[ex670c]

Traction can be defined as the

Co-efficient of friction (between a specific set of tyres and specific terrain) X Contact Patch/Foot print X Weight of Vehicle.

A.
If one or more components of traction (friction, footprint, weight)
decreases (less friction [mud etc], smaller footprint, less weight)
traction will decrease!
B.
If one or more components of traction (friction, footprint, weight)
increase in value (more friction [concrete etc], bigger footprint, more weight)
traction will increase!


what is TRACTION ? and why do we need traction?

[ex670c]

1) Why does a vehicle get stuck
i) The ground doesnot support the weight of the vehicle i.e mud, very soft sand or the contact patch doesnot float the vehicle.

ii) The vehicle gets caught is rocks/ledges/boulders

2) What to do.
i) If you cannot move forward try reversing
ii) If you cannot reverse tow the vehicle backwards.

If the vehicle get VERY/Badly stuck
i.e i)Side-wall Deep
ii) Differential Deep
iii) Chassis Deep

Then it will require a special tow i.e Recovery Technique.

[DirtyDan]

Arka, you use pulleys at times. When do you use them and what is their effect?

[ex670c]

1) Vehicle to Vehicle (Kinetic Energy or Tow)
2) Stand-Off i.e using a winch/crane etc

1) Vehicle To Vehicle
i) On hard ground a one-on-one tow will work.
ii) On Soft-Ground (Sand/Much) 2-1 Tow will work better.
This is because the additional weight of the towed vehicle is distributed on 2 vehicles, chances of the recovery vehicles digging in and much lesser.
Also you get better efficiency to overcome the RESISTANCE.

2) Stand-Off i.e using a winch
i) This technique is used when the the Recovery vehicle
a) Can get stuck (soft ground)
b) No room to move about
c) Inclines

[ex670c]

Surface resistance
A pull of 1/10 LW will cause a free wheeling truck to move on a hard, level surface.
A pull of 1/3 LW will cause a free wheeling truck to move on a softer surface, such as grass or gravel,
Damage resistance:
A pull of 2/3 LW will be required to move if the wheels cannot rotate (as if the brakes were fully applied), the pull required to overcome the resistance (drag) the truck id 2/3 or 67% of the LW. Damage resistance includes surface resistance (i.e. you only use one or the other)
Stuck (mire) resistance:
A pull of 100% of LW will be required if the truck is stuck to a depth of the sidewall on the tires.
A pull of 200% of LW will be required if the truck is stuck to the hubs.
A pull of 300% of LW will be required if the truck is stuck to the frame..
Mire resistance includes damage resistance (i.e. you only use one or the other)
Grade (slope) resistance:
Upgrade (vehicle has to be recovered up a slope or grade)
15 degrees - add 25% of LW
30 degrees - add 50% of LW
45 degrees - add 75% of LW
Vehicle recovery on level ground - no correction
Downgrade (vehicle has to be recovered down a slope or grade)
15 degrees - subtract 25% of LW
30 degrees - subtract 50% of LW
45 degrees - subtract 75% of LW
Final figure:
Add surface or damage or mire resistance and grade resistance, and this is your final figure or rolling resistance. This is the amount of pull the winch must apply in order to recover the stuck vehicle.

My MM540XD with normal load weighs in at 3500 lbs. I get stuck down a rock ravine that's about 45 degrees steep, and there are big rocks up to the frame hanging it up. Rolling resistance is 3500lbs x 3 + (3500 x 0.75) = 13125 lbs.

For a Vehicle -to-Vehicle pull a JEEP has a draw bar pull of 6000lbs so for the above case we will require 2-3 JEEPs.

But since this is uphill, its better to use a winch.

Now If my MM540XD has a 9000lbs winch we cannot pull it uphill, this is where we double the pull with a snatch block.

[ex670c]

The Sweetest Smelling Army in the world says..

RECOVERY

Reconnoiter Area – check the approach to the load,
Method of rigging,
And natural anchorage

Estimate Situation – Determine the load resistance,
And the capacity of the effort available.

Calculate Ratio – Calculate the approximate mechanical advantage for the rigging

Obtain Resistance – Compute the tackle resistance and the total resistance

Verify Solution – Compute the line forces to compare with the winch
And the dead line capacities.

Erect Rigging – Orient the crew, and instruct them to assemble the rigging,
And move back to a safe location.

Re-check rigging – Ensure that the rigging is erected for proper and safe operations.

You-are-ready – Signal the operator to apply power to the winch and recover the load.

[ex670c]

1) Calculate the Resistance. (for a MM540XD Stuck in Chassis deep mud.)

3500lbs X3 =10500lbs pull

2) Calculate pull/tow (each JEEP can pull 6000lbs)
10500 divided by 6000 = 1.45 JEEPs (1MM540XD + 1 MG413W )

SWAG rule says 2 JEEPs.

3) Rigging-
i) V - Or preferably an equilateral triangle.Thats how the vehicles are placed.
ii) Cover the Bumper with a mat to avoid the sling from getting chaffed/cut attach (2 10-12mm) polyprop Rope Slings (one for each recovery JEEP) on either chassis rails on the front bumper.
iii) Place the Recovery JEEPs at the pulling end of the V.
attach the tow cable to the rear pintle/tow hooks and lock them and secure them with nylon cable.
iv) attach the tow-cable to the polyprop rope sling one for each side and each JEEP. Ensure the cables are not entangled or CROSS each other.
v) Place a cable damper over each tow cable.
vi) clear all persons to twice the radius of the tow cable and far infront of the recovery JEEPs or Far behind the Recovered JEEP.
v) Recovery In-Charge briefs all the drivers about the procedure.
vi) Recovery In-Charge re-checks all rigging/lashing
vii) Checks all person are clear.
viii) Recovery In-Charge positions himself safely and Signals drivers to take in the slack and then gradually pull out the vehicle.

