freedyn2

Multibody dynamics for engineers – and for the assistants they work with.

freedyn2 simulates mechanical systems of rigid bodies, joints and forces: suspensions, drivelines, mechanisms, vehicles on roads, motorcycles. A model is a short text file; a viewer in the browser shows it, plays the results and lets you tune, compare and study variants; an AI assistant can build and change the model through the same tools you use.

What it is good at

• Models as text. A model is a YAML file of bodies, joints and forces, written by hand or by an assistant, reviewed like code, versioned with git. Every number can be a formula of named parameters: stiffness: =k_front.
• Checking before trusting. The viewer and freedyn2 check find what is physically impossible or likely a slip: inertia no body has, units, a joint far from its bodies, redundant constraints, a tire in the air. References (total mass, measured curves) are checked with every run.
• Interactive variant studies. Sliders for every parameter, runs compared to a reference with difference plots, a fan of seven variants simulated in parallel with key figures over the parameter, live eigenvalues and root loci while you move a slider.
• Fast and exact. The equations and their derivatives are generated symbolically (CasADi) and compiled to plain C; a small C++ runtime integrates them (HHT on a Lie group). A car with tires on an OpenCRG road takes about 0.15 ms per step; a fixed Newton mode does exactly the same work every step, for real-time use.
• Open. Apache 2.0. The successor of the FreeDyn freeware: its .fds models run as they are.
• Easy to install. A Windows installer for using it as a program (no Python needed), or pip install freedyn2 for use from Python, Jupyter and on Linux.

Where to start

• Getting started – install (Windows installer or pip), a first model, the viewer, in ten minutes.
• Tutorial – tune a quarter car over a bump: sliders, runs, a fan of variants, eigenvalues.
• Working with an assistant – let Claude or another MCP client build and examine models while you check and decide.
• Coming from FreeDyn – your .fds models in freedyn2, side by side with the legacy program.
• Coming from Adams – the concepts side by side.
• Model reference – every element and key.
• Examples – models to open, with live viewers on the page.

The way of working

freedyn2 is built for a division of labour: an assistant writes and changes the model, a person checks and decides. That only works if checking is cheaper than modelling, so the tools for looking at a model – the viewer, the checks, the comparison of runs, notes back to the assistant – are as important as the solver. You can of course write every model by hand; the file format is meant to be read and written by people.