Understanding Exhaust Hanger Materials

Exhaust hangers are critical components that secure the exhaust system to the vehicle undercarriage while allowing for movement and vibration dampening. They are typically made from rubber, metal, or a combination of both. Rubber hangers provide flexibility and vibration absorption, while metal hangers offer structural strength and support. The choice of material directly affects the system’s noise, vibration, and harshness (NVH) performance, as well as long-term durability.

Rubber hangers are commonly manufactured from ethylene propylene diene monomer (EPDM) or natural rubber (NR). EPDM offers excellent resistance to heat, ozone, and weather, making it a preferred choice for exhaust applications. Natural rubber provides superior elasticity and tear strength but may require additives to withstand high temperatures and environmental exposure. Metal hangers are usually made from stainless steel, carbon steel, or zinc-plated steel. Stainless steel offers the best corrosion resistance, while carbon steel is often coated for protection.

Proper testing of these materials before installation is essential to avoid premature failure, which can lead to exhaust rattles, misalignment, broken hangers, and even exhaust system collapse. This article provides a comprehensive guide to evaluating exhaust hanger materials using industry-standard methods and practical techniques.

Key Quality Parameters for Exhaust Hangers

Before diving into specific tests, it’s important to understand the properties that define a high-quality exhaust hanger. These parameters include:

  • Tensile strength: The maximum stress the material can withstand while being stretched before breaking. For rubber, this is measured in megapascals (MPa).
  • Elongation at break: The percentage increase in length at the point of failure. Higher elongation indicates better flexibility.
  • Durometer hardness: A measure of material resistance to indentation, typically on the Shore A scale for rubber. Exhaust hangers usually range from 50 to 70 Shore A.
  • Heat resistance: The ability to maintain physical properties under continuous exposure to exhaust heat (150°C–200°C).
  • Ozone and UV resistance: Resistance to cracking when exposed to ozone, which is present in the atmosphere and can degrade rubber.
  • Load-bearing capacity: The maximum static and dynamic load the hanger can support without excessive deformation or fatigue.
  • Corrosion resistance (for metals): The ability to withstand exposure to road salts, moisture, and high temperatures without rusting.

Testing should address all these parameters to ensure the hanger will perform reliably over the vehicle’s lifespan.

Pre-Installation Testing Methods

1. Visual and Dimensional Inspection

Begin with a thorough visual examination. Look for surface defects such as cracks, porosity, flash (excess material from molding), or non-uniform geometry. High-quality rubber hangers should have smooth surfaces, clean edges, and consistent thickness. Metal hangers must be free of burrs, sharp edges, and weld splatter. Use calipers or micrometers to verify critical dimensions (hole diameter, length, thickness) match the manufacturer’s specifications. Dimensional deviations can cause improper fit and early failure.

Pay particular attention to the center hole or mounting eyelets. Any irregularities can lead to uneven stress distribution during installation. A simple go/no-go gauge can be used to quickly check hole diameters. This inspection should be performed on a representative sample (e.g., 5% of a batch or a minimum of five hangers).

2. Durometer Hardness Test for Rubber

Hardness is one of the most important indicators of rubber quality. Use a Shore A durometer (per ASTM D2240) to measure the indentation resistance of the rubber. Place the hanger on a flat, hard surface and press the durometer’s indenter firmly into the rubber for a specified dwell time (typically 3 seconds). Record the reading. Acceptable values for exhaust hangers range from 55 to 70 Shore A. Softer hangers (below 50) may not provide adequate support; harder hangers (above 75) may not absorb vibrations properly and could transmit noise.

Take measurements at three different points on the hanger (both faces and edges) to check uniformity. Large variations (±5 points or more) indicate poor compounding or curing irregularities. Always test at room temperature (23°C ±2°C) and avoid testing on molded edges or flash lines.

3. Tensile Strength and Elongation Test

This destructive test determines the rubber’s mechanical integrity. Obtain a dumbbell-shaped test specimen cut from a flat section of the hanger (if possible) or use a sample from the same batch. Follow ASTM D412 for rubber testing. Clamp the specimen in a tensile testing machine (e.g., Instron or similar) and apply a constant crosshead speed (500 mm/min is common). Record the force at break and the elongation at break.

High-quality EPDM exhaust hangers should exhibit a tensile strength of at least 7 MPa and an elongation of 300%–500%. Lower values suggest poor compounding or poorly cured rubber. If the hanger is too elastic (elongation >600%) it may stretch under load and allow sagging. For metal hangers, tensile testing per ASTM E8 is applicable, but metal hangers are more often evaluated for yield strength and ultimate strength, typically >250 MPa for carbon steel used in brackets.

