RadControls for ASP.NET AJAX
Mouse wheel support is enabled by the TrackMouseWheel property when True (the default) and by the SmallChange property that controls the increments the slider moves in response to the mouse wheel. Populate the SmallChange property when you want the slider to respond in defined steps. For example, for a slider with MinimumValue and MaximumValue between 0-100 and SmallChange set to 5, the the slider can display values 0, 5, 10, 15, ... 95, 100. SmallChange defaults to 1.
<telerik:radslider id="RadSlider1" runat="server" trackmousewheel="true" smallchange="5" />