Proper placement of trailer axles can make the difference between a trailer that tows safely and easily and a trailer that is a danger to everyone around it anytime it is being towed. Too far to the rear and the percentage of the cargo weight that is carried on the trailer hitch is too high, causing undue wear on the tow vehicle's suspension and tires. Too far forward and the trailer tongue weight won't be enough, allowing the trailer to sway badly.
For Single Axle Trailers
Start by measuring the length of the trailer cargo box or platform, but do not include the trailer tongue. Multiply this length by .4. The resulting number is the distance from the rear of the cargo box to the centre of the axle. For example, the math for a 10-foot box is: 10 x.4 = 4. So, in this example, simply mount the axle so that the axle centre point is four feet from the rear of the trailer.
Double Axle Placement
The mounting point for a double axle set-up is calculated in the same manner as the single axle above, except that the calculated number is the centre point between the two axles. Using the same 10-foot cargo box as our previous example, we would make a mark four feet from the back of the trailer. This mark represents the centre of the equaliser for the front and rear spring. When this distance is known, the total length of the spring and half the equaliser is taken, and the other spring mount is welded at that distance from the centre mark..
By calculating the total length of all the axles, springs and equalisers to determine the centre of the assembly any number of axles can be added to a trailer. Simply mount the axles where the centre of the assembly matches the 40 per cent mark on the trailer frame. On a three-axle placement, for example, this would mean the centre of the middle axle was at a point 40 per cent in from the rear of the trailer.