Did some research a while back on forces exerted on Anchors and required force needed to extract a mired vehicle. I want to say pulling a vehicle on level smooth ground requires about 10% of the GVW (so an 8K lb vehicle would require 800 lbs of pull to move it.). If mired, it requires more:
Mire resistance:
- Wheel depth = 1x load
- Fender depth 2x load
- Cab depth 3X load
- Pulling opposite direction of travel can reduce the load by 10% (traveling in ruts)
- Moving wheels during extraction breaks suction and can reduce load but not in a dependable manner (40% for tracked vehicles)
- Example: 10 Ton tracked load mired to fenders, = 20T adjusted load. By moving wheels and pulling in opposite direction, this is reduced to 10 Tons
I also found the attached Army field manual for vehicle recovery. I found this very very helpful and easy to understand.
Be aware if you are using snatch blocks - you can end up putting double the load on an anchor in some situations. Combine a snatch block with a deeply mired load and you can accidently destroy your anchor which if it is your bumper could be costly and dangerous!
View attachment 828430
Using the diagram above I ran some numbers (I think I am correct but there are folks smarter than I am on this forum who I am sure will weigh in if I am not)
Variables:
Load 4 Tons (8,000 lbs)
Tackle resistance (10% of load per snatch block sheave)
Max Winch pull (6 tons, 12,000 lbs)
Pull per line = total load with resistance / # of lines
Load per anchor = load per line * # of lines to anchor
Situation 1: Single Line:
Mechanical Advantage = 1:1
Load on Winch: 8,000 lbs
Load on Anchor: 8,000 lbs
Situation 2: Double Line:
Mechanical Advantage = 2:1
# of snatch blocks: 1
Total load: 8,000 +800 {10% tackle resistance} = 8,800 lbs
Load per line: 4,400 lbs
Load on the Anchor: 8,800 lbs
Load on the winch: 4,400
Load on the truck anchor shackle: 4,400
Total load on the truck itself across all anchor points: 8,800
Situation 3 Triple Line:
# of snatch blocks: 2
Total load 8,000+1,600 {20% tackle resistance} = 9600
Load per line: 9,600/3=3,200
Load on Winch: 3,200 lbs
Load on Anchor 1: 6,400 lbs
Load on Anchor 2: 3,200 lbs
Load on Anchor 3: 6,400 lbs
Total load on vehicle (3,200+6,400)=9,600
Consequently, if you move anchors 1 and 2 to the same tree, the tree will see 9,600 lbs of pull (3,200 of the pull being felt by Anchor 2 and 6,400 being felt by anchor 1)
____________________________________________
I was actually quite surprised to find that the layers of rope (wire or synthetic) on the winch when pulling reduces the power of the winch.
Example with a 12K lb winch
Layer 1: 12,000 (100%)
Layer 2: 9,517 (79%)
Layer 3: 7,885 (66%)
Layer 4: 6,732 (56%)