mountain bikes. They have wider, treaded tires are less efficient than racing bikes at higher speeds on sealed roads. Gravel bicycles evolved from cyclo-cross bicycles and are drop-bar bicycles designed for riding on maintained but unsealed roads, often over very long distances. Frame The frame of a road bicycle must, according to the UCI regulations, be constructed using a "main triangle" with three straight tubular shapes—the top tube, down tube, and seat tube. These three tubes, and other parts of the frame, need not be cylindrical, however, and many racing bicycles feature frames that use alternative shapes. Traditionally, the top tube of a racing bicycle is close to parallel with the ground when the bicycle is in its normal upright position. Some racing bicycles, however, have a top tube that slopes down towards the rear of the bicycle; the "compact" frame geometry was popularized by Giant. Frame manufacturers are free to use any material they choose in the frame. For most of the history of road racing, bicycle frames were constructed from steel tubing, and aluminium and titanium alloys were also used successfully in racing bicycles. Racing bicycles in these three materials are still commercially available and are still used by some amateur racing cyclists or in vintage racing classes. However, virtually all professional road racing cyclists now use frames constructed from various carbon fiber composite materials, and a typical modern carbon fiber frame weighs less than 1 kg (2.2 lbs). Particularly since the introduction of carbon fiber frames, the shape of the tubes that make up the frame has increasingly diverged from the traditional cylinder, either to modify the ride characteristics of the bicycle, reduce weight, or simply achieve styling differentiation. However, a recent trend in road bicycle frame design is tubing claimed to reduce aerodynamic drag, adopting many design features from time trial bicycles, and the majority of top-level teams use bikes with aerodynamic tube shaping at least some of the time. Unlike mountain bikes, most road bicycles do not have a dedicated suspension. The majority of damping is provided by the tires, padding in a cyclist's clothing, seat, and handlebar tape. Wheels, frames, and forks provide a limited amount of flex, and manufacturers tune the thickness and construction of frame tubing to offer the best compromise between a stiff frame (desired for pedalling efficiency, particularly when sprinting) and ride comfort. Additionally, a few manufacturers offer racing bicycles