Interpretation ID: NJP Engineering Letter of Interpretation FMVSS No. 121signed

October 4, 2024

Mr. Nick Paulick, PE
Owner, NJP Engineering, LLC
P.O. Box 2161
Oshkosh, WI 54903 

Dear Mr. Paulick, 

This responds to your letter, dated October 31, 2022, regarding removing the third axle from a used three axle semitrailer and reducing both the trailer’s load-carrying capacity and its braking capacity. More specifically, we understand you to be asking whether the reduction in braking capacity would violate the “make inoperative” prohibition in 49 U.S.C. § 30122 with respect to Federal motor vehicle safety standard (FMVSS) No. 121, Air Brake Systems. 

Please note that our answer below is based on our understanding of the specific information provided in your letter. This interpretation letter does not have the force and effect of law and is not meant to bind the public in any way. This letter is intended only to provide clarity regarding existing requirements under the law and represents the opinion of the agency on the questions addressed in your letter at the time of signature. 

In your letter, you stated that you are requesting an interpretation regarding the modification of semi-trailers after first sale to add or remove axles to change their load-carrying capacity and the overall capacity of a combination motor vehicle. You note that you reviewed existing interpretations (specifically, our letter to John Paul Barber on May 24, 1993) and believe that the National Highway Traffic Safety Administration (NHTSA) “is not opposed” to aftermarket modifications such as removing an axle, “so long as they are done properly and that an explanatory label is affixed, advising the user of what these changes will mean in terms of gross weight rating.” 

You ask about the specific example of a three-axle semi-trailer with each axle having a gross axle weight rating (GAWR) of 20,000 pounds and the trailer having a gross vehicle weight rating (GVWR) of 75,000 pounds. You suggest the removal of one axle from the trailer, reducing its GVWR to 65,000 pounds. You further state that a “dataplate” would be permanently affixed to the modified trailer advising the user of the revised payload capacity. Because removing an axle from the trailer also removes the brakes associated with it, you ask whether the reduced “braking capacity” of the trailer would bring the vehicle out of compliance with NHTSA’s braking standards and potentially violate the “make inoperative” prohibition. For the purposes of this letter of interpretation, we understand “braking capacity” as used in your letter to mean the ability of the vehicle to meet the stopping distance requirements in FMVSS No. 121. 

By way of background, NHTSA is authorized by the National Traffic and Motor Vehicle Safety Act (Safety Act, 49 U.S.C. Chapter 301) to issue FMVSS that set performance requirements for new motor vehicles and new items of motor vehicle equipment. The Safety Act requires manufacturers to self-certify that their vehicles and equipment conform to all applicable FMVSS in effect on the date of manufacture. NHTSA also investigates safety-related defects. 

Your letter indicates that the modifications occur after first sale other than for resale.1 As you correctly note, the only legal requirement that is imposed on entities that make modifications to used vehicles is that a manufacturer, distributor, dealer, rental company, or motor vehicle repair business may not knowingly make inoperative any part of a device or element of design installed in furtherance of compliance with an applicable motor vehicle safety standard. 49 U.S.C. § 30122(b). A modifier subject to this prohibition must therefore ensure that the vehicle’s brakes and other systems remain in compliance with all applicable Federal motor vehicle safety standards once the modifications are performed. Civil penalties can be imposed under 49 U.S.C.
§ 30165(a) for violations of section 30112(b). 

We assume, for the purposes of this letter, that the trailer you intend to modify is equipped with an air brake system. FMVSS No. 121 establishes requirements for vehicles equipped with air brake systems. Under S5.3.1 of FMVSS No. 121, truck tractors must meet the stopping distance requirements specified in Table II of that standard, when loaded to their GVWR and tested using an unbraked control trailer.2 We note, however, that such requirements are applicable to the truck tractor. FMVSS No. 121 contains no stopping distance requirements applicable to trailers. FMVSS No. 121 does contain other requirements applicable to trailers, such as parking brakes, and certain as-equipped requirements should the trailer axles have brakes, such as that the braked trailer axles be equipped with anti-lock braking systems and have the required air reservoirs. 

We also assume for the purposes of this letter that the entity performing the modifications is subject to the “make inoperative” prohibition. You state that NJP Engineering provides engineering services for the evaluation of new and modified cargo tank motor vehicles. From this description, it is not clear what role NJP Engineering has in the actual performance of any modifications. We also note that even if an entity not subject to the “make inoperative” prohibition modifies the vehicles, such modifications may be subject to the laws and regulations administered by the Federal Motor Carrier Safety Administration and the States in which the vehicles are operated.

1 Because the trailer is a used trailer, you are not an alterer. An alterer is defined as a person who alters by addition, substitution, or removal of components (other than readily attachable components) a certified vehicle before the first purchase of the vehicle other than for resale. 49 C.F.R. § 567.3.
2 The following vehicles may meet the stopping distance requirements specified in Table IIa instead of Table II: Three-axle tractors with a front axle that has a GAWR of 14,600 pounds or less, and with two rear drive axles that have a combined GAWR of 45,000 pounds or less, that were manufactured before August 1, 2011; and all other tractors that were manufactured before August 1, 2013. For such vehicles, the analysis in this letter of interpretation is the same. 

In your letter, you state that a three-axle trailer would be modified to remove an axle and you ask about the applicability of the “make inoperative” prohibition with respect to the trailer’s braking capacity. FMVSS No. 121 does not contain stopping distance requirements applicable to trailers. Therefore, a reduction in braking capacity alone would not affect compliance with the FMVSSs and would not violate the “make inoperative” prohibition.3 Regarding other braking performance requirements applicable to trailers, such as parking brake requirements and as- equipped requirements for trailer axles, modifications to the trailer as stated must not take the trailer out of compliance with requirements that would be applicable to the modified trailer. You do not ask about, nor does this letter address, any other aspect of compliance with FMVSS No. 121, or other applicable FMVSS. 

I hope this information is helpful. If you have any further questions, please feel free to contact Eli Wachtel of my staff at (202) 366-2992.

Sincerely,
ADAM RAVIV
Adam Raviv Chief Counsel

Dated: 10/4/24
Ref: Standard No. 121

3 You note that you intend to affix a “dataplate” indicating the revised weight capacity of the trailer. We note that such a “dataplate” is not required by our regulations; however, we believe it is appropriate to add a label to the vehicle which indicates the appropriate loaded weight of the modified vehicle in situations where certain modifications that might be made to a used vehicle (e.g., adding or deleting an axle) which could make the originally assigned GVWR inappropriate for the vehicle as modified.