07-05-2018, 03:45 AM
What is the difference between tow straps and recovery straps and how are they most effectively used? Understanding the physics.
A tow strap is designed for towing a vehicle that is free to easily roll on a good surface.
A recovery strap is designed for pulling a vehicle out from being stuck which takes considerably more force.
The difference is the recovery strap is designed to stretch.
A chain is an example of almost no stretch. Let's compare.
A chain or a tow strap or a recovery strap can tow a vehicle. But, for recovery of a stuck vehicle, it's best to use a recovery strap.
Without stretch like the extreme example of using a chain, the force that can be used to pull out a stuck vehicle is limited mainly two ways:
1. The limit of traction of the pulling vehicle which might be in nearly the same mud or snow and ice or loose sand as the stuck vehicle, so not only does it need to have enough traction to not get stuck itself, it also has to have enough additional traction for the other vehicle and that it is stuck means even more traction which it might not have on a surface of poor traction.
2. Force. There is a limit of force that should be used to pull out the stuck vehicle.
If the required force can't be achieved with traction alone, having slack in the strap allows the pulling vehicle to gain some speed before encountering the resistance of the strap attached to the stuck vehicle. That short distance allows the pulling vehicle to gain momentum even when the low traction would be otherwise insufficient to getting both vehicles moving together as with a strap without slack. With a chain, the kinetic energy of the moving vehicle suddenly is imparted to the location on the vehicle to be recovered with a high peak of force of very small duration. Such high force of short duration is called impact. Impact has a tendency of being destructive and is not the sort of force conducive to moving things.
Let's examine that. Let's say that six thousand foot-pounds of energy is sufficient to move the stuck vehicle from being stuck. My rifle has more than six thousand foot-pounds of energy. If I were to shoot the stuck vehicle from behind, it would not budge the vehicle at all. Instead there will be a hole through it. That's an extreme example, but the energy of a moving vehicle suddenly transferred through a chain that suddenly becomes taught will likewise be impact. It can bend the stuck vehicle frame without moving it from being stuck and the tow vehicle frame can be damaged and it can even exceed the breaking strength of the chain. This experiment done in thought shows that impact or a high peak force over a very short duration is not useful for recovery and there is little benefit to getting this added short duration peak of force when taught chain force of just traction alone is insufficient.
Stretch is the key to recovery straps. They are designed to stretch by the choice of a material that stretches. Some tow straps also stretch and some have little stretch, so choose a recovery strap. Stretch works very powerfully for recovery by storing the energy of momentum when the slack is taken up. Instead of the tow vehicle coming to a sudden stop on a chain and useless and damaging impact imparted on both vehicles, the energy is stored to allow a much lower peak of force that is spread out over a much longer time duration and that is useful to pull out the stuck vehicle.
Now that we know that stretch works, we know we want to maximize stretch by purchasing the right recovery strap or straps. Pick one that is specifically a recovery strap rather than a tow strap. For safety, it has loops on the ends and no hooks to come dangerously undone and should be fastened properly. Remember the energy of momentum is stored in the stretch, so if a hook comes loose, it can have the energy of a bullet or more and act similarly (more on that in another post, if requested). A recovery strap of greater working load capacity than necessary will have reduced stretch, so more isn't necessarily better. A recovery strap of longer length will have more stretch and can store more energy from greater momentum without a greater peak force. That means less impact and more useful duration of pull which means it can pull out a vehicle that is stuck worse. Buy the longer length by choosing the longer recovery strap or several you can connect end to end with technique to not have a knot get too tight to come apart again (more on that in another post, if requested).
I have several recovery straps to connect end to end for both more stretch (more capacity to store the energy of momentum) and more length. The extra length can be advantageous in that it might allow the recovery vehicle to pull from different terrain for more traction instead of getting into the same slimy mud or loose sand or ice patch as the stuck vehicle. It will allow a vehicle with otherwise insufficient traction to pull out the stuck vehicle
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A tow strap is designed for towing a vehicle that is free to easily roll on a good surface.
A recovery strap is designed for pulling a vehicle out from being stuck which takes considerably more force.
The difference is the recovery strap is designed to stretch.
A chain is an example of almost no stretch. Let's compare.
A chain or a tow strap or a recovery strap can tow a vehicle. But, for recovery of a stuck vehicle, it's best to use a recovery strap.
Without stretch like the extreme example of using a chain, the force that can be used to pull out a stuck vehicle is limited mainly two ways:
1. The limit of traction of the pulling vehicle which might be in nearly the same mud or snow and ice or loose sand as the stuck vehicle, so not only does it need to have enough traction to not get stuck itself, it also has to have enough additional traction for the other vehicle and that it is stuck means even more traction which it might not have on a surface of poor traction.
2. Force. There is a limit of force that should be used to pull out the stuck vehicle.
If the required force can't be achieved with traction alone, having slack in the strap allows the pulling vehicle to gain some speed before encountering the resistance of the strap attached to the stuck vehicle. That short distance allows the pulling vehicle to gain momentum even when the low traction would be otherwise insufficient to getting both vehicles moving together as with a strap without slack. With a chain, the kinetic energy of the moving vehicle suddenly is imparted to the location on the vehicle to be recovered with a high peak of force of very small duration. Such high force of short duration is called impact. Impact has a tendency of being destructive and is not the sort of force conducive to moving things.
Let's examine that. Let's say that six thousand foot-pounds of energy is sufficient to move the stuck vehicle from being stuck. My rifle has more than six thousand foot-pounds of energy. If I were to shoot the stuck vehicle from behind, it would not budge the vehicle at all. Instead there will be a hole through it. That's an extreme example, but the energy of a moving vehicle suddenly transferred through a chain that suddenly becomes taught will likewise be impact. It can bend the stuck vehicle frame without moving it from being stuck and the tow vehicle frame can be damaged and it can even exceed the breaking strength of the chain. This experiment done in thought shows that impact or a high peak force over a very short duration is not useful for recovery and there is little benefit to getting this added short duration peak of force when taught chain force of just traction alone is insufficient.
Stretch is the key to recovery straps. They are designed to stretch by the choice of a material that stretches. Some tow straps also stretch and some have little stretch, so choose a recovery strap. Stretch works very powerfully for recovery by storing the energy of momentum when the slack is taken up. Instead of the tow vehicle coming to a sudden stop on a chain and useless and damaging impact imparted on both vehicles, the energy is stored to allow a much lower peak of force that is spread out over a much longer time duration and that is useful to pull out the stuck vehicle.
Now that we know that stretch works, we know we want to maximize stretch by purchasing the right recovery strap or straps. Pick one that is specifically a recovery strap rather than a tow strap. For safety, it has loops on the ends and no hooks to come dangerously undone and should be fastened properly. Remember the energy of momentum is stored in the stretch, so if a hook comes loose, it can have the energy of a bullet or more and act similarly (more on that in another post, if requested). A recovery strap of greater working load capacity than necessary will have reduced stretch, so more isn't necessarily better. A recovery strap of longer length will have more stretch and can store more energy from greater momentum without a greater peak force. That means less impact and more useful duration of pull which means it can pull out a vehicle that is stuck worse. Buy the longer length by choosing the longer recovery strap or several you can connect end to end with technique to not have a knot get too tight to come apart again (more on that in another post, if requested).
I have several recovery straps to connect end to end for both more stretch (more capacity to store the energy of momentum) and more length. The extra length can be advantageous in that it might allow the recovery vehicle to pull from different terrain for more traction instead of getting into the same slimy mud or loose sand or ice patch as the stuck vehicle. It will allow a vehicle with otherwise insufficient traction to pull out the stuck vehicle
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