beam slope in staff change

[Helge Kruse]

Lilypond calculates a very nice slope for the beams. But it fails when I 
add a staff change. What's wrong with my .ly?

How can I convince Lilypond to add a slope or tweak it by myself? I 
would like to get a layout in the second measure with slopes similar to 
the first measure.

Regards,
Helge


\version "2.14.2"

instrumentDrei = \relative c' {

\clef treble

\stemUp

r4 \times 4/6 { c'16 g e c g e }

g,4\times 4/6 { c''16 g e c \change Staff=four g e } \change Staff=three |

}

instrumentVier = \relative c' {

\clef bass

\stemDown

\times 4/6 { g,16 e' g c e g } c4

\times 4/6 { g,,16 e' g \change Staff=three c e g } c4 \change Staff=four

}

\score {

\new PianoStaff <<

\new Staff=three { << \instrumentDrei >> }

\new Staff=four { << \instrumentVier >> }

>>

}