In 1960, the United States began building a boxy armored personnel vehicle called the M-113. Over 80,000 have been built since then, and more than fifty nations have used them. There have been 40 variants of the 13-15 ton M113, for everything from air defense, command, carrying cargo and so on. In the 1980s, the 21 ton M-2 Bradley replaced M-113s as the principal infantry vehicle. The main reason for this was the need for a vehicle that could keep up with the M-1 tank, and a larger vehicle to carry more weapons (25mm cannon and anti-tank missiles.) But the M113 continues to receive upgrades. Currently, you can put a new suspension, engine (diesel/electric) and tracks (rubber) in a M113 and make it as nimble as the Bradley, and quieter. The move to introduce wheeled armored cars into rapid deployment brigades has run into these more effective (and lighter and cheaper) M113s, much to the chagrin of planners who assume that if it's better, it's gotta be brand new.