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FOUR POINT SUBFRAME – OR NOT?

When building an expedition truck a lot of important decisions have to be made. A decision that always provokes a lot of discussion is the million dollar question: four point subframe – or not?

Let me start by mentioning that I am not a technician – far from that. So, I do not have an engineering background or any technical knowledge whatsoever. Basically anybody with a technical background can blow me out of the water.

This article is purely meant to show the considerations we made based on past knowledge and our intended use of the expedition truck. 

Step 1: intended use of the truck

In my opinion the fist step in this decision making process is the intended use of the truck. This leads to a decision on the base truck (step 2) and so on. We already decided on the MAN HX60 so we leave step 2 out in this story.

Our intended use is to have a truck that can travel long distances in relative comfort on the road. Ground clearance and 4-wheel drive so we can drive relatively safe and competent in severe winter conditions. Low gear and differential lockers so we can reach ‘that’ special spot or can drive on unpaved roads – deserts (without trying to conquer the world).

So, pretty straight forward and not real extreme off-road driving.

Step 3: the box that will be mounted on the truck.

The choice of the type of box (build) will decide for a large part what subframe you will need. A lightweight box will probably need a four point subframe, otherwise it will come apart over time. A heavy duty box with steel elements can have much more strain than a lightweight box. So, you can choose between a standard bed from a 4×4 truck up to the four point subframe.

We have chosen a Box that Motorcraft Adventure Developments will build for us. It is a heavy duty box with lots of steel elements so it should be really rugged and sturdy.

Step 4: other factors

For us it is important that we have an easy to remove box on twistlocks. This way it is ‘load’ and therefore much cheaper in the Dutch road tax . A box on twistlocks can also easily get swapped onto another truck in case of a fatal breakdown.

Another very important factor is what you want to spend on a base – subframe? A tailor made rail-on-rail subframe will be around € 6K and a four point subframe around € 9K.

Step 5: pro's and contra's

Tailor made four point subframe: fantastic off road capabilities, little or no stress on the box, relatively light. Probably the only downsides are more body roll and that it is expensive.

Tailor made rail on rail subframe: decent to good off road capabilities, mediocre stress on the box, relatively light and cheaper than the four point. Downside: it is neither meat nor fish. Still spending a lot of money, why not move forward to the four point then?

Existing HX60 flatbed which includes a rail on rail subframe: decent to mediocre (good?) off-road capabilities. It was engineered this way by the specialists of MAN/Rheinmetall with twistlocks to carry a shelter box. But most of all: no extra cost. Downsides: the weight is about 1.000 kgs more than the above mentioned options. Probably a little more stress on the box than the tailor made rail on rail and certainly more stress than the four point subframe.

This solution is not for all trucks with existing flatbeds. It depends on the truck and the construction, strength and flexibility of the subframe. We used the existing bed with subframe of the Steyr 1291, bolted the Fokker shelter on it, and never had a problem.

Step 6: the decision

Construction of the rail on rail subframe with captive springs on a HX60.

We decided to use the existing flatbed of the HX 60. A very rugged platform to mount the shelter on. It is a rail on rail subframe with captive springs that was designed for military off road use. It already has twistlocks, so it should be the perfect base for our Box. 

MAN says the following about the frame:

A strong basis – the ladder frame. The body-friendly, low torsion ladder frame of the HX is based on the tried and tested series frame of the civilian TGA range and is designed for service with a large off road proportion – reliability built-in. It is made of high-strength, fine grain steel and has riveted and bolted cross members and a steel bumper. Due to the low torsion frame, comfortable and gentle transport is guaranteed. The lateral acceleration acting on the platform or body is low.

The box that Motorcraft is going to build for us will be heavy but very sturdy with lots of steel in it. The box should be strong enough to last on the flatbed (hopefully, that is what we have been told). The combination of the all steel floor of the HX and the steel framework of the Box will only make it stronger.

The weight will be relatively high but the HX60 has lots of reserves. Technically the HX is rated at 19 tonnes and we will be nowhere near that. An advantage of more weight is that the ride will be smoother as the leaf springs will have more to do.

When it comes to the downside of less off-road capability (compared to four point subframe): 1) driving skills can bring you a long way. 2) the HX has lockers all around so lifting a wheel should not be a major issue 3) we have the Rotzler winch as a last resort.

But most of all: choose your battles and engage your brain:
As slow as possible and as fast as necessary!

2 Comments

  • Scott
    Posted 19 April 2022 at 19:54

    Awesome article guys!

    I’m undecided on which direction to go with my HX60 overlander

    How does the cost compare having the heavy box constructed on the original flatbed as opposed to the bespoke rail on subframe and lightweight box?

    I’ve only enquired to Motorcraft regarding the latter. Now after reading your post I’m interested to hear more about using the original flatbed.

    • admin
      Posted 20 April 2022 at 07:59

      Thanks Scott!
      More and more people are building Boxes with twistlocks directly on the flatbed of the HX60. Our build with Motorcraft is still in progress but be will post on the ‘off road behaviour’ in the near future. Using the flatbed is much cheaper then a bespoke subframe. A lightweight Box was never an option for us as we like our stuff to be sturdy and rugged and we think the HX can handle (or even use) the weight as the springs are very stiff.

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