On 7/15/2022, DOT/NHTSA published a final rule (view it here) that makes significant changes to the requirements for rear impact guards (RIGs).   This rule is effective  1/11/23, for new vehicle manufacture. RIGS are also referred to as “underride” bumpers and these rules are applicable for all trailers, not just cargo tanks, with some exceptions (exceptions, like wheels-back vehicles, are essentially unchanged, though “low chassis vehicles” have been added as an exception). These rules are part of Federal Motor Vehicle Safety Standards (FMVSS) and can be found in 49 CFR 571.223 and .224.   The basic idea of the proposed changes is to increase the impact resistance.  The current rules are based on an impact from a passenger vehicle traveling at 30 mph.  The new rules increase that speed to 35 mph.  While that’s only 16.7% increase in speed, the energy from the resulting impact increases by 36%.    This change more or less aligns US rules with the Canadian underride rules which have been in place since around 2008.  

The proposed requirements would result in bumpers that:

  • Resist a uniform distributed load of 350 kN (78,683 lb), with no more than 125 mm (4.9″) of deflection and absorbing at least 20 kJ (14,751 ft-lb). OR resist a 700kN (157,366 lb) load with no more than 125 mm (4.9″) deflection (no energy absorption required for higher load).
  • Maintain ground clearance after the energy absorption test, not exceeding 560 mm (22″).
  • Not separate from their mounting structure after testing is completed.

NJP Engineering doesn’t have any specific commentary on these rules, other than the general commentary that:

  • Alignment with Canadian rules is a good thing.
  • DOT/NHTSA probably has understated the cost/safety impact of increased weight, particularly in the cargo tank industry.