Safety Affairs and Regulations
General Motors Corporation
30200 Mound Road
Warren, MI 48090-9010
Dear Mr. Bennett:
This responds to your request for the agency to clarify the thermal performance requirements in FMVSS No. 135, Passenger Car Brake Systems. Your specific question is what pedal force may or must be used during cold effectiveness testing of ABS-equipped vehicles for purposes of establishing allowable pedal force for thermal testing. As discussed below, it is our opinion that a vehicle must meet thermal test requirements at or below the average pedal force that achieves the shortest stopping distance during cold effectiveness tests conducted in accordance with the standard's test procedures.
The provisions in S7.5 set forth the cold effectiveness test for passenger car braking. That provision requires that the vehicle be capable of stopping within 70 meters from a speed of 100 kph with a brake pedal force that does not exceed 500 Newtons. Pursuant to S18.104.22.168, unless otherwise specified, the vehicle is to be stopped in the shortest distance achievable (best effort) on all stops.
As you correctly stated, the average pedal force used during the cold effectiveness test establishes the allowable average pedal force (and thus the stringency) for the hot performance test in S7.14 and the recovery performance test in S7.16. Specifically, S7.14 requires a vehicle with heated brakes to be capable of achieving at least 60% of the deceleration obtained during the best cold effectiveness stop, with an average pedal force that does not exceed the average pedal force recorded during that cold effectiveness stop, while S7.16 requires the vehicle to be capable of achieving between 70% and 150% of the deceleration obtained during the best cold effectiveness stop, with an average pedal force that does not exceed the average pedal force used during that cold effectiveness stop.
According to your letter, this test protocol is straightforward for testing non-ABS- equipped vehicles, but may be unclear with respect to testing ABS equipped vehicles. You stated that in testing ABS-equipped vehicles, GM rapidly applies and holds a constant 500 Newton pedal force throughout the cold effectiveness stop. You further stated that the presence of ABS allows the driver to apply a constant 500 Newton pedal force, stay within the standard=s wheel lock constraints, and meet the 70 meter stopping distance requirement. You stated that a problem arises because a lower pedal force could be used to match or possibly improve the stopping distance compared to a constant 500 Newton pedal force. This is so because at the 500 Newton level, the ABS would cycle to prevent excessive wheel lock, whereas a 400 Newton average level could result in an equivalent stopping distance if the driver modulated the braking force to avoid wheel lock and ABS cycling. You claimed that the pedal force difference would be unimportant for the cold effectiveness test since any force at or under 500 Newtons could be used to meet those requirements. However, it would be crucial in establishing the allowable pedal force and associated stringency for the hot performance and recovery performance tests. You stated that this could lead to compliance disputes between NHTSA and a vehicle manufacturer.
You suggested three alternatives to clarify the pedal force that may or must be used when conducting cold effectiveness testing of ABS-equipped vehicles:
(1) Allow a constant 500 Newton pedal force for cold effectiveness testing of ABS-equipped vehicles, notwithstanding the requirement in S22.214.171.124 to achieve the shortest possible stopping distance;
(2) Rewrite the thermal assessment provision of FMVSS No. 135 to use constant pedal force stops at the onset of the thermal sequence, rather than the pedal force obtained in the cold effectiveness stops, as the baseline for thermal performance assessment; or
(3) Stipulate that the pedal force used during cold effectiveness testing can exceed neither 500 Newtons nor the pedal force necessary to achieve the shortest possible stopping distance.
You recommended that NHTSA adopt Option #1 for a near term solution of this testing issue, and Option #2 as the longer term solution. You believe that Option #1 is practical, objective, repeatable, and provides a well defined pedal force constraint for the subsequent thermal tests. You acknowledged that there is a drawback to this option, i.e., that it may not result in a comparison of braking performance based upon the lowest possible average pedal force for the cold effectiveness stop. You stated that Option #2 would require rulemaking and the associated delay to implement. You stated that Option #3 would provide the intended "apples-to-apples" comparison of cold versus hot brake performance. You stated, however, that this option is not practical from a testing standpoint, since, for an ABS-equipped vehicle, a test driver could not be expected in the allowed six cold effectiveness stops to determine the minimum pedal force yielding the shortest possible stopping distance.
We anticipate that test drivers will utilize a variety of pedal forces during the six cold effectiveness stops in an effort to achieve the shortest possible stopping distance consistent with the test procedures. The average pedal force that resulted in the shortest stopping distance of
these six tests would be used to ascertain compliance with the thermal and recovery performance requirements under S7.14 and S7.16. If, as you suggest, the shortest distance can be achieved at more than one average pedal force level (e.g., if the ABS cycles at a variety of pedal forces below 500 Newtons, or the test driver is able to modulate braking forces to avoid wheel lock while matching the stopping performance of the ABS system), the vehicle must be capable of satisfying the thermal and recovery performance requirements at all such average pedal force levels.
This is consistent with the agency=s long-standing view that, as a general matter, when a standard does not specify a particular test condition, there is a presumption that requirements of the standard must be met at all such test conditions. This presumption may be rebutted if the language of the standard as a whole or its purposes indicate an intention to limit the unspecified test conditions to a particular condition or conditions. However, nothing about Standard No. 135 or its purposes provides a reason to limit the range of average pedal forces.
I hope you find this information helpful. If you have any other questions, please contact Edward Glancy of my staff at this address or by phone at (202) 366-2992.
Samuel J. Dubbin Chief Counsel