The physics of shear planes
When you mount a Heim joint in double shear, you are creating two “shear planes” for the bolt.
- Shear Strength: For inch-series automotive fasteners, SAE J429 defines the mechanical requirements for Grade 8 bolts, including high tensile and yield strength. In a double shear mount, the load is shared across two shear planes, which reduces bolt bending and allows the fastener to work closer to its intended shear-loading condition.
- Fatigue Prevention: In a single shear steering setup, every bump can apply bending load to the bolt. Over time, repeated bending can start fatigue cracks, especially near the thread transition or where the shear plane passes through the threaded section.
Where to use each setup
- Critical Links: Steering drag links, track bars, and 4-link mounts should always be double shear.
- Safety Washers: If you are forced to use a single-shear setup (like on some sway bar links), you must use a large “Safety Washer” (Article 86). This ensures that if the spherical ball ever fails, the link stays attached to the bolt rather than falling off.
Related Articles:
- Why You Must Use Safety Washers in Single-Shear Mounts.
- Double Shear Engineering: How bracket design eliminates bolt bending stress.
