Interpretation ID: nht95-3.38

TYPE: INTERPRETATION-NHTSA

DATE: July 5, 1995

FROM: John Womack -- Acting Chief Counsel, NHTSA

TO: Milford R. Bennett -- Director, North American Operations Safety, Affairs and Regulations, GM

TITLE: NONE

TEXT: Dear Mr. Bennett:

This responds to General Motors' (GM's) inquiry, raised in a June 16, 1995 meeting with the agency, regarding the test procedures in Federal Motor Vehicle Safety Standard No. 114 for determining whether a vehicle is in the "park" position. Under those p rocedures, a vehicle is deemed to be in park if it ceases rolling within 150 mm. You were concerned that different methods of measuring this distance could result in some vehicles not complying with the requirement that the vehicle roll less than 150 mm .

The short answer to your question is that the test procedure is a "static" measurement procedure. In other words, the agency will not measure the distance that a vehicle has rolled until after the vehicle has completely ceased moving.

This agency recently amended Standard No. 114, adding test procedures to determine whether the key can be removed when the transmission is in positions other than the "park" position and that the transmission remains locked in "park" after key removal. (60 FR 30006; June 7, 1995) NHTSA initiated that action in response to a petition from the automotive industry alleging that such a procedure was needed to make the standard objective.

Section S4.2.1(a)(3) of the amended standard specifies that "[each] vehicle shall not move more than 150 mm on a 10 percent grade when the transmission or transmission shift lever is locked in "park." To demonstrate that the vehicle is in "park" prior to attempting to remove the key, the test procedures in S5.2(e) and S5.3(b) both state:

Drive the vehicle forward up a 10 percent grade and stop it with the service brakes. Apply the parking brake (if present). n1 Move the shift mechanism to the "park" position. Apply the service brakes. Release the parking brake. Release the service bra kes . . . Verify that vehicle movement was less than or equal to 150 mm after release of the service brakes.

n1 The parenthetical reference occurs only in S5.3(b).

In the June 16 meeting with the agency, GM stated that vehicle movement could be measured in two different ways: dynamic or static. GM inquired as to which method NHTSA interpreted the standard as specifying, because the results using these two methods would be different. The "dynamic" method of measuring vehicle movement was described by GM as measuring the maximum play-out of a spool of wire attached to the front bumper after release of the service brakes. The "static" method would measure vehicle movement from a reference point on the wheels after the vehicle has come to a complete stop.

Under the dynamic method, a portion of the measured play-out would be due to the "rocking" motion of the vehicle's chassis on its suspension when the transmission engaged. The driveline components would also contribute some movement by temporarily stori ng some of the kinetic energy of the moving vehicle by flexing and twisting. However, both of these contributions to total rearward movement are temporary, disappearing after the vehicle comes to rest, as in the static measuring method. GM presented te st data for certain vehicles and theoretical worst-case calculations of static roll distance on non-production hypothetical vehicles and one test vehicle. The test data showed that dynamic measuring produces larger measurements of roll than does static measuring.

NHTSA interprets the limitation on vehicle movement specified in S4.2.1(a)(3) as referring to static movement. The agency did not contemplate using the dynamic method. The agency intends to measure only permanent components of total vehicle movement, u sing the "static" method. When conducting compliance testing, NHTSA will measure vehicle movement from a reference point such as the wheel centerline position. The starting time for the measurement will be at the moment before the service brakes are re leased. The ending time of the measurement will be when the vehicle has completely ceased moving, bouncing, and rocking (i.e., until the vehicle is again "static").

This agency believes that its confirmation that the static test method is the proper method should relieve any realistic concerns regarding compliance of the 1996 model year vehicles GM tested, and probably of any future vehicles as well. The actual test s GM conducted in preparation for the meeting with NHTSA all showed static roll distances well within the requirements of Standard No. 114.

I hope this information is helpful. If you have any further questions about NHTSA's safety standards, please feel free to contact Paul Atelsek of my staff at this address or by telephone at (202) 366-2992.