Interpretation ID: nht95-2.59

TYPE: INTERPRETATION-NHTSA

DATE: April 28, 1995

FROM: R F Tolley -- Senior Development Engineer, New Products Office, Magneti Marelli UK Ltd. Lighting Division

TO: Chief Council -- Office of Vehicle Safety Standards, NHTSA

ATTACHMT: ATTACHED TO 6/8/95 LETTER FROM JOHN WOMACK TO R. F. TOLLEY (A43; STD. 108)

TEXT: Dear Chief Council

Question concerning FMVSS108

External Aiming - Torque/Deflection Test

I would like to raise a question with you concerning the interpretation of the torque/deflection test contained in FMVSS108 section S7.8.5.1.(a).

I understand that this test is to take into account the weight of beam aiming equipment when aiming lamps. However I believe the instructions for performing the test are not sufficiently precise and can be interpreted in different ways.

Please see attached sketch which I have used to explain my query. In order to determine the downward force which is applied to the lamp, so as to cause the lamp aim to rotate downward, it is necessary to define the centre of rotation of the force. Sinc e torque = downward force multiplied by the distance to the centre of rotation of the force, then

downward force = Torque (20lbf.in) / distance to the centre of rotation of the force

My question is, where is the centre of rotation of the force taken to be? Is it?

a) the centre of the aiming pad pattern,

b) the point at which the axis (which passes through the centre of the aiming pad pattern and is perpendicular to the aiming reference plane), passes through the aiming reference plane,

c) the centroid of the reflector,

d) the centre of rotation of the headlamp assumed to be the centre of the pattern formed by the headlamp adjusters,

e) some other point.

With large, angled modern lamps the difference between the above points can be great, therefore it is important to select the correct point.

In summary my question is- When applying a downward force to create a downward torque (with respect to lamp aim), where is the centre of rotation of the force taken to be?

Attachment: FMVSS 108 EXTERNAL AIMING TORQUE DEFLECTION TEST

(Graphics omitted.)