Thanks for the remarks
On the 1st paper from Phase
it's not only bigger doesn't matter, it's even no deflector at all!
the reference [17] which can be downloaded with scihub, assumed that with a less aero shape of the car and higher clearance, deflector can be usefull
in the article, the comparison is only with and without deflector, but it doesn't say if a bigger deflector would be be better. The size may have been limited for design reason: too big, too ugly
On the 2nd article from Phase
increasing size is said to work but not too big. Study is with angular shape, and enclosed wheel
In the end, I probably made my deflector too big, I will cut some of the rigid tape
