Added Mass: Weight transfer to pin

[M and E]

I enjoy the systems side of RVing as much as I like the lifestyle (I love anything with wheels and a motor too). Anyhow, I’m not sure if this is widely known but there is a very basic calculation that can give you a really good idea of how much additional weight is transferred to the pin when you add weight to your rig. This is a basic calculation. Center of gravity, height of frame of the trailer, if you are running nose up (reduces weight transfer), nose down (increases weight transfer) or level, etc., all come into play.

You need to know these few basic things:

Wheelbase: Distance from the kingpin to the forwardmost axle.

Weight: The weight of the items you are adding

Weight Placement: The distance from the forwardmost axle to the center of the mass (stuff you are loading)

Overall length of the trailer: we’ll call it 38 feet for the 340RL

Here is the formula and it is called the “Lever Arm Formula”

Weight Transfer = (added weight x distance from wheelbase) / overall length of trailer

So if we add 1500 pounds of stuff (mass) 14 feet in front of the forwardmost rear axle (approximate distance from the axle to the garage) here is what it looks like.

(1500 x14) / 38 which calculates to 21000/38 = 552.68 (pounds of weight transfer to the pin for the added 1500 pounds)

Now let’s look at adding a washer and dryer and presume that in combination the washer and dryer are 400 pounds in aggregate. Those are placed, hypothetically, 24 feet in front of the forwardmost rear axle.

(400 x24) / 38 which calculates to 9600/38 = 252 pounds of weight transfer.

So the further forward the mass is in relation to the rear wheel the greater the percentage of weight transfer to the pin in this model.

Weight behind the forwardmost axle reduces the weight transfer. If you add weight behind the forward most axle, you can use the same formula but make the distance to the rear axle a negative number. Weight transfer from below the frame is calculated differently. Weight transfer in a bumper pull is calculated differently.

In a simple environment, lever, fulcrum, weight, this is highly reliable. In an RV it is a good ballpark estimate.

 

[George]

Thank you.
I added an aluminum toolbox to the trailer's rear accessory hitch on our 310RL to transfer some weight from the front compartments. I was able to add about 216# onto the hitch. I'll have to revisit that and finger out how much I unloaded the truck axle.

 

[Trent Mills]

Great post, thanks! Don't forget side to side is important too. When we weighed our rig (340RL) wheel by wheel the off driver side (kitchen side) rear wheel was at the 3500 lb max. The door side front wheel weighed out at only 2500 lbs.

 

[M and E]

Great post, thanks! Don't forget side to side is important too. When we weighed our rig (340RL) wheel by wheel the off driver side (kitchen side) rear wheel was at the 3500 lb max. The door side front wheel weighed out at only 2500 lbs.

That is super interesting. My neighbor is a retired GM engineer who gave me that calculation. I’m wondering if there is a calculation relative to disequal weight displacement and its influences on inertia, tracking, stopping, sway. I’ll ask.

 

[George]

After research and looking again at your post I came up with a slightly different methodology.
I'm not sure you should be using the overall length of the trailer since the Force is at the pin and the lever length is defined by the fulcrum and forces.
Just my $0.02. I'm not an engineer. Run this by your engineer neighbor please.
.
I'm a retired mechanical designer with nothing better to do 😜 so I created this spreadsheet and drawing. Unfortunately I can only attach a small subset of file extensions and neither "numbers" nor "xlsx" is one of them. 🤷🏼‍♂️

 

[M and E]

After research and looking again at your post I came up with a slightly different methodology.
I'm not sure you should be using the overall length of the trailer since the Force is at the pin and the lever length is defined by the fulcrum and forces.
Just my $0.02. I'm not an engineer. Run this by your engineer neighbor please.
.
I'm a retired mechanical designer with nothing better to do 😜 so I created this spreadsheet and drawing. Unfortunately I can only attach a small subset of file extensions and neither "numbers" nor "xlsx" is one of them. 🤷🏼‍♂️

I’ll run it past him (actually his wife too who was on the corvette program!) but this was his logic in the application of the lever arm formula. There are other variants of the formula but this was a simple method to get a ballpark and yours may be better. I do think his is simpler and probably reliable for the intended purpose; to produce a general idea of about how much weight was transferred to the pin by adding mass.

 

[M and E]

I’ll run it past him (actually his wife too who was on the corvette program!) but this was his logic in the application of the lever arm formula. There are other variants of the formula but this was a simple method to get a ballpark and yours may be better. I do think his is simpler and probably reliable for the intended purpose; to produce a general idea of about how much weight was transferred to the pin by adding mass.

Yup. Yours is more precise. This was his off the top of his head calculation in passing. I started looking into it more deeply and there are a lot of very simple formulas that can be applied. If I understand correctly, the model he gave me was an excerpt of another formula that considers weight on both sides of the fulcrum. Yours is precise the one I posted is general idea only.