Take a look at the torsion-free subframe for the floor of our truck house, this component was one of the bigger hurdles of the project thus far. This is an article explaining what it is, how it works and how we built it. A torsion-free subframe is really only for those of us that are building a vehicle that is designed to travel off-road such as a Unimog or any 4×4 truck, also known as an expedition vehicle. If you are building an expedition vehicle, a torison-free subframe is an essential component. A chassis that is unable to flex as required can snap or bend easily.
A torsion-free subframe is a system of brackets, usually, a four-point system, that allows the two chassis rails of a vehicle to flex independently of one another without transferring that torsion to the floor of the living space. The chassis rails of a truck are capable of a surprising amount of flex. For example, the chassis rails of our Isuzu FTS 700 are 855mm apart, when fully flexed they can have a height difference of 80mm from one another! Torsion free subframes come in a few different designs, some are fully torsion-free and some are partially torsion-free. Our system is a four-point system, which in theory should allow the rails to flex to their full amount while only transferring a small amount of torsion to our floor.
The design behind our torsion-free subframe is relatively simple, it’s basically two pivot points, one at the front of the floor and one at the rear, and a solid connection over the rear axle of the truck. Having two solid connections over the rear axle allows the floor to rock from side to side instead of being twisted. Which is what would happen if the floor was fixed directly to the chassis rails. This next picture should make it a bit clearer.
It’s better to over-engineer and accept a little bit more weight, than to under engineer and risk things breaking. This was certainly the case with the build of our torsion-free subframe. The parts were cut out of ¾” (19mm) mild steel plate. With a little help from my engineer friend Henry, we designed the components using Fusion 360 and had them water jet cut. The pivot points of the system are made up of 1 ¼” steel pins that fit into bronze bushes. To allow for a small amount of rotational motion in the centre connection as the chassis rails lift and lower, we designed a bracket that sits on a 10mm sheet of 80A teflon wrapped around a large piece of round bar.
The pivot connections, situated at each end of the sub-frame, sit on right-angle brackets that bolt to the exterior of the chassis rails with a piece of 80A teflon in between. We designed the subframe pivot connections to straddle the chassis rails in this way so we could fit water tanks between the chassis rails.
Take a look at the next two photos, this is the completed torsion-free brackets installed on our subframe. As you can see, the centre bracket allows a small amount of rotational movement as the chassis rails lift and lower, rocking the subframe from side to side on the pivot points at the front and rear of the subframe.
Now that we’ve had a chance to put out torsion-free subframe to the test, I can say that it works like a charm and I highly recommend incorporating this into your design, if you’re building an expedition truck.
The next couple of photos show the torsion-free subframe in action. The amount of lift in the rear tyre is approximately 500mm, it’s a bit hard to see from these photos, but there is almost no deformation in the subframe whatsoever.
Our torsion-free subframe was designed using Fusion 360, CAD software which is free for the hobbyist. When you purchase our torsion-free subframe plans you will receive the STEP files and DXF files. The STEP files are what is used to edit the design in a CAD program, the DXF files are what the cutting machine reads to cut the parts. Using Fusing 360, you can modify the STEP files to suit your chassis. This will be as simple as confirming the height of your chassis rails to modify the height of the centre connection chassis plate. Then confirm the width of your chassis to modify the length of the lower half of the pivot connections. You should also confirm that the mounting brackets including bolt heads are not going to interfere with anything on your truck, such as the leaf springs or fuel tanks.
Our torsion-free subframe plans do not come with certification and each person should conduct their own stress modelling, taking into account materials used and maximum load potential.
Download our Torsion-Free Subframe plans here