Overland Truck Subframe Guide

The subframe is a critical part of any expedition vehicle build. Providing the all-important interface between the vehicle and the habitation module.

Serving two critical functions that are almost contradictory. It needs to hold the habitation box securely to the vehicle while remaining flexible.

 

Most large off-road vehicles utilise a ladder chassis design that comprises of two 'C' section rails that cross members join these rails together. You then bolt all the components that make up the truck to this substructure.

 

By design, the ladder chassis will flex/twist. The flex/twist acts as a secondary shock absorber protecting the vehicle's components. It also adds articulation in the axels, increasing the vehicle's off-road capability.

Rail on Rail

This consists of two rails positioned on the chassis, one on either side. These are rigidly fixed at one end of the vehicle and then fastened at the other with spring bolts.

 

Using spring bolts allows the subframe to lift away from the chassis a restricted amount allowing for the chassis to flex underneath the subframe.

 

The rail on rail is well suited to either smaller vehicles like the Iveco Daily or for vehicles that will have limited off-road use and don't need the performance of a multi-point system. 

Advantages 

  • Cheaper to Manufacture/Buy

  • Better load distribution onto the chassis - because of the larger contact area, the rail on rail distributes its load evenly across the chassis, so on vehicles with a lighter chassis, a rail on rail is often a better choice. 

 

Disadvantages

  • Limited travel by spring bolts means a more rigid structure and therefore reduced off-road performance.

  • Higher maintenance as they can be prone to corrosion between the rails due to debris ingress

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Four-point torsion-free

Our four-point torsion-free sub fame utilises three pivot points attached to the chassis rails at four positions, a pivot at the front and the rear of the chassis, mounted longitudinally. The third is in the centre mounted laterally. Our pivots utilise maintenance free nylon bushes, allowing them to be used in the most extreme conditions. 

 

Advantages

  • Almost unlimited amounts of free movement due to the use of rotation pivots

  • Ultimate off-road performance

 

Disadvantages

  • Isolated load distribution - due to the reduced contact area, the vehicle chassis will experience more significant point loading. This means a four-point subframe is not suitable for lighter duty chassis 

  • Due to the complex nature and multiple materials and components, the four-point subframe is more expensive to manufacture and therefore buy.

 

So what happens if you use a rigid subframe?

You have two main issues if you don't decouple the habitation module from the torsional forces applied to the chassis during off-road use. Firstly the habitation body module acts as a "splint" to the chassis and limits its articulation and, therefore, its off-road performance, Secondly, and more importantly, those torsional forces are then applied to your habitation module structure. This will put significant loads onto the habitation module, and it will end up prematurely failing. This could be a minor failure such as water ingress or major fractures that would render it unsafe for road use.

 

So what is the solution? The solution is a dynamic fixing between the vehicle's chassis and the subframe.

 

Here at Motorcraft, we offer two solutions designed to suit your vehicle with your needs in mind. You can either have the rail on rail subframe or the four-point subframe.

 

All our subframes are made in-house specifically for your vehicle by our team of skilled fabricators. The components are then sent out for zinc coating to give the steel protection against the elements ensuring your subframe will last a lifetime.

To see more of our subframe products clean here