When you’re working post-storm to remove heavy limbs or trunks, especially with negative rigging (where the load is dropped and caught by the rope), understanding the forces at play is critical. One wrong move, and the gear you trust could be pushed beyond its limits—leading to catastrophic failure.
⚙️ Dynamic Load Formula (Imperial Units)
To estimate the peak force on your rigging during a negative catch, you can use this simplified formula:
Where:
- F = Peak dynamic force (in pounds of force, lbf)
- W = Weight of the log (lbs)
- H = Distance the log (center of mass) falls before the rope catches it (feet)
- D = Distance the rope stretches or the system decelerates the load (feet)
🧮 Real-World Example
Scenario: You’re lowering a 1,320 lb (≈600 kg) log. It free-falls 5 feet before the rope begins to slow it. The rigging system stops it abruptly in just 1 foot.
Plug it into the formula:
🪢 How Does This Compare to Samson Stable Braid?
| Rope Size | Break Strength (ABS) | WLL (1:5 SF ASNI) | OK for 14,520 lbf? |
|---|---|---|---|
| 1/2″ | 10,400 lbf | 2,080 lbf | ❌ Too low |
| 5/8″ | 16,300 lbf | 3,260 lbf | ❌ Still low |
| 3/4″ | 22,000 lbf | 4,400 lbf | ❌ Unsafe |
| 1″ | 38,000 lbf | 7,600 lbf | ✅ Marginal, but exceeds WLL |
Even though the 1″ Samson Stable Braid has a break strength of 38,000 lbs, the Working Load Limit (WLL) at a 5:1 safety factor (recommended by ANSI Z133) is only 7,600 lbs. Our calculated peak force of 7,920lbs exceeds this, highlighting how dangerous shock loading can be.
🕊️ Why “Letting It Run” Saves Gear—and Lives
The key factor in reducing peak force is stopping distance. The shorter the deceleration, the higher the force.
Let’s change the stopping distance from 1 foot to 4 feet:
Now we’re well under the WLL of a 1″ rope.
That’s the power of “letting it run”—allowing the load to slow over a longer distance rather than catch abruptly. Techniques like friction devices, natural crotches, or even tying off higher to introduce stretch help lengthen the stopping distance, drastically reducing the forces involved.
🌳 Final Thoughts
Storm rigging isn’t just about strength—it’s about strategy. Knowing how dynamic forces scale with drop height and stopping distance can make the difference between a safe operation and catastrophic gear failure. When possible:
✅ Use the strongest rope appropriate for your load
✅ Avoid hard catches
✅ Use friction to increase stopping distance
✅ Train your crew on the mechanics of dynamic force
“Let it run” isn’t laziness—it’s physics working in your favor.
*** ALL CALCULATIONS ARE ESTIMATES AND SHALL NOT REPLACE OFFICIAL TRAINING OR INDUSTRY STANDARD SAFETY PRACTICES. ANCHOR POINTS AND ROPE CONDITION MUST BE TAKEN INTO CONSIDERATION DURING ANY RIGGING OPERATIONS.
Well done!