Sorry it took a bit for me to get back in here. Been slammed at the shop.
So, BS (breaking strength) vs WLL.
WLL = BS / a safety factor.
Yes, WLL is used to provide a "safety factor"
so a piece of equipment is not stressed to the point where there could be a failure which could = damage to equipment or worse, injury or death.
In off-road recovery gear BS is used often, and almost always if not always with winch lines. Samson's Amsteel Blue, which is a Dyneema SK75 fiber, and a very popular raw line material for splicing winch lines, has a WLL of 1/4 the BS. So, a 3/8" line with an average BS (ABS) of 19,600 lbs has a WLL per Samson of 4,900 lbs. To add to that, synthetic line should not be shock loaded, and Samson spec's a shock load to be a quick loading of the line at anything more than 10% of the WLL.
Now why not put a synthetic line on a winch that has a WLL that is equivalent to the rating of the winch? Let's look at the popular Warn M8000, or many other 8,000 lb winches that hold 100' of 5/16" line rated with an ABS of 13,700 lbs, or 85' of 3/8" line rated with an ABS of 19,600 lbs. To get a WLL on the line of 8,000 lbs you would need a line with an ABS of 32,000 lbs at Samson's 4:1 WLL/safety factor rating. An ABS of 32,000 lbs would necessitate 1/2" diameter line with an ABS of 34,000 lbs. Using 1/2" diameter line you would only be able to fit around 40' on the drum with about 10' of that reserved for leaving on the drum so the line attaches to the drum properly, which leaves you with around 30' of working line length. That's not very much.
So why do you often see WLLs in rigging gear and ABS/BS in off-road components such as winch lines and straps? With rigging/lifting gear if a failure occurs someone could very likely be injured or killed. In off-road if a failure occurs someone could also be killed or injured, but the likely hood is less. Using a WLL on winch line would take any of the modern and common winches and make them basically useless because there would be so little line available.
Now Southeast Overland recommends a 2:1 safety factor on winch line strength vs winch rating. This is to work in a safety factor. Winches often only work at a few thousand pounds of their rating, which effectively gives you your 4:1 WLL. You do need some safety factor because if you dynamically load your winch line you can cause a failure.
Also think about straps, especially snatch straps and kinetic recovery ropes. A polyester tow strap might only stretch a few percent, a nylon snatch strap 10-12 percent, and a kinetic recovery rope around 30%. A key point here that many don't realize is that 30% stretch for a kinetic recovery rope, or any strap, is at the BS/ABS. You don't get 30% stretch out of a 7/8" recovery rope if you pull on it with your hands, only if you yank on it with your vehicle. If it stretched so easy with your hands it would break way too soon.
Something else to think about is the use of rigging hardware in the off-road recovery industry. The shop uses Excel hooks on our winch lines and Van Beest shackles in our recovery bags because they are high quality. The hooks we use on our winch lines, both 5/16" and 3/8" have a BS of 25,200 lbs. The calculated WLL at a 4:1 safety factor is 3.15 tons or 6,300 lbs. Some only go up to a hook with this rating on a 3/8" line but we like to see a beefier hook used on both the 5/16" and 3/8" lines. We upsize the hook for 7/16" lines to a BS/ABS of 42,200 lbs, a calculated WLL of 10,600 at a 4:1 safety factor. This upsizing of hooks costs a significant bit more than the next step down in hooks, but we hate seeing undersized hardware, and especially hooks, on winch lines. Arguably, a smaller hook can be used on 5/16" line, especially synthetic which recoils less than steel cable, but we prefer to go beefier. You don't want a piece of hardware breaking on you and potentially recoil. Now - you could even argue to go with a heavier hook so the hook's WLL = the rating of a winch, plus an additional safety factor of 2-3 to account for dynamic loading of the line. I've been there when I was "getting funky" to get up a slick hill, had my winch spooling in, I overdrove the rate that the winch could spool in, gained ground, got "extra funky" and slid back hard on the line. Before the line went taught I called out to my passenger (who is my wife) that "the line is about to break." I was wrong, but was surprised that I was wrong. In that position there wasn't much that I could do to avoid that situation as it was a long winch, a steep slick hill, no good ground anchor, and there wasn't a good opportunity for a double line pull...
Speaking of steel line, take a look at some of the hooks, and their gates, that come on steel cable as stock equipment on some winches. In our judgement they are undersized.
So - with all that said - I absolutely do not think MasterPull was being underhanded, deceitful, <fill in the blank with a synonym) with how they spec their lines. MasterPull knows their stuff. They sell a ton more equipment than Southeast Overland does. I trust them and the shop sells their Superline and Superline XD lines.
The whole rating of WLL and safety factors pulled over into off-road gear is a bit different than how it is used in rigging, especially lifting, but we still need to be very careful. Too many people buy recovery gear and have no idea of how to use it. I've seen some horrible examples. I even tried to correct a neighbor of mine before but "he knew what he was doing!" and used a smaller than 1/2" shackle to set up a snatch strap. Since he wouldn't listen I made sure everyone else around was waaaaaaaaaaay clear. Winching is the same - too many people just hook up a line and spool in - bad things happen that way. I've been yelled down, even basically challenged to a fight, because someone was doing something so stupid winching they were putting many spectators at risk. Big Boy thought he knew better. Luckily nothing broke, but it so could have.
Whew - that was a lot to type! I'm sure I left some things out. Shoot any other questions at me you might have.
