Head Restraints as per MSA Blue book.
K 13. HEAD RESTRAINTS WHEN REQUIRED TO BE FITTED=
Head restraints must be capable of restraining a 17kg mass decelerating at 5g. Dimensions to be 10cms x 10cms 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 5cm of the driver’s/passenger’s helmet when they are normally seated.