The units of measure once you complete the kinetic energy equation comes to (Lb*ft^2)/(sec^2), which is the Imperial unit of measure for a Joule. A Joule is not a measure of force, but energy, a cumulative measure of force applied over a distance to get from point A to point B. Your intuition is correct though, you can divide the calculated Joules by the amount of stretch in the kinetic rope and calculate the amount of force applied to the rope.I finally got around to finding a calculation for the Force a truck might apply when using a Kinetic Rope.

Kinetic Energy is equal to (Mass divided by Two) times (Velocity Squared). So, (m/2)*(v^2). I'm using pounds, foot-pounds and Feet per Second.

The recommended speed to pull away with a Kinetic Rope is between 2 and 5 MPH, which is 3 to 7.5 feet per second.

My truck weighs about 5,500 lbs when loaded for Recovery work. And we'll use 4 MPH for this example.

Therefore, (5,500lbs/2) * (5.5fps^2) = 83,187 ft-lbs of kinetic energy (force). EIGHTY-THREE THOUSAND POUNDS !!

It's a good thing that the rope absorbs some of this energy, and does not apply the rest rapidly.

Because NONE of the other components (soft shackles, D-rings) can withstand that level of force.

(edited)

AND, typical Kinetic Ropes will Stretch as much as 4 feet. So ... we would divide the force by that much ... cuz it's lbs over some number of feet?

(not just one foot ... ?) Resulting in an overall force of perhaps 20,000 lbs over the distance the rope stretches ... ?

Am I doing this calculation correctly?

footnote: The amount that a kinetic rope might stretch in use was taken from the Project Farm episode where this was tested.

Something you can also do is calculate the spring constant for the kinetic rope by dividing the calculated force by the distance of rope stretch. This will assist in determining the "stiffness" of the kinetic rope. Different manufacturers have different stiffness's for their ropes, which greatly affect the felt tug of a vehicle when doing a pull. The lower the spring constant, the lower the felt tug or acceleration when pulling someone out, which can greatly reduce the stresses on a vehicle when yanking another out. However, if you were to do a practical test, you would only use the first measured values since the kinetic ropes tend to loose their elasticity in repeated tests and do not come back to zero stretch immediately, resulting in the need of a resting period to get "stretchy" again.

Great thoughts vezePilot, it's been a while since I looked at kinetic rope tests.