using additional flexing points or vibration-absorbing materials at key points in the frame for additional comfort. Wheels and tires Most road bicycles use 700C bicycle wheels (622 mm bead seat diameter), with matching tyres that are 23-28 mm wide. Wider tires became more common over the 2010s, as more sophisticated measurement showed that wider tires increase rider comfort, improve grip, and decrease rolling resistance without unduly affecting aerodynamics. The wheels greatly affect the performance of a racing bike. The rim of the wheel can be shaped for greater aerodynamic efficiency making a triangular cross-section to form a teardrop with the tire. For aerodynamics and rotating weight, it is generally better to reduce the number of spokes in the wheel. For high-end wheelsets, the spokes can be shaped to have a bladed cross-section, further reducing wind resistance. The most common wheel material for professional racing is now molded carbon fiber rims being a popular choice for pro-level racers and enthusiasts. Aluminium rims are still widely used outside racing and by some less well-funded amateur racers. Carbon fiber rims are lighter than the same shape in aluminium, allowing riders to choose "deeper", more aerodynamic rims without an unacceptable weight penalty. Race-grade wheelsets are very expensive and often fragile. Riders who race often choose to own at least two pairs of wheels: a heavier, more durable, and cheaper wheelset for training, and a lighter, more aerodynamic wheelset for racing. Racers with sufficient resources may have multiple racing wheelsets to choose from depending on the course and weather conditions; deeper rims lose their aerodynamic advantage, and are hard to control, in high crosswinds, and on mountainous courses the lightest possible wheelset may be preferred by some riders. To reduce both air resistance and rolling resistance on the road, tires are lightweight, narrow, and have a thin, smooth tread. Track racing tires can be inflated up to circa . Tires for road racing used to be inflated to pressures almost as high in the belief that this minimised rolling restistance. However, it was discovered that on real road surfaces, much lower tyre pressures or around , depending on rider weight and conditions, result in both a more comfortable ride and lower rolling resistance. Until recently, most racing bikes used tubular tires which have no beads: they are sewn around the tube and glued to the rim. These tires provide an advantage in weight (lacking the