Tow strap vs. Kinetic rope

[R_Lefebvre]

I just checked with the manufacturer of my original tow strap (Fiberlink) to get the MBS and WLL along with suggestions for it's use. The guy on the phone said the strap had about a 15% stretch factor with a 6,667 lb WLL and a MBS of 20,000 lbs. He said it could be used "like a rubber band" to pull a truck out of mud but the strap it's self says not to yank with it. The strap has a pretty low WLL and I can see it easily breaking if used in a kinetic recovery. It seems perfectly safe and reasonable to use as a "tow strap" in a static recovery however. Combine that with the limited stretch and you have a something that people might use as a kinetic strap but isn't. I have the idea that most of the youtube videos are shot using straps very similar to mine.

I am starting to believe people associate this type of recovery with this type of strap. So any other product designed for a kinetic recovery would have similar properties of the simple strap you see used improperly for kinetic recoveries. You will NEVER, I repeat for emphasis NEVER see me use the Fiberlink strap in a kinetic recovery. I see the two different products as different as night and day. I have complete confidence in the Masterpull rope to be used in a Kinetic recovery. The phone call to Fiberlink cemented my believe that people are just using their junk for the wrong use and it is giving the style of recovery, not the misuse a wrong name. IMHO.

You're still missing the message.

First, I agree with what you're saying, that there are proper and improper uses for different tools. I think the typical "snatch straps" are fine for some light tugging, but not superman yanks as sometimes seen. A higher quality rope is better for that.

But any kinetic recovery will always have an element of risk. With a winch, you have a very good idea what the maximum force number will be. With a kinetic recovery, you have no idea what the force number will be. Ever. Period.

 

[michaelgroves]

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.

I presume you mean when it reaches the end of the slack! For your energy equation to hold good, the truck must freewheel (frictionlessly, of course), so that it is using only its known kinetic energy at the end of the slack. If, as in the real world, the driver continues to apply throttle, then the energy in the rope will be supplemented by the additional pull of the engine. (Which would of course be a very significant increaase in the force and energy!).

Given that, your e=mv^2 equation obviously yields the same result as my equations in my post above.

This stored energy relates to the force exerted on each end by the following: energy = 1/2 * average force * distance. The distance is how far the strap stretches. The average force is assuming the rope exerts constant force. Because it's force exerted most closely resembles a linear relationship to the stretch, the average force should be multiplied by 2.

energy = force x displacement.

Why do you use e=1/2 * f * s, and then double it? It gets the same result, but I don't follow the logic of the "average" force part. We are only interested in the terminal force and terminal energy, surely?

 

[R_Lefebvre]

These equations are only illustrative of the relative forces involved in kinetic recovery using various straps. You'll never come close to the real number because there are way too many variables we don't know. If the stuck vehicle moves at all, all these equations go out the window. If the tow vehicle keeps applying power, forget about it. When a strap is declared "15% stretch", is that 15% at the WLL, or the MBS?

I wouldn't spend too much time trying to calculate this. It's really more for entertainment value than education, because you'll just never know.

BTW, anybody seen the Mythbusters where they tried to test a myth that a cable snapping could cut a person in half if struck with it? I forget the details but they set up a pig carcass and a worst case scenario, using a bigger cable than we do, and they weren't able to do it. IIRC.

 

[JamesDowning]

I presume you mean when it reaches the end of the slack! For your energy equation to hold good, the truck must freewheel (frictionlessly, of course), so that it is using only its known kinetic energy at the end of the slack. If, as in the real world, the driver continues to apply throttle, then the energy in the rope will be supplemented by the additional pull of the engine. (Which would of course be a very significant increaase in the force and energy!).

Given that, your e=mv^2 equation obviously yields the same result as my equations in my post above.

Indeed. It does simplify the mathemetics a bit to assume the driver only uses momentum to provide the snatch. This is the way I begin all snatch recoveries, as it doesn't shock load the driveline. Only if necessary will I still apply gas after the snatch occurs. I'd rather bounce off the rope a few times than over-drag the other vehicle.

The other element here, is if you are on soil with a low shear strength (such as sand), you can dig yourself into trouble if you continue to apply power.
... But proper recovery technique is not the question at hand, eh?...

energy = force x displacement.

Why do you use e=1/2 * f * s, and then double it? It gets the same result, but I don't follow the logic of the "average" force part. We are only interested in the terminal force and terminal energy, surely?

