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On road towing

By Dean Mellor, 04 Aug 2011 Gear

Tech: On-road towing

State road and transport authorities treat towing trailers as child’s play, but shouldn’t an articulated combined mass of up to four-and-a-half tonnes travelling at 100km/h be left to those with extensive training?

State road and transport authorities treat towing trailers as child’s play, but shouldn’t an articulated combined mass of up to four-and-a-half tonnes travelling at 100km/h be left to those with extensive training?

I’ll never forget the first big trailer that I towed. I was in my mid-20s and I’d not long been working on a then-popular four-wheel drive magazine. The editor wanted to run a towing comparison test and he wanted me to do the driving.

Up until that point I had towed nothing bigger than my dad’s box trailer, from home to the tip, at a speed of little over 60km/h. But this test was going to involve a tandem-axle trailer with a massive boat on top, weighing some 2000kg all up, which was nearing the stated towing capacity of some of the vehicles on test. And our test loop would involve city driving, a wickedly long and steep descent, a power-sapping hill climb and a quiet stretch of long, flat road where we could measure the performance and braking power of all seven vehicles.

The boat itself was brand new and worth several times my annual salary. Unfortunately, the trailer was a rusty old thing that had launched many a boat into salty water, so we had to fix a few things up first, such as repack the wheel bearings, fix the override brake piston and replace some globes, before we deemed it roadworthy.

Before each test drive, I carefully hitched the trailer, checked that the lights were working and then a colleague with a notepad and a stopwatch and I would head out on to public roads to start the testing procedure.

Initially, I was almost solely focussed on the mirrors to make sure I didn’t clip a gutter with the trailer wheels or veer out of my lane. Luckily my colleague in the passenger’s seat reminded me that there was traffic in front of us and that I should spend more time focussing my attention that way.

Around town I soon learnt how to look well ahead and predict the cycle of the traffic lights. More importantly, I also learnt about extended braking distances, the beneficial effect of ABS and the importance of removing the bung from a boat in inclement weather when it’s on the trailer.

Once on the open road I soon discovered what trailer sway was all about and the best way to control it once it had started; we didn’t have an anti-sway device nor a weight-distribution hitch fitted to the rig. I also learnt a lot about what gear I should select well before a big hill (up or down) and that other traffic would often want to desperately get in front of me and then slow right down once they had. I’ve since performed many towing tests and now consider myself to be a reasonably competent driver with a trailer hitched up, but the fact that I learnt all of this on a public road is somewhat worrying.

In this day and age of occupational health and safety, I doubt that the company I then worked for would let me out on to the road under such circumstances without first sending me on an accredited course on towing trailers. As a private road user, however, with most big four-wheel drives I can still legally tow a tandem-axle trailer weighing more than 2000kg with nothing more than a standard driver’s licence. A national set of towing guidelines was agreed to between all states and territories back in December 1998. These guidelines, published by the National Transport Commission, state that: “A motor vehicle with a Gross Vehicle Mass (GVM) not exceeding 4.5 tonnes must not, without the approval of an authority, tow a trailer with a mass (including any load) exceeding: The capacity of the towing apparatus fitted to the vehicle, or; A relevant maximum trailer mass specified by the vehicle manufacturer.”

Basically, this means that you cannot exceed the towing capacity of your vehicle as stated by the vehicle manufacturer or the capacity of your towing hitch. It should also be noted that you should never exceed the Gross Combined Mass (GCM) of your 4X4, which is the total mass of the loaded vehicle and the loaded trailer. Finally, you should never exceed the maximum weight as stated by the trailer manufacturer. So long as you don’t exceed the above weight limits, you are free to travel on the nation’s roads at the posted speed limit, except in Western Australia where the maximum speed for vehicles towing trailers is 100km/h.

