so when is enough too much when winching or tugging out?

[DaveInDenver]

Speaking of aftermarket bumpers, mine arrived with grade-five bolts for mounting, three or four on each side (mount positioned perpendicular to the bumper, so it would be a straight shear test if the bumper were pulled directly forward). I'd have to do the math again to be sure if it were three or four per side, but the total shear strength of that many bolts was just about equal to the theoretical pulling power of a 12k winch, without much in the way of wiggle room.

I'm reading between the lines, but are you thinking of the bolts in a bumper mount in simple bearing shear based on their cross section?

When you bolt on a bumper or recovery point the bolt is clamping two surfaces, the bolt itself is primarily in tension. The torque specification along with the surface area of the interface is how you determine the full shear. The arrangement of the bolt pattern also factors into the total load capacity of the connection. A group of 4 bolts arranged in a square resist a single shear greater than the sum of just 4 individual bolts. As you might expect the base materials matter here, because the steel of a bumper may easily be assumed to tear before the bolts fail, for example.



Consider that Warn (or whomever) is selling a 12k winch that can be mounted feet down and is held fast that way with just 4 x 3/8"or 10mm bolts in single shear. Mounting a bumper with a similar number of bolts may be fine or may not. Everyone should give 10% of the thought about winching and recovery mount ratings as is being done here, so I'm not sure either way but tend to believe most bumpers are overbuilt (which if you have airbags should be another concern).

 

[s.e.charles]

trying to follow along here. "...the surface area of the interface..." does this mean the head area of the attaching bolts, or the 'plate - to - plate' square inches of two components touching? if, from the drawing, it means bolts, can the washers just be increased in thickness & diameter (fender washer style)??? then torque could be increased to an upgraded bolt and - hey! - what effect does all this have on airbags? I remember the ARB brochure mentioning they were the only (front) bumper 'air bag rated' or something similar.

 

[DaveInDenver]

and - hey! - what effect does all this have on airbags? I remember the ARB brochure mentioning they were the only (front) bumper 'air bag rated' or something similar.

The airbag compliance means that the timing is supposed to remain the same. This is a big unknown when you install an aftermarket bumper and supposedly ARB crash tests theirs to demonstrate their bumpers don't change anything. Most (and I can only say for my Tacoma) trucks have inertial sensors that will work regardless of the bumper you have but if the hell-for-stout bumper you put on doesn't collapse or distort then the airbags will trip different than the factory intends. Part of the crash response is the plastic bumper and subframe getting torn apart and if that doesn't happen the energy has to go somewhere.

 

[DaveInDenver]

trying to follow along here. "...the surface area of the interface..." does this mean the head area of the attaching bolts, or the 'plate - to - plate' square inches of two components touching? if, from the drawing, it means bolts, can the washers just be increased in thickness & diameter (fender washer style)??? then torque could be increased to an upgraded bolt

I'm not a mechanical engineer by trade, so hopefully someone knowledgeable will explain it. But my understanding is that a bolted clamped connection in shear the area is distributed through the bolt/nut, washer and material. So therefore adding a larger diameter and/or thicker washer and increasing the torque (tension) on the bolt should increase the shear resistance.

I believe this is the basis for the slightly raised ring under a good quality cap screw, so the clamping force is uniform and consistent, or why a flanged bolt might be used or specified in critical applications.

 

[SDDiver5]

OMG!!! don't get into the "pulley thread " whatever you do . . . .

ps: in case you do: https://www.expeditionportal.com/forum/threads/people-think-pulleys-increase-pulling-power.191591/ but don't say I didn't "warn" you. nyuk nyuk

haha. Given the response, I assume this was an expo bickering thread? Cant wait to read it. Thanks for the link!

 

[s.e.charles]

just the fellas having a bit of fun. can't be out wheelin' all the time.

 

[kbroderick]

I'm reading between the lines, but are you thinking of the bolts in a bumper mount in simple bearing shear based on their cross section?

