5.4 Brake Shoes, Linings & Friction Codes
Key Takeaways
- Fabricated steel brake shoes comprise a cylindrical curved table, a central structural web, and an anchor/roller crown engineered to withstand extreme mechanical clamping loads.
- SAE J661 edge codes denote normal (cold: 200°–400°F) and hot (400°–600°F) friction coefficients using letter classifications (e.g., FF rating equals 0.35 to 0.45 coefficient of friction).
- Brake linings contaminated with grease or oil CANNOT be cleaned, solvent-washed, or torched; they must be replaced in complete axle sets to eliminate severe friction fade.
- Under 49 CFR 393.47(d)(2) and CVSA Out-of-Service Criteria, non-steering-axle air drum brake lining is condemned below 1/4 inch (6.4 mm) measured at the shoe center, or when worn flush with a rivet head or down to the wear indicator.
Last updated: August 2026
1. Fabricated Steel Shoe Construction & Sizing Standards
Commercial vehicle drum brakes utilize fabricated steel brake shoes engineered to support friction lining blocks and withstand extreme mechanical clamping and thermal expansion forces during heavy braking cycles.
+-----------------------------------------------------------------------------------+
| FABRICATED BRAKE SHOE ANATOMY |
+-----------------------------------------------------------------------------------+
| [ Shoe Table: Cylindrical Curved Steel Friction Support ] |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| [============ Riveted / Bonded Friction Lining Block ===================] |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| | | |
| [ Anchor End ] <====== [ Structural Steel Web Plate ] ======> [ Roller Crown ] |
| (Anchor Bushing) (Roller Pocket) |
+-----------------------------------------------------------------------------------+
Anatomy of a Commercial Brake Shoe
- Shoe Table: The curved outer cylindrical steel plate that provides the mounting foundation for the friction linings. The table must maintain precise radial curvature to ensure full-surface lining-to-drum contact.
- Shoe Web: The heavy vertical steel backbone welded to the underside of the table. The web contains precision-machined cutouts for the anchor pin bushing at the heel and the cam roller pocket / crown at the toe, as well as holes for return springs.
- Standard Q vs. Q-Plus (Extended Life) Shoes:
- Standard Q-Type: The legacy industry standard utilizing flat cam rollers and open-ended anchor pin pockets.
- Q-Plus (Q+) Type: Introduced thicker friction linings (especially at the center crown), stepped cam rollers with retaining clips, and enclosed anchor pin ears to dramatically extend service reline intervals and prevent hardware dislodgement under high vibration.
Brake Shoe Sizing Dimensions
Brake shoe assemblies are designated by their Nominal Drum Diameter × Shoe Width:
| Brake Shoe Size | Typical Axle Location | Vehicle Application Classification |
|---|---|---|
| 15.0" × 4.0" | Steer Axle | Medium-duty straight trucks (Class 6/7) |
| 16.5" × 5.0" | Steer Axle | Standard Class 8 highway linehaul tractors |
| 16.5" × 6.0" | Steer Axle | RSD-compliant (Reduced Stopping Distance) Class 8 tractors |
| 16.5" × 7.0" | Drive & Trailer Axles | Standard Class 8 heavy-duty industry workhorse (Q-Plus) |
| 16.5" × 8.625" | Drive Axles | Severe-duty, refuse, cement mixers, heavy haul |
2. SAE J661 Edge Codes & Friction Ratings
Commercial brake linings are certified and stamped along their exposed edge with a standardized alphanumeric code defined by SAE J661 (Chase Machine Testing Procedure). This code conveys the manufacturer, friction compound formulation, and two mandatory friction coefficient letters.
