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Frederic

Towing capacities and loads

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As we sometimes involve in off-road recoveries, i wanted to have a look at what the loads are when doing tows or recoveries in the desert with our vehicles.

After all, we are all committed in working safely, and if i can speak for the Off-road club we constantly look at improving things as we should learn and keep learning. 

The biggest source of information on this topic can be found on the forums of companies and people who are working in the recovery and towing of heavy vehicles on the highways such as trailers, trucks, boats, and others. But the same calculation can be used for towing / recovering in the desert and is a good guideline to quickly calculate the TOTAL LOAD that will be involved before starting the recovery.

The most important thing to calculate is the resistance of the vehicle to be towed. It is a well known rule that to pull a vehicle on asphalt, you only need to apply 5% of the total weight to move that vehicle. So for example if you'd push a Pajero SWB of 1600 kg on the road, it would take you 80kg to get it rolling. 

As you can probably imagine, the surface resistance is totally different on sand, and even worse when the wheels are buried in, and much worse if the car needs to be towed uphill. This is called the gradient resistance.

What is resistance? Resistance is the amount of force required to move an object, given its condition and environment. It's written as a percentage of the weight that is being moved, which may be less than or more than the object's weight, depending upon conditions. That's because different surface environments, the condition of the load and the casualty, and terrain can increase or decrease resistance. Is the casualty stuck in mud? Do you have to move the casualty up hill? Both of these situations will increase the resistance to your efforts, and thus increase the amount of force you'll have to apply. 

So below the article i found that explains resistance: 

PS Disclaimer this article is taken from the certified training courses of Wreckmaster.com. I cannot take personal responsibility for the accuracy. 

 There are 4 types of resistance encountered in our industry.

  1. ROLLING RESISTANCE
  2. MIRE RESISTANCE
  3. GRADIENT RESISTANCE
  4. DAMAGE RESISTANCE

What is rolling resistance? Rolling resistance is the force it takes to move a rolling object, such as a wheel. (Remember when you were a kid and you coasted your bike down a hill? Eventually you'd slow down—and that's because of the forces that contribute to rolling resistance.) Forces that affect rolling resistance include deformation of the wheels, the surface the object is rolling on, wheel diameter, speed, and the load on the wheel. In the towing industry, we refer to a vehicle as either "rolling hard" or "rolling soft." A vehicle is considered "rolling hard" if it's sitting on a hard, flat, level surface such as concrete and has all of its tires inflated, wheels rolling freely, and its transmission in neutral. It requires 5% of the casualty's total weight to move something that's rolling hard. A vehicle is "rolling soft" if it's on a soft surface such as grass or gravel. It takes more force to move an object that's rolling soft—15% of the total weight of the casualty.

TOTAL WEIGHT  x  0.05  =  "ROLLING HARD" RESISTANCE

TOTAL WEIGHT  x  0.15  =   "ROLLING SOFT" RESISTANCE

 

What is mire resistance? Mire resistance is created when a wheel or load is sunk into the dirt, gravel, mud, sand or other soft surface. The deeper it's sunk, the more force you'll need to move it. If it's sunk up to the lower part of the wheel ("tire mire"), you'll add an amount of force that's equal to 75% of the casualty's weight. If it's sunk up to the bottom of the wheel rims ("wheel mire"), add 100% of the casualty's weight. If it's sunk up to its body ("body mire"), add 150% of the casualty's weight.

TOTAL WEIGHT  x  0.75  =  "TIRE MIRE" RESISTANCE

TOTAL WEIGHT  x  1.0  =  "WHEEL MIRE" RESISTANCE

TOTAL WEIGHT  x  1.5  =  "BODY MIRE" RESISTANCE

 

What is gradient resistance? Gradient resistance is the force created by gravity when moving a load up or down a grade. It must be added or subtracted from the total surface resistance. Add it when you're moving the object uphill, and subtract it when you're moving the object downhill.

TOTAL WEIGHT  x  0.25  =  RESISTANCE AT GRADIENT OF 15° 

TOTAL WEIGHT  x  0.50  =  RESISTANCE AT GRADIENT OF 30° 

TOTAL WEIGHT  x  0.75  =  RESISTANCE AT GRADIENT OF 45° 

 

What is damage resistance? Damage resistance is the force that resists the movement when the rolling object is damaged, for example, the wheels won't turn freely or the object has missing wheels. Damage resistance is always calculated at the same rate, regardless of surface conditions. It is two-thirds of the total weight of the object you're moving.

TOTAL WEIGHT  x  0.666  =  DAMAGE RESISTANCE

 

Using all the calculations above, we can calculate the total resistance required to move the casualty.

 

HOW TO CALCULATE TOTAL RESISTANCE

1. Figure out the static weight of the load. The static weight includes all equipment, luggage, fuel, and anything else the vehicle may be carrying. You may need to adjust your static weight to compensate for weight transfer if there is more than one surface, for example the casualty is mired in mud and then will be on grass. The weight transfer number is added to the static weight, and it's calculated using the same gradient resistance numbers as we showed above (e.g. multiply static weight by 0.25 for a gradient of 15°). This adjusted number is the one you should use to calculate the surface and gradient resistance in step 2 and 3 below.

