What is the JL Pulling Capacity? (Recovery, Not Towing)

[Reinen]

A situation arose which made me wonder exactly what the JL pulling capacity is. I'm not talking about towing. That's well documented. I'm talking about recovery capacity. As in dropping into 4WD LOW with F&R lockers and pulling all you can. It occurred to me during this recovery that I don't really know how much force my Jeep could potentially exert and if I could possibly exceed the working load limit of the recovery points/shackle/snatch strap or chain.

Yes, I'm a geek and like to do the math. Especially when failure could mean a shackle rocketing into your head. This isn't the kind of "hold my beer" moment I want to go out in.

As you can see the recovery from the muddy wash was successful. Obviously it will depend upon how much purchase you can get on the terrain. Looking at the footage I'm actually torqueing my tires slightly so it must be a sizable amount of force. But how much force could it potentially be? IDK.

Does anyone know if someone has hooked a JL to an immovable object, pulled and measured the load force on the chain or strap? What amount of force could I be wielding here?

In this situation we opted for chain as there were horses in the trailer. Didn't want to jerk them too much with a snatch strap. If chain failed we'd unload the horses. If still stuck we'd bring out the snatch strap and give it a good yank. But chains worked.

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[Willysfan]

Nice job on that recovery. And nice 2 door. As for your question, I have no idea but I’m interested as well.

 

[rickinAZ]

I guess that it depends on your engine:

 

[C.Sco]

Not a physicist, but assuming you have a 3.6 Rubicon using 4-Lo, then you're putting out a crazy amount of torque to the hubs, something like 250 ft/lbs x 4 hubs, so if your tire radius is about 1.5 feet then by my math that's something like 170 ft/lbs at each tire, 675 ft/lbs of torque to the ground in total.

The 4xe and 392 are even crazier, like 1250 ft/lbs of torque to the ground.

I have no idea what that translates to in pulling force though, I exhausted my napkin math skills. But in any case, the amount of torque you're putting to the ground is almost certainly more than the tire/surface friction can handle, regardless of what your tires are and what the surface is. I can't think of any scenario where traction is so good that more torque would actually deliver more pulling power, other than maybe if you fitted your jeep with tracks.

 

[Reinen]

I guess that it depends on your engine:

I was in full control of how fast my wheels spun and could easily break them loose with a 2.0 turbo. At that point I don't think engine HP and torque matters anymore. It's exceeding your ground surface traction so it's enough.

At faster speeds it would matter when you don't have the mechanical leverage of 4WD LOW, but that's not what we're doing here.

 

[driventoadventure]

Not a physicist, but assuming you have a 3.6 Rubicon then you're putting out a crazy amount of torque to the hubs, something like about 250 ft/lbs x 4 hubs, so if your tire radius is about 1.5 feet then by my math that's something like 675 ft/lbs of torque on the ground. The 4xe and 392 are even crazier, like 1250 ft/lbs of torque to the ground.

I have no idea what that translates to in pulling force though, I exhausted my napkin math skills

It's a division... Multiplication only happens at the gearbox. Then the tractive force is divided by the surface area and it's friction coefficient. If he's in 4low, is something on the order of (tq x drivetrain efficiency x final gear ratio for the current gear) * friction coefficient (.35 assuming dirt) to get torque to the ground.

(edited to reflect better accounting of friction coefficient)

 

[C.Sco]

It's a division... Multiplication only happens at the gearbox. Then the tractive force is divided by the surface area and it's friction coefficient. If he's in 4low, is something on the order of (250 x drivetrain efficiency x final gear ratio for the current gear) / tire area in sqft x friction coefficient. Since 4low is 4:1, basically it cancels to (250 x ~.9 x differential ratio) / single tire contact area x around .6 (assuming dirt) to get torque to the ground.

I'm sure you're right - what does that formula work out to for torque to the ground then? Assuming a V6 rated at 260ft/lbs at the crank

 

[driventoadventure]

I'm sure you're right - what does that formula work out to for torque to the ground then? Assuming a V6 rated at 260ft/lbs at the crank

Damnit, was working the math, on mobile (dmv fun times) and tabbed away to verify a thought, then lost the reply. I will redo it all from home this afternoon, but before loses it was looking about 2000ish to the wheels. Seems wrong so I would need to verify stacking loses, good excuse to brush some dust off some memories.

