Introduction
Tractor trailers, also known as semi-trucks or 18-wheelers, rely on air brakes to safely come to a stop when traveling at highway speeds. Air brakes use compressed air to actuate the brake pads, which clamp down on the brake rotors to slow the spinning wheels. Air brakes are used instead of hydraulic brakes because they provide superior stopping power for heavy vehicles.
In this comprehensive guide, we will explain how tractor trailer brakes work in detail, covering the air compressor, air tanks, brake chambers, slack adjusters, brake pads, and rotors. We’ll also discuss foundation brakes, ABS, brake bias, and brake maintenance best practices. Read on to learn everything you need to know about this critical truck and tractor trailer safety system.
Overview of a Tractor Trailer Brake System
A tractor trailer brake system is made up of the following core components:
- Air compressor – Generates compressed air which actuates the brakes
- Air tanks – Store compressed air for the brake system
- Foot brake valve – Sends air to the brakes when the driver presses the brake pedal
- Trailer hand valve – Allows manual application of the trailer brakes independent of the tractor brakes
- Brake chambers – Use compressed air to create mechanical force and push the brake pads against the rotors
- Slack adjusters – Automatically adjusts slack in the brake assemblies
- Brake pads – Friction material that clamps down on the rotor surface
- Brake rotors – Steel discs attached to the wheel hubs that the brake pads squeeze to slow the wheels
In addition, tractor trailers are equipped with ABS (anti-lock braking system) to prevent wheel lockup and loss of steering control during hard braking. The tractor has a separate brake system from the trailer, and brake bias valves balance the braking force between tractor and trailer for optimized stopping distance.
Now let’s look at each of these components in more detail.
Air Compressor
Tractor trailer air compressor
The air compressor is the heart of the air brake system. It is typically engine-driven, meaning it runs off the truck’s engine to build compressed air. The air compressor will fill the air tanks up to the governor cut-out pressure, typically around 120 PSI.
The air compressor will kick back on when the air tank pressure drops to the cut-in pressure, usually around 100 PSI. This cycle repeats during driving to maintain the air pressure that feeds the brake system.
Air compressors are either single stage or two stage. Two stage compressors are more common on heavy duty vehicles as they build air pressure faster.
Air Tanks
Tractor trailers have multiple air tanks that act as reservoirs to store compressed air. They ensure there is ample air volume available when the brakes are applied, even if the compressor needs to kick back on.
There are separate air tanks for the front brakes, rear brakes, parking brakes, and accessories like air horns. The front brake and rear brake tanks are connected by an equalization line that balances pressure between them when braking.
A one-way check valve allows air flow from the compressor into the tanks, while blocking reverse flow. The governor cut-out valve vents excess pressure above 120 PSI to prevent over-pressurization.
Here are the common air tanks found on a tractor trailer:
- Front brake tank – Feeds air to the front brakes
- Primary rear brake tank – Feeds air to the rear trailer brakes
- Secondary rear brake tank – Supplements the primary tank for the trailer
- Parking brake tank – Applies the spring parking brakes when air pressure drops
- Accessory tank – Supplies air for horns, etc.
- Wet tank – Catches moisture from the compressor
Proper air tank drainage is essential to avoid moisture contamination in the brake system. Tanks should be drained daily.
Brake Chambers
Brake chambers convert compressed air into mechanical force to actuate the foundation brakes. They are mounted to the axles or wheel ends.
When the driver presses the brake pedal, compressed air enters the chamber and pushes against an internal diaphragm. The pressure on the diaphragm extends a push rod out of the chamber.
As the push rod extends, it activates the slack adjuster and camshaft brake assembly, pushing the brake pads against the rotor. When air pressure is released, return springs retract the push rod and release the brakes.
There are two main types of brake chambers:
- Type 30 chambers – 30 cubic inch volume, used on front axles
- Type 30/30 chambers – 30 cubic inch service side and 30 cubic inch emergency side, used on rear axles and trailers. The emergency side activates the parking brakes when air is lost.
Slack Adjusters
Manually adjusting slack with a brake adjuster tool
Slack adjusters connect the brake chamber push rod to the S-cam brake assembly. They provide leverage to increase the applied brake force.
The slack adjuster’s critical role is to automatically adjust for wear and “slack” in the brake system as the brake pads erode over time. The slack adjuster will rotate slightly when the brakes are applied to take up any looseness, keeping the same pad clearance and force.
Manual slack adjustment is still necessary periodically using a brake adjustment tool. The brake stroke should be checked at every tire rotation or oil change. Excessive manual readjustment can indicate a problem with the self-adjusting mechanism.
Foundation Brakes
Foundation brakes are the actual braking mechanism that clamps brake pads onto a spinning rotor surface to create friction. They turn the compressed air’s energy into braking force at each wheel end.
S-cam drum brakes were previously common, but now disc brakes have become standard on most tractors and trailers for better stopping performance. Disc brakes have separate brake pads for each side of the rotor, while drum brakes share a common shoe.
Disc brake components:
- Brake pads – Consist of friction material mounted to metal backing plates. Pads squeeze the rotor to create friction.
