Handbook of Winching Techniques

THE THEORY OF WINCHING
To get the best from your Superwinch and auxilliary equipment requiers some understanding of the mechanics involved
in winching. For winching purposes the resistance to motion of a vehicle is dependant on 4 main factors:
(i) The inherent resistance to movement of the vehicle.
(ii) The total weight of the vehicle.
(ii) The nature of the surface to be transited.
(iv) The gradient up which the vehicle is required to be moved.
(i) The inherent resistance of a vehicle depends on the state of the tyres, friction in the drive-chain (which will
causedrag), the weight of the vehicle, and whether the vehicle has sustained any damage to its running gear. For our
purposes, we will assume that the vehicle is in good working order and has all tyres inflated to teh recommended
pressures - a flat tyre will cause considerable drag, and it may be advisable to change a tyre that is deflated before
commencing recovery operations.
(ii) The weight of the vehicle, includes all equipment, luggage, fuel, passengers and stores etc. aboard the vehicle.
(iii) The nature of the surface to be traversed is the largest variable in the winching operation. A vehicle in good
running order on metalled surface will only require a force of about 4% of its total weight to induce motion, whereas a
vehicle to be recovered from a bog will require a pull equivalent to about 50% of the total weight of the vehicle. The
table below shows that different surfaces require proportionate efforts to produce vehicle movement.
Type of Surface Effort required to move
Vehicle as a fraction of total weight
Hard metalled road 1/25 total weight
Grass 1/7 total weight
Sand (hard wet) 1/6 total weight
Gravel 1/5 total weight
Sand (soft wet) 1/5 total weight
Sand (soft/dry/loose) 1/4 total weight
Shallow mud 1/3 total weight
Bog 1/2 total weight
Marsh 1/2 total weight
Clay (clinging) 1/2 total weight
A simple calculation will slow that approximate rolling resistance of an undamaged vehicle on a flat surface can be
predicted e.g. the pull required to move a vehicle weighing about 4500 lbs along a flat sandy beach of hard wet sand.
Weight of Vehicle (lbs) = 4500 lbs = 750 lbs
co-efficient of resistance of hard wet sand 6
However, as all surfaces are not flat, the calculation must therefore include the gradient resistance co-efficient.
(iv) Gradient Resistance. The gradient up which a vehicle is to be moved may only cover a short distance, over the
total distance of pull, e.g. a ditch or rock, or it may cover a long climb up a hill. Even for a relatively short upward pull,
gradient resistance must be taken into account. For practical winching purposes, gradient resistance can be taken as a
1/60th of he weight of the vehicle for each degree of the slope, up to 45 degree incline.
Gradient x Weight of Vehicle
60
e.g. for a 15 degree slop, gradient resistance will be 15/60 of the weight of the vehicle, which is 1/4 the vehicle weight.
For an incline over 45 degrees the gradient resistance will be equal to the total weight of the vehicle. That the slope to
be negotiated to all intents and purposes is only 1ft high will make no difference to the calculations, and should be
borne in mind when pulling vehicle over ridges. If we combine the weight of the vehicle, the type of surface to be
transited and the gradient to be overcome we get the calculation.
THE THEORY OF WINCHING
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