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I noticed images in Gallery for Wheeled Vehicle where the chassis includes lego pieces. Similarly, can teams use the non-electric and non-electronic pieces from Robotics kits to build Wheeled Vehicle? Materials are not explicitly mentioned in the rules manual. A confirmation would be very helpful to guide our teams.

You're right - build materials are not mentioned in the rules, therefore you can use whatever materials you like.

The general rules do say that a device must be built by the team members, so showing up with a purchased car that you've attached an elastic band to might get frowned at by an ES. But using lego bricks, vex chassis kits etc. is fine.