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A SIX WHEELER CAN SOLVE THE UTE GVM ISSUE

An extra axle makes all the difference

Adding a third axle to a 4WD ute can increase its gross vehicle mass to 4495kg - car driver's licence limit - or higher in some cases. The additional axle carries some of the additional weight and, being braked, increases stopping power. 

 

There are two ways to add a third axle: it can be  driving axle, making the 4x4 into a 6x6, or it can be a so-called 'lazy' axle, converting the vehicle to a 6x4. There's no doubt that a 6x6 has superior off-road tractive ability to a 6x4, but there can be on-road compromises.  The close-coupled rear driving axles need a centre differential or driveline and tyre wear can be horrendous. 

In on-road driving the differences between a 6x6 and a 6x4 are less noticeable. Most conversions are done to part-time 4WDs, so on high-friction surfaces a 6x4 configuration is normally driven as a 6x2: middle axle driving and the steer axle and lazy axle non-driving. a 6x6 configuration is normally driven on road as a 6x4: steer axle non driving and the two rear axles driving. In some situations, such as sandy or loose-gravel roads the double rear driving axles can have a tractive effort advantage over the lazy axle layout. However, the driver of the lazy axle machine can always bring in the front driving axle for additional drive on low-friction surfaces.

There's no doubt that a full 6x6, four-differential, three-axle ute conversion is the ultimate, but it's a very expensive business. For most people, a much lower cost 6x4, three-axle conversion will suffice, especially where the primary goal is to increase payload space and mass.

Typical 6x4 applications are crew cab ute conversions, allowing a five-seat ute to carry a large slide-on camper, or fifth-wheeler, or to tow a large caravan with 350kg or more ball weight.

The team at Six Wheeler Conversions has been converting vehicles to six wheelers since 1985, with 1300+ vehicles now in service all over Australia. The team now operates from a new, modern Toowoomba workshop.

When OTA spoke to Mike and Julian at Six Wheeler Conversions we were impressed with their frankness. They know that a six wheeler is not necessarily the right solution for everybody and if they think people are better off with something else, they tell them so.  An example is where a customer needs little or no off-road ability and can tow a fifth wheeler or large caravan behind a light truck.

The foundation of their conversions is a 6x4 lazy axle set up. The original driveline and axles are left largely untouched, other than for eye mounts for the lazy axle springs being attached to the rear axle housing. The third-axle springs connect to those eyes and to the lazy axle. Between the third leaf spring eyes are trunnion mounts that are connected to the chassis, behind the original rear axle.

In action the original rear axle behaves normally and as it oscillates up and down the lazy axle springs pivot around the trunnion bushes: as the original rear axle moves upwards in a bump movement that upwards action forces the lazy axle downwards, keeping it in road contact. As the original rear axle drops into a hole, that action forces the lazy axle upwards, endeavouring to keep the driving axle in contact with the ground.

The dimensions of the lazy axle spring are calculated to give a roughly 60:40 weight distribution to the two rear axles, so the driving rear axle always has more weight than the lazy, on a flat surface. A side benefit of this weight distribution is minimal effect on the turning circle.

Six Wheeler Conversions says this system has been refined over 30 years of operation on many load carrying vehicles,  

Many different models of 4WDs have been converted over the years, but there are four current models that have Federal Second Stage Manufacturer approvals for new, unregistered 4WDs. Although these are type-approved at GVM and GCM ratings there are some individual compliance requirements that vary by state.

The four Second Stage Manufacturer models are Ford Ranger, Mazda BT-50, Isuzu D-Max and the Toyota LandCruiser 79 series. Under separate approvals there are conversions for Holden Colorado, Nissan Navara and many others.

Vehicles can also be fitted with patented 'Eye-Tie' ute trays. These trays allow any load to be tied down securely within the tray from an array of Eye-Tie points all around each tray. There's also a recently released the 'SupaTub'; a 2.4-metre extended styleside cargo body that suits Rangers and current BT-50s.

Six Wheeler Conversions feature:  increased payloads, up to 1.5 tonnes; increased GVMs, up to 4495kg on a car licence, or higher on the right vehicle, on a light truck licence; increased towing capacity, up to 4.5 tonnes; longer trays, up to 3.5m on single cabs;  more stability on and off road; improved dynamic safety;  better braking ability, thanks to a braked third axle; improved ride and comfort and, probably, better off-road ability and fuel economy than a fully (or overloaded) 4x4.

Six Wheeler Conversions start from $16,000 plus GST.


 


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