There is no definitive answer to this question as there are a number of factors that can influence the speed of a XC bike. However, some key elements that are often cited as contributing to the fast speeds achievable on XC bikes include their lightweight construction, efficient pedal-assist systems and low rolling resistance tires. Let's take a closer look at each of these in turn.
Firstly, XC bikes are typically made from lightweight materials such as carbon fiber or titanium which helps to reduce their overall weight. This is important as it means that less energy is required to propel them forwards, resulting in higher average speeds. Secondly, many XC bikes come equipped with pedal-assist systems which provide an extra boost of power when needed, making it easier to maintain high speeds for extended periods of time. Finally, XC bikes often have tires with low rolling resistance; this means that they require less effort to keep them moving forwards and again contributes to higher top speeds being achievable.
In conclusion, there is no single factor that makes XC bikes fast but rather a combination of several different elements working together. By taking advantage of lightweight materials, efficient pedal-assist systems and low rolling resistance tires, XC bikes are able to achieve high speeds and perform exceptionally well on cross-country courses.