ECE regulation 13H specifies that a laden vehicle must be capable of being held on a 20% gradient for 5 minutes with a maximum force of 400N applied at the manually operated hand lever. (1). However, anecdotal reports suggest that when an unattended vehicle fails to remain stationary, the period of time which has lapsed may be more than 5 minutes, and the gradient on which the event occurs may be less than 20%. A survey of drivers and driving instructors indicated that as many as 13% of respondents had experienced such an event (4).
The aim of this study is to explore whether rear brake type and the effects of brake cooling affects the ability of the parking brake system to hold the vehicle stationary in real life driving conditions. The relevance of this in relation to driver interaction with the parking brake system and parking practice is then considered.
Testing was conducted with a total of 53 vehicles, 26 fitted with rear disc brakes and 27 fitted with rear drums on 3 gradients of <5%, 10% and 20%. Vehicles were parked facing down the gradient in neutral and the parking brake lever was pulled up to the lowest ratchet position to hold the vehicle stationary. The rear brake temperature was recorded before driving a predetermined route of around 3 miles and afterwards at three 5 minute intervals. A "roll" was recorded when contact was made with the chock positioned in front of the rear wheel.
The initial stopping temperature after driving the set route was significantly greater for the disc brakes (mean 430C) than the drums (270C), p=0.001, but after 5 minutes the difference had greatly reduced. In 62% of the vehicles observed fitted with disc brakes, the ratchet position of the parking brake lever to hold the vehicle stationary on parking was lower after driving the route i.e. when the brake temperature was increased.
Overall 63% of the vehicles fitted with rear disc brakes and 4% of the vehicles fitted with drums rolled. Nine of the 10 vehicles fitted with disc brakes tested on the 20% gradient and 5 of the 9 vehicles tested on the <5% gradient rolled as the brake temperature returned to ambient in under 15 minutes.
This paper presents the findings of a study which was part of a wider project to explore the factors associated with vehicle roll away from an ergonomic and human factors perspective. Previous papers by the authors have provided an overview of the potential factors but this paper focuses on the performance of the parking brake system in near normal driving conditions.
Unlike previous engineering studies conducted under laboratory type conditions this study employed drivers in their own vehicles which introduced a complexity of variables and experimental constraints. However the results provide sufficient evidence that further work in this area is required.
Conclusions: Failure of the parked unattended vehicle can result in property damage, injury or even fatality. The seemingly simple task of applying the parking brake requires some understanding of how the system works and what factors could contribute to failure of its intended function. The results suggest that where the parking brake system utilises disc brakes, it may be more susceptible to the effects of brake cooling. Should the driver only pull the lever up to the minimal position to hold the vehicle and not park in gear, it would appear that there is the potential for the vehicle to roll as the temperature of the discs return towards the ambient.
Valerie G. Noble, Richard J. Frampton, John H. Richardson - Design School, Loughborough University