The following challenges of existing capo designs were identified:
Most capos use a fret as a substitute for the nut. This kind of set-up can be excellent for intonation accuracy, as exemplified by Brian May’s preferred ‘zero fret’ approach. However, for finger picked arpeggios on the open strings of an acoustic instrument, they have less ‘ring’.
Many capo designs are not shaped to match the radius of the fingerboard very accurately. There have been some very welcome developments in this area, with manufacturers providing inserts for the capo arm, or complex compensation mechanisms. But these are the exception.
The use of plastics and rubber in capo design. Whilst it is accepted that rubber is very important for the protection of the instrument, and has been used to make up for inaccuracies referred to in the previous point, it is not a sonorous material and can deaden the sound.