Understanding Towbars and Couplings

by admin

Those of us who tow a boat often take for granted the tow bar and trailer attachment on the back of our vehicles. We buy a vehicle and we expect it to be right. Well while 90% of the time it probably is, it may not always be the case. So let’s take a quick look at the nuts and bolts so to speak of tow bars and tow bar components. First your vehicle. Is it suitable to tow the weight your boat? The tow rating of your vehicle will vary of course depending on facts such as vehicle weight and engine power.

Most vehicles have tow ratings given to them by the manufacturer specifying the gross trailer weight braked, unbraked, or both, that the vehicle can safely tow. Although the law does not require these tow ratings to be followed, the NZ Transport Agency recommends that they are taken into account. Also, the law requires that every light vehicle and trailer combination must be capable of stopping within a distance of seven metres from a speed of 30km/h. In effect, this means that the maximum allowable weight of an unbraked trailer is limited by the weight and braking ability of the vehicle being used to tow it. Most modern sport-utes such as an Isuzu D-Max and an Isuzu MUX SUV, both are powered by grunty 3-litre turbo diesel and have a tow rating of 3500 and 3000 kgs respectively in a braked trailer format. Unbraked they are both 750 kgs. I will not go into the issues about braked trailers and all the regulations concerning that, that’s a whole new story, but suffice to say if your boat and trailer combo has a gross laden weight of 2,000kg or less then you do not require brakes on the trailer. This is provided the combination of vehicles can be safely stopped within 7m from a speed of 30kph.If the gross laden weight exceeds 2,000kg, you must have brakes on the trailer. And if you go over 3500 kgs, then that’s a whole new set of braking issues.

Tow Bars

Tow bars are not required to be rated and certified on light vehicles. Why the NZTA do not implement a tow bar certification for light vehicles is not clear. However while it may prove unpopular due to the added costs, it would ensure certainly that every tow bar fitted was up to a certain standard. Certifications on tow bars are put on them by the vehicle manufacturer and are not subject to any rigid testing by the NZTA. It is important also to have the tow bar fitted by a qualified person. 

Companies such as Auckland based Best Bars go to extraordinary lengths to ensure their tow bars are safe, robust and secure. Best Bars Ltd have been designing & manufacturing tow bars for over 30 years and are approved suppliers to many motor companies & new vehicle distributors, both in New Zealand & Internationally. The tow bar must also be correctly fitted so that it transfers the towing forces to the structure of the towing vehicle without any distortions of the tow bar or the towing vehicle’s bodywork/structure. In other words, you can’t simply bolt it to the bodywork! I know of a case where the tow bar was bolted through the panel steel of the boot floor on a brand new Ford and it broke free while being towed. The owner was towing the boat, a reasonably heavy Smuggler 6.3 on a tandem axle trailer, got the wobbles on and the result was the boat and trailer with the drawbar stillattached  careened across the grass and down a bank. The bolts, which were all still attached to the tow bar mounting plates. They had just ripped out of the floor pan. The towbar should be bolted through the most structurally sound part of the vehicle that’s practical, such as the subframe.

In the case of the Isuzu D-Max and MUX, there are two vertical and two horizontal bolts into the rear of the chassis either side and further three bolts attaching to the side plates. There’s no way this is going to come away in a hurry…if at all. The stats speak for themselves. According to the LTSA, on average, seven people a year are killed and 45 seriously injured in crashes involving a light vehicle towing a trailer (the word trailer includes caravans, as well as boat, horse and garden trailers). Incorrect loading is a factor in around 27 light vehicle crashes a year including one death and five serious injuries.


How often have you been following behind a trailer that is swaying behind the vehicle towing it? If it goes too far and the trailer pivots on the tow bar, the towing vehicle starts to sway from side to side. The boat and trailer either rolls or jackknifes and the results can be fatal. It’s around 80 km/h that this usually happens. If the trailer starts to sway, do not apply your brakes, instead, remove your foot from the accelerator and allow the vehicle to slow down. This is usually caused by having too much weight concentrated towards the end of the trailer. 

You need to have sufficient downwards force on the vehicle tow bar at the point of attachment, equal to about 5% to 10% of the weight of the load being towed. As with my Isuzu DMax that is rated for 3500 kgs towing capacity, the maximum tow ball mass load or the amount of weight on the tongue is 350 kgs. Conversely, the Isuzu MUX being rated lower at 3000 kgs has a tow ball mass load of 300 kgs. That 5-10% is a good  guide for larger boats, such as a trailer boat over 5.5m, but if you have a light tinnie or RIB of 4.5 – 5m, then it’s not so important as they are out of the wind force and tend to track easier behind the car. I used to tow a very light inflatable Thundercat that my boys raced, which was reasonably light on the drawbar and it towed fine at speed. If you do have a problem with too much weight aft, consider simply moving any tote tanks, fish or dive gear as far forward as you can before leaving the ramp. Putting the full fish bin forward will do the trick nicely. 

In defence of the trailer manufacturers, they supply a trailer to the boat manufacturer that is suited to a standard boat without all the accessories and don’t have any idea about how heavy or where that weight will be in the boat when you get it. They can only make an educated guess. When you add a heavier 4-stroke outboard, dual batteries, etc., the balance can be all out. Fortunately, today’s modern trailers allow you to easily slide the front post and the axles fore/aft to correct the balance. This is something that if you are a DIY sort of guy you can easily do, or if not, then take it to your local trailer guy and they can adjust it for you.

Towballs & Couplings

The coupling on the trailer must have a manufacturer’s rating appropriate for the gross laden weight of the trailer and be compatible with the tow ball size. There are two sizes of tow balls in use, the older and somewhat unique to NZ, 1 7/8 inch diameter ball and the more internationally accepted 50mm diameter tow ball. I have also seen the larger 70mm balls from the US, which are popular on the bigger imported trailer boats such as from Sea Ray, Bayliner and Grady White.

 If you are like me and tow various trailers then using a multi-sized coupling is a significant advantage.  I use a duo-ball that easily allows me to change from 1 7/8” to 50mm by simply changing the ball. I also have a multiple size coupling on the trailer that lets me use both 1 7/8″ and 50mm tow balls by just lifting and rotating the handle 180 degrees. The danger here is (and I have done it), is to drop a 50mm coupling onto a 1 7/8” ball. While the weight of the drawbar will hold it down for a while, the first time you go over a bump or maybe brake hard, the trailer is likely to jump off and be left hanging by the chains. I say chains because it is recommended that any boat trailer over 2000 kgs should have double chains. Also, the D-shackles and chains should be of a suitable size to handle the weight of your rig. Look for yellow or blue coloured shackle pins for the higher rated D-shackles. However all that changed for me at the recent Hutchwilco New Zealand Boat Show, when I called into the Ashburton based CM Trailer Parts stand. On display for the first time was their all new MultiFit Master Auto Select Coupling, that also carried off the prestigious Local Innovation Award. The unique patented design of the MultiFit Master Auto Select Coupling eliminates any risk of mismatching a vehicle’s tow ball with a trailer coupling, resulting in a safe and reliable connection every time. This design solution effectively addresses a major safety issue that is on the rise in New Zealand and around the word – trailers separating from the tow vehicle at speed. In addition the unit comes with a built in noise dampening device that eliminates any coupling rattle and a locking pin. Rated to 2500 kg, the Multifit Master is arguably the safest multi-fit coupling on the market.

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