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Air Brakes: Air Brake Performance

23 questions in the bank cover this topic. Everything below is drawn from the sources those questions cite.

Sources: 49 CFR 393.40, 49 CFR 393.42, 49 CFR 393.44, 49 CFR 393.52, 49 CFR 571.121, TC 21-305-20 para 3-45

What you need to know

Air is compressible and has to travel the length of the system before the chambers move the shoes.

Nothing slows the vehicle during that delay, so the distance it covers is added to total stopping distance. A hydraulic system has no equivalent. The distance covered during that delay is called brake lag distance, and it is a fourth component on top of perception, reaction and braking.

Vehicles equipped with air brakes take additional time to stop because it takes the air time to travel through the system to engage the brakes. This is called brake lag. The distance the vehicle travels during the brake lag is called brake lag distance.

TC 21-305-20 para 3-45

393.52(a) measures braking force, deceleration rate and stopping distance from 20 miles per hour, against the table in 393.52(d).

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

49 CFR 393.52

393.52(b) requires the emergency system alone to stop the vehicle from 20 mph within the table distance, with no other brake system applied.

It must work by itself, because in a real emergency the service system has failed.

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

49 CFR 393.52

393.52(c)(1) specifies a hard, substantially level, dry, smooth surface free of loose material, and the vehicle must stay inside a 12-foot lane.

The standard removes surface variation so the result measures the brakes.

Any test must be made with the vehicle on a hard surface that is substantially level, dry, smooth, and free of loose material

49 CFR 393.52

393.44 requires the system to be built so a broken front brake line does not take the rear brakes with it.

A bus full of passengers losing all braking from one burst line is the failure this prevents. That is the point of front brake line protection: a broken front line cannot cost you everything.

On every bus, if equipped with air brakes, the braking system shall be so constructed that in the event any brake line to any of the front wheels is broken, the driver can apply the brakes on the rear wheels despite such breakage.

49 CFR 393.44

393.42(a) requires brakes acting on all wheels.

The narrow exceptions include three-or-more-axle trucks built before 25 July 1980, certain light trailers, and some driveaway-towaway configurations.

Every commercial motor vehicle shall be equipped with brakes acting on all wheels.

49 CFR 393.42

393.42(b)(1) exempts three-or-more-axle trucks and truck tractors manufactured before 25 July 1980, though they must still meet the 393.52 performance requirements.

The axle count and the build date both have to apply.

Trucks or truck tractors having three or more axles and manufactured before July 25, 1980, are not required to have brakes on the front wheels.

49 CFR 393.42

393.40(a) states the basic duty as being adequate to stop and hold.

Holding is why a separate parking brake system is required alongside the service brakes.

Each commercial motor vehicle must have brakes adequate to stop and hold the vehicle or combination of motor vehicles.

49 CFR 393.40

All four reduce the friction available at the tire, so the same brake application produces less deceleration and the vehicle travels further.

Gravel is easy to overlook because the surface looks dry.

Most vehicles can be stopped on a dry road surface within the distance required by state laws. However; snow, ice, rain, and gravel reduce the vehicle's stopping ability and increases the distance necessary to bring the vehicle to a complete stop.

TC 21-305-20 para 3-45

FMVSS 121 S5.3.3.1(a) allows 0.45 second for trucks and buses.

Trailers get longer allowances, up to 0.60 second, because the air has further to travel. That interval is brake lag, and the vehicle covers ground throughout it.

With an initial service reservoir system air pressure of 100 psi, the air pressure in each brake chamber shall, when measured from the first movement of the service brake control, reach 60 psi in not more than 0.45 second in the case of trucks and buses

49 CFR 571.121

FMVSS 121 S5.3.3.1(a) allows 0.60 second for a trailer not designed to tow another air-braked vehicle, against 0.55 for a converter dolly and 0.45 for trucks and buses.

More length and more couplings mean more lag.

0.55 second in the case of trailer converter dollies, and 0.60 second in the case of trailers other than trailers designed to tow another vehicle equipped with air brakes

49 CFR 571.121

FMVSS 121 S5.3.4.1(a) allows 0.55 second for trucks and buses.

Release timing matters because brakes that release slowly drag, overheat and fade, and on a combination a slow-releasing trailer fights the tractor.

