Interpretation ID: nht92-4.22

DATE: 09/01/92 EST

FROM: PAUL GOULD -- SENIOR ENGINEER, FRICTION MATERIALS, LUCAS HEAVY DUTY BRAKING SYSTEM

TO: PAUL RICE -- NHTSA

ATTACHMT: ATTACHED TO LETTER DATED 11-19-92 FROM PAUL J. RICE TO PAUL GOULD (A40; STD. 121)

TEXT: I have recently been conducting dynamometer tests to FMVSS 121, here at Cwmbran, and I have some questions which I would like to pose to you as a matter of clarification on the actual meaning of "Average deceleration Rate" and its tolerance.

Taking the Brake Power Test, as an example, the FMVSS states: S5.4.2 "shall be capable of making 10 consecutive decelerations at an average of 9fpsps".

When conducting such tests on our dynamometers, we would carry these out in Constant Torque Mode. The dynamometer is given a deceleration to achieve, and the pressure is modulated around that figure depending on the frictional variations during the stop.

For the Brake Power Test, the dynamometer would attempt to achieve 2.7 m/s2 (in SI units) or 26.49%g.

A typical result obtained from out tests is: Stop 1 26.55%g Stop 2 26.17%g Stop 3 25.93%g Stop 4 26.10%g Stop 5 26.13%g Stop 6 26.05%g Stop 7 25.85%g Stop 8 25.96%g Stop 9 25.94%g Stop 10 25.63%g

This represents a control capability to within 5% (although on the low side).

The FMVSS does not however state this either as a minimum or maximum deceleration.

The points on which I require clarification are:

1) Results presented in this way appear to be lower than required for FMVSS, however, given that only a 5% shift exists are these acceptable, bearing in mind that the more crucial requirements are the pressure limitations and the Hot Stop deceleration rate.

2) It is my interpretation that the deceleration rate is only a Target in order to fade the Linings, and to within an error of 5%, our method is acceptable, rather than aim for a higher deceleration rate, which may mean much higher average deceleration than that stated in the FMVSS. This is also not strictly correct. I also wish to add that during the testing, the pressure utilised was well within the FMVSS demands.

I am sure that it is just a matter of interpretation, but it is vital to clear this up for future testing commitments.