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This must consist of composite honeycomb structures solidly attached to steel tube structures on each side of the car. These tube structures must conform to the material specifications given in Article 253-8.3.3 of Appendix J, with the exception of the dimensions of the tubes, which must measure at least 30 x 2 mm. These structures must be fixed to the main structure of the car. The minimum thickness of the composite panels is 15 mm, and they may be mounted on either side of the tubes. The outermost part of the protection must be situated at the level of the centre of the wheel hubs, over a minimum length of 60 % of the wheelbase. This protection must extend outwards on both sides at least as far as the vertical planes passing through the middle of the foremost part of the rear tyres and through the middle of the rearmost part of the front tyres, but not further than the vertical planes passing through the outside of the foremost part of the rear tyres and through the outside of the rearmost part of the front tyres. The space between this protection and the bodywork must be entirely closed.