"To stiffen the suspension" means to make the suspension system of a vehicle more firm or rigid. This helps to reduce the amount of movement or bouncing when driving over bumps, resulting in a smoother and more controlled ride. Full definition
Track mode stiffened the suspension so much as to be uncomfortable. (cnet.com)
On the road, the C 300 hustles along quite briskly when the accelerator is mashed, especially when the Dynamic Select drive mode selector is toggled to Sport +, which stiffens suspension damping and delivers a sharper throttle response and more direct steering. (autofocus.ca)
Tapping the «S» transmission button also stiffened the suspension, offering more body control as a result. (automobilemag.com)