SAE J661 BRAKE LINING EDGE CODE ANATOMY
[ MFR-CODE ] - [ FORMULATION-ID ] - [ F F ]
| |
Normal (Cold) Friction Rating --------+ | (200°F to 400°F)
Hot Friction Rating --------------------+ (400°F to 600°F)
Friction Coefficient ($\mu$) Classification Table
| Friction Code Letter | Coefficient of Friction Range ($\mu$) | Performance Characteristics |
|---|---|---|
| C | $\mu \le 0.15$ | Extremely low friction (rarely used) |
| D | $0.15 < \mu \le 0.25$ | Low friction rating |
| E | $0.25 < \mu \le 0.35$ | Medium-low friction rating |
| F | $0.35 < \mu \le 0.45$ | Standard Heavy-Duty Commercial Truck Rating |
| G | $0.45 < \mu \le 0.55$ | High friction rating (severe-duty / vocational) |
| H | $\mu > 0.55$ | Extremely high friction (specialty / racing) |
| Z | Unclassified | Non-graded compound |
Interpreting Two-Letter Friction Ratings
- First Letter = Normal (Cold) Friction: Measured under moderate operating temperatures between 200°F and 400°F (93°C to 204°C).
- Second Letter = Hot Friction: Measured under severe thermal braking temperatures between 400°F and 600°F (204°C to 316°C).
- Common Ratings:
- FF Rating: Coefficient of friction between 0.35 and 0.45 under both cold and hot operating temperatures. This is the industry-standard balance for on-highway linehaul tractors.
- EF Rating: Medium friction when cold (0.25–0.35) rising to high friction when hot (0.35–0.45).
- GG Rating: High friction (0.45–0.55) across all temperatures, common in severe-duty refuse and transit bus operations.
[!CAUTION] Axle Friction Matching Mandate: Always ensure that brake shoes installed on the left and right wheel ends of the same axle have identical SAE friction edge codes and lining formulations. Mixing friction compounds across an axle (such as an FF shoe on the left and a GG shoe on the right) produces severe brake pull, wheel lockup, and violent tractor jackknifing.
3. CVSA Lining Wear Limits & Contamination Rules
Roadside enforcement agencies and fleet maintenance guidelines enforce strict numerical discard limits for brake shoe friction linings to prevent foundation metal-to-metal contact.
+-----------------------------------------------------------------------------------+
| BRAKE LINING WEAR & DISCARD LIMITS |
+-----------------------------------------------------------------------------------+
| [ Worn Lining Profile ] |
| | |
| v |
| +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+ |
| | Top Friction Surface | |
| | | |
| | (Rivet Hole) (Wear Indicator Groove) | |
| | | | | | |
| | | | v | |
| | | | <--- 1/16" Min ----> [ === ] | |
| | v v | |
| | [ Rivet Head ] <------------- 1/4" (6.4 mm) CVSA DISCARD ----+ |
| +---------------------------------------------------------------+ |
| | Steel Shoe Table | |
+-----------------------------------------------------------------------------------+
CVSA Out-of-Service Discard Criteria
A commercial vehicle foundation drum brake is placed Out of Service if:
- Lining Thickness Limit: The friction material is worn to less than 1/4 inch (6.4 mm / 0.250 in.) for an air drum brake, measured at the shoe center as specified by 49 CFR § 393.47(d)(2), or worn to the wear indicator if the lining is marked with one. (A steering-axle shoe carrying a single continuous strip of lining is condemned at 3/16 inch / 4.8 mm.)
- Rivet Exposure Limit: The lining is worn flush with any rivet head, or rivet head depth is less than 1/16 inch (1.6 mm) from the friction surface.
- Wear Indicator Depth: On linings molded with visual wear indicator slots, the lining has worn to less than 1/16 inch remaining in the groove.
Hydrocarbon Contamination: The Zero-Tolerance Replacement Rule
When an inner wheel hub oil seal, wheel bearing grease seal, or camshaft spider seal fails, lubricating oil or chassis grease migrates directly into the foundation brake cavity.
flowchart TD
Leak[Oil or Grease Leak Detected on Linings] --> Action{Attempt to Clean or Burn Off?}
Action -->|STRICTLY PROHIBITED| Prohib[Linings are porous -> Oil soaks into core]
Prohib --> Hazard[Thermal Boiling -> Catastrophic Brake Fade & Fire]
Action -->|CORRECT ACTION| Replace[Replace Brake Shoes in Complete Axle Sets]
Replace --> SealFix[Replace Failed Wheel Seal & Clean Hub/Drum Cavity]
Why Contaminated Linings Cannot Be Saved:
- Commercial friction blocks are manufactured from porous composite matrices (organic binders, synthetic fibers, metallic particles, and friction modifiers).
