C: Calculate
‘C’ STANDS FO 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:
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