The phrase "to strengthen the chassis" means making the structure or main framework of something (like a car or a building) stronger and more stable. Full definition
The Innova Crysta is based on a more strengthened chassis with wider cross members. (rediff.com)
The Ram 2500 HD provides the kind of strengthened chassis and up - rated suspension system that is so necessary when hauling or towing a heavy load, but it's not over-sprung to the point where it's impossible to drive when empty. (autobytel.com)
To this end Rolls - Royce's engineers have stiffened and strengthened the chassis where necessary. (evo.co.uk)