Project: Limiting Factors For Improving Heavy Goods Vehicle Brake Performance

Reference: S284B/VD

Last update: 25/06/2004 10:46:16


The project objectives are:

* to identify and explain the factors that limit the braking performance of HGV's (e.g. tyre grip, load retention, vehicle suspension limitations, vehicle stability etc);
* justify the implications of addressing the factors identified above with particular emphasis on the interaction between these factors;
* determine the current level of braking performance in HGV's; and
* propose a programme of testing, for the second phase of this project, that will examine the predominant factors that limit the braking performance of HGV's.


Accident statistics have shown that although the accident rate for HGV's is low, accidents involving this category of vehicle are more likely to lead to fatal injury for the occupants of vehicles from other categories. Additionally, HGV accidents may cause extensive road congestion, even when injuries may be less serious.

The Department is concerned that the standards for HGV braking systems have not advanced as rapidly as that of cars and passenger carrying vehicles. Different theories for this apparent lack of progress have been put forward by the industry and the vehicle manufacturers/designers.

This research will deliver proposals for the examination of the most significant factors that restrict higher levels of deceleration being introduced for HGV's. It will also consider the domino effect, where improvements in one aspect of the design/construction of a vehicle lead to changes in other areas.

This is the first stage of a multi-stage research project. This stage is concerned with a review of the factors that may limit improvements in the braking performance of HGVs. However, it includes a limited amount of testing of typical HGV's in order to assess actual braking performance against the prescribed braking performance of Community Directive 98/12/EC.


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

Contract details

Cost to the Department: £152,859.00

Actual start date: 02 January 2001

Actual completion date: 23 January 2004


PR/SE/543/02. Factors That Limit Improvements in Commercial Vehicle Brake Performance. Final Report
Author: TRL Ltd
Publication date: 01/05/2004
Source: Contact:

Summary of results

  1. Two vehicle types were modelled: a 13 tonne two axle rigid and a 41 tonne articulated vehicle comprising a 3 axle tractor with a 3 axle semi-trailer. These closely matched vehicles from the test programme. Stopping distances from the simulation at 30km/h, 60km/h and 90km/h were within 5% of those recorded in the track tests for the equivalent articulated vehicle. For the rigid vehicle the values were within 12% but this was a consequence of the poor track performance (see below) rather than a failing of the simulation.

    The main findings of the simulation were that a reduction in stopping distance of up to 20% could be achieved by reducing the response time from 0.5s to 0.2s. Wheel locking was less prevalent with air suspension than with steel spring suspension and in contrast to the track tests stopping distances when unladen were shorter than laden.

    Results from the track tests showed that the braking performance of current vehicles varied greatly. Current type approval standards were not met by the worst performance but the best vehicle substantially exceeded them with a mean fully developed deceleration (MFDD) of 7.55 m/s2. The two rigid vehicles gave the worst performance and the best performance was given by the six axle 44 tonne articulated combination fully laden. The tractor was fitted with EBS and disc brakes and the trailer with discs.

    Other findings of note were that suspension type and, in turn, oscillation, affected stopping distance. Air suspension was notably superior to the mechanical spring type for which severe oscillation occurred on a rigid vehicle. Measurements in a load restraint showed that small improvements in brake performance could disproportionately and thus, greatly increase the force on load restraint systems. Education of drivers and operators was considered essential.

    Reliability was poor with three serious brake defects found on the six test vehicles. One defect with an ABS system caused random brake failure and was thus very dangerous; the fault could not be identified. Other problems occurred with ABS systems.