Simple physics for horizontal force is F=M*A, and falling force is F=M8G.
If my math is correct a rig traveliong .5 mph would have an impact force of 2.933lbs, i.e. F=4000*0.73333333(.5mph in ft/second). However seeing how most trail impacts often from lurching, which could easily be 5mph, a plastic skid plate would have to stand up to at least 29,333lbs of force, or F=4000*5mph(7.3333333 ft/second).
Dropping onto a rock would be F=4000*9.8m/s/s(32ft/s/s). A 4000lb rig dropping for a 1/10 of a second would have 12,800 lbs of force, i.e. F=4000*3.2(32ft/s/s*.1).
