ScrollViewer: how set SmallChange value

on system.windows.form the SmallChange value is set as:
scrollableControl.HorizontalScroll.SmallChange = 50

how can i do it the same on WPF System.Windows.Controls.ScrollViewer?

The ScrollViewer control encapsulates ScrollBar the ScrollBar a
RangeBase.SmallChange, bud how can i set this values?

thanks
anton

i have found the solution to set the SmallChange
ScrollBar sb = MyScrollViewer.Template.FindName("PART_VerticalScrollBar",
scrollMy) as scrollBar;
sb.Maximum = 600;
sb.SmallChange = sb.Maximum / 20;
BUT...!!!
sb.SmallChange can set now bat have no effect on scrollbar
however sb.Maximum is function correcly.

My problem is that i need fixed size (30 pixel) when scrolling with scroll
button.

have anybody an idea why SmallChange have no effect
or make it otherwise.

thanks
anton

i have found a solution to set the SmallChange value
double d = 30.0;
ScrollBar sb = MyScroll.Template.FindName("PART_VerticalScrollBar",
MyScroll) as ScrollBar;
sb.SetValue(ScrollBar.SmallChangeProperty, d);
or
sb.SmallChange = d;

BUT!....

sb.SmallChange or sb.SetValue(..) don't have effect
where sb.Maximum = 100 is work correctly.

have anybody an idea why?

my problem is that the scrollbar should have a fixed scrolling size when
click on scrobar because my lines are 30 pixel height.

thanks
anton