Federal Chain Requirements
- Determine the minimum ultimate strength required for safety devices and attachments based on towed vehicle weight.
- Position two safety chains correctly relative to the tow vehicle centerline and attachment points.
- Apply 49 CFR 393.70 requirements to the selection and installation of safety devices.
- Tape measure — Ensure chain attachment points are equally distant from the tow vehicle centerline.
- Weight scale or data plate — Identify the gross weight of the towed vehicle to determine required ultimate strength.
- Reference guide: 49 CFR 393.70 — Verify federal compliance for coupling devices and towing methods.
The safety device and its attachments must have an ultimate strength not less than the gross weight of the towed vehicle(s).
Two chains or one chain may be used. When two are attached at separate points, they must be equally distant from, and on opposite sides of, the tow vehicle centerline.
How It Works
Strength Rating Compliance
The safety device and its attachments must have an ultimate strength not less than the gross weight of the towed vehicle(s). This ensures the device can withstand the forces of the trailer in the event of a primary coupling failure.
Chain Configuration Options
Check the applicable coupling provision and its conditions in current 49 CFR 393.70 and the equipment instructions. Do not treat a summary of chain configurations as permission to substitute a different arrangement merely because its strength rating appears adequate.
Symmetrical Attachment
When two chains are attached at separate points, they must be equally distant from, and on opposite sides of, the tow vehicle centerline to ensure balanced load distribution during a disconnect.
Diagnostic Chart
| Symptom | Check | Spec / Expected | Likely cause & fix |
|---|---|---|---|
| Safety chain ultimate strength is lower than the trailer gross weight. | Compare the rated ultimate strength of the chain and its attachments against the gross weight of the towed vehicle. | Ultimate strength must be not less than the gross weight of the towed vehicle(s). | Undersized safety devices were installed that do not meet 49 CFR 393.70 requirements. Basis: 49 CFR §393.70 Coupling devices and towing methods |
| Two chains are attached at different distances from the center. | Measure the distance from the tow vehicle centerline to each chain attachment point. | Attachment points must be equally distant from, and on opposite sides of, the tow vehicle centerline. | Improper installation of attachment points on the tow vehicle. Basis: 49 CFR §393.70 Coupling devices and towing methods |
| Safety device attachments are weaker than the chain itself. | Verify the ultimate strength rating of both the safety device and its individual attachment hardware. | The safety device and its attachments must have an ultimate strength not less than the gross weight. | Failure to account for the strength of hooks, bolts, or other attachment hardware. Basis: 49 CFR §393.70 Coupling devices and towing methods |
At the Bay: Diagnose the Case
CASE 1 / 3
Safety chain ultimate strength is lower than the trailer gross weight.
- Test
- Compare
- Correct
Select the next check
Basis: 49 CFR §393.70 Coupling devices and towing methods. A likely cause is a hypothesis; confirm it with the applicable service procedure before repair.
Common Bay Mistakes
- ✕ Choosing a chain arrangement by strength alone without confirming the applicable coupling provision and equipment instructions.
- ✕ Neglecting the strength of the attachments (hooks/brackets) while only checking the chain itself.
- ✕ Measuring chain distance from the edge of the bumper instead of the vehicle centerline.
- ✕ Using a safety device with an ultimate strength exactly equal to the trailer weight without verifying if it is 'not less than' the gross weight.
Quick Check
Practice only — not scored.