With an initial service brake chamber air pressure of 95 psi, the air pressure in each brake chamber shall, when measured from the first movement of the service brake control, fall to 5 psi in not more than 0.55 second in the case of trucks and buses

49 CFR 571.121

FMVSS 121 defines it by speed as well as force: 85 psi in an output circuit, or maximum treadle travel, within 0.2 seconds.

It is the standard's definition of braking as hard and as fast as the driver can. Measured from the start of the application.

Full-treadle brake application means a brake application in which the treadle valve pressure in any of the valve's output circuits reaches 85 pounds per square inch (psi) within 0.2 seconds after the application is initiated, or in which maximum treadle travel is achieved within 0.2 seconds

49 CFR 571.121

FMVSS 121 S5.3.1.1 tests from 60 mph on a surface with a peak friction coefficient of 1.02.

Note this is the vehicle manufacturing standard. The in-service test a roadside inspector applies is 393.52, which uses 20 mph.

Stop the vehicle from 60 mph on a surface with a peak friction coefficient of 1.02

49 CFR 571.121

393.40(f)(1) requires operative brakes to survive a failure anywhere in the operating mechanism, with the single exception of the service brake actuation pedal or valve itself.

One broken part must not leave the vehicle with nothing. The service brake pedal or valve is the one excepted part.

If the brake systems required by § 393.40(a) are interconnected in any way, they must be designed, constructed, and maintained so that in the event of a failure of any part of the operating mechanism of one or more of the systems (except the service brake actuation pedal or valve), the motor vehicle will have operative brakes

49 CFR 393.40

393.52(a)(1) sets braking force as a percentage of gross weight.

Expressing it as a fraction of weight makes the standard scale with the load, which a fixed force figure would not.

Developing a braking force at least equal to the percentage of its gross weight specified in the table in paragraph (d) of this section

49 CFR 393.52

393.52(a)(4) permits a performance-based brake tester for vehicles over 4,536 kg, summing the braking force at each wheel as a percentage of gross weight.

It measures what the brakes actually deliver without needing a test track.

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

49 CFR 393.52

FMVSS 121 S5.3.6 requires at least three of four consecutive stops within the 12-foot lane with no part leaving the roadway.

Allowing one miss acknowledges real-world variability while still demanding consistency.

each truck tractor shall stop at least three times within the 12-foot lane, without any part of the vehicle leaving the roadway

49 CFR 571.121

FMVSS 121 S5.3.6.1 uses a wet curve at 0.55 friction, roughly half the 1.02 of the dry stopping distance test.

The curve plus low grip is where a vehicle without ABS would spin, which is what the test is for. Its peak friction coefficient must be 0.55.

stop the vehicle from 30 mph or 75 percent of the maximum drive-through speed, whichever is less, on a 500-foot radius curved roadway with a wet level surface having a peak friction coefficient of 0.55

49 CFR 571.121

FMVSS 121 S5.3.1 requires six stops with at least one inside the table distance.

Repeating allows for brake temperature and surface variation across the sequence in Table I.

When stopped six times for each combination of vehicle type, weight, and speed specified in S5.3.1.1, in the sequence specified in Table I

49 CFR 571.121

FMVSS 121 S5.4.1.1 specifies 125 to 200 degrees Fahrenheit before each stop.

Brake friction varies with temperature, so the starting point is controlled to make results comparable.

With an initial brake temperature between 125 °F. and 200 °F., conduct a stop from 50 m.p.h., maintaining brake chamber air pressure at a constant 20 psi.

49 CFR 571.121

FMVSS 121 S5.4.1.1 steps the pressure up in 10 psi increments across six runs, starting at 20 psi.

That maps how retardation force relates to applied air pressure rather than giving one number.

Repeat the procedure six times, increasing the brake chamber air pressure by 10 psi each time.

49 CFR 571.121

393.40(f)(3) gives buses this route.

393.44 ensures a broken front brake line still leaves the rear brakes usable, which achieves what the interconnection rule is trying to guarantee. Specifically 393.44 for front brake line protection and 393.52(b) for emergency brake performance.

A bus is considered to meet the requirements of § 393.40(f)(1) if it meets the requirements of § 393.44 and § 393.52(b).

49 CFR 393.40

Check yourself

Answers are in the practice test for this topic.