Project: Brake Compatibility of Vehicle Combinations

Reference: S291B/VD

Last update: 18/08/2003 12:53:19


The objectives of this project are to:

* compare the findings of earlier work on this subject with the results of comparative testing of a vehicle combination whose trailer is engineered to satisfy the latest UNECE braking standards (Regulation 13.09). These standards will be mirrored by the EU Directive;
* examine the stability of the combination under the range of achievable decelerations over a wide range of vehicle speeds;
* deliver data concerning the magnitude and direction of the forces developed at the towing vehicle/trailer coupling; and
* additionally, as a secondary objective, the project may examine the merits of early brake warning for following motorists by analysis of accelerator movement.


Poor compatibility results in premature degradation of the braking system, which should in turn affect stability, efficiency and safety. In addition the EU and UNECE have introduced new brake fade test performance requirements. Investigations need to be made to discover if any consequential changes to the design of the brake influence the suitability of the current compatibility corridors.

This project involved testing a vehicle combination repeatedly over a pre-defined high mileage route to simulate normal use. The braking system on the test vehicle was set at differing levels of threshold pressure within the envelope permitted by current legislation. Interspersed with this, the brake performance was monitored by means The braking systems of tractors and trailers that form HGV articulated combinations are designed around the fact that they are individual vehicles, both required to meet minimum standards of performance. To ensure that each part of the combination does its share of the braking work the EU and UNECE specify performance requirements, used to type approve the braking systems, which contain controls called compatibility corridors. These compatibility corridors ensure that the braking of the individual vehicle elements of the combination is within a specified range, to ensure that under braking the combination remains stable. However, the Department has received evidence of braking incompatibility between tractors and trailers that could lead to vehicle combinations, in certain circumstances, being potentially dangerous.

Static brake tests monitered brake performance, where brake lining wear was measured on each axle. Analysis of this data was used to establish the ideal condition for balanced and stable braking, with an acceptable level of lining wear. Once the ideal conditions were established, new and more precise performance corridors were defined. This will allow the braking effort of the vehicle combination to be divided more evenly, which will improve vehicle to trailer compatibility and consequently vehicle stability.


TRL Limited
Crowthorne House, Nine Mile Ride, Wokingham, Berkshire, RG40 3GA
+44 (0)1344 773131

Contract details

Cost to the Department: £318,000.00

Actual start date: 01 October 1997

Actual completion date: 30 April 2000


PR/SE/605/99. Compatibility of Heavy Vehicle Combinations. Final Report.
Author: TRL Ltd
Publication date: 01/05/2000
Source: Contact

Summary of results

  1. The following recommendations came from this work:

    * The compatibility corridors should be narrowed and the difference in threshold pressure controlled;
    * the correction factors producing the trailer corridor should be set and a tolerence established;
    * the use of an adjustable predominance valve should be phased out;
    * vehicle manufacturers should produce simple tables indicating the correct brake temperature distribution applicable to each loading condition; and
    * evidence should be provided to show that the theoretically derived regulatory requirements are met in practice.