Partial Testing

1.0 Introduction

As explained on the Responsibilities page and elsewhere on this site, before new bus models can be acquired by FTA grantees, they must undergo full testing and the grantee must possess the applicable Bus Testing Report (wise grantees also study the report carefully).  In the early days of the Bus Testing Program, a previously-tested bus model that was subsequently produced with major changes would generally have been required to complete another full test.  It quickly became apparent that a more flexible approach was needed.

FTA introduced the concept of partial testing in the July 28, 1992 “Reinstatement and Modification of Interim Final Rulemaking” (IRM) for the Bus Testing Regulation (57 FR 33394).  The IRM defined partial testing as “the performance of those bus tests which may yield significantly different data from that obtained in previous bus testing conducted at the bus testing facility.”  Partial testing was offered as a means to reduce the testing burden on manufacturers of previously-tested bus models that were subsequently produced with major modifications, while complying with the intent of the Bus Testing Regulation.  The partial testing concept allows modified versions of previously-tested buses to undergo only those tests in which the data would be expected to be substantially different from the original test data. 

Without the accompanying Full Bus Testing Report, a Partial Bus Testing Report is intrinsically incomplete.  When an FTA grantee acquires a bus model covered by a Partial Bus Testing Report, they must certify that they possess not only the Partial Testing Report, but also the “baseline” full Bus Testing Report for the originally-tested configuration of that model.  In some cases, such as when partial testing is conducted because a bus model is offered with a different chassis or other major component but that component has been tested in a similar bus by another manufacturer, the grantee may also need to obtain one or more full and/or partial testing reports of the(se) similar bus(es) that taken together present all the data that would have been included in a full test report of the bus model in question.

FTA published in the May 26, 1993 Federal Register (58 FR 30213) a notice of availability of a draft document titled, “Testing Requirement Guidelines for Five- and Four-Year Buses; and Partial Bus Testing Procedures for All Bus Categories.”  A final version of the 1993 Guidelines was never published, however FTA has regularly consulted the draft guidelines in making partial testing determinations.  FTA has reviewed and updated the 1993 draft guidelines, and the changes are reflected below.  These Partial Testing Guidelines supersede the 1993 guidelines and constitutes FTA’s current guidance for partial testing.  Partial testing that may be required due to changes to buses or situations other than those discussed below may need to be reviewed on a case-by-case basis by requesting a determination on Bus Testing requirements from the FTA Bus Testing Manager.

2.0 Partial Testing Guidelines

FTA has identified a set of general partial testing guidelines that are applied equally to all buses.  We have also identified other partial testing guidelines whose application depends on the expected service life of the bus and/or the manufacturing techniques employed in its production.

2.1 General Partial Testing Guidelines

Major changes to a previously-tested bus that may require partial testing typically fall into four main categories:

1. Changes in components.
2. Changes in design or configuration.
3. Changes in legal rating of chassis, major structure, or systems.
4. Changes in structural materials and/or manufacturing methods.

The individual tests that would be included in the partial testing required depend on which tests we would reasonably expect to obtain significantly different test data compared to when that bus model was previously tested.

2.1.1 Changes in Components

We use the term “components” to refer to major components such as engines, transmissions, axles, etc.  In general, a change of components will not require a previously-tested vehicle to require partial testing at the Bus Testing Center provided all of the following criteria are met: 

• The new component(s) has been previously tested on a comparable vehicle at the Bus Testing Center; and
• The installation of the new component(s) will not result in any significant structural modifications to the vehicle; and
• We would not reasonably expect the installation of the new component(s) to result in significantly different data from that previously obtained at the Bus Testing Center.

2.1.2 Changes in design or configuration

Examples of major changes in design or configuration include:

• Change from one type of fuel system to another (e.g., diesel to CNG).
• Change to/from space frame from/to semi-monocoque.
• Change to/from front-engine from/to rear-engine.
• Addition of doors, dropped floors, or other significant structural irregularities.
• Changing from a two-piece to a one-piece windshield.
• Addition or deletion of a tag axle.
• Significant increase or decrease in length (generally >4 feet) or width (generally >10 inches).
• Significant increase in center of gravity or decrease in roll center height (generally >6 inches).
• Significant change in major load paths (e.g., revised suspension geometry, added weight on the roof).

2.1.3 Changes in legal ratings

Major changes in the limiting ratings legally assigned to components, assemblies, or the overall vehicle by their respective manufacturers may require additional testing.  Examples of major changes in ratings include changes by more than 10 percent in:

• Gross axle weight rating,
• Engine power or torque ratings, and/or
• Gross vehicle weight rating.

