Tow strap vs. Kinetic rope

[AndrewP]

1-I think it's really cool that MasterPull is here on this thread. props, because I'm sure some bashing will/has occurred.

2-I have used one of those super yankers that belonged to a friend (to pull his FJ40). they are the real deal and I wish I had one. I also have a couple of recovery straps, and the stretch in the yanker is better and feels like less of a shock load when brought quickly into tension.

3-Say what you will, but it seems like no recovery is perfect and bends the "rules" a bit. I can remember one where my winch was pulling on a strap, shackled to another strap, shackled to a chain, attached to a bumper recovery point. It was all we could do at the time, and the choice was leaving an 80 series sunk in the snow for the rest of the winter. It's nice to theorize about what should be done, but it seems like some carefully considered improvisation always happens. In a perfect world, you'd always pull with your winch in a nice safe and slow manner, with the winch aligned and spooled perfectly, and attached to recovery point that cannot be broken. Lacking that, straps or the super yanker is a nice solution.

 

[michaelgroves]

So for instance: a 20% stretch, 30 foot long strap will fail at 36ft of length (6ft of stretch)?

On our ropes, yes that is how we mark them. Ours are marked for 30% stretch so a 30ft rope becomes a 39 foot rope.

-Alex

Fail, as in "break", or fail as in "stop stretching"?

 

[UK4X4]

Just a question- not loaded in any way.

A truck 1000kg in weight drives away using a non stretchy tow strap(all straps stretch)at 15 mph he comes to the end of the slack

what was the force exerted ?

Now repeat with a stretchy tow rope say with 20% stretch

what force was exerted ?

me being a traditional engineer would look up the formula on google, guestimate it

Then go into into the yard and test it using a strain guage...being that I don't have one

This question is to our budding rocket spring engineers and their years of universty education ?

 

[Antichrist]

Fail, as in "break", or fail as in "stop stretching"?

Yeah, I was wondering the same thing. Breaking when it reaches its maximum stretch doesn't seem too good to me.
Also, the amount of stretch is dependent on the load applied to a given rope/strap so I don't think you can say categorically that a rope will stretch x%.
I think you'd have to say, for each size rope/strap, x% stretch at y% rated capacity.

 

[michaelgroves]

Just a question- not loaded in any way.

A truck 1000kg in weight drives away using a non stretchy tow strap(all straps stretch)at 15 mph he comes to the end of the slack

what was the force exerted ?

Now repeat with a stretchy tow rope say with 20% stretch

what force was exerted ?

me being a traditional engineer would look up the formula on google, guestimate it

Then go into into the yard and test it using a strain guage...being that I don't have one

This question is to our budding rocket spring engineers and their years of universty education ?

Does he freewheel when he comes to the end of the slack, or does he continue to use the throttle? (I'll assume he freewheels). How long is the rope? (I'll assume 10m). I'll also assume the strap stretches exactly by its 20% with that mass and velocity, and that the stretch decelerates the truck linearly wrt time.

Force=mass x acceleration. His first run has no stretch, so the deceleration is almost instant, so the force tend to infinity. Something breaks.

For the second run, a=(v^2)/2s, where a=acceleration, v=velocity=6.7metres per second, s=displacement=2m.
So a=11.2m/s/s.
So the force is 11,200N (or the equivalent of about 1.2 tons).

Arithmetic, formulae and logic only as good as my Monday brain!


Note: The energy in the rope is a function of the force acting over a distance. This means that if the truck free-wheels as soon as the slack has been taken up, and only uses its momentum to stretch the rope, then only the speed and weight of the truck are relevant to the stored energy, not the amount of stretch. (A little stretch will mean a lot of force over a short distance, a lot of stretch will mean less force over a bigger distance.)

 

[michaelgroves]

Yeah, I was wondering the same thing. Breaking when it reaches its maximum stretch doesn't seem too good to me.
Also, the amount of stretch is dependent on the load applied to a given rope/strap so I don't think you can say categorically that a rope will stretch x%.
I think you'd have to say, for each size rope/strap, x% stretch at y% rated capacity.

On thinking about it, I suppose everything breaks when it stops stretching!

I would guess that the stretch % must be the amount the rope is designed to stretch, at its rated capacity. Any futher force on the rope may stretch it a bit more, but will start to damage it? I wait with bated breath for words from the horse's mouth!

 

[Antichrist]

On thinking about it, I suppose everything breaks when it stops stretching!