Ahh! You're right. Why did I halve it at first...? I must have confused the energy of a spring equation (1/2 * k * s^2) with the energy of force equation (f * s). Crap... anyways, moving on.

If we use the f * s equation, it implies that the force is constant over the distance 's'. That is a problem, because in our scenario it is not.

In our scenario, the strap/chain/etc most closely represents a linear spring. At displacement = 0, force = 0, and the force line increases at some unknown, approximately linear, rate from there.

The full energy equation is an integral of the force equation over the displacement:

.

All we are looking for is the max force though. If you compare the force vs. distance graph of a constant force and a linear force over the same displacement range from the origin, both with the same total areas (or energy)... you will find the 'max force' of the linear force graph = twice that of the constant force graph. Thus the final * 2.

So if you don't mind, I will amend my response below:

(hopefully my explanation makes sense, I am having trouble putting my thoughts into words here)

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, the truck 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 instance 2, we will use a dynamic strap, which will stretch about 6 feet.

5.7 kJ / (6 ft) * 2 = 2,089 lbf (well within the safe range of most straps)

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.

 

[muskyman]

trying to put specific numbers to this stuff without a discussion of the materials used in each product is just funny and goes even farther twards fooling oneself that they can control the amount of force during a dynamic recovery.

 

[JamesDowning]

What would be really nice would be having a force vs. elongation graph for a given strap. It would be interesting to see how the strap acts after reaching the yield point. Does it plastically deform or just fail?

However, knowing what we do, we can make educated assumptions and mathematically simplify the system with a macro viewpoint.

The biggest problem is that we do not know exactly how much these ropes elongate under a given amount of force. That's why I think manufacturers should move away from the seemingly arbitrary % stretch rating and report actual stretch constants to relay the strap's physical qualities.

 

[muskyman]

What would be really nice would be having a force vs. elongation graph for a given strap. It would be interesting to see how the strap acts after reaching the yield point. Does it plastically deform or just fail?

However, knowing what we do, we can make educated assumptions and mathematically simplify the system with a macro viewpoint.

The biggest problem is that we do not know exactly how much these ropes elongate under a given amount of force. That's why I think manufacturers should move away from the seemingly arbitrary % stretch rating and report actual stretch constants to relay the strap's physical qualities.

I agree that a elongation graph would be a good thing and a safety measure that would be good for the users.

One of the problems I have seen with these ropes is that they will elongate much more then the listed percentage that they are rated for and because elongation is the main factor in stored kinetic energy the potential for damage and injury goes way up.

The first kinetic rope I ever used that was listed as a kinetic rope was one that we used for pulling stuck garbage trucks loose with at a transfer station 25 years ago. Luckily the pull points on chassis' used for garbage trucks are huge because the amount of energy you could store and transfer were ridiculous. That rope could very easily be elongated to 2x its length without failure and in doing so a case W31 could extract a garbage truck that weighed 2x what the case front loader weighed.

I also would add to this thread that straps are getting a bad rap here for sure. I have a "kinetic strap" that was sold as a kinetic recovery device and works great for recovery and the amount of shock it transfers seems to be very minimal and has never torn anything off a recovered truck. I also used to have a 60' long recovery "snap strap" and used it to recover trucks for 20 years without ever tearing a recovery point off. That strap infact was very smooth and progresive and worked wonders because it was long enough to keep the recovering truck out of the soup while doing the recovery.

Like anything else in life moderation is key!

 

[muskyman]

I thought I would add this just for fun. Here is where I bought my 60' strap all those years ago. They have actual data on there straps.

Now these are old school recovery straps for sure but they do actually give you the numbers to see and man oh man those are some huge numbers if you are tossing to much speed into the mix:Wow1:

If someone was really willing to run the real numbers on some of the new style Kerr out there that have huge elongation factors the numbers would astound you as well.

 

[michaelgroves]

trying to put specific numbers to this stuff without a discussion of the materials used in each product is just funny and goes even farther twards fooling oneself that they can control the amount of force during a dynamic recovery.

No, I think both James and I were very specific about the limitations of the calculations. Apart from the mental stimulation, it is often useful to calculate something using a simplified model, and then discuss the upwards- or downwards-effect of real-world complexities. At least you start off with a base of numbers, and some idea of where the unknowns lie.

In this case, the caclulations usefully illustrate, if nothing else, how a small change in speed results in a vast increase in energy in the rope.

 

[Antichrist]

I thought I would add this just for fun. Here is where I bought my 60' strap all those years ago. They have actual data on there straps.

