What do you do when you need more carrying capacity than a 4X4? You add an axle and make a 6×6. we review the 6×6 Australia Mazda BT-50.

THE RSPCA EXISTS to prevent cruelty to animals, but there’s no such institution for 4X4s. But if such an organisation existed then it would be kept very busy with a series of overloaded wagons and utes, mostly illegally so, and lots of cases of cracked chassis. 
 
The problem is a perfect storm of factors that contribute to trouble. The first is that today’s utes and wagons are very powerful, safe and comfortable handlers, so you can load them up (and over) yet they still move, unlike days of old when an overloaded vehicle couldn’t get out of third gear. Then there’s payload ranging from 600-800kg for wagons, and 900-1100kg for utes, numbers which aren’t huge, but combine that with the third factor of people wanting to take more and more gear and you end up with vehicles attempting to carry weights well beyond their design capacity. The gear we take is getting heavier too, as lightness is something of a foreign to concept the 4WD industry.
 
The problem is ‘solved’ by running very stiff springs, and in the case of utes, with unfeasibly large overhangs placing significant amounts of weight a long way behind the rear axle. The chassis, brakes, engine and everything else isn’t designed for that load so we end up with things breaking…and then owners complain their vehicle is weak. There’s a reason the GVM (gross vehicle mass) is a maximum, not the equivalent of a yellow advisory speed sign.
 
There are a number of solutions to the problem which are described at the end of this article, but one is to simply make a 6X6 – six wheels, six wheel drive – out of a 4X4. And if that’s what you want to do, then 6X6 Australia can help, as they have converted hundreds of vehicles in Australia and overseas, ranging from Nissan Patrols to Toyota 70 Series and now BT-50 and Toyota 200 Series. To find out more, we spent a day with Bob James, the founder and brains behind 6X6 Australia.
 
 

Body and chassis

A 6X6 is going to be longer than a 4X4, so the chassis has to be extended. The chassis rails are cut, and an extension of around 1200mm long is welded in place, then reinforced with a RHS (rolled hollow section) steel strengthener, and additional flate plate reinforcements at critical points.  
 

 
 
Ute cabin bodies are typically left as-is, and a longer new tray or cargo system can then be added – take your pick of anything from a tray to a service body. Wagons are a different matter. These are typically converted into a dualcab ute body by removing the cargo area, and then with the chassis extension there’s plenty of space for a ute body of some description, unlike a usual wagon-to-ute conversion where the tray has to be very small as it’s mostly behind the rear axle.
 
Only vehicles with a separate chassis can be converted – this means a vehicle which relies on its steel chassis for its load bearing strength and has a body that is non-structural sitting on the top of the chassis. This is as distinct from a monocoque construction where the body directly contributes to the overall strength and rigidity of the vehicle, which means changing the shape is a much more complex affair than merely extending two long, thick chassis rails. 
 

The 6X6 driveline – challenges and solutions

These 6X6s leave the original drive configuration as unchanged as possible. The original rear axle is used as the third axle, and a second axle is sourced. The differential is modified to take an output shaft and drive the third axle – so if it’s a part-time 4X4 vehicle like the GU Patrol which is 4X2 onroad, 4X4 offroad then the conversion is 6X4 onroad, 6X6 offroad. A constant 4X4 vehicle like the 200 Series is 6X6 at all times, but its centre differential lock is unchanged. Some 6X6 offroad vehicles are 6X2 onroad, which can lead to traction problems as the third axle is not always in firm contact with the ground. Still, a 6X2 avoids problems with windup.
 
Windup is a problem that every vehicle which drives more than one axle faces, whether it is a 4X4 or, 6X6, or even an 8×4. In short, every time a vehicle goes around a corner the front axle travels further than the rear. If both axles are driven, then you need something between the axles that allows both to be driven, but driven at different speeds otherwise on high-traction surfaces the transmission “winds up” which leads to a breakage. The same problem is found when the second and third axles are driven on a 6X4, albeit to a lesser degree as those two axles are closer together than the first and second axles.
 
The problem of windup on a 6X6 between the front axle and the rear two is taken care of by the donor vehicle’s base 4WD transmission – this is typically either part time 4WD where the front axle is simply disconnected from the driveline, or a centre differential that allows both front and rear axles to be driven but at different speeds around a corner. So the 6X6 challenge is all about windup between the second and third axles.
 
Ideally, you’d want some means of driving the second and third axles on a 6X4 which permits them to be driven at different speeds, just like how the centre differential works on some 4X4s. That would reduce tyre wear through avoiding scrubbing, improve the steering and provide additional traction over a 6X2 drive.
 