4. Heat Aging and Ozone Resistance Test

Exhaust hangers operate under constant heat and are exposed to ozone from the environment. To simulate heat aging, place rubber samples in an air-circulating oven at 100°C for 70 hours (per ASTM D573). After removal, allow them to cool for 24 hours at room temperature, then re-test hardness and tensile properties. Acceptable change in hardness is ±10 points; change in tensile strength should not exceed 30% reduction. Materials that become brittle or sticky have failed.

Ozone resistance testing follows ASTM D1149. Mount rubber samples under mild tensile strain (20% elongation) and expose them to ozone at 50 pphm (parts per hundred million) at 40°C for 72 hours. Inspect for cracks at 2x magnification. No visible cracks constitute a pass. This test is especially important for rubber hangers used in regions with high air pollution or strong sunlight exposure.

5. Load and Fatigue Testing

Load testing confirms the hanger’s ability to support the actual exhaust system weight without excessive deformation. Secure the hanger in a test fixture that mimics the vehicle mounting points. Apply a static load equal to 1.5 times the expected maximum load (to include a safety margin) and measure the deflection after 1 minute. The hanger should not show permanent set (i.e., it returns to near-original shape when unloaded). Permanent set exceeding 5% indicates poor elastic recovery.

Fatigue testing evaluates long-term durability. Use a dynamic test rig that cycles the hanger through a defined displacement amplitude (e.g., ±5 mm at 2 Hz) for 100,000 cycles. Inspect for cracks, tears, or excessive wear. High-quality hangers should survive 500,000 cycles or more without failure. For metal hangers, apply cyclic loading in a servo-hydraulic machine to simulate road-induced vibrations. Weld joints and brackets must pass a minimum of 1 million cycles without crack initiation.

Advanced Testing for Metal Hangers

Metal hangers, whether solid brackets or combined rubber-metal components, require additional scrutiny. Corrosion resistance is a primary concern. Perform a salt spray test (ASTM B117) by exposing metal samples to a salt fog environment for 96 hours. Stainless steel (grade 304 or 316) should show no red rust; zinc-plated parts should not exhibit white or red rust before 48 hours. For coated parts, cross-hatch adhesion tests (ASTM D3359) and coating thickness measurements (using magnetic gauge) ensure the protective layer is intact.

Weld quality is another critical factor. Examine welds visually and use dye penetrant inspection (PT) to detect surface cracks. For critical load-bearing brackets, ultrasonic or radiographic testing (UT or RT) may be justified. Dimensional accuracy of mounting holes and alignment tabs must be held to ±0.5 mm to ensure proper fit with the exhaust system. Metal fatigue can be exacerbated by sharp corners; ensure all edges have a radius of at least 0.5 mm to reduce stress concentrations.

Tips for Sourcing Quality Exhaust Hangers

Testing is essential, but starting with reputable suppliers reduces risk. Look for manufacturers that comply with ISO 9001 or IATF 16949 quality management systems. Request material certifications and test reports (mill certificates for metals, compound data sheets for rubber). For rubber hangers, ask for compound number and test data on hardness, tensile, and heat aging. Verify that the supplier uses ozone-resistant EPDM with proper antioxidant packages.

When sourcing metal hangers, specify the exact grade of stainless steel (e.g., AISI 304) or coating thickness for plated parts. Avoid severely price-cut components from unknown sources—they often use recycled rubber with poor heat resistance or thin coatings that fail quickly. Consider independent third-party testing for high-volume purchases. Organizations like ASTM International and SAE International provide standards that can be referenced in purchase orders.

For technicians performing in-house testing, inexpensive tooling like a durometer, calipers, and a simple loading fixture can catch most defects. More advanced tests (tensile, heat aging, salt spray) may require partnering with a local materials testing lab. The Quality Magazine guide to selecting a test lab offers practical advice on vendor qualification.

Conclusion

Testing exhaust hanger materials before installation is not an optional step—it is a critical quality assurance practice that prevents costly rework, safety hazards, and vehicle downtime. By systematically applying visual inspection, durometer hardness testing, tensile and elongation measurements, heat and ozone aging, and load/fatigue testing, technicians can confidently select materials that will perform reliably under the demanding conditions of the exhaust environment. For metal components, corrosion resistance and weld integrity tests further reduce risk.

Remember that testing is only as good as the standards you follow. Adhere to recognized ASTM or ISO protocols, document results, and use the data to build a quality history for your suppliers. Investing a few minutes per batch can save hours of troubleshooting and parts replacement down the road. With the methods outlined here, you are equipped to verify that every exhaust hanger you install meets the high standards required for safety and longevity.