2. Calculate the surface resistance. The surface resistance is either rolling or damage or mire resistance, whichever is the largest number. 

3. Add or subtract the gradient resistance. Add it if you're moving the casualty uphill. Subtract it if you're moving the casualty downhill.

 

THREE SIMPLE EXAMPLES

A 20,000-pound vehicle with four fully functioning wheels is to be towed up a 45° slope on a paved road. The total resistance would be (0.05 x 20,000) [surface resistance] + (0.75 x 20,000) [gradient resistance] for a total of 16,000 pounds of resistance. The same vehicle moved over a level grass surface would create much less resistance: (0.15 x 20,000) [surface resistance] + 0 [gradient resistance] for a total of 3,000 pounds. If the same vehicle became mired up to its body, the resistance would increase to 1.5 times its weight (1.5 x 20,000) [surface resistance] or 30,000 pounds of resistance.  

As above calculation is a bit tedious to do when you're standing in the middle of the desert sweating profusely and discussing with other members, i can highly advise you to install the APP called Wreckmaster, which has this calculator module built in. To have the app fully functioning the cost is around 90 dirhams. Than you can use the calculator with all functions.

Once you start the app, you only need to fill in the vehicle static weight (includes fuel and equipment), and the angle of the slope. fill in 15° for uphill and -15° for downhill. Then choose the type of resistance (from hard surface to mire body depth, this means a vehicle which is completely buried up the chassis).

1* Hard grass, gravel, concrete.... 

2* Soft grass, gravel

3* Damage

4* Mire tire depth ( a car little bit bogged down in the sand).

5* Mire wheel depth (a car where the wheels are buried in the sand).

6* Mire body depth (a care that has its body/chassis stuck on sand).

In our Off-road scenarios, options 4,5,6 will be applicable.

Let's try an example:

A 1700 kg car is stuck and has to be towed on an uphill slope of 20°. The wheels are buried in (scenario 5). The result is that you will have a total resistance of 2409 kg

In that case, a car like Nissan Xterra that has a theoretical towing capacity design of 5000lbs or 2267 kg, is not designed to tow this vehicle upwards. Of course these values are merely an indication which have a safety margin, but it explains the total forces required to pull that vehicle upwards. 

The difference between towing and pulling/tugging is important, because the strain on the equipment is much higher because the total load presents itself in one peak, not in a gradual increase like you would have when towing or winching. 

I hope this article provides some clarification on the subject, the app might help us to quickly determine the total loads and severity of the situation. 

 

 

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Superb research and fact finding to help everyone with the ballpark numbers before we carry out any recoveries.

Also any idea, if Xterra would have would used both side tow points, would have helped...?

Also any vehicle towing capacity and pulling capacity is same?

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11 minutes ago, Gaurav said:

Superb research and fact finding to help everyone with the ballpark numbers before we carry out any recoveries.

Also any idea, if Xterra would have would used both side tow points, would have helped...?

Also any vehicle towing capacity and pulling capacity is same?

Thanks buddy. The tow capacity of a vehicle is a theoretical calculation of the load that a vehicle should be capable to tow, like a boat or a trailer. This is taking into consideration that the car should be able to be strong enough to take that load onto hills, so engine and transmission power comes into play. As you won't find a pulling capacity of a car on the OEM specs, i think it's safe to use the towing capacity as a good starting point. After all, the trailer hitch is also mounted to the engine frame, just as the towing or recovery points. 

It would definitely have helped to spread the total load over 2 side tow points to evenly distribute the total load. But again these are indications and ballpark numbers. 

My proposal would be in case of certain recoveries, to use the calculator as a kind of rating from easy to severe. In the classification of serious or severe, it becomes advisable to:

1) Spread the load between 2 tow points.

2) Use the towpoint safety straps.

3) Do a buildup of several small pulls.

4) Look for alternative solutions.

 

Edited by Frederic
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For the people that have some spare time, this link below is like the bible of recovery:

http://www.pirate4x4.com/tech/billavista/Recovery/

The indicated mire resistances are even rather higher here than the previous article:

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Edited by Frederic
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I haven’t seen this mentioned yet. Giving a sharp tug or snap may very well exceed the rated capacities. One of our resident off roaders who has lost his front end in more than one occasion can maybe comment on this 😬

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12 minutes ago, Barry said:

I haven’t seen this mentioned yet. Giving a sharp tug or snap may very well exceed the rated capacities. One of our resident off roaders who has lost his front end in more than one occasion can maybe comment on this 😬

One time it was me pulling him from a soup soft sand pocket as he ignored my radio of "Don't Come".

On third little harder tug frontend was out, it was because of back design partially as hook was not on frame.

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