The engineers at 6X6 Australia have a solution, and it is a drive overrun system they call a power divider, as pictured below:
 

The third axle is free to rotate faster than the second axle, either going forwards or in reverse. This means when the vehicle turns a corner the scuffing on third axle’s tyres is minimised and the vehicle is temporarily a 6X2. However, the third axle cannot rotate any slower than the second…so the moment there is any traction loss the power divider locks up and the vehicle is back to 6X4, or 6X6 if the front wheels are also driving. As the second and third axles are close together there doesn’t need to be much allowance for different wheel speeds, and the divider is designed to lock up after about only 1inch of tyre rotation on third axle. The overall concept is roughly equivalent to automatically locking mechanical cross-axle differentials, and automatically unlocking mechanical front hubs on 4WDs. Speaking of cross-axle locking differentials, these can be installed on any or all of the three axles if they are available for the vehicle. 

The electronic aids; braking, stability and traction control

The braking system needs modification too. Drum brakes as typically found on utes can be retained, or replaced with disc brakes. If the vehicle already has disc brakes then another set of brakes is sourced for the third axle and fitted.
 
The electronic systems present a challenge as they are only designed for a four-wheeled vehicle. The solution is to “siamese” from the third, OEM axle, to the second axle, so whatever happens to the third axle also happens to the second axle.
 
There’s four primary electronic programmes to consider; ABS, EBD, stability control and brake traction control. These are related but distinct electronic driving aids, all of which rely on sensors to pick up dynamic movement such as wheel speed, yaw and pitch. 
 
The ABS system detects individual wheel speeds and when one wheel begins to rotate slower than the rest the system identifies it as about to lock up and briefly reduces brake pressure on that wheel alone, preventing a lock. In a six wheeler, the third axle is the most likely to lock up under braking due to the the forwards weight transfer – same as the rear wheels being less likely to lock in a four wheeled car – so it makes sense for the third axle to control what happens to the second axle. For example, if the left rearmost wheel is about to lock then it, and the middle left wheel will have its brake pressure reduced by ABS. That’s not a perfect system as it doesn’t necessarily follow that the two rearmost wheels need the same amount of brake pressure, but given the two wheels are close together it is a reasonable compromise to avoid massively expensive electronic re-engineering.
 
EBD is electronic brake distribution and this works by channeling more braking power to the axle that needs it, typically the rear. The system is unchanged in a 6X6 operation, and again driven off the rearmost axle. If the system detects that the front wheels are locking it will send more brake force to the rear, and given the rears now have less work to do as there’s four of them then they can accept more braking power. Again, ideally each of the four rear wheels would be individually controlled, but the design is effective as it is. 
 
Then there’s stability control, which is explained here. A brief overview; stability control systems detect oversteer and understeer, then individually apply brakes to keep the vehicle going in the direction the steering wheel is pointing. The siamesing system works well here, as if the inside wheel rear needs to be braked then it’s best that both inside rear wheels are braked. The only slight disadvantage is that because the system thinks there’s just one rear wheel the initial braking pulse may not be accurate, perhaps over-compensating, but as it is a closed-loop system it will instantly sense the change in vehicle attitude and adjust very quickly, measuring the braking force by the results on the car’s attitude. The yaw sensor would also be slightly thrown out as the vehicle’s yaw characteristics and centre of gravity would have changed, but again the closed-loop system helps compensate. Finally, the 6X6 has considerably more lateral grip than the 4X4 so the chances of stability control kicking in are lower. Overall, no real concerns with stability control operation and the 6X6 itself is certainly more stable than its 4X4 donor.
 
Finally, brake traction control. Again we have an explanation here, but in brief this system detects wheelspin and brakes individual wheels to direct torque to the non-spinning wheel. This is where the siamesing is a disadvantage, because if the one of rearmost wheels spins relative to its partner on the same axle, then the wheel on the second axle ahead of the spinning wheel will also be braked…and that’s potentially not necessary and will slow the vehicle down. Conversely, that wheel could spin and not be given a control sign to be braked. But in practice, given the huge traction from the 6X6 system, the fact the four wheels are very close together, and that the electronics are closed-loop it is likely these issues are more theoretical than practical. Unfortunately, our test drive was nowhere near extensive enough to explore the issues fully, and they would only be an issue – if at all – in extreme circumstances. There’s also the option of cross-axle differential locks which would solve the brake traction control issues, although there’s pros and cons of traction control vs lockers.

Suspension

The suspension is a clever design and different to most 6X6s.  It looks like this:
 
 

The two rear axles are attached to an arm (in green) that pivots around a central point (yellow). The combination of axles and arms is known as the bogey.
 
The design means that as one axle moves in one direction, the other one is forced in the opposite direction, helping keep all four wheels on the ground, not unlike how a live axle works, but longitudinally not laterally. Here’s a photo of the arms:
 

The photo below shows the A-arms which connect to the top of the differentials (not shown on the diagram above). The axles can pivot laterally and longitudinally as they are mounted by a bearing to the A-arms. The triangular mount point for the bogey is visible below.  The steering diagram further down shows the A-arms in light grey.
 