Ha! I remember them, hadn't thought about them in years. I actually had one of their straps but It went walkabout some years back. I'm thinking it must have been stolen when most of my tools were stolen while under Atlas Van Lines "care" when I moved to Atlanta from Wisconsin.

 

[muskyman]

No, I think both James and I were very specific about the limitations of the calculations. Apart from the mental stimulation, it is often useful to calculate something using a simplified model, and then discuss the upwards- or downwards-effect of real-world complexities. At least you start off with a base of numbers, and some idea of where the unknowns lie.

In this case, the caclulations usefully illustrate, if nothing else, how a small change in speed results in a vast increase in energy in the rope.

I agree with you Michael that this is a very important point. I was not really laughing but just thinking that the specific numbers being posted with the equations gets kinda funny as pulling out a calculator on the trail is not likely to happen.

My point was that the numbers just have to many missing variables. Thats why I posted that simple grid. it actually shows how crazy the numbers get with small changes and they address that there are lots of variables at play.

The one thing I look at in all this is that a light weight low mass strap might actually be the best solution because it will fail in a less dynamic way before the levels of stored kinetic energy surpass the working load limits of most recovery points are reached. The 2" kinetic strap I carry these days is very soft in application of power and if I were to need more pull to recover a truck then it can apply I would be looking at other options for sure. I like to go to my hydraulic winch real fast in most cases anyways because I can recover almost anything in a very controlled way with out any drama.

 

[muskyman]

Ha! I remember them, hadn't thought about them in years. I actually had one of their straps but It went walkabout some years back. I'm thinking it must have been stolen when most of my tools were stolen while under Atlas Van Lines "care" when I moved to Atlanta from Wisconsin.

I have known Randy that owns that company for years, he is a funny guy with a kinda hillbilly wheeler thing going on but he also has 3 decades of off road recoveries to draw on when he makes recommendations.

 

[R_Lefebvre]

Just to throw another unknown variable into the mix...

The calculations being used are attempting to calculate the force on the *pulling* vehicle. The energy imparted onto the stuck vehicle will be significantly less due to the hysterisis inherinent in this elastomer rope. The full maximum force imparted on the strap by the tow vehicle will be applied to the stuck vehicle should it remain stuck. But once it starts to move and the strap is shrinking again, the force it returns will be much lower.

The same factor probably makes the failure of the strap itself somewhat safer. In fact, it's probably in large part what makes the failure of a synthetic winch cable so much safer than a steel cable. The hysterisis in a steel spring is much lower than a polymer fiber.

 

[muskyman]

I agree with you Rob, the hysteresis effect will change the forces each end of the strap or KERR sees as the forces ramp up and drop off based on factors like the resistance to being stuck of the truck to be recovered and traction of the recovering truck.

This is exactly why dynamic recovery has inherent dangers because all these factors are unknown for the most part. This is exactly why experience on the trail is so important. Many times the only way to make a good judgement as to the form of recovery to choose is by having been part of many other similar recovery efforts before.

The part that always scares the hell outa me is when I am on trail rides with people that have the cash to buy all the best equipment and now by default think they are experts. Having the correct equipment is only part of the solution, knowing how and when to use it is the real key.

Last year I was on a trail ride and responded to a call over the radio for "more highlifts" When I arrived on the scene many well meaning individuals had a range rover classic precariously perched on multiple highlifts and sandladders allready. We removed all those and placed two small rocks in front of the correct tires and the RRC drove out of the situation under its own power. Simple solutions often win when it comes to off road recovery.

 

[JamesDowning]

True, there are many unknown variables due to the dynamic nature of the process.

The thing I took from it all was that there are two main factors that contribute to the force of the recovery. Speed, and elongation of the strap.

That is exactly why it's so important to get a strap properly rated for the vehicle weight. For example if you purchased the 6" wide strap listed in the table above and performed a 5mph recovery, that strap would not stretch much, and thus just produce an abrupt but high amount of force... not useful. A stretchier strap (like a 2") produces a longer, lighter pull.

The stretch also greatly has to do with how long your strap is. For example, 10% of 10 feet is 1 foot of stretch. 10% of 30 feet is 3 feet of stretch. A 30 foot strap gives more elongation, and thus stretches the force over a longer amount of time to create a more useful and lighter recovery force.

At least, that was my take away.

(Also, referencing the above table, doesn't 15mph seem high for an average pull? Maybe it's just me.)