CAD
We use Solidworks CAD for all our design work, this allows us to design accurately, checking clearances before anything is committed to metal. It also allows us to provide you with screenshots and renders so that you can see what your build will be like and let you make any tweaks that you would like.
Finite Element Analysis (FEA) is a type of simulation that allows us to test the strength of our designs, thus we are able to optimise them and keep the weight down. We always back this up with sound engineering principles and years of experience.
We have a good relationship with the technical team at Mercedes Special Trucks and we have Mercedes CAD for most of the newer Unimog models. We have also developed our own CAD models for the older series which allows us to work quickly, efficiently and accurately.
We can also get 2D drawings or, if newer, 3D CAD of most trucks from Mercedes, MAN, Scania, Mitsubishi and IVECO.
Manufacturer Support
We have access to bodybuilder information from Mercedes, MAN, Scania, Mitsubishi and IVECO which allows us to design any components to the manufacturer’s specifications. This is particularly important for the subframes where an incorrect design can cause damage to the truck’s chassis.
We can leverage our contacts within the manufacturers to gain information or further clarification on design specifications or restrictions.
Testing
On top of our years of experience we have carried out extensive testing on multiple aspects of the build, notably the subframe/box connection, the box construction, and the thermal properties of the box.
As a result we believe we have achieved the optimal balance of strength, weight and thermal efficiency which gives the ideal base for any expedition vehicle.
Habitation Box Construction
All our habitation boxes are constructed entirely from high strength composite GRP and XPS foam panels with GRP capping inside and out. No metal or timber frames are used in their construction. This allows the panels to be highly insulative and also eliminate cold bridging whilst keeping the weight down.
The composition of our composite panels has been specifically optimised for use on expedition vehicles where they need to be able to withstand much harsher environments than a standard composite panel.
Both inner and outer skins are made from a 2mm woven GRP panel which offers a much stronger and more impact resistant surface than with regular GRP sheet.
We use two layers of XPS foam with staggered joints ensuring that there are no weak or cold spots.
Our standard panel thickness is 64mm for walls and 84mm for the floor.
The R-value of the 64mm panels is approximately 2.42 m2.k/W and is 3.22 m2.k/W for the 84mm panels. This gives an incredible amount of insulation and based on the size of a typical Unimog habitation box the heat transfer is approximately 16 W per °K temperature difference internal to external.
Doors and windows of course add to this but rest assured it is designed for all climates.
Adhesive
Our habitation boxes are of a fully bonded construction, no screws, bolts or rivets. This allows us to keep the GRP skin of the boxes intact and sealed, giving much fewer areas for potential water ingress, which we have found is the main killer for habitation boxes.
We have done more adhesive testing than we ever thought we would when we started this and are confident that we have settled on the absolute best option on the market.
We use SABATACK 750XL for bonding of all GRP-GRP connections in the box. It is an UV-Stable MS polymer based adhesive which is both high strength and flexible. It has a temperature resistance of -40°C to +120°C so is suitable whether travelling within the cold of the arctic or the heat of the desert. It is widely used in the commercial vehicle industry and is a tried and tested adhesive that we have utmost confidence in.
GRP Bolts
The all-important connection between the composite box and the subframe is achieved using a number of GRP bolts. They are fitted into specially designed recessed spreader plates to reduce intrusion into the interior space.
These bolts have a thermal conductivity over 100 times less than steel, so this effectively eliminates all cold bridges in the connection.
Subframe
A subframe is the metal structure that supports the habitation box and fixes it securely to the truck’s chassis. It is crucial that it is properly designed to avoid potential damage to both the habitation box and the vehicle.
A flexible chassis is a feature on almost all offroad trucks, especially Unimogs, by twisting the chassis allows a greater difference in angle between the front and rear axles, increasing the articulation over what is afforded by the suspension. The flexibility also helps to absorb bumps in the road and gives a better ride.
For an expedition vehicle it is essential that the forces created by the flex of the chassis are not passed through the subframe and into the habitation box where it is likely to cause damage and structural failure. It is also important that the subframe allows the truck’s chassis to flex as designed, else it will restrict its articulation ability and offroad performance.
Some trucks are designed to require an additional frame that sits atop the chassis rails to give the chassis its designed flexibility, this usually forms part of the standard back body. This additional frame prevents the chassis from overtwisting during extreme articulation and experiencing damage as a result. The additional stiffness is also designed to give the correct driving characteristics for the vehicle.
If required, the stiffness of this frame must be replicated when removing the back body. This can be achieved by either the fitting of a rail on rail subframe with correctly specified fixing or by an intermediate frame in combination with a 4-point torsion free subframe.
All our subframes are of tried and tested designs that have been optimised through years of real-world experience and computer simulations.
Our subframes are designed to offer sufficient support to a fully composite box so that it does not need any metal framework or any frames embedded in the floor. This massively improves the insulation of the composite box and when used in conjunction with our non-metallic bolt attachment system it completely eliminates any cold bridges through the floor of the box. This makes temperature control of the box easier and removes cold areas that can get damp through condensation.
Additionally, as no metal frames are needed in the box, the centre of gravity is lowered which will make day to day driving more comfortable and increase the vehicle’s maximum rollover angle in extreme offroad scenarios.
There are two types of subframe that we offer on builds, a 4-point torsion free frame or a rail on rail frame.
4-Point Torsion Free Subframe
A 4-point subframe is one of the original features on all SBU Unimogs and is one of the reasons that they excel offroad. A 4-point subframe offers no additional torsional stiffness to the chassis so the chassis is allowed to twist significantly. This, coupled with the suspension allows all four wheels to remain in contact in the ground even in extreme terrain, improving traction and stability.
The other benefit of this is that no torsional forces from articulation are passed on to the subframe and consequently into the back body or habitation box. This ensures that the box isn’t stressed and will have the longest possible lifespan.
We use a 4-point subframe on all our Unimog builds, utilising the original Mercedes mounting points, thus the Unimog is just as capable with the habitation box as it was with the original back body.
A 4-point subframe is also an option for a truck, though they have to be approached differently to the Unimog. We have found from a mix of testing and bodybuilder information that most offroad capable trucks are designed to have a back body mounted directly to the chassis. This adds significant torsional stiffness to the chassis and can prevent it from sustaining damage in extreme offroad scenarios.
As such, if the truck is going to be used extensively offroad we recommend the combination of an intermediate frame and a 4-point subframe.
An intermediate frame sits directly on top of the chassis and adds the required stiffness. We usually mount it with the original mounting brackets and hardware for the back body and size it to either the bodybuilder specifications or to replicate the stiffness of the original body.
The intermediate frame contains the mountings for the 4-point subframe and this sits above, protecting the habitation box from the torsional forces.
If the truck is going to be used primarily on road and light offroad then it may be suitable to forgo the intermediate frame.
Rail on Rail Subframe
A rail on rail subframe is another way of isolating the box from most of the torsional forces and allowing some chassis movement to improve articulation whilst still supporting the chassis.
A rail on rail frame sits directly on top of the chassis and incorporates some sprung mountings to allow relative movement. The springs have a greater allowable deflection at the front of the subframe as the relative movement is greater.
With a rail on rail subframe it is a balance of reducing the forces on the box and supporting the chassis. This is done by tailoring the strength of the subframe and the strength of the mounting springs to suit the vehicle.
It is never able to reduce the forces in the way that a 4-point subframe does, so we don’t recommend it for serious or extended offroad situations.
A rail on rail is best suited to a truck that is going to be used primarily for road and soft-road scenarios. It is a widely used design and offers a good balance between performance and price.