Technically I suppose that's correct. I guess what I'm saying is that hopefully if they are designed to stretch 20% before breaking, that proper use would rarely, if ever, stretch them over 15%. To pick a percentage at random.

 

[R_Lefebvre]

7wt: Let me try to put it to you this way. I have a Superwinch EP9 on my truck. It has a stalling torque of about 9000lbs. As long as I have suitable recovery points, I know that the failure mode of this system is safe. ie: the winch will stall out before any of the recovery points fail. So, I'm still stuck, but I'm safe.

With a strap, you don't know what the forces are. Say you've got a nice 10,000lb recovery point on the front of the stuck vehicle. Now, how fast can you drive with a KERR before you reach the limits? You have no idea. So you do a little tug, and the other truck is still stuck. So you give a little more, still stuck. You go a little more, still stuck. The fact is, you don't know just how hard you can pull, until something lets go. And when it lets go, it'll be BIG, because of the energy stored. That is the risk of these things.

You can be as careful as can be, or as smart as can be, but you're not psychic, and you don't know how hard you can yank on something without it breaking because you can't KNOW the actual force number you are imparting.

Unfortunately, this is a very common issue. My first big wheeling trip, I was out with what I thought was the premier club in Ontario. I got myself stuck in a mud hole which had a log across the end. The log was positioned such that it was at about the mid-point of my front wheels, so it was a dead stop. The first thing the leaders did is hook a tow strap up to my factory recovery point up front (weak!) and gave me a huge yank from an XJ to get me over that log. In hindsight, it was ridiculous. I had wanted to winch, but they didn't want to take the time, and I didn't know any better. We're all lucky nothing let go.

I find this very common, to use the strap first, and I don't know why. Now, I have used the strap first, but only under certain conditions. ie: I had a stock JK on stock tires stuck in a silly little mud patch. Hook up the strap, to his stock HOOK (for shame!) and a little "nudge" with the strap and out he came. I wouldn't use it for much more than that, but it also would have been silly to set up for a winch.

Just a question- not loaded in any way.

A truck 1000kg in weight drives away using a non stretchy tow strap(all straps stretch)at 15 mph he comes to the end of the slack

what was the force exerted ?

Now repeat with a stretchy tow rope say with 20% stretch

what force was exerted ?

me being a traditional engineer would look up the formula on google, guestimate it

Then go into into the yard and test it using a strain guage...being that I don't have one

This question is to our budding rocket spring engineers and their years of universty education ?

Couple problems here. We do not know the STIFFNESS of either of these two ropes. The percent stretch is not the stiffness (modulus of elasticity) it is the ultimate breaking stretch (ie: strain at point of tensile failure). So, you can't calculate the force in either case.

In fact, for a given vehicle, travelling a given speed when the slack is taken up, a "low stretch rope" might impart the exact same force as a "high stretch rope", if their stiffness were the same. Remember, "% stretch" is a measure of how far a rope can stretch before it breaks, NOT a measure of how far it will stretch for a given load.

This gets back to my point above. You must use straps with caution, because NOBODY knows what forces are going on inside, EVER. At best it is a guess based on experience. At worst, it is a gross underestimation based on ignorance.

 

[7wt]

Unfortunately, this is a very common issue. My first big wheeling trip, I was out with what I thought was the premier club in Ontario. I got myself stuck in a mud hole which had a log across the end. The log was positioned such that it was at about the mid-point of my front wheels, so it was a dead stop. The first thing the leaders did is hook a tow strap up to my factory recovery point up front (weak!) and gave me a huge yank from an XJ to get me over that log. In hindsight, it was ridiculous. I had wanted to winch, but they didn't want to take the time, and I didn't know any better. We're all lucky nothing let go.

I find this very common, to use the strap first, and I don't know why. Now, I have used the strap first, but only under certain conditions. ie: I had a stock JK on stock tires stuck in a silly little mud patch. Hook up the strap, to his stock HOOK (for shame!) and a little "nudge" with the strap and out he came. I wouldn't use it for much more than that, but it also would have been silly to set up for a winch.

Was it a tow strap or a kinetic rope? I am taking you at your work when you say tow strap and not kinetic rope. Let me be clear, I don't EVER want to be yanked with a tow strap. It is dangerous and rough on the vehicles involved. The rope is a different story. I too too had a bad taste in my mouth after several tow strap recoveries in the past. I hated the things and always feared them. However after using a proper kinetic rope, my view has changed. I am starting to think people are getting the two crossed. A tow strap should never be used for anything other than a static pull.

