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To cut a long story short I work in recovery and part of the job requires you to have to undergo official training called I.V.R.

So clearing out my work jacket i came across the winching guide and though some might find it helpful

So here go's

Sorry if some parts not needed

Rolling resistance

Smooth road w ÷ 25
Grass max: w ÷ 7 min: w ÷ 4
Gravel max: w ÷ 7 min: w ÷ 5
Beach shingle w ÷ 3
Sand max: w ÷ 6 min: w ÷ 2
Mud max: w ÷ 3 min: w ÷ 2
Soft clay w ÷ 2

BOG to axle w × 1
BOG to wheel top w × 2
BOG to rad. Top w ×3

W = weight

Winch power required =
rolling resistance + damage resistance + gradient resistance

Damage resistance = weight × number of damaged wheels ÷ total number of wheels

Gradient resistance = weight ÷ 60 × slope in degrees

This is the basics of the I.V.R.

Hope you find it useful

2,384 Posts
So a quick question if you have some expertise in this area...

When you winch your vehicle, it's pretty common wisdom that inserting a snatch block to create a 2:1 line pull halves the load on the winch.

In fact I was just reading my own course notes from the Army course I did (oh such a long time ago), I read that in fact they say the advantage is only 1.5:1. If you want to halve your winch load you need a three line pull.

Where is the extra load going? Friction losses? Safety margin? According to this my 1600kg Tirfor winch will only safely pull 2400kg with a two line pull (well, more, because 1600kg is its vertical hoist capacity, but you get the idea...)
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