Complex sentence: They typically have a modified mountain bike frame with a tall head tube to provide an upright riding position, 26-inch wheels, and 1.75 or 1.95-inch (45-50 mm) smooth or semi-slick tires.
Split into simpler sentences: They typically have a modified mountain bike frame with a tall head tube to provide an upright riding position. They are maybe fitted with 26-inch wheels and 1.75 or 1.95-inch (45-50 mm) smooth or semi-slick tires.