Thread: Winching Formulas

Originally Posted by Hymie

Has anybody read Australian Four Wheel Drive Monthly, particularly Pat Callinans article on how to winch and calculate how much force is needed to extricate a bogged Forby?
What a load of rubbish!!
Read Pats "formulas" for calculating required pulls then sit back and consider a few thing I'm going to teach y'all.
Pat is right on a few things, there are 4 types of resistance to take into consideration.
1. Rolling Resistance, this being the resistance to motion given the surface the vehicle is on, ie, Bitumen, Grass, Sand or Mud
2. Gradient Resistance, the effort required to overcome gravitys pull on an incline
3. Damage Factor, did you damage anything that might increase resistance tomotion, rip a tyre off a rim, bend a steering arm etc.
4. Add a safety margin.

To work out rolling resistance, take the weight of your vehicle and divide it by a. 25 for bitumen or concrete
b. 10 for gravel
c. 7 for grass
d. 5 for sand
e 2 for mud

To work out Gradient resistance,
a. calculate the slope
b. Factor 1/60th of a tonne for every degree of slope

To work out a Damage factor,
throw in 25% of the vehicles mass for every damaged wheel.

Safety Factor is 25% of the total of all of the above.

This will give you a Total Pull Required. (TPR)

Sooooooo.
Using the example in the magazine of a 3.3 tonne GU Towning a 1 Tonne Camper trailer Bogged to the chassis.

RR= 4.3/2= 2.15 Tonne
GR=0
DF=0
Total =2.15 Tonne
Safety Factor 2.15/4=.53 Tonne

TPR=2.68 Tonnes

A whole lot less than the 12.9 Tonne quoted!

Lets do another,

Blknight.aus has a fully loaded Big Red of around 3 Tonne stuck on a muddy slope of around 40 degrees. ( sorry Dave, hope you don't mind)

RR= 3/2 = 1.5
GR= 3000kg/60*30=1.5 (1500Kg)
DF=0
Total = 3.00
Safety Factor = 3.00/4=.75
TPR=3.75 Tonnes

Now you know why Tirfors are so much fun

dont mind at all, your numbers are on the money as per the Raeme book of recovery...

what was in the mag that pat had done differently?

Originally Posted by Blknight.aus

dont mind at all, your numbers are on the money as per the Raeme book of recovery...

what was in the mag that pat had done differently?

Should be, I helped proofread it...
Pat had a formula where if a vehicle is bogged to the rims add 100%
To the Hubs 200%
To the chassis 300%
Some weird 25 percent for every 15 degree of slope formula
and so on...

Good thing Pat doesn't design shear pins

An 8mm 8.8 shear pin would never be a problem.

Originally Posted by Hymie

Should be, I helped proofread it...
Pat had a formula where if a vehicle is bogged to the rims add 100%
To the Hubs 200%
To the chassis 300%
Some weird 25 percent for every 15 degree of slope formula
and so on...

Thanks Hymie.

You have just given me another reason as to why I don't bother to read the damn magazine. Qualifying the facts as you have done doesn't seem to be in their vocabulary.

And little wonder I sold the tirfor! I'm an old codger and it did seem a lot of hard work - now I know it WAS hard work!

I think its more important to take on board all of the information that you can, and then make your own judgements on its relevance or value rather than cut off an information conduit based on a human error. Monthly amongst the other mags have a lot of value despite their errors.

JOOC...

i wonder if you did the numbers he was using up like a table and did the same for the Raeme book maths if his wouldnt look the same but cover lower values with a broad spectrum. Sort of like a ready reckoner for when you just dont have the time to do the math...

I have not read the article but there is so much differing opinion on calculations and safety margins it is not funny. In emergency response rope rescue we work with a 8:1 safety margin so there is no possibilty of failure. Excessive in some people views, Yes but likliehood of equipment failure is absolutely minimal even with damaged equipment. Some people operate a lot closer to the equipment tolerances with success. Who is right or wrong. Neither really but I know what safey margin I would follow.