What is a defining characteristic of a ladder frame design?

A defining characteristic of a ladder frame design is the presence of two parallel main rails that run along the length of the vehicle. This structure provides a robust foundation for the vehicle's body and suspension components, allowing for better load distribution and strength. Ladder frames are commonly used in trucks and some SUVs because they can effectively support heavy loads and withstand rough terrain.

The parallel main rails are a critical design feature because they contribute to the overall durability and stability of the vehicle. In contrast, the other features mentioned do not specifically define a ladder frame. High rigidity can be found in various frame designs, not just ladder frames. Lightweight construction is more associated with unibody designs, which utilize fewer materials for structure. A seamless body structure pertains to monocoque or unitary designs where the body and frame are combined, contrasting with the distinct, separate components of a ladder frame.