2 words: Unsprung Weight.

Nung magpalit ka ng mas malaking yantas, naging mabigat tuloy ang buong assembly ng gulong. When this happens, the natural frequency of the suspension changes due to the change in the unsprung weight. Unlike a change in the "sprung weight", a change in the unsprung weight will affect the momentum and inertial forces involved in the up and down motion of the suspension and the axle/s.