A racing bicycle, also known as a road bike, is a bicycle designed for competitive road cycling, a sport governed by according to the rules of the Union Cycliste Internationale (UCI). The UCI rules were altered in 1934 to exclude recumbent bicycles. The most important characteristics about a racing bicycle are its weight and stiffness[citation needed] which determine the efficiency at which the power from a rider's pedal strokes can be transferred to the drive-train and subsequently to its wheels. To this effect racing bicycles may sacrifice comfort for speed. The drop handlebars are positioned lower than the saddle in order to put the rider in a more aerodynamic posture. The front and back wheels are close together so the bicycle has quick handling. The derailleur gear ratios are closely spaced so that the rider can pedal at his or her optimum cadence. Other racing bicycles, especially those used in time trialling, prioritize aerodynamics over comfort. Bicycles for racing on velodromes are track bicycles; bicycles for racing offroad are mountain bicycles, cyclo-cross bicycles or cycle speedway bicycles; bicycles that race according to the rules of the International Human Powered Vehicle Association include faired recumbent bicycles which, on flat ground, are the fastest bicycles in the world.[1][2][3][4] Recumbents were excluded from the UCI definition of a bicycle on 1 April 1934. Time trial bicycles are a subset of racing bicycles that are designed for time trial events. The UCI rules for these bikes are slightly less prescriptive than those for "massed start road races" (see rules 1.3.020 to 1.3.023). Triathlon bicycles are governed by International Triathlon Union (ITU) rules, which allow more recent technological developments than do the UCI rules. The frame of a racing bicycle must, according to the UCI regulations, be constructed using a "main triangle" with three straight tubular shapesthe top tub , 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.[citation needed] 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,[citation needed] 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 racing bicycle frame design is tubing claimed to reduce aerodynamic drag, adopting many design features from time trial bicycles. While many professional riders use such bicycles, as of 2012 they have not been universally adopted in the professional peloton.[citation needed] Such frames are typically slightly heavier than comparably-priced frames without aerodynamic shaping, and reviews of such bicycles have indicated that ride and handling characteristics of many have been inferior to more conventional bicycles.[5]