Cross country (XC) mountain bikes are designed primarily around the discipline of cross country racing, placing emphasis on climbing speed and endurance, and therefore demanding lightweight, efficient bikes. In the 1980s and early 1990s, XC mountain bikes typically consisted of a lightweight steel hardtail frame with rigid forks. Throughout the 1990s XC bikes evolved to incorporate lightweight aluminum frames and short travel (65 to 110 mm) front suspension forks. Recently full suspension designs have become more popular among racers and enthusiasts alike, and the use of advanced carbon fiber composites has allowed bike designers to produce full-suspension designs under 10 kilograms (22 lb). In recent years 29" wheels have largely replaced the original standard of 26"; the US men's and women's marathon cross-country races were won on 29ers in 2009 and 2010. The geometry of Cross Country bikes favours climbing ability and fast responses over descending and stability, and as a result typical head angles are 70–71°. Although intended for off-road use, Cross Country bikes are not designed for use on steep or particularly rough terrain. Put in terms of rider emphasis, XC bikes are designed for approximately 80% uphill or flat riding, and 20% downhill.