2.1.4 Changes in structural materials and/or manufacturing methods

Major changes in chassis or body structural materials and/or manufacturing methods may require additional testing; examples include:

• Change to/from metal, composites, wood, plastic, etc.
• Changing to/from forgings from/to castings.  However, FTA may accept detailed laboratory proof testing in lieu of additional Altoona testing in cases where the change is confined to a specific discrete part whose exact loading conditions can be accurately simulated in the laboratory (e.g., an axle or a suspension kingpin).
• Change in major load-bearing components from mild steel to stainless steel (stainless steel is a different material than mild steel, with different mechanical properties and more stringent manufacturing requirements).  Changing from stainless steel to mild steel will generally not require additional testing as mild steel is typically more forgiving of manufacturing variations.

In addition, materials changes to some non-load-bearing parts or components may also require additional testing, if the material change could impact their performance in fulfilling an important function.  For example, changing to/from a metal roof from/to a plastic roof could impact the roof’s performance in resisting water intrusion and may affect the interior noise.

2.2 Specific Partial Testing Guidelines

The general partial testing guidelines may be supplemented by additional guidelines particular to the service life category and the construction method of the bus. 

2.2.1 Variations by Service Life Categories

The service life categories defined in the Bus Testing Regulation (49 CFR Part 665) are:

• Minimum service life of 12 years or 500,000 miles – typified by heavy duty large buses, approximately 35-40 foot, as well as articulated buses.
• Minimum service life of 10 years or 350,000 miles – typified by heavy duty small buses, approximately 30 foot.
• Minimum service life of 7 years or 200,000 miles – typified by medium duty mid-size buses, approximately 25-35 foot.
• Minimum service life of 5 years or 150,000 miles – typified by light duty mid-size buses, approximately 25-35 foot.
• Minimum service life of 4 years or 100,000 miles – typified by light duty small buses, cutaways, and modified vans, approximately 16-28 foot.

Due to their lower cost, shorter design life, and industry practice of offering several variations based on the same bus body model, FTA treats buses in the 4-year and 5-year service life categories differently in determining partial testing requirements than it does for buses in the 7-year and higher service life categories.  See the discussion of the “family of vehicles” concept below.

2.2.2 Bus Construction

Vehicles built by final stage manufacturers on unmodified third-party chassis are vehicles built by adding a proprietary body to a chassis, where the chassis is produced by an independent third party and offered in the same configuration to multiple bus manufacturers.  In general, partial testing determinations will consider the fact that manufacturers of such buses do not have direct control over the design, engineering, production, or quality assurance of the chassis.  This category has historically been exemplified by light- and medium-duty buses in the 4-, 5-, and 7-year service life categories, but in recent years has occasionally included buses in higher service life categories. 

In the 1993 draft Partial Testing Guidelines, FTA established the concept of a “family of vehicles.”  This concept, which is only applicable to buses in the 4-year and 5-year service life categories, is offered to ease the testing burden on manufacturers of small body-on-chassis type buses, who frequently offer what is essentially the same bus body shell or a closely related derivative on various similar third-party cutaway chassis.  Under the family of vehicles concept, FTA generally permits a manufacturer of a bus body that has been tested on one unmodified third-party chassis to offer the same or closely related body built on another similar unmodified third-party chassis that has been tested at the Bus Testing Center (on one of its own buses or on a competitor’s similar bus), without requiring additional testing.  This includes changes in bus length, such as from one standard wheelbase offering to another.  The family of vehicles concept does not apply to buses in the 7-year or higher service life categories.

Vehicles built by final stage manufacturers on modified third-party chassis are constructed in the same manner as that described in the paragraph above, but the bus body manufacturer has independently modified the third-party chassis.  In general, FTA will treat such vehicles as vehicles built by final stage manufacturers on unmodified third-party chassis, provided that any modifications are performed in strict compliance with the chassis manufacturer’s official chassis modification guidelines.  If the modifications are not performed in strict compliance with the chassis manufacturer’s official chassis modification guidelines, then FTA would consider such a vehicle to have a proprietary chassis.

Vehicles built with integrated design or proprietary chassis are vehicles built completely by the primary manufacturer using a chassis integrated with the bus body, or a separate chassis built in-house by the manufacturer.  Chassis built by others for the exclusive use the by manufacturer using its proprietary designs and subject to its own internal quality assurance may also fall into this category.  The discriminating factor is whether the bus manufacturer is directly responsible for the design, engineering, production, and quality assurance for the chassis.  In general, partial testing determinations will consider the fact that manufacturers of such buses do have direct control over the design, engineering, production, or quality assurance of the chassis (if a chassis can be separately identified from the rest of the bus). At the time of this writing, this category is primarily represented by large heavy-duty buses and small mass-produced vans. 

Unmodified mass-produced vans are categorically exempted from testing (full or partial testing) by the Bus Testing Regulation.  However, FTA notes that this exemption historically assumed that unmodified mass-produced vans would only be offered in the 4-year, 100,000-mile service life category.  Therefore, FTA now clarifies that unmodified mass-produced vans are categorically exempted from testing by the Bus Testing Regulation only in the 4-year, 100,000-mile service life category; unmodified mass-produced vans offered in the 5-year, 150,000-mile (or higher) service life category are subject to testing.