Weight Transfer Formula?

[WreckerOp911]

Good Afternoon-
I apologize if my post is on the wrong forum.... I have been searching & searching for an answer & a theory on weight transfer-

Bare with me, hope I can make this make sense.... I am starting my training to work on a Rotator Wrecker which is a tow truck equipped with 360 deg rotating boom that can be used as a crane.

During any operation you need to know how much a vehicle weighs..... When a vehicle has landed on its passenger side, the vehicle needs to be uprighted back onto its tires. During this operation you will be flipping the car back over and will be using the passenger side tires (front & back) as a fulcrum. Since the vehicle will not be completely suspended in the air, the boom will not be lifting 100% of the vehicles weight.

During this lift, depending on the angle of the vehicle will determine how much weight is being lifted. (IE: 15 Degrees = ?? % of Vehicle Weight / 30 Degrees = ?? % of Vehicle Weight / 45 Degrees = ?? % of Vehicle Weight ) During the lift, as vehicles is lifted up, the weight will get lighter or heavier??

How do I figure out how much weight or % of vehicles weight am I lifting since the weight transfer will vary depending on the angle of the vehicle due to the fact that one side of the vehicle is kept in contact with the ground-

I been told that there are ballpark numbers out in the tow recovery world but I
want to know the theory / physics of why this is happening?

If anyone knows how to explain this or if you can point me to the direction of where I can find the info out myself, that would be great!! Thank You & apologize for such a long drawn out question....

 

[Baluncore]

Welcome to PF.

The vehicle will roll or hinge over a fulcrum line, that joins the two tyres that contact the ground. The forces will depend on how you attach the lifting wire, and the direction of the pull.

The weight that must be lifted will depend on the position of the centre of mass of the vehicle. The force needed will start out at about half the vehicle weight, it will reduce to zero, before the forces change direction and the vehicle falls for the second half of the roll.

You must control that second part of the roll, after the forces reverse, and begin to increase again.

The forces will rise to a maximum of about half the vehicle weight, as it lands on its wheels. That is when you will wish you had applied the emergency brake, and chocked the wheels. If you do not control the descent and landing, it may continue to roll over onto the other side. That is especially true when the ground has a slight side slope.

As a practical experiment, get a brick, rest it on one side, then lift it, or pull it with a finger, to make it roll over, to lie flat on the ground.

 

[berkeman]

I am starting my training to work on a Rotator Wrecker

During any operation you need to know how much a vehicle weighs.....

There should be some book or catalog that has typical GVWs for various vehicles, no? Is that not included in your training materials and references?

And as Baluncore says, you will need to be able to support up to half of that GVW plus impulses during the rotation.

 

[Baluncore]

During the lift, as vehicles is lifted up, the weight will get lighter or heavier??

If you do it right, the forces will become less, and should not exceed half the weight of the vehicle.

Any formula will be made more difficult by the many variables involved in the recovery roll. The complexity of the equation will rise more rapidly than you can compute the forces. At some point, if you rely on a mathematical equation, you will miss something important, and the recovery will not go well, as more damage will be caused to the vehicle being recovered.

You must familiarise yourself with the principles of levers, the number of falls on a pulley block, and the vector sum of forces, then you will be able to estimate the forces needed for any step of a recovery. Those principles will also allow you to quickly optimise the positioning of the recovery vehicles. Those are all things you learn while playing with bricks as a child, through experience on the job, or by observing a skilled operator, and analysing their work.

The position, or height, of the centre of mass will be important, and depend on how the vehicle is loaded. The tipping point will depend on that mass distribution.

The lifting attachment point will also be important, we might assume you will pull on the wheels that are in the air, in which case the pull must be significantly horizontal to lower the forces. If too much high-pull by the rotator is horizontal, the vehicle will slide towards the winch, rather than have the tyres grip on the ground, and so begin the vehicle roll.

 

[jack action]

I want to know the theory / physics of why this is happening?

How about doing a Free Body Diagram?

You have to know where the CG is located, where the pulling force is applied, and at which angle. Then, you have two reaction forces at the tires: one vertical force from the ground and a lateral one from tire friction.

When the vehicle will begin to move, then an angular acceleration will be introduced in the dynamic process.

 

[Lnewqban]

 

[WreckerOp911]

I appreciate all the responses!!

