Document ID: chunk:federal_register_of_legislation:F2024C00147:body:0:p69
Version: federal_register_of_legislation:F2024C00147
Segment Type: other
Provision Reference: 
Character Range: 223744–226679

shall have not less than 64.5 square centimetres of reflective surface and an average radius of curvature that is not less than 1,200 mm.
6.                                          ADDITIONAL EXTERNAL REAR VISION MIRRORS
6.1.                                    Mirrors in addition to those required by this Rule may also be fitted, provided that:
6.1.1.                              they do not protrude or project beyond the vehicle more than allowed for other mirrors required by this Rule.
6.1.2.                              they do not obstruct or interfere with the field of view requirements provided by mirrors fitted to meet the specified requirements of this Rule.
6.1.3.                              if the mirror is the only mirror on the left-hand side it shall be flat or convex with an average radius of curvature of at least 1,200 mm.
6.2.                                    Additional mirrors need not meet the requirements of Clause 2.4.2.2 except as specified in Clause 6.1.3 above.

       Annex 1

       (EXTRACTED FROM FMVSS 111 "REAR VIEW MIRRORS", SECTION 12)

       S12  Determination of Radius of Curvature.

       S12.1   To determine the average radius of curvature of a convex mirror, use a 3-point linear spherometer, which meets the requirements of S12.2, at the 10 test positions shown in Figure 1 and record the readings for each position.

       S12.2   The 3-point linear spherometer has two outer fixed legs 1.5 inches apart and one inner movable leg at the mid- point. The spherometer has a dial indicator with a scale that can be read accurately to 0.0001 inches, with the zero reading being a flat surface.

       S12.3   The 10 test positions on the image display consist of two positions at right angles to each other at each of five locations as shown in Figure 1. The locations are at the centre of the mirror, at the left and right ends of a horizontal line that bisects the mirror and at the top and bottom ends of a vertical line that bisects the mirror. None of the readings are within a 0.25 inch border on the edge of the image display.

       S12.4   At each position, the spherometer is held perpendicular to the convex mirror-surface and a record is made of the reading on the dial indicator to the nearest 0.0001 inch.

       S12.5   Convert the dial reading data for each of the 10 test positions to radius of curvature calculations using the following formula.

        where  R = Radius of curvature

        H = Linear displacement of the centre probe

        C = Chord Length = 1.5 inches

       S12.6   Calculate the average radius of curvature by adding all 10 radius of curvature calculations and dividing by ten.

       S12.7   Determine the numerical difference between the average radius of curvature and each of the 10 individual radius of curvature calculations determined in S12.5.

       S12.8   Calculate the greatest percentage deviation by dividing the