 
Steering is always going to be a 6X6 problem, because the rearmost wheels will tend to drag around corners. This leads to excess tyre wear, reduced fuel efficiency and stress on the steering. But here again 6X6 Australia has an answer.
 
When the vehicle corners, say to the left, there’s body roll to the right. The suspension geometry is arranged so that on the right side of the vehicle the distance between the second and third axle increases, and on the left, it decreases.  Somewhat exaggerated, it looks like this:
 
 
 
The effect is that the rear wheels help steer the vehicle, and help avoid the 6X6 issues of tyre scrub on the rear four wheels. Changing suspension while cornering to help steer in this way is not unusual for vehicles – many cars are designed with camber changes for example – but it is for 6X6 conversions. The system appears to be effective, but on a offroad sideslope would tend to move the vehicle down-slope. The concept of what is in effect all wheel steer is not unusual either, and Porsche have employed just such a system in the latest 911 although that’s computer controlled, whereas this one works just off weight transfer.
 
The two photos below are of the BT-50 pictured in the title. Bob jacked up one side of the vehicle to simulate weight transfer, and the results are below, showing the rear-steer system.
 
 

 
Any leaf springs are replaced with coils. There are many advantages of coil springs compared to leaves; there’s negligible friction in coil spring operation so the damping can be left entirely to the shock absorber which can be precisely valved for different rod speeds, for example corrugations are high rod speeds, dropping off rocks is low speed. The net result is better ride and handling.
 
Leaf springs also do not locate the axle very well – by definition, as the location system is designed to bend as it is a spring. You only need to look under the average leaf-sprung ute to see asymmetric shock absorbers (one ahead of the axle, one behind) which is done to combat torque twist under acceleration. In contrast, a coil system has a series of separate arms (such as Panhard rods or Watts linkages) to locate the axle which are far more effective. These arms also allow the suspension geometry changes under suspension compression and extension which assist with the steering. 
 

What about the rest of the components?

Components such as the transmission and cooling are left standard, as is the power. There’s another set of brakes so those are upgraded in line with the vehicle.

But now the vehicle has an extra 250-400kg to lug around, and can be loaded to its new GVM of an additional tonne or more. This will place extra stress on the transmission components, but in some ways the vehicle is designed for it.

The new GVM (maximum the vehicle can weigh) is 4495kg for versions that can be driven on a car license, and the GCM (maximum all up weight of vehicle and trailer) for all of the vehicles is a lot more than 4495kg. Also, the new tare (unladen) weight is well below the original vehicle’s GVM.

However, loading the vehicle to its maximum and then towing the maximum would be pushing well out of the original design limits, which would mean wearing out components quicker that usual.

Driving impressions

We had a decent dirt-road and bitumen drive in a 2012 BT-50, which had a bullbar, snorkel and the 6X6 conversion but was otherwise stock. Climbing into the cab you’d never know there was an extra set of wheels, and indeed there’s not much difference once you’re under way. The 6X6 conversion adds around 250-400kg, so a stock BT converted to 6X6 weighs around the same as one modified for offroading with the usual accessories.
 

Around twisty bitumen there’s no real difference either, until you start to push it a bit harder. That’s when you notice the vehicle feels a bit more planted, more stable. It doesn’t understeer any worse than a normal BT-50 though, thanks to the self-adjusting rear suspension. 
 
It’s on dirt roads that the car starts to become seriously impressive. When Bob was driving he took the BT up to around 90km/h and wildly waved the steering wheel back and forth. Takes a bit to scare me in a car but that made me nervous! Yet the BT just followed the control input and obeyed, no drama at all. I did the same on bitumen and dirt with the same result, much the consternation of our back-seat passenger…
 
Oh, and I forgot to mention something. Take a look at this:
 
 
Yes indeed, we cruised around with that on the back, a full 1275kg of load, plus the car’s barwork at the front. That’s way more payload right there than just about anything else on the market.
 
Even with that load, the handling, ride and control were truly impressive, because the load was able to be carried ahead of the rear axles, and the four wheels at the back provided considerable stability. We didn’t have a chance to do any offroad work, but having driven Mercedes G-Wagen 6X6 vehicles and a Pinzgauer then I am sure this 6X6 would easily beat 4X4s in the rough.
 
We didn’t get a chance to drive it unladen either. However, it should do quite well as much of the load is taken by four springs which can each be softer than just two. Therefore, you wouldn’t expect the harsh ride typically found with unladen heavy-duty utes.The use of coil springs over leaves would help too, as would the greater directional stability.
 