As far as not knowing what the forces are, I'll give you that. No way to ever really tell but practice and knowledge of your gear goes a long way. I don't drive with a heavy foot. If I am using a kinetic rope and it requires a heavy foot to free someone then I will be the first to admit I chose the wrong tool for the job.

 

[R_Lefebvre]

Actually, I'm not really sure anymore. When I bought it, it was labelled as a "Recovery Strap" or something like that. Might have even said "Kinetic" on it. Until this thread, I didn't know any better. Regardless, it's the white 3" wide strap that everybody knows as a "snatch strap", and that's what everybody uses it for. It is not the yellow stiff "rope" that is usually labeled as a tow strap.

It's definitely somewhat stretchy, and I have used it to "nudge" people, but I'd never use it for a superman recovery like that MP video. Especially after reading this.

The basic point is the same. Unless you put a force gauge on your setup, you have no idea what the forces are, and therefore have no idea if you are overloading your recovery point rated for "X", until it's too late.

Yes, experience goes a long way to make them work well, and I don't think anybody here is arguing against that. What they are arguing against is their use as a "go to" tool, and with blind ignorance about how they work.

I've seen people yanking with TOW straps before. The short stiff yellow rope usually with 2 hooks. While doing recovery work at a Pro Rally, the back markers were helping eachother out of snow banks by yanking eachother with these things. Scary.

 

[michaelgroves]

Couple problems here. We do not know the STIFFNESS of either of these two ropes. The percent stretch is not the stiffness (modulus of elasticity) it is the ultimate breaking stretch (ie: strain at point of tensile failure). So, you can't calculate the force in either case.

In fact, for a given vehicle, travelling a given speed when the slack is taken up, a "low stretch rope" might impart the exact same force as a "high stretch rope", if their stiffness were the same. Remember, "% stretch" is a measure of how far a rope can stretch before it breaks, NOT a measure of how far it will stretch for a given load.

Cunningly, I (expressly and magically) assumed away these problems, as well as another couple of issues, for the purposes of my calculation...

I defined the stretch as linearly decelerating that weight of truck, at that initial speed, over the given distance of the stretch.

I suppose in general terms, it bears pointing out that the force (and the energy) will always be proportional to the weight of the towing truck, but will go up as the square of the towing truck's speed. (In both cases, plus a considerable margin if you continue to apply throttle once the slack has been taken up).

What this means in practice is that each little increment in run-up speed can make a very big difference to the forces involved. Add 25% to your run-up speed, and you're adding nearly 60% to the energy!

 

[michaelgroves]

Actually, I'm not really sure anymore. When I bought it, it was labelled as a "Recovery Strap" or something like that. Might have even said "Kinetic" on it.

Definitely, a rope/strap should be designed for kinetic recovery if it is to be used for thaat purpose! Not only that, but it should be roughly matched to the weight of the recovery truck.

Too little stretch will impart too much force and just break something. The whole idea of a kinetic strap is to take advantage of the tow vehicle's momentum, without it being offset by the stuck vehicle's inertia.

Too much stretch, and there will be less force applied to the stuck vehicle, but huge energy stored in the rope. If something does break, it's going into orbit!

 

[dzzz]

So the safest for people and vehicles may be one that gives only enough to not overstress both vehicles, but isn't designed to store any more energy than necessary.

 

[7wt]

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.

 

[JamesDowning]

Just a question- not loaded in any way.

A truck XXXXkg in weight drives away using a non stretchy tow strap(all straps stretch)at XX mph he comes to the end of the slack

what was the force exerted ?

Now repeat with a stretchy tow rope say with 20% stretch

what force was exerted ?

A similar question to this was brought up a while back on another forum I frequent. It involved a similar scenareo... but with a dynamic strap, a static strap, and a chain. Everything stretches some... this minute amount of stretch is what actually makes the differences into how much total force is transferred. If you guys don't mind... I'll quote myself below... hehe:

Dynamic vehicle recovery is all about managing energy. We'll use a standard 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 = 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.

In instance 1, the truck used a static strap, which we will assume stretches only 4 inches before reaching equilibrium.

5.7 kJ / (0.5 * 4 in) * 2 = 50,144 lbf (enough to easily snap the strap or rack your frame)

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

5.7 kJ / (0.5 * 6 ft) * 2 = 4,178 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 * 0.5 in) = 401,152 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... didn't want the meaning of this post to become lost in conversions!).

Cheers guys! :smiley_drive: