Kinetic Recovery Rope Sizing Guide
In addition, there are no industry standards for kinetic ropes and many companies are using different factors to calculate the recommended rope size. No wonder everyone’s confused.
The purpose of this article is to clear the smoke and provide you with the knowledge to properly size a kinetic rope for your vehicle–and application. It is pretty in-depth so if you just want a quick answer see my blog post, What Size Kinetic Recovery Rope Do I Need?
I will cover the following topics in this article.
- Why size doesn’t matter
- Why Working Load Limit is more important than size
- What Safety Factor is and how it affects sizing
- What Dynamic Load is and how it affects sizing
- Why you don’t want to oversize your kinetic rope
- How your application affects kinetic rope sizing
1This is also true of winch cables, but we’ll leave that for another time.
Size Doesn’t Matter
For the first time in history in which any man has uttered this phrase, it’s true. Before you run out and buy a 7/8″ recovery rope because that’s what you where told you needed, you need to understand why size doesn’t matter.
For some reason companies have decided to use size to correlate kinetic recovery ropes to vehicle weight. This works ok for high-quality manufacturers since each size will translate to the same Minimum Breaking Strength. Once I looked past the top-tier companies I found quite different results.
Standard is the rating the top tier companies all rated that size rope—the 7/8″ rope varied slightly for some reason, but not more than 200lb. I won’t name the companies with higher ratings but will say I highly suspect their ratings, and would avoid them. By the way, Company B is very well known.
1 This number looked reasonable at first glance, but then I noticed it was Average Breaking Strength, which means the MBS is well below the standard.
Anyone who bought from Company D is going to be in for a very unpleasant surprise. This is a perfect example of why you don’t want to buy a kinetic recovery rope based on size.
Working Load Limit and Safety Factor
Working Load Limit is the maximum mass or force which a product is certified to support in general service. While Minimum Breaking Strength is determined through testing, WLL is a calculated number. It is derived by applying a Safety Factor (SF) to the Minimum Breaking Strength (also called Mean Tensile Strength, MTS).
WLL should always be greater than the weight of the vehicle being recovered. This is why it’s so important to understand what it is—and how it is calculated since different companies use different SF’s.
Safety Factor is the ratio between Minimum Breaking Strength and Working Load Limit. The purpose of SF is to prevent a user from overloading their rope/sling in a real-world environment past the MBS.
MBS ÷ SF = WLL
Since there are no industry standards for the SF of kinetic recovery rope you will find even the top-tier companies recommend different Safety Factors.
Let’s look at an example. Two companies have a 7/8″ kinetic rope with an MBS of 28,600lb. One uses an SF of 5:1 and the other uses 3:1.
- 28,6oolb ÷ 5 = 5,720lb WLL
- 28,600lb ÷ 3 = 8,600lblb WLL
That’s a pretty big difference. So why the large variance in Safety Factors?
Most vendors use the international standards set by the Cordage Institute, which sets standards for the industrial overhead lifting industry. They recommend a Safety Factor for polyester/polyolefin dual fibre rope “in the general range between 5:1 and 12:1.” It is important to note this standard is for rope with minimal elasticity rather than kinetic rope with a 30% stretch.
Although the Cordage Institue stands as a general standard many different markets have evolved to create their own SF standards based on unique applications. The off-road industry uses a 5:1 Safety Factor in general. I presume they used the lower end of the range since the application is much less demanding than overhead lifting.
Since the Cordage Institute standards don’t apply to kinetic recovey rope different companies put more or less weight on different sizing factors. Some of the issues they consider are the risk of oversizing, the application and dynamic loads.
For more information on Minimum Breaking Strength, Working Load Limit and Safety Factor see my article on Calculating Working Load Limit for Recovery Gear.
Dynamic Load is any load that is nonstatic, such as a wind or moving live load.
When you are using a kinetic recovery rope you introduce dynamic loads which will be far larger than the weight of the vehicle alone. The same is true of using a tow strap or snatch strap. What’s important to note is the dynamic load is actually less with a kinetic rope.
The following information is quoted from an analysis of dynamic load of a recovery strap done by TrailX at Offroad Trailblazers.
It gets a little wonky so feel free to skip the methodology and go right to the summary. I’ll summarize the salient points at the end. First though a few definitions.
- The Kinetic Energy (KE) of an object is the energy that it possesses due to its motion.
- A Joule is a unit of energy equal to the energy transferred to an object when a force of one newton acts on that object in the direction of the force’s motion through a distance of one-meter. A kiloJoule (kJ) is 1,000 Joules.
- Pound-force (lbf) is equal to the gravitational force exerted on a mass of one avoirdupois1 pound on the surface of Earth.
1No that’s not a typo, and I’m not even going to try to explain what it means.
“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 × velocity2.”
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 the vehicle 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 a constant force, which ours does not.”
“Because the 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.”
Tow Strap with 2″ Stretch (less than 1%)
5.7 kJ / (4 in) × 2 = 25,072 lbf — enough force to snap a strap
30′ Snatch Strap with 6 Feet Stretch (20%)
20′ Kinetic Rope with 6 Feet Stretch (30%)
5.7 kJ / (6 ft) × 2 = 2,089 lbf — well within the safe range
30′ Kinetic Rope with 9 Feet Stretch (30%)
5.7 kJ / (9 ft) × 2 = 1,392 lbf — better yet
The math clearly shows that a tow strap (<1% stretch) will have a much higher Dynamic Loading than a 30′ snatch strap (20% stretch) or a 20′ kinetic rope (30% stretch). The dynamic load would even be less on a 30′ kinetic rope thanks to the additional length providing more stretch.
Dynamic Loading is the one factor that actually provides some insight into the argument about what Safety Factor should be used for kinetic recovery rope. Tow straps have been used for years to recover vehicles. Since we don’t see a lot of breakage—if any—when properly used and of quality manufacture, we can reasonably state the 5:1 SF is sufficient.
Therefore we can safely use a lower Safety Factor for kinetic rope while maintaining the kinetic effect (as discussed in the next section) and still preventing breakage. How much lower is a question for further analysis, but the quality companies that use a SF of 3:1 or 4:1 are likely well within the safe range.
This analysis also makes it obvious why kinetic straps/ropes should always be used in dynamic vehicle-to-vehicle recoveries.
And there’s one other thing we can learn from this analysis. It’s a commonly held belief that you size the recovery gear for the vehicle being recovered. In the case of a kinetic rope though you should size the rope to the pulling vehicle, as they are the ones generating the energy that will transfer through the system.
Oversizing Kinetic Rope
Not necessarily. There is a major disadvantage of going with a larger rope than necessary. For example, if you’re using a 7/”8″ rope to pull out a 5,000lb vehicle it will not stretch as much as a 3/4″ rope would in the same situation. Therefore you are losing some benefit of the kinetic effect.
According to Advanced Synthetic Rigging, which recommends a 3:1 SF, “We have had more than a few returns of large Kinetic Ropes that ‘didn’t work’ when the reality was they went against our advice and bought an oversized rope.” Other companies have reported the same issue.
I have discussed this with several recovery experts and they agreed with my own experience that oversized ropes do not provide the expected kinetic effect.
So while safety is a good thing, it can be overdone.
Cartoon used with the permission of Advanced Synthetic Rigging and hopefully will make up for the agony I just put you through on Dynamic Loading.
The other factor affecting the recommended size of a kinetic rope is the application. Interestingly enough, the biggest application issues are a result of inexperienced users. Beginners put much more stress on their recovery gear than more experienced off-roaders. There are a variety of reasons for this but the major ones are improper rigging1, using the wrong tool for the job (using a kinetic rope when a winch is more appropriate) or not considering environmental issues.
The major environmental issue affecting recovery is resistance. There are three types of resistance that affect the load placed on recovery gear during operation. They are rolling resistance, mire resistance, and gradient resistance. In extreme recovery situations, we adjust for these resistances by applying a multiplier to the weight of the vehicle being recovered.
Rolling resistance is the force it takes to put a vehicle in motion. On a hard, flat surface (such as concrete or asphalt), you would multiply the vehicle weight by 1.05. On grass or gravel, use a factor of 1.15.
Mire resistance accounts for how deeply the tires are buried. If the vehicle is mired in mud to a depth that covers the lower part of the wheel, use a factor of 1.75. If wheels are mired in mud up to the bottom of the wheel rims, double the amount of the vehicle’s weight.
Gradient resistance accounts for the force of gravity pulling against the stuck equipment. The steeper the slope, the greater the stress exerted on the towing vehicle. For a 15 degree angle add 25% to rolling resistance, for 30° add 50% and for 45° degrees add 75%.
Now that we better understand about sizing kinetic rope, let’s have a look at two companies that provide a Kinetic Recovery Rope Sizing Chart. Each using very different Safety Factors.
1For example, side-loading a shackle will decrease it’s Working Load Limit by 50-70%.
