13 Diagram 66b.- c. 2.10. Appendix 13 Diagram 66. a.- c Material specification: 2.11. Tube 2.12. Cold Drawn Seamless Unalloyed Carbon Steel Minimum Yield strength 350N / mm2 2.13. Minimum dimensions 35mm diameter x 2.5mm wall thickness 2.14. Box Section( Structural) 2.15. Unalloyed Carbon Steel 2.16. Minimum Yield Strength 350N / mm2 2.17. Minimum dimensions 35mm square x 2.5mm wall thickness
2.18. Any mounting holes drilled in the seat rails must have bushes installed, these must be fully welded. The bushes must be profiled to prevent them being pulled through, alternatively they may be of ' top hat ' type inserted from below. Minimum bush diameter 13mm for M8 bolts, 15mm for M10 bolts. Multiple bush position are permitted.
2.19. The minimum overall length of the " U " shape extruded section is 50mm with a 2.5mm wall thickness. 2.20. Each mounting point must be capable of withstanding a force of 15000N applied in any direction.
2.21. The seat must be attached to the supports via 4 mounting points, 2 at the front and 2 at the rear of the seat, using bolts with a minimum diameter of 8mm and reinforcements integrated into the seat.
2.22. The minimum thickness of the supports and counterplates is 3mm for steel and 5mm for light alloy materials. The minimum longitudinal dimension of each support is 6cm.
3. Headrests
3.1. On all Vehicles where it is not mandatory it is strongly recommended that a head restraint in the form of a headrest be fitted as Head Restraints below.
4. Head Restraints 4.1. Head Restraints when required to be fitted, must be capable of restraining a 17kg mass decelerating at 5g.
4.2. Dimensions to be 100mms x 100mms and located such that the Driver’ s / Passenger’ s, head / helmet is restrained and cannot move past it under rearward forces or be trapped between the rollbar and the head restraint. It is recommended that it be within 50mm of the Driver’ s / Passenger’ s helmet when they are normally seated.
5. Harnesses( Seat Belts)
5.1. All seat safety belts must be complete units sourced from a recognised manufacturer and fitted in accordance with the manufacturers’ instructions, ASN recommendations or FIA requirements.( Diagrams 49, 50, 51, 52 and 65).
5.2. Where the Vehicle manufacturer’ s standard safety belts and associated fitments are not utilised, bolts must be steel and of a minimum 7 ⁄ 16in UNF or M12( grade 8.8) or, for an FIA homologated harness, as specified by the harness manufacturer.
6. Three point
6.1. Either one diagonal shoulder strap and one lap strap, or two merged shoulder straps and one lap strap bearing an ECE ' E ' mark. With three anchorage points on the Chassis / body shell or roll over bar of the Vehicle on either side and to the rear of the Driver’ s seat.
7. Four point 7.1. Two shoulder straps and one lap strap, with four anchorage points on the Chassis / body shell or roll over
Motorsport UK 2025 National Competition Rules Version 11
Chapter 7 Appendix 7- Seats Harnesses and Headrests 271