I know chassis flex is what helps make speed, but it also has to wear chassis parts.
Lf rf spindles, axles, king pins....did i miss any?? when should these items be replaced on newer chassis???
& what happens to them over time??
Almost every stressed component will change with age. Castor blocks, king pins, spindles, rear axles, seat struts, seats, etc. All of these things tend to slow a kart over time. However, for racing at most levels (except the most competitive), the highest stressed items will do (spindles, king pins, heims, RR seat strut, rear axle, etc.)
Common sense, that is the strut that holds the seat up when the kart transfers the weight from left to right side in the turns. the left strut will probably never get stressed anywhere near as much as the right one gets.