I know a big issue with my question is there are a lot of variables; GVWR of vehicle, position of vehicle, angle of boom, CG, surface resistance, etc and etc - As for the uprighting of reefer truck video, that is exactly what I am asking about -

All of this came about because I came across an old document from a recovery company that gave a chart with a semi (doesnt matter on vehicle) on its side, and it had different angle lines (15%, 30%, 45%, 60%, 75%, 90%) simulating has the vehicle is uprighted it becomes lighter due to 'weight transfer' from the top being transferred to the tires (fulcum point). The problem is the document is old & was probably scanned so you cant make out any of the numbers in relation to the % of the vehicle.

There were ballpark numbers in relation
to what percent the vehicle was at during the lift - But the main points were that when uprighting a vehicle on its sides:
1. When determining vehicles weight, when the vehicle is on its side take vehicles weight & cut in half.
2. As you lift the vehicle, the weight seen on the hook / cable is going to decrease as vehicle is uprighted


I am looking for explanation / physics behind weight transfer and what the ballpark percentages that can be assigned to the angle.

Thank You!

 

[Baluncore]

2. As you lift the vehicle, the weight seen on the hook / cable is going to decrease as vehicle is uprighted

It seems you are lifting the roof of the vehicle upwards, to put it back on its wheels. There is little structure to attach to on the roof, so you should be attaching to the wheels or axle and pulling horizontally.

I am looking for explanation / physics behind weight transfer and what the ballpark percentages that can be assigned to the angle.

As I suggested earlier, get a brick, and roll it over with your hand. Find the balance point. That is a good way to get a feel for, and see how, the weight is transferred during a roll.

If a vehicle was 2.5 m, (8 ft), wide and high, with the centre of mass 1.25 m, (4 ft), above the road, then the weight at each end of the roll would be half the weight of the vehicle.
There would be a tipping point, with zero-weight, at 45°.

 

[jrmichler]

How do I figure out how much weight or % of vehicles weight am I lifting since the weight transfer will vary depending on the angle of the vehicle due to the fact that one side of the vehicle is kept in contact with the ground-

How about doing a Free Body Diagram?

You need to learn Free Body Diagrams (FBDs) in order to figure this out. An FBD shows the contact point between the vehicle and ground, the center of mass of the vehicle, and the location and direction of the pull. An example FBD is shown below.

In this example, the cable is attached somewhere near the chassis and wrapped around the side of the vehicle. The pulling force is at the point where the cable separates from the vehicle. The center of gravity (CG) is the location of the center of mass of the vehicle. A full cement truck has a high CG, an empty semi trailer has a low CG. For the purpose of the FBD, the entire weight of the vehicle is assumed to be at the CG.

The weight of the vehicle is multiplied by the distance from the CG to the contact point. Weight in pounds times distance in feet results in a moment in ft-lbs. A 20,000 lb vehicle with a CG distance of 4 feet will have a moment of 80,000 ft-lbs.

The pull force can be in any direction. The pull force creates a moment in the opposite direction of the moment due to the weight of the vehicle. When the pull moment is greater than the CG moment, the vehicle starts to raise. The task is to find the pull force to raise the vehicle. First find the distance from the pull force to the contact point. The pull force distance is the perpendicular distance from the line of pull force to the contact point. If the pull force is vertical or horizontal, it is easy to measure. If not, you need to be very careful to get it right.

The calculation is: Pull Force = Weight of vehicle * CG Distance / Pull force distance

Sample calculation:
Weight of vehicle = 20,000 lbs
CG distance = 4 feet
Pull force distance = 9 feet
Pull force = 20,000 X 4 / 9 = 8,900 lbs

As the vehicle raises, the CG distance decreases. When the CG is directly over the contact point, the pull force decreases to zero.

 

[WreckerOp911]

Thank You to all for helping me and giving me some awesome topics to research more. The response regarding the brick, and the FBD, it was one of those moments where my mind clicked!! Response on picking location, top vs bottom - Everyone had great stuff!

I work in public safety and I been doing this work for some time now and I am working towards my certification process to be qualified to operate our Wrecker. Every written response all had things that I have been doing. The Youtube video, that is a common pick that we get called out for. Lot of things to consider: CoG, Pick Points, Pulling vs Lifting, Angles of Winch Lines, Angle Forces, Shackle Forces, Moving vs Non Moving Pulleys, etc etc! I know I will be coming back here for future help! Thank You to all for all the responses!