Interpretation ID: nht94-4.58

TYPE: INTERPRETATION-NHTSA

DATE: October 21, 1994

FROM: Scott E. Peters -- Director, Regulations & Compliance, U.S. Electricar

TO: Phil Recht -- Chief Counsel, NHTSA

TITLE: Interpretation Regarding Tire Loads for Electric Vehicles

ATTACHMT: ATTACHED TO 2/13/95 LETTER FROM PHIL RECHT TO SCOTT E. PETERS (STD. 110)

TEXT: U.S. Electricar is a California corporation which manufactures electric vehicles, including the Electricar Pickup (converted Chevrolet S-10) and the Electricar Sedan (converted GEO Prizm). The Electricar Pickup is fully FMVSS certified while the Electri car Sedan is currently built under NHTSA temporary FMVSS exemption 92-3 for low-emission vehicles. We are aggressively pursuing development of the Sedan and expect to achieve full FMVSS certification within several months.

As you are aware, production electric vehicle development is fairly recent and most of the Federal Motor Vehicle Safety Standards were not written with electric vehicles in mind. We occassionally come across a Safety Standard provision which does not se em applicable to electric vehicles because of their special operating characteristics and limitations compared to traditional internal combustion powered vehicles. Section 4.4.2 of Standard No. 110 is such an example, and we believe this section is not relevant to our particular electric passenger car (the Electricar Sedan), with a speed and speed/endurance limitation substantially below almost any internal combustion powered passenger car.

The purpose of Standard No. 110 is to ensure proper tire selection in order to prevent tire overloading, and thus prevent tire failure. Section 4.2.1 requires that vehicle maximum load on each tire shall not be greater than the maximum tire load rating as specified in one of the tire industry publications listed in Standard No. 109. This section alone prevents tire overloading as long as high speed operation is not a factor.

Section 4.2.2 of Standard No. 110 states that the normal load on the tire shall not exceed the test load used in the high speed performance test specified in S5.5 of Standard No. 109. The test load is 88 percent of the tire's maximum load rating. As sp ecified in S5.5.4 of Standard No. 109, tires at this test load must operate at speeds of 75 mph for 30 minutes, 80 mph for 30 minutes and 85 mph for 30 minutes. It is our understanding that the purpose of Standard No. 110, S4.4.2 is to ensure against ti re failure due to prolonged operation at speeds in the range of 75 mph or higher.

It is our interpretation that Standard No. 110, S4.4.2 is not intended to apply to the Electricar Sedan and other electric passenger cars in which it is physically impossible to operate at high speeds for an extended duration. The Electricar Sedan is ba rely capable of reaching a speed of 75 mph, and could not maintain this speed for more than a few minutes due to the extremely high power requirements and limited energy stored on-board in the vehicle's batteries.

Electric vehicles are intended for use in urban areas with air quality problems and are not suitable for operation at prolonged freeway speeds because of their range limitations. To maximize range and reliability in the Electricar Sedan, the top speed i s limited by software in the vehicle's electronic power control unit. The drag limited speed (non-software governed) is also below most of the "high speed" figures cited in Standard 109, S5.5.4.

Optimizing tire size is an ongoing challenge in electric vehicle development. Because of the weight added in the electric conversion process, original tires must generally be replaced with larger tires which have a higher load capacity. Larger tires on c onverted electric vehicles have the disadvantages of increased rolling resistance and reduced tire clearance and turning radius compared to the original vehicle. Increased rolling resistance lowers the vehicle operating range, a factor which is critical in the acceptance of electric vehicles by both fleet users and the general public. Section 4.2.2 of Standard 110 is based on the load-carrying capacity of tires at high speeds and would require the use of tires which are larger than those needed for co mpliance with S4.2.1 and larger than we believe are required for the safe operation of an electric passenger car with limited speed and speed/endurance capability.

Would you please review Standard 110, S4.2.2 in light of the performance limitations of our electric passenger car and provide us with your interpretation as to the applicability of this section to the Electricar Sedan described above.

Please contact me if I can provide additional information for your consideration of our request.