There is a frictional grip between the tyres and the road depending on the nature and condition of both. The braking force at any wheel cannot exceed the available adhesion or that wheel will lock and the retardation will be reduced, resulting in a lower value of efficiency than would be the case if the wheels do not lock. However, since retardation is at a peak just before the wheel locks, efficiency is at a maximum at that point and therefore when a slide results during a braking test the maximum efficiency will have been reached and may have been recorded.
In order to achieve the maximum braking possible on any surface, the output of each brake must be directly in proportion to the load carried by the wheel concerned. Since both wheels on one axle carry nominally the same load, brake output is considered in terms of axles and axle loading. However, the load on the various axles of a vehicle, at the time when the brakes are applied, depends partly on the particular loading of the complete vehicle and partly, because of the weight transfer during braking, on the deceleration actually achieved. It can be shown that the effect of weight transfer is more severe on short wheel based vehicles compared with longer wheel based vehicles under similar loads.
If the maximum possible braking is to be achieved, the braking ratio between different axles needs to be variable; although various ways have been developed for arranging this, these are only an approximation to the ideal requirements.
Usually, therefore, the wheels of one axle will be slightly overbraked and will lock before the other wheels are developing the maximum useful braking force. This will affect braking on very good and on very bad surfaces. On a good surface rear wheel locking might limit the braking efficiency to about 85%, while on a bad surface, front wheel locking might limit the efficiency to 15%.
This takes effect between the tyres and the road; it can be measured by replacing the normal road surface with an alternative surface capable of measuring the forces acting on it. This necessitates using special equipment in a garage or testing station and rules out testing at will during the course of period of general running. Brake force measuring equipment is of two types – roller and platform.
The roller type equipment tests the individual axles in turn, either both wheels at once or more usually separately. The wheels stand on pairs of rollers which are driven at low speed by an electric motor; when the brakes of the vehicle are applied, the motor works harder to continue to turn the rollers and a load cell built into its mountings indicates the braking force on a dial in front of the operator.
With the second type there are individual platforms, flush with the floor and able to move slightly against the restraint of load measuring cells. The vehicle is driven onto the platforms and braked so that an individual reading is obtained for braking force contributed by each wheel. Initial speed is necessarily low and some practice is necessary to enable the driver to make effective tests.
With both types of tester, the total braking force recorded is divided by the gross weight of the vehicle to give the required efficiency reading. Equipment of this kind enables the output of individual brakes to be compared and so can assist in diagnosing faults. The use of the roller type tester calls for no special skill on the part of the driver and a high degree of accuracy is obtained.
Deceleration is the direct effect produced by the braking force – a loss of speed at a particular rate – and it can be measured with the vehicle by means of a decelerometer. The essential part of a decelerometer is either a pendulum, a spring mounted mass or a volume of fluid contained in a U-shaped tube inside the case of the instrument.
When the instrument and the vehicle in which it is carried move at a constant speed, the moveable part occupies a constant neutral position and a zero reading is given by some means on a scale visible to an observer. As soon as the vehicle decelerates, the pendulum, mass or fluid moves forward against its restraint, the amount of movement depending on the magnitude of the deceleration, and a reading is given on the scale.
A decelerometer can be used at any speed and provided the brakes are applied long enough to enable the instrument to give its reading, it is not necessary to keep the brakes on until the vehicle stops. Such an instrument is very suitable for routine testing of brakes to ensure that the required minimum efficiency is being obtained. It cannot, however, identify an individual brake which is not functioning correctly.
Some decelerometers will show if there is any variation of deceleration during a brake application. Statutory tests are concerned with the achievement of a certain minimum value and the observer can note that this requirement is satisfied. Some instruments can be set to indicate the maximum value achieved and retain the reading until reset.
Decelerometers are widely used for brake and brake lining testing by development and test engineers in the automotive industry.
The function of the vehicle brakes is to control the speed of the vehicle on hills, to reduce the speed when required and to stop the vehicle altogether and hold it stationary. How well a set of brakes fulfills this function depends on many factors, one of which – road surface condition – is in no way under the control of the driver. Other factors such as tyre condition and gross vehicle weight are not directly related to the design and condition of the brakes, although they are the responsibility of the driver.
The ability of the brakes to perform their function is popularly known as braking efficiency and in most countries legally enforceable regulations require that all road vehicles have an efficient braking system. In assessing braking efficiency, it is usual to consider the effect which the brakes achieve when they are applied.
