393.52 Brake performance.§ 393.52 Brake performance.
(a) Upon application of its service brakes, a motor vehicle or combination of motor vehicles must under any condition of loading in which it is found on a public highway, be capable of -
(1) Developing a braking force at least equal to the percentage of its gross weight specified in the table in paragraph (d) of this section;
(2) Decelerating to a stop from 20 miles per hour at not less than the rate specified in the table in paragraph (d) of this section; and
(3) Stopping from 20 miles per hour in a distance, measured from the point at which movement of the service brake pedal or control begins, that is not greater than the distance specified in the table in paragraph (d) of this section; or, for motor vehicles or motor vehicle combinations that have a GVWR or GVW greater than 4,536 kg (10,000 pounds),
(4) Developing only the braking force specified in paragraph (a)(1) of this section and the stopping distance specified in paragraph (a)(3) of this section, if braking force is measured by a performance-based brake tester which meets the requirements of functional specifications for performance-based brake testers for commercial motor vehicles, where braking force is the sum of the braking force at each wheel of the vehicle or vehicle combination as a percentage of gross vehicle or combination weight.
(b) Upon application of its emergency brake system and with no other brake system applied, a motor vehicle or combination of motor vehicles must, under any condition of loading in which it is found on a public highway, be capable of stopping from 20 miles per hour in a distance, measured from the point at which movement of the emergency brake control begins, that is not greater than the distance specified in the table in paragraph (d) of this section.
(c) Conformity to the stopping-distance requirements of paragraphs (a) and (b) of this section shall be determined under the following conditions:
(1) Any test must be made with the vehicle on a hard surface that is substantially level, dry, smooth, and free of loose material.
(2) The vehicle must be in the center of a 12-foot-wide lane when the test begins and must not deviate from that lane during the test.
(d) Vehicle brake performance table:
|Type of motor vehicle||Service brake systems||Emergency brake systems|
|Braking force as a percentage of gross vehicle or combination weight||Deceleration in feet per second per second||Application and braking distance in feet from initial speed at 20 mph||Application and braking distance in feet from initial speed of 20 mph|
|A. Passenger-carrying vehicles:|
|(1) Vehicles with a seating capacity of 10 persons or less, including driver, and built on a passenger car chassis||65.2||21||20||54|
|(2) Vehicles with a seating capacity of more than 10 persons, including driver, and built on a passenger car chassis; vehicles built on a truck or bus chassis and having a manufacturer's GVWR of 10,000 pounds or less||52.8||17||25||66|
|(3) All other passenger-carrying vehicles||43.5||14||35||85|
|B. Property-carrying vehicles:|
|(1) Single unit vehicles having a manufacturer's GVWR of 10,000 pounds or less||52.8||17||25||66|
|(2) Single unit vehicles having a manufacturer's GVWR of more than 10,000 pounds, except truck tractors. Combinations of a 2-axle towing vehicle and trailer having a GVWR of 3,000 pounds or less. All combinations of 2 or less vehicles in drive-away or tow-away operation||43.5||14||35||85|
|(3) All other property-carrying vehicles and combinations of property-carrying vehicles||43.5||14||40||90|
Notes: (a) There is a definite mathematical relationship between the figures in columns 2 and 3. If the decelerations set forth in column 3 are divided by 32.2 feet per-second per-second, the figures in column 2 will be obtained. (For example, 21 divided by 32.2 equals 65.2 percent.) Column 2 is included in the tabulation because certain brake testing devices utilize this factor.
(b) The decelerations specified in column 3 are an indication of the effectiveness of the basic brakes, and as measured in practical brake testing are the maximum decelerations attained at some time during the stop. These decelerations as measured in brake tests cannot be used to compute the values in column 4 because the deceleration is not sustained at the same rate over the entire period of the stop. The deceleration increases from zero to a maximum during a period of brake system application and brake-force buildup. Also, other factors may cause the deceleration to decrease after reaching a maximum. The added distance that results because maximum deceleration is not sustained is included in the figures in column 4 but is not indicated by the usual brake-testing devices for checking deceleration.
(c) The distances in column 4 and the decelerations in column 3 are not directly related. “Brake-system application and braking distance in feet” (column 4) is a definite measure of the overall effectiveness of the braking system, being the distance traveled between the point at which the driver starts to move the braking controls and the point at which the vehicle comes to rest. It includes distance traveled while the brakes are being applied and distance traveled while the brakes are retarding the vehicle.
(d) The distance traveled during the period of brake-system application and brake-force buildup varies with vehicle type, being negligible for many passenger cars and greatest for combinations of commercial vehicles. This fact accounts for the variation from 20 to 40 feet in the values in column 4 for the various classes of vehicles.
(e) The terms “GVWR” and “GVW” refer to the manufacturer's gross vehicle weight rating and the actual gross vehicle weight, respectively.