The trick here is to totally get of the clutch in 2 wheel revolutions or to take up the slack and alternately pump the accelerator and the clutch till the clutch can be released.

CAUTIONS
1) Check the Equipment
2) Use gloves to handle ISW Ropes
3) Check the lashings
4) Clear all persons
5) Brief the drivers
6) Check Cable Dampers

[DirtyDan]

I worked near a big airport for awhile. One day a big airliner went about 100 meters off the end of the runway into soft mud and immediately sank to half and 2/3 the height of its tyres. The plane stayed erect on its gear, just the wheels and tyres had some minor damage.

But the recovery effort was amazing to watch (beat working anyway). There were 4 huge recovery vehicles on hand. I asked and each one of those cost $400,000 to $700,000.

Using winches, just as Arka describes but on a larger scale, they edged, cajoled and tweaked that thing out of the muck and back on the runway in about 10 hours. There were all kinds of geologists (soil) and aircraft engineers there doing calculations. They did it a couple of centimeters at a time, continually checked for stress on the landing gear and tow points and recovery equipment. They continually also dug by power shovel around the landing gear to ease resistance. This was dangerous as the plane might collapse upon the shovels. It is an intriguing fine art when done right. It is a dangerous cartoon when done wrong.

[DirtyDan]

Originally Posted by DKG

With a 4 inch wide canvas belt this is not much of a problem during towing. But when you winch with a wire rope, this becomes mandatory as failure to add this protective element can prove fatal. Also people need to stay clear of the line of pull.

I have no evidence to support this but I think if you get hit hard with a broken canvas strap you are not going to survive. Its kind of like...would you be deader if they dropped ten tons of feathers on you or ten tons of bricks. No doubt some graduate physics twit will see this thread and devise a test using cadavers, monkeys, condemned prisoners and write his thesis on the results. He will then get a job making more money than me and I will be p*ssed off.

Between the 3 mediums, canvas, nylon and steel, I would think that canvas would be the weakest and most prone to water damage and rot. I think in any case I would be standing way the h*ll away from the lines.

I saw a disgusting newsreel not long ago. Groups of college men were having a tug of war using a nylon rope.
there were about 300 men on each side. For those of you who do not know, a "tug of war" is a contest in which 2 groups of equal numbers hold onto a rope and try by force to pull the other side across a center point, sometimes a mudhole. The contest is over when all the men on one side are pulled over. Well, using a nylon rope these guys tugged and pulled for many minutes. The nylon rope stretched. Then one side gave up abruptly. The nylon rope quickly resumed its original length and lopped off the fingers of many men holding it in the process. Lesson? Stressed lines of any kind are dangerous to the point where they can kill you.

[RedMM340]

This discussion is a good one, especially regarding the safety aspects of recovery. This can be a very dangerous affair. A cable snap can be fatal.

I refer you to the attached pic. of the way I did a recovery pull near Kaza last summer. A jacket is being used as a damper on the cable, all humans are standing clear of the wire cable, anchor vehicle has brakes locked, pulled vehicle is in neutral with motor off.

I agree with what has been said about Nylon straps. They do stretch, and can snap back with incredible force. I once was pulling out a stuck jeep with my jeep and the nylon strap which had steel hooks at the ends, slipped off my vehicle. The steel hook shot back with incredible energy, and hit the other vehicle on the windshield, causing it to shatter.

[vinod_nookala]

Arka,

Lot off technical stuff appreciated!This calls for a session on OTR recovery techniques in chennai.:-)

[jaysmokesleaves]

Arka CHIEF,

That is some good technical stuff about recovery. Im sure theres more valuable information to come from you. PLease be kind enough to add photos of recovery situations, some close ups of rigging for towing, how to etc etc. Im sure it will be of great help to me and others interested in the topic and its practical applications.

Thanks a recovery ton for sharing.
J

[tsk1979]

Arka did you see the video?
The Gypsy clutch was burning as the pathfinder was completely bogged down.
Is it advisable to use a relatively weak and low torque vehicle to try and tow out a 2 ton SUV?
I am guessing a diesel would have done a better job here.
At 4L, even at idle the diesel would have had adequate torque.

Other offroading videos I have seen of elephants getting stuck, the towing vehicle is usually a bulldozer or a tractor or even a crane. 2 ton SUVs are the worst things to get stuck in I guess

[shazikon]

This is a very informative thread..Nice to see so much offroading experience in the forum.
Here is a link just to substantiate safety during snatch.. Snatch recovery
keep it going fellas.. i am enjoying the discussion..

[susan3004]

Originally Posted by DirtyDan

Arka, you use pulleys at times. When do you use them and what is their effect?

We have been using pulleys to apply more strength to the vehicle being towed.
I will try to explain without a photograph(please excuse me as my written skills are on the lower side).
The equipment for towing out a stuck vehicle is a longer cable(generally steel) with hooks on either side and an independent pulley with a hook for attaching on the vehicle which is stuck.
As I mentioned, the pulley is securely affixes on the affected vehicle and the cable rope is passed through the pulley.
One side of the cable is attached to the vehicle attempting this recovery and the other end it tied with a tree which is ahead of the affected vehicle and about in the same line as the vehicle attempting recovery.
Now, when the vehicle attemps a recovery, the force which is applied on the stuck vehicle doubles up and even loaded vehicles stuck are pulled out.(Don't ask me to explain the law of physics please, but this happens)
If I sum it up, the requirement is a longer cable. A pulley and of course,a tree or a place which you can secure the other end. There are times when a tree iis not in the vicinity, in that case another vehicle can be used as an anchor.
And in case another vehicle is also not present then Murphys' law may be applied."When something has to go wrong, it will go wrong."