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Be safe out there Part 2: How to calculate resistance for towing and recovery

Posted by Justin Cruse on January 29, 2018

In part two of this five-part article series, we recap WreckMaster's approach to increasing safety and efficiency in the towing and recovery industry, then focus in on step two: calculating resistance. We teach you about the four types of resistance, then show you how to calculate each so you can figure out the total resistance required to recover a casualty. Let's get started.

 

THE DISCIPLINE

Towing, recovery and vehicle incident management are all inherently risky activities. WreckMaster trains towing and recovery operators and vehicle incident responders to use a routine that minimizes their risk and increases their efficiency and effectiveness. We call this approach 'The Discipline.'

The Discipline has five steps, with each step corresponding to one of the letters in the word 'SCENE.' Practiced together, the steps will dramatically reduce dangerous, embarrassing, time consuming and costly towing and recovery mishaps. Our goal is success the first time, every time.

 

S stands for “SURVEY”

C stands for “CALCULATE”

E stands for “EXPLAIN”

N stands for “NOs”

E stands for “EXECUTE”

 

Last month we covered step 1: 'SURVEY.' In this step, the operator introduces him or herself to the person in charge at the scene, examines the condition of the casualty and the load, establishes static weights, and assesses surface environments and the terrain in order to develop a plan.

 

 

'C' STANDS FOR CALCULATE

To successfully move a load you must employ a certain amount of force. To apply the correct amount of force the first time, you'll have to do some math to figure out the resistance. The calculation may seem complicated, but it's critically important to get it right. Calculate it incorrectly and you can overload your rigging, wire rope and recovery equipment. You may also overestimate your truck's ability to anchor the pull.

The good news is that you don't have to do the math on your own. Resistance calculators are available in the WreckMaster store, on Google Play and the App Store.

 

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. 

 

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.  

 

This may seem very complicated, but practice makes perfect!

 

Next time we'll tackle the third step—'E' for 'explain.'

 

Topics: Useful towing tips, Tow truck safety

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