I’ve finally found the time to assemble the components of the torsion free subframe for the floor of our truck house, this component was one of the bigger unexpected 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. So, first things first –
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 living space. The chassis rails of trucks 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 axel of the truck. Having only one solid connection 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 nerdy DJ friend Henry, we designed the components using sketch up 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 twist in the center connection, we designed a bracket that sits on a 10mm sheet of 80A teflon.