Hi Folks,

I recently weighed my camper on a weighbridge in order to determine its overall empty weight and the load on the drawbar. The process involved driving the trailer onto the scales with the vehicle off the scales but the drawbar still connected. Then the drawbar is disconnected (with the jockey wheel down) and the second figure is obtained. The second figure is the total empty weight and by subtracting the first figure, the load on the drawbar is obtained. So you can check to see that you are not towing too much weight overall, and you can see what the load on the drawbar is, which may also have legal limits.
When you head off on a trip, with the camper loaded, you need to weight it again to see if you are within limits. Bit of a nuisance, and costs about $20 each time, more if you want an "official" weight.
And what if you are planning to make some changes? Let's say you are thinking of putting a second battery somewhere on the trailer. You can add the weight to your existing data to get the new total load, but what effect will it have on the drawbar weight? If you put it in front of the axle, it will increase the drawbar weight, and if you put it behind the axle, it will reduce the drawbar weight- but by how much?
The attached spreadsheet allows you to calculate (roughly) the impact of such changes.
The spreadsheet uses the tailgate of the trailer as a measurement reference point. Any point will do, but I chose the tailgate as most equipment will be added forward of this. However, be aware that if you bolt something onto the back of the tailgate (eg another spare), you need to enter the distance from the reference point as a negative number.
You need to know your trailer's axle weight and drawbar weight from a measurement on a weighbridge as described above. You also need to know the distance from the reference point to the axle, and from the reference point to the trailer coupling on the drawbar.
Enter these figures on the LHS of the spreadsheet and it will then calculate the position of your trailer's center of gravity (cg) and the distance of the cg from the axle and from the drawbar.
When you load your trailer, you add the weight of the item and it's distance from the reference point and the spreadsheet re-calculates the position of the cg and the axle and drawbar weights respectively.
If the drawbar weight becomes negative, it means it will lift up when disconnected from the car!
Most items are usually stored in the same place when you go camping, so once you have entered the usual items the calculator will be set up for most trips. But if you decide to modify your trailer, you can add (or remove) the weights at the respective distances and see what effect it will have on your drawbar weight before you go ahead and do it.
The accuracy of the spreadsheet is limited by the accuracy of your distance measurements and weights. Exact figures aren't necessary but at least it gives you an idea of what is likely to happen if you make a major change.
The spreadsheet is protected so you can't accidentally erase cell formulas, but there is no password, so those of you who want to tinker with it can do so if you wish.
Hope you find it useful,
Cheers,

C00P
Drawbar Calculator.xlsx