Understanding the Two Types of Brake Adjusters

The two types of brake adjusters are manual adjusters and automatic (self-adjusting) adjusters. In practice, these appear as “slack adjusters” on heavy-duty air-brake systems and as star‑wheel adjusters on passenger-vehicle drum brakes. Both serve the same purpose: maintaining the correct clearance between the brake shoes or pads and the braking surface to ensure consistent, safe stopping performance.

What Brake Adjusters Do

Brake adjusters compensate for normal wear in brake linings. As friction material wears, clearance grows; adjusters reduce that gap so pedal or brake lever travel remains within spec and braking force arrives promptly. The mechanism differs by vehicle type, but the function is the same—keeping brake response predictable and compliant with safety standards.

The Two Types at a Glance

Brake adjusters fall into two broad categories, each defined by how they are set and how they maintain the proper shoe-to-drum or pad-to-rotor clearance over time.

• Manual adjusters:
  In passenger cars with drum brakes, this is typically a star-wheel screw you turn during service to set the shoe-to-drum clearance. In heavy-duty air-brake systems, it’s a manual slack adjuster that requires periodic technician adjustment to set pushrod stroke correctly.
• Automatic (self-adjusting) adjusters:
  These mechanisms take up wear automatically during normal brake operation. In passenger cars, a self-adjusting lever and cable advance the star wheel under certain brake applications. In trucks and buses with air brakes, automatic slack adjusters (ASAs) continuously maintain correct pushrod stroke without routine manual tweaking.

While both types aim for the same outcome—proper brake clearance—automatic systems reduce the risk of human error and help maintain consistent performance between service intervals.

Key Differences

Choosing or servicing the right adjuster hinges on understanding how they differ in operation, upkeep, and regulatory context.

• Adjustment method: Manual adjusters require periodic, deliberate adjustment by a technician; automatic adjusters self-correct during normal brake use.
• Maintenance burden: Manual systems demand regular checks; automatics still need inspection but not routine adjustment if functioning properly.
• Failure modes: Manual adjusters risk being neglected or misadjusted; automatic adjusters can mask underlying mechanical issues if someone repeatedly “cranks” them to compensate for worn or seized components.
• Regulation (air brakes): In the U.S. and Canada, automatic slack adjusters have been required on most new air‑braked vehicles since the mid‑1990s (FMVSS 121 in the U.S.), shifting best practice away from manual slack adjusters in commercial fleets.

In short, automatic adjusters improve consistency and compliance, but both types rely on proper inspection to catch wear, corrosion, or linkage problems.

Where You’ll Find Them

Both passenger vehicles and commercial vehicles use adjusters, but the hardware and terminology differ with the braking system.

Passenger Vehicles (Hydraulic Drum Brakes)

Older or basic drum-brake systems often use a manual star-wheel adjuster accessed through a backing plate slot during service. Many modern drum setups incorporate a self-adjuster: a lever-and-cable or lever-and-gear mechanism that advances the star wheel when brakes are applied (often during reverse stops or under specific pedal applications). Disc brakes generally don’t use external “adjusters” in the same sense; they rely on caliper piston seals and parking-brake mechanisms to self-compensate.

Commercial Trucks and Buses (Air Brakes)

Air-braked axles use slack adjusters at each brake chamber. Manual slack adjusters require scheduled manual adjustment to set pushrod stroke. Automatic slack adjusters (ASAs) self-maintain stroke, improving consistency and helping fleets meet regulatory requirements. In North America, ASAs have been standard on new air‑braked heavy vehicles since the mid‑1990s, and safety agencies warn against manually adjusting ASAs to “fix” stroke—doing so can conceal mechanical defects.

Maintenance and Safety Tips

Whether manual or automatic, adjusters must be inspected to ensure the braking system stays within specification and operates safely.

• Inspect regularly: Check for corrosion, seized components, damaged return springs, and proper lubrication per manufacturer guidance.
• Verify stroke/clearance: On air brakes, measure pushrod stroke against spec; on drum brakes, confirm correct shoe-to-drum clearance and even contact.
• Don’t mask problems: Never “dial in” an automatic slack adjuster to compensate for worn cams, bushings, or out-of-adjustment foundation parts; fix root causes.
• Follow torque and procedures: Use the correct tools and methods when setting manual adjusters; improper adjustment can cause drag or delayed braking.
• Document service: Keep records of inspections and adjustments to track trends and comply with fleet or regulatory requirements.

Consistent inspection and correct procedures prevent brake fade, pulling, and non-compliance, and they extend component life.

Summary

Brake adjusters come in two types: manual adjusters and automatic (self-adjusting) adjusters. Passenger-vehicle drum brakes use manual or self-adjusting star wheels, while heavy-duty air-brake systems use manual or automatic slack adjusters, with automatics now standard on most modern commercial vehicles. Regardless of type, proper inspection and maintenance are essential for safe, reliable braking.

What are brake adjusters?

A brake adjuster is a component in drum braking systems that maintains the correct distance between the brake shoes and the brake drum, compensating for lining wear to ensure consistent braking performance. It operates manually with a tool like a brake spoon to turn a star wheel or automatically via a mechanism that engages during reverse stops or when the emergency brake is applied. Properly functioning adjusters prevent excessive brake pedal travel, reduced braking efficiency, and increased stopping distances. 
This video demonstrates how drum brakes work and the role of the adjuster mechanism: 1mBleepinJeepYouTube · Oct 25, 2013
How it works

• Drum Brakes: Adjusters are primarily found in drum brakes, where brake shoes press against a rotating drum to slow the vehicle. 
• Compensation for Wear: As brake linings wear, the gap between the shoes and the drum increases, requiring more travel for the brake pedal and potentially leading to less effective braking. 
• Automatic Adjusters: These systems use a mechanism, often a lever and ratchet, that rotates a star wheel to lengthen the adjuster. This lengthens the brake shoe assembly, bringing the shoes closer to the drum. This usually happens when the vehicle is stopped in reverse or when the emergency brake is applied. 
• Manual Adjusters: These require a technician to use a special tool (like a brake spoon) inserted through a rubber plug in the brake drum’s backplate to manually turn the star wheel and adjust the shoe position. 

Why it’s important

• Safety: A properly adjusted brake system ensures effective braking and prevents dangerously long stopping distances. 
• Performance: It maintains optimal braking performance by keeping the brake shoes and drum at the correct distance. 
• Efficiency: Automatic adjusters reduce the need for frequent manual adjustments, improving safety and lowering maintenance costs. 

What are the different types of brake adjusters?

The document discusses three types of brake slack adjusters: manual, automatic, and S-ABA (Self Setting). Manual adjusters require manual rotation to adjust the brake lining gap during wear, while automatic and self-setting adjusters rotate automatically.

What are the two types of Slack adjusters?

Slack adjusters are available in two types: manual slack adjusters and automatic slack adjusters. Manual slack adjusters compensate for the distance in the brake linkage by adjustments made with a wrench. Automatic slack adjusters remove the slack in the brake linkage without the need for manual adjustment.

What are the two types of brake controllers?

Two Basic Types of Brake Controllers. There are many trailer brake controller options on the market, each with its own bells and whistles – from smart wireless controllers to integrated OE-style models. But, virtually all of them can be placed into two basic categories: time-delay and proportional.