The one major drawback is maneuverability. The 6X6 is significantly longer (around 6.5m for the BT-50 compared to 5.3m stock), and the turning circle is greater. Not a huge amount though, maybe another 1 to 1.5 metres; the exact figure hasn’t been measured. Still, I would be happy taking it into tight High Country tracks, where it’d be a bit more work in some instances than a 4X4, but still usable, far more so than say a tall light-truck 4X4, camper trailer or bigger American truck. 
 
Overall, the 6X6 is easier to drive than the equivalent 4X4 on bitumen and dirt roads. It is certainly easier to drive than a trailer, and much easier to maneuver. I can’t really think of a reason not to recommend a closer look if you want greater bulk and weight carrying capability from your 4X4.
 
 

6X6 and 4X4 comparative specifications

• A 150kg allowance for a tray had been added to the 6X6 models. The 4X4 models come with trays. All tare weights are approximate and dependent on the exact model.
• LC79 rear axles widened to match front!
• Light truck versions require a Light Rigid license to drive.
• No light truck version for the Ranger/BT-50.
• The greater payload for the 6X6s means the maximum towing weight is more realistic than the 4X4 (more on that here.)
• The length of the vehicle is increased by about its chassis extension. No figure is given as the exact length is dependent on tray or tub dimensions. However, the BT-50 is 5300mm long so a 1200mm extension makes it 6500mm. A bullbar would increase that slightly, as would a custom rear bar.
• All weights below in kg.

 

	LC79 singlecab ute	GU Patrol Cab Chassis (coil)	Ranger PX / BT-50 dualcab	LC200 GXL
	4X4	6X6	4X4	6X6 (car)	6X6 (LT)	4X4	6X6	4X4	6X6 (car)	6X6 (LT)
Tare	2065	2640	2135	2700	2700	2250	2530	2740	3150	3150
GVM	3300	4495	3150	4495	5200	3200	4495	3350	4495	6200
GCM	6800	7995	6300	7995	8500	6000	7995	6850	7995	8500
Payload	1235	1855	1015	1795	2500	950	1965	610	1345	3050
Trailer braked	3500	3500	3200	3500	3500	3500	3500	3500	3500	3500
Chassis extension (mm)	–	1000	–	1000	1000	–	1200	–	1000	1000

what are the alternatives to a 6×6?

Forty thousand dollars is what you’re looking at for a conversion, so let’s look at the alternatives. Clearly if you are considering a 6X6 then you must have a lot of weight or bulk to carry. Taking the BT-50 as an example, you’re raising the payload from 950kg to nearly 2000kg, and getting a true 3500kg towing capacity as distinct from the rather misleading capacity quoted by Mazda and Ford. 
 
So here’s what else you could do if the standard car isn’t up to your job. All of the options below have as an advantage extra load carrying capacity in both bulk and weight which is why they’re listed.

Trailer

Caravan or large camper trailer.
 

 
Pros

• You can disconnect the trailer from the towcar returning the vehicle to its agile original.
• Huge range of trailers, you can rent, own more than one.
• Cheapest option.
• Simple, just requires an electric brake controller and upgraded suspension.

Cons

• Significant fuel consumption penalty.
• Reduces offroad capability.
• Trailers cannot be towed everywhere, either by limit of the vehicle capability, or by law.
• Not as easy to drive onroad.

Light truck

An Iveco Daily, Mitsubishi Canter or other light 4WD truck.
 

Image: http://www.traveltrucks.com.au/
 
Pros

• Robust vehicles built for heavy duty cycles.
• Forward control cab gives a good length-to-cargo-space ratio.
• Several companies offer prebuilt options.

Cons

• Relatively poor handling on road.
• Size, especially height.
• Safety and mod cons are nowhere near smaller vehicles.
• Limited selection of vehicles.
• Vehicles heavier than 4.5 tonnes GVM require a Light Rigid license.

Extended 4X4

Extend your 4X4 chassis.
 

Image: http://www.creativeconversions.com.au
 
Pros

• Cheaper than a 6×6 for an extended length.
• Easiest way to get a ute version of a wagon eg. 200 Series, wagon Patrol.
• Based on common vehicles so parts and servicing are easy.

Cons

• Poorer offroad capability with an increased wheelbase, usually extra weight with no extra tyres.
• GVM (Gross Vehicle Mass) unchanged unless additional work done.
• Tow rating likely to be unchanged.

Larger ute

American style ute such as an F250, Ram, or Tundra as pictured below.
 

 
Pros

• Used as the manufacturer intended, no changes.
• Potentially luxurious interior.
• Usually high payload and tow rating compared to smaller utes, wagons and extended wagons.

Cons

• Taller, wider and longer. Not for tight tracks.
• Rare vehicles so parts an issue at times.