Rear Impact Guards: The Four Measurements That Decide

Four measurements decide rear impact guards. Only the 22-inch height moves.

Rear Impact Guards: The Four Measurements That Decide

Of everything an inspector can measure on a trailer, the rear impact guard is one of the few things an owner-operator can check alone, in the yard, with a tape measure. No rack, no scanner, nothing to take apart. Four measurements and one height rule, each written with an exact number in the federal regulation.

What follows is what 49 CFR 393.86 says, without interpretation. It is worth knowing for one practical reason: three of those four measurements depend on the equipment installed and do not change on their own, but the fourth moves without anybody touching the guard.

Which units it applies to

The rule splits in two.

Paragraph (a) covers trailers and semitrailers with a gross vehicle weight rating of 4,536 kg (10,000 pounds) or more, manufactured on or after January 26, 1998. Those must have a guard meeting FMVSS 223 as it was in effect when the vehicle was built, and the vehicle with the guard installed must meet FMVSS 224.

Paragraph (b)(1) covers the rest: vehicles manufactured after December 31, 1952 other than those post-1998 trailers. Those need a guard when the rear bottom edge of the body or chassis sits more than 76.2 cm (30 inches) above the ground when the vehicle is empty. Truck tractors are excluded from that paragraph.

Both cases carry express exemptions: pole trailers and driveaway-towaway trailers as defined in 390.5, and under 393.5 pulpwood trailers, low chassis vehicles, special purpose vehicles, wheels back vehicles and road construction controlled horizontal discharge trailers.

The four measurements, with numbers

Width. The outermost surfaces of the horizontal member must reach within 100 mm (4 inches) of the side extremities of the vehicle, and must not extend past them. Not short, not over.

Height above ground. The bottom edge of the horizontal member cannot sit more than 560 mm (22 inches) above the ground at any point across the full width of the guard. Rounded corners may curve upward within 255 mm (10 inches) measured from the side tangent planes.

How far back. At any height of 560 mm (22 inches) or more, the rearmost surface of the guard must be within 305 mm (12 inches) of the rear extremity of the vehicle.

Depth of the member. The horizontal member must have a cross-sectional vertical height of at least 100 mm (3.94 inches) at any point across the width of the guard.

The measurement that moves on its own

Here is what makes this worth a periodic check instead of a one-time look when the trailer was bought.

Width, distance to the rear extremity and member depth are geometry of the guard itself: if nothing hits it and nobody swaps it for one that does not belong there, they stay put. The 22-inch height is different, because it is not measured from the trailer. It is measured from the ground.

Anything that changes the ride height of the unit changes that number without anybody touching the guard. Tired suspension. A spring replaced with a different rating. Air bags that no longer lift the same. A tire size change that looked minor at the time. In every one of those cases the trailer sits lower or higher, and the guard goes with it.

So measure it with the unit as it actually runs — empty, on level ground — instead of trusting that it was right the day it was bought.

How to check it in the yard

  • Level ground, empty unit. A sloped yard is enough to throw the reading off, and paragraph (b) speaks specifically about the vehicle unloaded.
  • Measure at several points across the width, not just the center. Both limits, the 22 inches and the member depth, are written as at any point across the guard. A bowed member passes in the middle and fails at the end.
  • Check both ends against the side of the trailer. The 4 inches of clearance apply on each side, and that is where an old dock hit shows up.
  • Write it down with the date. Measure before and after a suspension or tire change and you have the number for how much it moved, which is exactly what the eye does not catch.
  • If the trailer predates 1998, do not assume the same rule. That one runs on the 30-inch rear bottom edge test, which is a different measurement and a different criterion.

The preventive read

This is a tape-measure check, and that is the whole point of it: it needs no shop and no special tool. What is worth understanding is what knocks it out of range, because when the height goes out of spec it is almost never the guard that changed — the unit sat down. That same settling is what starts wearing tires unevenly and loading the suspension harder than it should.

If the measurement shows the trailer riding lower than it used to, the guard is the symptom and not the problem: what needs looking at is suspension, springs and ride height. That is handled at The Truck Savers, in Houston, Dallas and Monterrey.

Original source

Rule text verified directly against the eCFR, 49 CFR 393.86, Rear impact guards and rear end protection.