Now I don’t know about you, but I don’t at all like the idea of an inexperienced driver in control of an articulated vehicle weighing up to 4500kg travelling at 100km/h; especially a vehicle that might not be properly set up for the task at hand. Nevertheless, the rules are the rules, so here are some tips on how to properly set up your rig for safe towing.

Even if your trailer weighs less than 750kg, it will still have an impact on the driving dynamics of your 4X4, affecting its handling, performance and braking characteristics, especially if it’s a lightweight vehicle. The last point is possibly the most important one, as trailers that weigh less than 750kg are not legally required to be fitted with brakes, so the four brakes fitted to your vehicle will be working overtime to haul up the combined mass of vehicle and trailer. But at this trailer weight, towing is a relatively straightforward procedure and, so long as you’re aware of your trailer and the fact that it takes a tighter arc when cornering than your vehicle, and you can competently reverse it without backing into something or someone, you shouldn’t get into too much trouble.

It’s when your trailer exceeds 750kg that you have to take much more of an interest in correctly setting up your vehicle and trailer. Balancing your vehicle and trailer is of critical importance and if you intend to tow a big, heavy trailer, you’re best off doing it with a big, heavy 4X4. Past experience has taught me that vehicles with wide tracks and long wheelbases usually make the most stable towing platforms, but modern electronic traction aids such as stability control have made many vehicles with narrower tracks and shorter wheelbases just as stable when towing. Some modern stability programs even have special modes for towing applications.

Part of the balancing act is achieving the correct towball weight. Too much weight can stress components such as the tow hitch and the vehicle’s rear suspension, as well as take too much weight off the tow vehicle’s steering axle, thus affecting the vehicle’s ability to steer safely. Too little towball weight, or even a negative load, can cause the trailer to swing and even reduce the grip of the vehicle’s rear wheels. A general rule of thumb is that you want around 10 percent of the overall trailer weight to rest on the towbar. This can be achieved by balancing the trailer itself by moving its contents – such as gas bottles, spare wheel, water tank etc – fore or aft. If this is not possible, such as in the case of a boat trailer, you can use an adjustable weight distribution hitch with longitudinal torsion bars. The 10 percent rule is not a fixed one, however, and you should never exceed the towball capacity stated by either the vehicle or the towbar manufacturer.

Many new trailers are fitted with electric brakes that require the tow vehicle to be fitted with a brake controller. The brake controller allows you to adjust the trailer brake timing and pressure. Ideally, you want the trailer brakes to actuate just before the vehicle brakes do, which will help the combination pull up in a straight line. By adjusting trailer brake pressure depending on the load in the trailer, you’ll be able to get the maximum possible braking pressure without locking the trailer wheels, which can result in trailer swing.

If you don’t get the vehicle and trailer balance right, or the brake bias right, the rear wheels of the tow vehicle can lock up under braking, which can result in a dangerous jackknife situation. Fortunately, this scenario is rare these days thanks to anti-lock braking systems.

Depending on your choice of tow vehicle and the weight of your trailer, you may need to upgrade the rear suspension to cope with towing duties. Many modern 4X4s will sag significantly at the rear-end when even a moderate towball load is applied, upsetting the balance of the vehicle and leaving the headlights pointing up in the trees. Heavy-duty rear springs can prevent this sag, as can the fitment of airbag suspension or airbag helper springs.

If you have everything properly set up, your towing combination should be perfectly stable on the road, but there are a number of factors that can still lead to instability such as trailer sway, including poor trailer design, excessive speed, abrupt steering manoeuvres, aggressive braking and acceleration, crosswinds and even oncoming traffic such as large trucks. If trailer sway is an issue with your towing set-up, you should look at products on the market such as sway control bars, friction hitches and weight distribution hitches.

As you can see, there’s a lot more to safely towing a trailer than simply hitching it to the back of your rig. If the insurance implications of an illegally set-up towing combination don’t scare you, then the safety implications should. So far we have only discussed towing trailers on the road. Off-road towing is a whole different kettle of fish, which we’ll discuss in the next issue.