When you bolt on a bumper or recovery point the bolt is clamping two surfaces, the bolt itself is primarily in tension. The torque specification along with the surface area of the interface is how you determine the full shear. The arrangement of the bolt pattern also factors into the total load capacity of the connection. A group of 4 bolts arranged in a square resist a single shear greater than the sum of just 4 individual bolts. As you might expect the base materials matter here, because the steel of a bumper may easily be assumed to tear before the bolts fail, for example.

View attachment 459014

Ah. So I was missing something. And that reminds me that I need to retorque those mounting bolts.

Consider that Warn (or whomever) is selling a 12k winch that can be mounted feet down and is held fast that way with just 4 x 3/8"or 10mm bolts in single shear. Mounting a bumper with a similar number of bolts may be fine or may not. Everyone should give 10% of the thought about winching and recovery mount ratings as is being done here, so I'm not sure either way but tend to believe most bumpers are overbuilt (which if you have airbags should be another concern).

I'm 100% convinced my bumper is overbuilt; in hindsight, I wish I had managed to find a similar one in aluminum rather than plate steel. The mounting I'm less sure about.

 

[Umbrarian]

if vehicle #1 gets stuck, and vehicle #2 cannot move #1 with either winch or strap, is it ever acceptable for vehicle #3 to latch on to #2 with the intention of doubling the pull power?

For me it is never acceptable to latch #3 onto #2. Dangerous and inefficient. I have always had #2 & #3 latch to different tow hooks on #1.

 

[s.e.charles]

okay; that's helpful. I've been trying to find youtube examples of good extraction but man weeding through the yay-hoos and tallywhackers is quite a chore.

even the guys who lose their trucks at the boat launching ramps have such a cavalier attitude. good for 'em if they can kiss away that much money and not givahoot. i'm certainly not in that position financially. maybe they cry once the camera stops rolling? I dunno . . .

 

[DaveInDenver]

Seems to me the question depends. I've understood that it's OK to anchor a vehicle (#3) to another (#2) who's winching a stuck one (#1) with the intention of preventing the winching one (#2) from moving towards the bogged one (#1).

If you're using two active vehicles then #3 may be placing an unnecessary stress on #2 attempting to double the force. But I can see in theory how it might work. It's essentially a double locomotive train, isn't it? Obviously snatch straps would be impractical in the scenario.

Connecting to two recovery points could work but coordinating the two pulling vehicles so that they aren't working against each other might be a challenge. Even a small difference in angle would result in a force trying to spread the stuck vehicle's frame apart rather than pulling it forward. Perhaps rigging a sling between the two points with a common connection to the two pulling, like a big 'X', would equalize.

In any case as long as all the component ratings are sufficient then I don't see why it wouldn't be OK. That's the real rub, the middle vehicle and the stuck vehicle would need a substantial connection.

 

[Umbrarian]

If you're using two active vehicles then #3 may be placing an unnecessary stress on #2 attempting to double the force. But I can see in theory how it might work. It's essentially a double locomotive train, isn't it?

Doubling can only be achieved if both are connected to the #1. Otherwise, #3 cannot add to pulling of #1 without first pulling #2.

 

[DaveInDenver]

What I'm picturing in the multiple pulling vehicle is distribution, like a equalized collection point in climbing anchors.

 

[DaveInDenver]

Doubling can only be achieved if both are connected to the #1. Otherwise, #3 cannot add to pulling of #1 without first pulling #2.

I'm thinking of it as sum of forces, e.g. like a tug-o-war. That has a common connection through the rope. If you have 200 N of force on each side there's a zero net force and equilibrium. Add a little 50 N puller to one side and you get a net 50 N force to that side and they win. That would the case of adding multiple vehicles to a common point (the stuck vehicle), right?

But the example of a multiple engine locomotive as the supposed configuration. There's no direct connection between the lead engine and the cars behind the helper, so something is lost in the transfer through the intermediate helper but it should still be an additive result and a positive net force, similar to a tug-o-war.

 

[s.e.charles]

it does seem that a common point would need to be identified in order for a double-header to help most efficiently. but in the event of a catastrophic "stuck", where maybe the vehicle is in danger of sliding off into the abyss or the tide is coming in, secondary damage may be the lesser of evils (ex: frame dis-alignment)