TRAILER WEIGHT AND BALANCE ESTIMATE

This page aims to show how you can perform a detailed weight and balance estimate for a small trailer. It starts with the most basic calculation and works up to the sort of methodical approach I would recommend.

To keep it easy to read, only an extract of a typical weight estimate is shown for each example.

Longitudinal Datum

In order to find the longitudinal centre of gravity (LCG) of the trailer, we need to have somewhere from which to measure the longitudinal position of each item. When you are choosing this point, or 'datum', consider the following:

• It is better to pick a datum at, or beyond, one end of the trailer, so that all measurements from the datum are positive. If you put your datum in the middle of the trailer, some measurements will be positive and others will be negative - and eventually you will make the mistake of swapping a positive and a negative measurement.
• It is better to pick a datum that is 'fixed in space', rather than being fixed to some part of the trailer - for example, if you pick the hitch ball as the datum and then later extend the tongue, all the measurements will change.

Basic Calculation

The most basic calculation just adds up the weight and moment of each item like this:

Key:

• Weight - Total weight of item or items
• Lever - Distance from longitudinal datum to centre of gravity of item
• Moment - Weight x lever
• Total weight - Total of weight column
• Total moment - Total of moment column
• LCG - Total moment / total weight

You need to estimate the position of the centre of gravity of each item, to measure the 'lever'. For simple objects, like timbers or rectangular pieces of plywood, this is easy - it is the middle of the object. For more complex shapes, look at this link. You need to exercise some judgement about really complex objects - for example a rubber torsion axle has almost all of its parts forward of the wheel centre, so you need to guess where the resulting centre of gravity is. Guessing at this level of detail is still more accurate than just hoping the trailer will balance, so go ahead!

Development 1

As a weight estimate, the table above has some drawbacks. There are two objectives that the table should meet:

• It should be relatively easy to check if the weight estimate is correct - so 'hidden' calculations are undesirable.
• If design changes occur, it should be as simple as possible to update the weight estimate table.

To meet these objectives, we want to make 'visible' as much of the data and calculations as possible. Firstly, it is sensible to state clearly how many items are included in each line, by adding a 'number off' column.

Key:

• Weight each - Weight of a single item
• Line weight - No off x Weight each

Development 2

Rather than just give a weight, it would be useful to split this into a quantity and a unit weight (the weight for a quantity of one unit). For example, it is more useful (and easy to check) to specify 48.4in of tubing at 0.261lb/in, if we want to either change the type of tube or its amount. This makes the following weight estimate table look more complicated, because less is hidden.

Development 3

One final addition is to specify what each item is made of, as shown in the following table - this makes it much easier to enter new items since the same material probably already exists in the table.

An example of a completed weight estimate can be seen here.

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