Master Pull Sizing Guide
Master Pull provides a range of vehicle weights for each size recovery rope. For example, the 7/8″ rope is recommended for a vehicle weighing between 3,500lb and 7,500lb. In conversations with them, they made it clear the chart is intended as a guideline and should be adjusted based on vehicle size and application.
The median Safety Factor they use is 5:1. If you look at the range though the Safety Factor varies widely. For example, on the 7/8″ rope it is 8:1 for a 3,500lb vehicle and 3.8:1 for a 7,500lb vehicle.
Note: Although it’s counter-intuitive a larger vehicle does not need as big a Safety Factor as a smaller vehicle.
Let’s take my Jeep as an example and see where I land on the chart. The Gross Vehicle Weight of my 2019 4dr Rubicon is 4,800lb (curb weight + fuel + passengers + cargo). According to the chart, I’m slightly below the center of the range for the 7/8″ rope.
To look at it another way, my Safety Factor would be 6:1 with the 7/8″ rope and 4:1 with the 3/4″ rope.
Since recovery ropes are only manufactured in limited sizes this is a common issue. A 13/16″ rope would be about perfect if I wanted a 5:1 Safety Factor, but there is no such thing. You’ll just have to look at the numbers and decide which side you want to lean towards based your application.
Advanced Synthetic Rigging Sizing Guide
ASR also shows a range of vehicle weights for each size recovery rope though they do not list the weights, but rather just provide vehicle icons. As mentioned earlier they recommend a Safety Factor of 3:1 in order to maintain the kinetic effect of the recovery rope.
The numbers on the chart are for MBS of the recommended size rope.
According to this chart my Jeep should use a 7/8″ rope. I’m a little confused by that since the Safety Factor would be 5.9:1* and ASR recommends a Safety Factor of 3:1.
So far we’ve got a pretty wide range in recommended Safety Factors. Let’s look at couple more top-tier suppliers and see if we can clear things up a little.
|Bubba Rope||3.5:1 – 4:1|
Well that wasn’t much help.
If you’re more confused than before looking at all the information, join the club. The problem is, there are not only no standards for kinetic rope but there is no publically available testing to quantify the effects of oversizing, Dynamic Loading (although the analysis by TrailX helps a lot) and application.
I can say that I have seen oversized ropes ( in most cases 1″) that did not provide the expected kinetic effect. I can also say the vast majority of kinetic recovery ropes I’ve seen break when used properly were cheap Chinese ones. In talking with other knowledgable off-roaders their experiences were about the same.
Bottom line, based on the data and empirical data I would go with a 3:1-4:1 Safety Factor.
- If you have a 2dr Jeep go with a 3/4″ rope.
- The same for a light 4dr Jeep.
- If you have a heavy 4dr Jeep go with the 7/8″.
- If you’re a mudder, I would go 7/8″ instead of 3/4″.
- For the Gladiator a 7/8″, or 1″ for mud.
In my case, I don’t do any heavy mudding and I have a winch and plenty of other recovery gear options. Most importantly though, my expectation of the kinetic rope is it will get me, or someone else, out of most situations. When other factors like weight or terrain suggest I might be pushing the 3/4″ rope’s abilities I use another recovery option. It’s just one tool in my toolbox.
This section is slightly off-topic, but if you’re trying to understand how to size your kinetic rope then you might also wonder about what length to buy. The important thing to know is more nylon between the two vehicles will provide more stretch and thus help transfer the energy more effectively and require less effort to extract.
Before you run out and buy a 30′ kinetic rope though consider the following. In many recovery situations, the terrain (trees, rocks, winding trail) won’t allow enough room for the longer recovery rope. So the choice depends on where you typically wheel, and only you know the answer to that question.
One solution to this issue is to go with a 20′ kinetic rope and a 10′ kinetic bridle like the one Master Pull sells. The bridle can be used to extend the reach to 30′ in emergency situations* or allow you to use two recovery points. The benefit of the latter is it will reduce stress on the individual recovery points.
*I would not use this as my go-to solution since you are adding more points of failure.
- Discussions with Factor 55, Southeast Overland and Advanced Synthetic Rigging
- Cordage Institute, Polyester/Polyolefin Fiber Rope Standard
- Advanced Synthetic Rigging, Production Testing Data and Kinetic Recovery Rope Info and Usage
- Offroad Trailblazers, Dynamic Vehicle Recovery
- Masterpull, Super Yanker Sizing Guide (note: this chart was provided to me prior to publication)