Dynamic Recovery:A dynamic recovery derives its forces from the kinetic energy of a moving mass. Because of the method, it is important to ensure you are using an appropriate dynamic (stretchy) strap. Dynamic Recovery Straps: A dynamic recovery can be a simple method, provided that you have a second vehicle present. Dynamic recoveries derive the recovering force from the vehicle’s kinetic energy. The kinetic energy is the energy that a vehicle stores when it gains speed. That speed is transferred into the strap, which acts like a spring and stretches. When the strap stretches it provides a force that is exerted on both ends of the strap equally. Because of the high levels of momentum involved, it is important to visualize where the vehicle will move once unstuck. You don’t want the stuck vehicle to slam it’s rear axle into a rock during the pull. If that is a possibility, it may be best to perform a static pull that can be more controlled.
As you guessed it, the key component to a dynamic recovery is the strap. This is one instance where it is not helpful to oversize a component too far. If the dynamic strap does not have an appropriate amount of stretch to it, it will act more and more like a static component (see the example below for details as to why more stretch is good). In general, a 2” wide strap is properly sized for most vehicles under 6k lbs. For a larger vehicle, such as a large pickup or camper, a 3” strap may be better suited. It’s a good idea to size the strap to the pulling vehicle, as they are the ones generating the energy that will transfer through the system. All vehicles are meant for forward progress. The suspension and gearing is designed to handle forward stresses. If a recovery is performed in reverse, stresses are transferred differently and can potentially cause axle wrap, stress suspension components, or pull out a driveline. If necessary, turn your vehicle around so you are facing away from the stuck vehicle. This allows you to see where you are going, makes it easier to control the vehicle, and allows proper forward gear to be selected. Because there is an energy storing component in the system, it is extra important to create a ‘closed system’ if possible. A closed system means that the system could vibrate or shake and not allow the strap to fall off, or misplace itself. Closed system components include D-rings, shackles, strap eyes, or hooks with spring retainers. An open hook is not desired in most situations, and a trailer ball hitch should never be used. Due to these high stresses, it is important to ensure your components are strong enough for the recovery. Again, compare the dynamic strap to a spring. If you stretch a spring and let it go, it will fly back towards itself. The dynamic strap will do the same. When making your strap connections to the vehicles, ensure they are frame mounted, and sturdy. Good straps will come with protective coverings on their loop-ends, and sometimes along their length. Make sure these are located where the strap rubs against anything. This can prevent serious damage to your strap. To begin a dynamic recovery, start with a few feet of slack in the rope between the vehicles. The recovering vehicle will gain speed until reaching the end of the strap. At that point the recovering vehicle should let up on the gas a bit, and allow the momentum of the vehicle to produce most of the recovering forces. If the pull does not recover the vehicle, do NOT spin your wheels at the end of the strap, this defeats the purpose of a dynamic recovery and adds stresses to the driveline. Back up to get slack in the strap, and try again with more speed. Because the dynamic strap produces unknown forces and stresses that vary depending on vehicle speed, strap stretch, and amount of “stuckage” it makes sense to begin a dynamic recovery process with a slow, light pull. If the first pull doesn’t recover the stuck vehicle, try again with a slightly faster pull. Slightly increase the speed each time until the vehicle releases, or until you feel the pull is becoming too harsh. If you do not feel safe with the speed, do NOT try a “superman pull”. Instead, try combining a passive recovery method with a dynamic recovery. It can be helpful for the stuck vehicle to slowly spin their tires during the recovering process. It is important not to spin the tires excessively fast, as this will shock load the drive-train once the tires regain traction. It can also be helpful to aid the process by digging out the soil in front of the stuck vehicle’s tires. Below is a mathematical example to show why it is important to use a stretchy strap in-between the vehicles when performing a dynamic recovery.
|The effects of using a non-dynamic connector:|
We’ll use an average 5,000 lb vehicle as our example recovering vehicle. The two vehicles are attached with the strap, loose at first. The recovering vehicle proceeds forward with a bit of gas, reaching only 5 mph when reaching the end of the strap. At this point the vehicle has gathered kinetic energy equal to 1/2*mass*velocity^2.KE = 0.5 * 5000 lb * (5 mi/hr)^2 = 5.7 kJ We will assume for this instance that the stuck vehicle will remain stuck and will not budge (worst case)… so all of the recovering vehicle’s energy transfers into the strap and is turned into elastic potential energy. This stored energy will be equal to the kinetic energy that the truck had. This stored energy relates to the force exerted on each end by the following: energy = average force * distance. The distance is how far the strap stretches. The average force is assuming the rope exerts constant force, which ours does not. Because it’s force exerted most closely resembles a linear relationship to the stretch, the average force should be multiplied by 2 to get the maximum exerted force (which is all we are interested in here)… assuming the system reaches equilibrium without failure. In instance 1, we will use a dynamic strap, which can stretch about 6 feet.
5.7 kJ / (6 ft) * 2 = 2,089 lbf (well within the safe range of most straps)
For instance 2, we used a static strap, which we will assume stretches only 4 inches before reaching equilibrium.5.7 kJ / (4 in) * 2 = 25,072 lbf (enough to snap a strap or possibly rack your frame)
For the last instance, what if we used a chain, which has extremely minimal stretch. So we will say 0.5″…5.7 kJ / (0.5 in) * 2 = 200,576 lbf (you will certainly break something!!) So, I hope this gives you a real world, numerical understanding of why dynamic straps should ALWAYS be used in dynamic vehicle-to-vehicle recoveries.
* I did not show unit conversions for the sake of simplicity (there were a lot)