- Petroleum hydrocarbons soak deep into the microscopic pores of the lining material.
- Prohibited Practices: Technicians must NEVER attempt to wash oil-contaminated linings with solvent, brake cleaner, or degreasers, and NEVER burn off oil using an oxy-acetylene torch.
- Physical Consequence: Solvent washes only remove surface film. During subsequent highway braking, internal friction temperatures reach 400°F–600°F, boiling the trapped oil back out to the friction interface. This creates a liquid boundary layer that causes complete loss of braking friction (catastrophic brake fade), severe brake pull, and potential wheel fires.
- Mandatory Action: Saturated or contaminated brake shoes MUST BE CONDEMNED AND REPLACED IN COMPLETE AXLE SETS along with repairing the defective oil seal.
4. Heat Checking vs. Structural Fractures & Shoe Distortion
During foundation brake teardown, the technician must distinguish between acceptable thermal surface wear and dangerous structural condemnations.
+-----------------------------------------------------------------------------------+
| LINING DEFECT DIAGNOSTIC MATRIX |
+----------------------+-----------------------------+------------------------------+
| DEFECT TYPE | PHYSICAL CHARACTERISTIC | DISPOSITION / ACTION |
+----------------------+-----------------------------+------------------------------+
| Surface Heat Checks | Fine, hairline cracks | **ACCEPTABLE**; continue in |
| | (< 1/16" wide, surface only)| service if thickness > 1/4" |
+----------------------+-----------------------------+------------------------------+
| Through-Fractures | Cracks extending completely | **CONDEMN SHOE**; replace |
| | through lining to shoe web | complete axle set |
+----------------------+-----------------------------+------------------------------+
| Rivet-to-Rivet Cracks| Cracks propagating between | **CONDEMN SHOE**; lining may |
| | two adjacent rivet holes | shear and break away |
+----------------------+-----------------------------+------------------------------+
| Missing Lining Chunks| Friction material broken or | **CONDEMN SHOE** if missing |
| | spalled away from table | area exceeds 1.0 sq inch |
+----------------------+-----------------------------+------------------------------+
| Rust-Jacking | Rust swelling under lining, | **CONDEMN SHOE**; core table |
| | distorting lining curvature | is corroded and unserviceable|
+----------------------+-----------------------------+------------------------------+
Shoe Core Distortion & Rust-Jacking Inspection
- Rust-Jacking: In harsh winter operating environments with road de-icing chemicals, moisture penetrates between the steel shoe table and the riveted lining blocks. Severe rust scale forms and expands (rust-jacking), exerting upward pressure that lifts the lining, cracks rivets, and distorts the arc contour.
- Shoe Core Re-Use Standards: When relining brake shoes, inspect stripped shoe cores with a precision contour gauge:
- Measure table flatness across the width (maximum allowable twist/warpage: 0.010 inch / 0.25 mm).
- Inspect anchor pin holes for elongation or egg-shaping (maximum allowable clearance: 0.005 inch).
- Any shoe core with cracked weld beads between table and web, stretched anchor holes, or heavy rust scaling must be scrapped.
Test Your Knowledge
A commercial brake lining block is stamped with the edge code AB-450-FF. What do the letters FF signify according to the SAE J661 standard?
A
B
C
D
Test Your Knowledge
During a wheel-end inspection, a technician discovers that an axle wheel hub oil seal has leaked gear oil across the brake shoes and linings. Which of the following is the proper repair procedure?
A
B
C
D
Test Your Knowledge
What is the CVSA / 49 CFR 393.47 Out-of-Service minimum lining thickness for a non-steering-axle air drum brake, and where is it measured?
A
B
C
D
Test Your Knowledge
While inspecting foundation brake shoes during a preventive maintenance inspection, a technician observes fine, hairline cracks less than 1/16-inch wide on the surface of the lining that do not extend to the steel shoe table. What does this indicate?
A
B
C
D