Constant Speed Friction Coefficient Testing of Brake Friction Pads

The constant speed friction coefficient test is the primary laboratory-scale method for evaluating the friction performance of automotive brake pads and linings across a controlled temperature range of 100 °C to 350 °C. Performed on a constant-speed (Chase-type) friction tester in accordance with standards such as GB 5763, ISO 7881, and JIS D4411, it quantifies the friction coefficient, thermal fade behavior, recovery characteristics, and wear rate of friction materials. This article explains the test principle, sample requirements, procedure, measured parameters, acceptance criteria, and typical industrial applications.

Technical Overview

The constant speed friction coefficient test is a standardized tribological evaluation in which two small friction material specimens are pressed against a rotating cast iron disc under a fixed contact pressure while the disc temperature is raised in discrete steps. Because the sliding speed, load, and contact geometry are held constant, any variation in the measured friction force directly reflects the intrinsic friction behavior of the material as a function of temperature. The test produces a friction coefficient versus temperature profile that reveals how a brake pad performs during cold braking, normal operation, and severe high-temperature service.

This method is the industry benchmark for friction material characterization worldwide. In China it is specified in GB 5763 (Brake linings for automobiles), and equivalent small-sample constant speed methods are described in JIS D4411 and ISO 7881. Accredited third-party laboratories, friction material manufacturers, and OEM technical centers rely on the test for formulation development, batch quality control, and product certification support.

• Small-sample screening test using two 25 mm × 25 mm coupons
• Constant contact pressure of 0.98 MPa applied to the specimens
• Temperature steps at 100, 150, 200, 250, 300, and 350 °C
• 5,000 disc revolutions run at each temperature stage
• Maximum and minimum friction coefficients recorded at every stage
• Wear rate determined from specimen thickness loss over the total sliding distance

Testing Samples

Disc Brake Pad Coupons

Coupons are cut from the friction block of a finished disc brake pad, with the backing plate and any shims or underlayers removed. Each coupon is surface-ground to obtain two flat, parallel faces and is prepared to the standard test dimensions of 25 mm × 25 mm with a friction layer thickness typically in the range of 5 mm to 7 mm. Two coupons are required for each test so that they are mounted diametrically opposite each other in the specimen holder.

Drum Brake Lining Coupons

Coupons for drum brake linings are cut from the curved lining segment and flattened by grinding to the same 25 mm × 25 mm × 5–7 mm geometry. The curved-to-flat conversion must be performed carefully so that the original friction surface, heat treatment state, and porosity of the lining are preserved and representative of the delivered product.

Development and Reference Materials

For research and development purposes, coupons may be cut directly from laboratory-molded friction plates or plaques, allowing newly developed formulations to be screened before full-scale pad production. Certified reference friction couples may also be tested periodically to verify the calibration and repeatability of the testing machine.

Sample Conditioning

All coupons are cleaned to remove cutting fluids, dust, and oils, then conditioned in a controlled laboratory environment before testing. Initial thickness measurements are taken at multiple defined points on each coupon using a micrometer, and, where required, the samples are weighed so that thickness and mass loss can both be evaluated after the test.

Testing Items

The constant speed test yields a complete friction and wear characterization of the friction material under standardized thermal loading. The principal measured and reported items are:

• Friction coefficient at each specified temperature stage: 100 °C, 150 °C, 200 °C, 250 °C, 300 °C, and 350 °C
• Maximum friction coefficient at each temperature stage, indicating friction sensitivity and potential for grab or over-braking
• Minimum friction coefficient at each temperature stage, indicating fade margin and braking stability
• Fade behavior: the decline of the friction coefficient as temperature rises toward 350 °C
• Recovery characteristics: the ability of the friction coefficient to return toward its baseline value after cooling from the highest temperature
• Wear rate, expressed as volumetric loss per unit of friction work (typically ×10⁻⁷ cm³/(N·m)), calculated from thickness loss, contact load, and total sliding distance
• Post-test surface inspection for cracks, blistering, delamination, charring, or material transfer to the disc

Testing Methods

Apparatus

The test is performed on a constant speed friction tester consisting of a rotating friction disc manufactured from specified cast iron, a specimen holder that presses the two coupons against the disc with a controlled normal pressure of 0.98 MPa, a force measurement system with a calibrated load cell, an electric heater to raise the disc temperature, and a thermocouple mounted in close proximity to the friction track for accurate surface temperature control. A revolution counter and data acquisition system record the friction force continuously throughout the run.

