I tried to attach my external Crossover monitor to my Lenovo Yoga 2 Pro with help from this page on notebookcheck.com.
My monitor seems to be able to display only 2560x1440 at 60.0, xrandr shows this output:
xrandr
Screen 0: minimum 320 x 200, current 3200 x 1800, maximum 32767 x 32767
eDP1 connected primary 3200x1800+0+0 (normal left inverted right x axis y axis) 293mm x 165mm
3200x1800 60.0*+ 48.0
2048x1536 60.0
...
DP1 disconnected (normal left inverted right x axis y axis)
HDMI1 connected (normal left inverted right x axis y axis)
2560x1440 60.0 +
That page on notebookcheck.com sais in german:
xrandr shows the possible resolutios of your monitors.
Now enter
cvt 2560 1440 30
that will generate the modeline "2560x1440_30.00" :
# 2560x1440 29.94 Hz (CVT) hsync: 43.95 kHz; pclk: 146.25 MHz
146.25 2560 2680 2944 3328 1440 1443 1448 1468 -hsync +vsync
Create the new resolution with
xrandr --newmode "2560x1440_30.00" 146.25 2560 2680 2944 3328 1440 1443 1448 1468 -hsync +vsync
and add it with
xrandr --addmode HDMI1 "2560x1440_30.00"
I tried that but the monitor doesen't work correct: first it seems to work, but when you close any window on it, a shadow of that window stays on the screen. after moving some windows around the scereen is full of artefacts
I also tried 60Hz but that was even worse
