My door frames are also 21cm. That doesn't fit! Unfortunately. The Multi-Gym Pro (and with it the hand bars), then hangs at an angle, not horizontally. I modified mine a bit, so it works now, but only for very slim hands. So I wouldn't recommend it for 21cm door frames.

I would also say 13-17 cm is optimal. However, the support on the upper edge of the door (frame) is also important. This should also not be smaller than 1.5 cm.