 

[Zandcwhite]

I think in a steady pull the best you'd do is 100% of the vehicle weight on a dry surface. A coefficient of friction of 1 means at best the curve generated would be the weight of your vehicle before you broke traction. In most off road recovery situations you're looking at far less traction. Now start adding in momentum and those loads can quickly far exceed the breaking strength of the recovery gear. Slow and steady pull I wouldn't worry about breaking properly rated tow straps. Now that being said I won't use chains or metal shackles in any recovery anymore. Things can fail well below their ratings due to wear or manufacturing defects. Why take the risk when soft shackles, synthetic winch lines, straps, and kinetic recovery ropes are readily available? Always follow safety protocols, never put anyone in the path of the pull, use a winch weight, etc. Remember the "stuck" vehicle in this situation might weight 20klbs, but you don't need to pull 20k lbs as the tires roll, the vehicle is helping, etc. You aren't lifting it off the ground you are pulling it on wheels. You aren't generating 3klbs of force when you push a stalled civic out of the road by hand for the same reason.

 

[JeepinPete]

By my math, for a manual transmission, it works out roughly as follows:

260 fl-lbs * 5 (1st gear) * 4 (low range) * 4.1 (diffs) / 4 (# of tires) / (37"/2/12) = 3457 lbs force per tire.

If you assume coefficient of friction = 1, then your pulling force is going to be limited to the weight of the jeep.

Pete

 

[Wabujitsu]

I think in a steady pull the best you'd do is 100% of the vehicle weight on a dry surface. A coefficient of friction of 1 means at best the curve generated would be the weight of your vehicle before you broke traction. In most off road recovery situations you're looking at far less traction. Now start adding in momentum and those loads can quickly far exceed the breaking strength of the recovery gear. Slow and steady pull I wouldn't worry about breaking properly rated tow straps. Now that being said I won't use chains or metal shackles in any recovery anymore. Things can fail well below their ratings due to wear or manufacturing defects. Why take the risk when soft shackles, synthetic winch lines, straps, and kinetic recovery ropes are readily available? Always follow safety protocols, never put anyone in the path of the pull, use a winch weight, etc. Remember the "stuck" vehicle in this situation might weight 20klbs, but you don't need to pull 20k lbs as the tires roll, the vehicle is helping, etc. You aren't lifting it off the ground you are pulling it on wheels. You aren't generating 3klbs of force when you push a stalled civic out of the road by hand for the same reason.

Zach, unless the stuck vehicle is buried to the frame in deep mud, or if you are pulling uphill. In those cases, you have other variables to overcome.

 

[Zandcwhite]

Zach, unless the stuck vehicle is buried to the frame in deep mud, or if you are pulling uphill. In those cases, you have other variables to overcome.

And you still can't pull more than 100% of your vehicle weight unless you are using momentum. Uphill still means far less than the weight of the towed vehicle unless it has no wheels and that hill is vertical? Buried in the mud is a whole other animal and the reason you want a winch rated at least double the weight of your vehicle in my opinion. You can still be so buried that you're not coming out without multiple tractors but that's not the point of the OPs question.

 

[Wabujitsu]

And you still can't pull more than 100% of your vehicle weight unless you are using momentum. Uphill still means far less than the weight of the towed vehicle unless it has no wheels and that hill is vertical? Buried in the mud is a whole other animal and the reason you want a winch rated at least double the weight of your vehicle in my opinion. You can still be so buried that you're not coming out without multiple tractors but that's not the point of the OPs question.

Zach, points taken. Thanks.

 

[C.Sco]

And you still can't pull more than 100% of your vehicle weight unless you are using momentum.

The thing that confuses me still is things like the picture below, the Tundra was able to start pulling it without any momentum and the space shuttle definitely weighs more than 100% what the Tundra weighs:

Or this Toureg pulling a 747 on what appears to be wet tarmac:

 

[zouch]

that’s just about the rolling resistance of the load, not its weight.

The thing that confuses me still is things like the picture below, the Tundra was able to start pulling it without any momentum and the space shuttle definitely weighs more than 100% what the Tundra weighs:

Or this Toureg pulling a 747 on what appears to be wet tarmac:

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