- Brake caliper – Holds the pads in place and contains hydraulic pistons that clamp the pads onto the rotor when actuated. Air disc brakes use brake chambers instead of hydraulic pistons.
- Brake rotor – A steel disc attached to the wheel hub that rotates along with the tire. The pads clamp down on the rotor to slow its rotation.
- Caliper slide pins – Guide the caliper assembly so that it floats and centers on the rotor as the pads wear.
- Carrier plate – Bolts the caliper assembly to the axle or brake spider.
Brake Bias
Tractor trailer with front axle brake bias valve
Brake bias refers to the front-to-rear braking force distribution. If too much force is applied to the tractor drive axles or trailer axles, those wheels can lock up resulting in a loss of steering control.
To prevent this, brake bias valves automatically adjust and limit air pressure to the front brakes to maintain the ideal brake bias. Front brake limiting valves may also be used.
Ideally the tractor drive axles should lock up just before the trailer axles. Brake bias is tuned using these valves to shift the center of braking force rearward for maximum straight line braking without wheel lockup.
Brake Bias Adjustment
The brake bias may need adjustment if the tractor or trailer wheels are locking up prematurely during hard stops. Here are some of the bias tuning options:
- Front axle limiting valve – Reduces front brake pressure
- Variable load valve – Lowers rear brake pressure under light loads
- Double check valves – Limit air pressure to the trailer to prevent aggressive trailer braking
Brake bias is optimized when minimal steering effort is required during an emergency stop without wheel lockup.
Anti-Lock Braking System (ABS)
Key components of a tractor trailer ABS system
ABS prevents the wheels from fully locking up during hard braking. This gives the driver maximum stopping power while maintaining vehicle steerability.
Here’s how ABS works:
- Wheel speed sensors monitor the rotational speed of each wheel
- When a wheel begins to lock up and slip, the ABS module detects this change in wheel speed
- The ABS module then quickly decreases,
- The ABS module then quickly decreases, increases, and re-applies brake pressure multiple times per second to stop the wheel from locking.
- This pulsing action allows the wheel to continue rotating right at the point of maximum braking force, while preventing a full skid.
- ABS rapidly pumps the brakes up to 15 times per second to maximize stopping ability without lose of steering control.
- When activated, the driver will feel pulsing in the brake pedal and hear ABS operation noises. This is normal and indicates the system is working to prevent wheel lockup.
- The ABS controller monitors data from all wheel speed sensors to identify when and which wheels need modulated pressure to avoid lockup.
- Tractor trailers usually have ABS systems on all axles to deliver total braking stability. The tractor and trailer ABS are electronically integrated.
- Trailers will have an ABS control unit with valves to modulate trailer brake pressure as needed. The tractor ABS module manages the overall system.
- ABS is a major safety advancement, helping drivers avoid jackknifing and maintain maneuverability during hard stops, especially on wet or slippery roads.
- Warning lights on the dash alert drivers of any detected ABS faults that should be addressed promptly to keep this crucial system functioning properly at all times.
- Like all safety components, ABS systems require periodic inspection and maintenance to stay in peak operating condition. Technicians need to thoroughly diagnose and repair any ABS faults or codes.
Conclusion
Tractor trailer air brake systems allow heavy trucks to stop safely and effectively. The air compressor charges the air tanks which feed the brake chambers that actuate the foundation brakes at each wheel end. Slack adjusters compensate for brake wear, while ABS prevents wheel lockup during hard stops.
Proper brake system maintenance and performance tuning through brake bias adjustment helps tractor trailers achieve short, straight stops in all conditions. While air brakes may seem complex, their rock-solid design has made them the standard in trucks and tractor trailers worldwide. Understanding how the components work together delivers insight into keeping these heavy vehicles safe on the highway.
Frequently Asked Questions
Q: How do air brakes work differently than hydraulic brakes?
A: Hydraulic brakes use brake fluid under pressure to apply the brakes. Air brakes use compressed air stored in tanks to power the brake chambers which activate the brake mechanisms. Air stores energy easily but is compressible, while hydraulic fluid is incompressible but requires high pressure pumps and lines.
Q: Why do air brakes require more maintenance than hydraulic brakes?
A: The many air hoses and connections in air brake systems can leak over time, causing drops in air pressure. Air tanks must be drained of moisture buildup daily. Air brakes also have more components like brake chambers and slack adjusters that require servicing.
Q: What happens if air pressure is lost in the tractor trailer brake system?
A: Most tractors and trailers have spring brake chambers that apply the parking brakes if air pressure drops below a certain PSI. This prevents a runaway vehicle scenario. The parking brakes will remain engaged until air pressure is restored.
Q: How often should tractor trailer brake adjustments be checked?
A: Experts recommend checking brake adjustment and stroke at every oil change or tire rotation. The slack adjusters should maintain the brakes with minimal manual readjustment needed. If excessive manual adjustment is required between service intervals, there may be an issue.
Q: What is the main purpose of brake bias adjustment?
A: Brake bias tuning ensures ideal front-to-rear braking balance. If the tractor or trailer brakes too aggressively, those wheels can lose traction. Proper brake bias prevents wheel lockup and loss of steering control during hard stops.