonne (20.6Kn) – an ‘ideal scenario’ in terms of a snatch recovery.
The second recovery was undertaken with the Pajero bogged on a short incline (less sand depth than the level location, but the wheels again pointed straight-on), and the recovery vehicle snatched the bogged Paj forward and over the incline. The figure recorded in this scenario was 3.16 tonne (31.6Kn), showing how factors such as gravity/ incline influence load on the recovery points, shackles and the snatch strap.
This also explains the high load rating figures on aftermarket recovery points, snatch straps and shackles – and why you should fit and use them.