Why are there two sizes of towballs in New Zealand…and does it matter which one I use?

In a rule change a number of years back the Government introduced the 50mm towball, in line with the European regulations. About 95% of towballs now sold are 1 7/8” diameter, and the remainder are 50mm diameter. 1 7/8” is the most common towball size used on vehicles in NZ, while 50mm is standard in Europe. There is not a lot of difference between the two sizes – a whole 2.375mm, to be exact!

This has brought about a lot of confusion as while a 1 7/8″ coupling will not fit on a 50mm towball, the      1 7/8″ towball fits into the 50mm trailer coupling easily but is a loose fit, which is very dangerous and must not be done.

It must be realised that a towball and coupling are critical components which are engineered to high standards with very little tolerances or margins for error. The two items form the crucial link between the tow vehicle and the trailer – any incompatibility between the two could have serious consequences. With that in mind, it is critical the correct pairing is used. After all, appearing in court because your trailer detached due to using an incompatible coupling & towball configuration and caused an accident is an outcome that no one wants.

In most situations, the towball and coupling will be 1 7/8”, so you are good to go! All towballs have their ball size stamped into the top of the ball, so ensure that you check that your coupling is the correct size before you hitch up and drive off.

However, if a trailer is towed by multiple vehicles (a trailer from a hire centre, for example), they cannot be sure what size towball their customer will have on their vehicle.

There are a few options to overcome this issue safely:

• Trojan® developed a Duofit® coupling. This works, although it does require the operator to select the correct ball size position and seat the adjustor bolt. This might sound simple, but we have seen multiple instances of trailers being towed with the incorrect setting applied.
• CM Trailer® developed their Multi-fit®. This will automatically adjust to suit the size towball, although market feedback we've recieved suggests care must be taken when using, especially on heavy trailers and in wet conditions.
• A Trailparts® Autofit® coupling is the industry leading coupling. Developed in conjunction with hire companies who had seen enough accidents caused by fitting couplings to the wrong sized towball, this coupling is impossible to fit incorrectly. Just drop the coupling head over the towball and it will automatically select the right ball size position. It even takes up slop if the towball is worn. It is easy to use in all conditions.

Also as an option for vehicle owners that tow multiple trailers, they may opt for 2 towballs – 1 of each size and swap them accordingly. This is time consuming and also requires tools to be carried. As a premium option there is a Convert-A-Ball® and a Duo-Ball®, which are a fixed shank sold with both a 1 7/8” ball and a 50mm ball, which allows users to switch ball sizes quickly and with minimum tools.

And as always - if unsure which size ball you have or what coupling you need - ask a local engineer or contact a company who sells them and get their advice. When it comes to couplings, hitching and ensuring your trailer is properly and safely connected, it is always better to be safe than sorry!