Trailer Design

1. Brakes
The type of brake fittings need to comply with VSB01 Standard as well as practical uses. For example, an override coupling would not be recommended for 4WD use as the braking cannot be controlled when travelling up and down steep roads causing it to shunt off and on. Trailer weight will also affect the type of brakes needed.

2. Mudguards
These need to be selected based on the diameter of the outside of the tyre as well as the width. The wrong mudguard and tyre width combination can cause the tyres to fit outside of the mudguards. Please allow room between the hub face and chassis to fit brakes.

3. Overall Width
Leave enough room for accessories. 15mm wide clearance lights can make a trailer exceed the maximum legal width of 2500mm. Also caravan accessories, such as awnings, can push this dimension past its legal limit.

4. Height
Consider height limitations such as bridges and the height of garage roller doors to fit the trailer. Be careful that the trailer isn’t too top heavy or is made unstable from wind.

5. Stability
Avoid, at all costs, putting heavy items too far to the back of the trailer – behind the rear axle. Too much rear weight can unexpectedly make the trailer sway violently out of control. The heaviest items going into the trailer should be placed in the centre and slightly toward the front of the trailer, without compromising the weight down-force on the tow ball. Leave enough weight allowance to account for additional items such as toolboxes already on the drawbar, and other items and accessories.

6. Axle Centre Position and Tow Ball Down-force
Position the axles as far back as possible. The further back the axles are, the more stable the trailer will be. However, be aware that moving the axle too far back can cause the load in the trailer to overload the tow ball down-force limit as well. A combination of both of these is required to ensure everything is aligned.

7. Length and Overhang
Avoid making the trailer too long. When choosing to make a trailer longer, rather than wider, be mindful of rear overhang. There is a limit to how long a standard trailer can be made, so please be mindful of your trailer length. Consider choosing a tri-axle setup as it decreases the overhang past the rear axle and increases stability and spreads the centre weight, creating less down-force on the tow ball.

8. Weight
Make sure the towing vehicle can handle the tow ball down-force as well as the pulling weight. For details related to trailer weight and engineering requirements, please consult VSB01 or an engineer.

9. Corrosive Environment
Do you need to use galvanised or Zinc Tech parts?

10. Width and Driver Fatigue
Avoid making the trailer too wide, where possible. Towing wider trailers requires a higher level of concentration than usual, this can cause fatigue.

11. Chassis Clearance for Wheels
Leave enough room for chassis clearance inside the mudguards. Often customers end up having to alter the chassis width or compromise on their favourite alloy wheels because they have purchased the wrong combination of wheels, mudguards and brakes. Give yourself some extra space when building, or enquire first. A 10-inch wide mudguard (260mm) will not fit a 250mm wide tyre – this leaves only 10mm between the tyre and the chassis, which is not enough. This could lead to the tyre rubbing out on the chassis.

Below is the minimum formula that should be used for standard rims for the tyres to be flush with the outside of the mudguard.

260 mm      Wide tyre
+ 25mm     Inner mudguard to tyre clearance (40 – 50mm for 4WD).
+ 5mm       Outside mudguard to outside of tyre clearance.
_________
290 mm     Minimum mudguard width.
_________

Allow another 10 – 20mm on the outside, if possible. A wider gap can also be used toward the front.
Not working it out on paper or not consulting an expert is the difference between an easy, professional job and one which is troublesome.