if the bicycle is streamlined, the calculation of lateral and drag forces becomes more complex; a proper treatment involves considering the forces on the surfaces like the forces on sails. The drag coefficient depends on the shape of the object and on the Reynolds number, which itself depends on . However, if is the cross sectional area, can be roughly approximated as 1 for usual cycling speeds of a rider on an upright bicycle. Rolling resistance The power for overcoming the tires' rolling resistances is given by: where g is gravity, nominally 9.8 m/s^2, and m is mass (kg). The approximation can be used with all normal coefficients of rolling resistance . Usually this is assumed to be independent of (speed of the bicycle on the road) although it is recognized that it increases with speed. Measurements on a roller-mechanism give low-speed coefficients of 0.003 to 0.006 for a variety of tires inflated to their maximum recommended pressures, increasing about 50% at 10 m/s. Climbing power The vertical climbing power on slope is given by . This approximation approaches the real solution for small, i.e. normal grades. For extremely steep slopes such as 0.35 the approximation gives an overestimation of about 6%. As this power is used to increase the potential energy of bike and rider, it is returned as motive power when going downhill and not lost unless the rider brakes or travels faster than desired. Power for acceleration The power for accelerating the bike and rider having total mass m with acceleration a and rotationally also the wheels having mass is: The approximation is valid if is assumed to be concentrated at the rims and tires and these are not slipping. The mass of such wheels can thus be counted twice for this calculation, independent of the wheels' sizes. As this power is used to increase the kinetic energy of bike and rider, it is returned when decelerating and not lost unless the rider brakes or travels faster than desired. Total power where is the mechanical efficiency of the drive train described at the beginning of this article. Given this simplified equation, one can calculate some values of interest. For example, assuming no wind, one gets the following results for power delivered to the pedals (watts): 175 W for a 90 kg bike + rider to go 9 m/s (32 km/h or 20 mph) on the flat (76%