UGV Chassis Design: Structural FEM & Manufacturing Trade-Off Study

An autonomous mountain mowing UGV needed a wheel-drive module chassis capable of carrying two heavy-duty motor units and a large lithium battery pack, surviving off-road traction and slope loading, and — just as importantly — being something the client's own team could actually build. With welding capability and workshop resources still being scoped, the chassis design had to remain open to more than one manufacturing route without compromising on structural performance or serviceability.

That constraint shaped the whole project: rather than committing to a single chassis concept, two structural architectures and two candidate materials were developed and evaluated in parallel, each optimised on its own terms, so the client could make an informed manufacturing decision backed by real structural data rather than a single fixed proposal.

Two structural concepts were developed side by side: a fully welded flat-plate chassis, with every panel manufacturable on simple 2D CNC or waterjet equipment, and a bent-sheet variant that consolidated several flat plates into fewer formed panels, reducing both part count and total weld seam length at the cost of requiring sheet-bending capability. Each concept was carried through in both steel and aluminium, since a hybrid material chassis was ruled out early on structural and corrosion-compatibility grounds — steel and aluminium simply don't weld to each other, and mixed-material fastening at the chassis base introduces a long-term corrosion risk the design couldn't justify accepting.

A thickness parametric FEM study was then run across both material candidates under the full set of governing load cases, with the explicit optimisation goal of minimising total chassis mass while keeping every node below yield strength at a 1.5 safety factor and controlling overall deflection. The results settled the material question decisively: the aluminium variants showed extensive node failures across the parametric sweep, driven by the drastically reduced yield strength in the heat-affected zone around welds — a real degradation that a simpler design check might have missed — while the steel variants converged cleanly to a feasible, mass-optimised thickness distribution. That comparison, run consistently across the same load cases and acceptance criteria for both materials, gave the client a directly comparable, evidence-based answer rather than a materials selection based on assumed properties.

Beyond the core structural trade-off, the chassis was developed with hands-on serviceability and integration constraints built in from the start. The motor-to-chassis interface was designed around threaded mounting plates rather than a pure bolt-and-nut assembly, chosen specifically because it offered the most reliable load transmission path while remaining something the client's own team could execute without specialised tooling. The battery bay was designed as a full drawer assembly using telescopic slides sized to the battery pack's actual footprint, allowing a 95 kg battery to be serviced without disturbing the surrounding structure, and the chassis top plate was deliberately specified as removable rather than welded, giving direct maintenance access to the internals without compromising the welded structural shell beneath it.

Every design decision in this phase was made with the next stage of the programme in view: the manufacturing route selection (welded-plate versus bent-sheet) was left explicitly as a client decision point gating the transition into detailed manufacturing drawings, ensuring the structural work delivered here would translate directly into a build-ready package once that choice was made, rather than requiring rework.