Test Conditions

• Specimen quantity and size: 2 coupons of 25 mm × 25 mm each
• Contact pressure: 0.98 MPa
• Disc speed: fixed rotation at the speed specified by the standard (approximately 420 r/min on GB-type machines), producing a constant sliding velocity across the specimen faces
• Temperature stages: 100 °C, 150 °C, 200 °C, 250 °C, 300 °C, 350 °C
• Duration per stage: 5,000 disc revolutions, with continuous friction force recording

Test Procedure

The two conditioned coupons are mounted in the holder and a short run-in is performed to ensure full, uniform contact with the disc. The disc temperature is then raised to 100 °C and stabilized, the load is applied, and the disc is run for 5,000 revolutions while the friction force is recorded. On completion of the stage, the temperature is raised to the next level and the sequence is repeated through 150 °C, 200 °C, 250 °C, 300 °C, and 350 °C. After the highest temperature stage, the disc is allowed to cool so that the recovery of the friction coefficient at the baseline temperature can be assessed. Finally, the coupons are removed, allowed to return to room temperature, and re-measured for thickness and mass to determine wear.

Data Processing and Evaluation

The friction coefficient is calculated as the ratio of the measured friction force to the applied normal force, and both the maximum and minimum values within each 5,000-revolution stage are reported. The results are plotted as a friction coefficient versus temperature curve, which is the primary deliverable of the test. Wear rate is calculated from the mean thickness loss of the two coupons divided by the product of the normal load and the total sliding distance. Acceptance criteria depend on the product class defined in GB 5763: for most classes, the friction coefficient at each temperature stage must remain within a band of approximately 0.25 to 0.65, and the wear rate must not exceed the class limit tabulated in the standard. A well-balanced material shows a flat, stable μ–T curve with minimal fade at high temperature and consistent values between the maximum and minimum readings at every stage.

Applications

Constant speed friction coefficient testing supports every stage of the friction product lifecycle, from material research to market surveillance:

• Formulation development: rapid screening of resin, fiber, filler, and lubricant combinations during friction material research
• Competitive benchmarking: objective comparison of a product against reference materials or competitor samples on a common test platform
• Production quality control: routine batch release testing of disc pads and drum linings to verify consistency of the manufacturing process
• Certification and compliance support: generation of standardized friction and wear data for product type approval, market access documentation, and customer specification sheets in domestic and export markets
• Supplier qualification: verification that incoming friction materials from new suppliers meet declared friction classes before series production
• Failure analysis: diagnosis of field complaints such as brake fade, extended stopping distance, or accelerated lining wear by reproducing the thermal conditions in the laboratory
• Academic and tribological research: fundamental studies of temperature-dependent friction mechanisms in polymer-matrix composite materials

FAQ

Q: Which standards govern the constant speed friction coefficient test for brake pads?

A: The test is specified in GB 5763 (Brake linings for automobiles) in China, with closely related small-sample constant speed methods described in JIS D4411 and ISO 7881. These standards harmonize specimen dimensions, contact pressure, temperature stages, and evaluation criteria.

Q: What sample size and quantity are required?

A: Two coupons of 25 mm × 25 mm each, with a friction layer thickness of approximately 5 mm to 7 mm, are cut from the finished pad or lining and surface-ground flat before testing.

Q: Why is the test run at temperatures from 100 °C to 350 °C?

A: This range simulates the thermal spectrum from normal urban braking to severe downhill or high-speed deceleration events. It allows the fade resistance of the material at elevated temperature and its recovery after cooling to be quantified, which cannot be judged from room-temperature friction alone.

Q: What do the maximum and minimum friction coefficient values mean?

A: Within each temperature stage, the friction force fluctuates as the friction film forms and evolves. The maximum value indicates the peak braking aggressiveness, while the minimum value indicates the worst-case braking capability. A large spread between them signals instability, even if the average value is acceptable.

Q: How does this test differ from a full-scale inertia dynamometer test?

A: The constant speed test is a small-sample screening method that isolates the intrinsic friction properties of the material under fixed, controlled conditions. An inertia dynamometer test evaluates the complete brake assembly on a real disc or drum under vehicle-representative speed, torque, and pressure cycles. The two methods are complementary: the constant speed test is faster and cheaper for material ranking, while the dynamometer validates system-level performance.

Q: How long does a standard test take?

A: A complete test sequence through all six temperature stages, including heating, stabilization, and post-test measurements, is typically completed within a few hours, making the method well suited to high-throughput quality control.

Q: Can the test predict noise, vibration, or judder problems?

A: No. The test measures friction coefficient and wear only. Noise, judder, and pedal feel phenomena depend on the complete brake system and require dedicated NVH testing and dynamometer programs.