There is no concept of a timer in friction drive pulleys which are generally flat faced and are rather left uncrowned. Such a recommendation can be explained by a Belt Technologies engineer familiar with metal belt dynamics. However, full radius and / trapezoidal geometries may be used if crowned. Although the former is less stressful on the belt it is difficult on the machine therefore costly.
The latter is more cost effective and works well too but should be avoided in case of high belt tensile loads which might result in the rise of stress at the crowns transition points between the angled flats. Although blending the points may prove beneficial it does not eliminate high stress risers.
These pulleys can be designed as rolls the width of which is narrower than the belt running on it.