invention the air guide 8 particularly the lower part 8 1 thereof which corresponds to the main air inlet 7 1 may thanks to suitable positioning and immobilizing means house a cassette 20 of mobile shut off flaps which are controlled by a lateral control device 21 fig2 and 5 such a cassette may be in the form of a rectangle as can be seen in fig4 it is also possible to provide another cassette in the upper part 8 2 but the lower flow rate passing through that does not necessarily justify such an approach the lateral control device 21 is supplied by a cable not depicted passing through a lateral opening likewise not depicted in the lower box section 8 1 of the air guide fig5 schematically depicts a slightly different embodiment again showing the beam 4 and the lower box section 8 1 situated under the beam and opening into a manifold 9 of the size of the radiator 3 in this solution the upper box section has simply been omitted the mobile flaps cassette 20 is housed in the single lower box section 8 1 against a peripheral end stop 22 sealed against air lines 23 24 indicate the potential regions at which the air guide can be attached to the beam 4 lines 23 and or in the lower part to the spoiler of the bumper 6 lines 24 fig6 shows the rear part of a preferred embodiment of air guide of the invention intended to straddle the impact beam not depicted and made.