The action of applying the brakes sets up a force effective at the road surface, which acts in the opposite direction to the motion of the vehicle and causes it to slow down or decelerate. This deceleration is normally compared to a standard value (the acceleration due to gravity, g) and reported as a percentage of “g”. By defining braking efficiency in such a way, directly comparable standards of braking can be established for differing classes of vehicle. Dependent on whether metric or imperial units are used, “g” may be 9.81m/sec/sec or 32 ft/sec/sec in absolute terms.
Vehicle construction legislation normally requires various classes of vehicle to have a braking system capable of producing a specific minimum deceleration. In modern vehicles the braking system is designed so that, provided the vehicle is correctly maintained and the driver applies sufficient pedal pressure, the minimum braking efficiency will be achieved irrespective of whether the vehicle is loaded or not.
Only in cases where the brakes are poorly maintained or where the vehicle is loaded significantly above the maximum gross design weight, will the weight of the vehicle affect the braking efficiency and the brakes be unable to achieve the minimum required efficiency. Within the above limits, considerations of the weight of the vehicle can be ignored, since for a particular minimum efficiency required by the regulations, the ratio “Braking force achieved to gross vehicle weight” will be constant.
There are a number of factors which need to be taken into account when brake testing if the conclusions which are reached are to be accurate. The more important of these are commented on as follows:
1. Reaction Time
When brake efficiency is determined by measuring braking force or deceleration, reaction time is not involved. When either stopping time or distance is measured, depending on the method used, reaction time may influence the measurement.
A typical minimum reaction time with an alert driver can be as low as 0.5 sec. If this were included with the actual stopping time, it would influence considerably the estimate of brake efficiency being made. It is important to include reaction time when, for road safety purposes, estimates are being made of stopping distances to be published in the Highway Code, but it must not be allowed to influence tests of the brakes themselves.
2. Braking on Gradients
Although it is more usual to conduct brake tests which are carried out on the road on a level surface, equally accurate results can be obtained on a constant incline, the means of making allowance being very simple. The severity of a gradient can be expressed as a decimal by calculating the sine of the angle of the slope which will be a number between 0 and 1.
The significance of this result is that it gives the force acting to push the vehicle down the slope as a proportion of the gross weight. For example if a vehicle is standing facing down a 1 in 8 slope, the gradient may be described as 1/8, 0.125 or as 12.5% and the force acting down the slope is 1/8 of the vehicles gross weight.
If then the braking efficiency is determined by measuring either deceleration, stopping time or stopping distance, the result will be 0.125 too low and can be corrected to level road conditions by adding 0.125 or 12.5%. Similarly, a rising gradient helps a vehicle stop and the result obtained must be corrected by deducting from it the measure of the gradient.
3. Weight Transfer
Weight transfer during braking varies the axle loading and so affects the adhesion available. It also affects the reading of decelerometers of all types very slightly if the suspension is such that it allows the body of the vehicle to tip forward significantly when transfer takes place. For most vehicles this error may be ignored.
4. Wheel Locking
If one or more wheels lock, the overall efficiency recorded will be less than that which would have been indicated if locking had just been avoided. Since, as has already been noted, brake tests should only be made under suitable conditions, this state of affairs should only arise at high decelerations and brakes should be released immediately to avoid unnecessary tyre wear.
5. The effect of Speed
Any effect is very small and the results achieved may be assumed to be independent of the test speed used over the range 0-40 mile/hr (0-64 km/h).
6. Brake Fade
True fade is a loss of brake output due to overheating of the brake linings. Modern drum brake linings are little affected by heat until operation temperatures exceed 350-440 °C while disc brake linings are more heat resistant.
To exceed these temperatures a vehicle must be driven very hard and even then the onset of fade is very slow. Brake linings also lose their friction if they become soaked in either hydraulic fluid or lubricating oil, or if linings get wet. Recovery from immersion in water is usually fairly rapid but if linings have become oily they must be replaced and the discs/drums cleaned.
Stopping time is directly related to speed, but stopping distance varies as the square of speed – i.e. if initial speed is doubled, stopping time is doubled, but stopping distance is multiplied by four.
Braking force, deceleration, stopping time or stopping distance can all be used as a measure of braking efficiency; in the case of the first of these, it is necessary to know the gross weight of the vehicle and for the third and fourth, the initial speed must be known. Since these four quantities are inter-related, they will all lead to the same conclusion provided that all the measurements are accurate. The practicability of using measurements of these quantities as a means of assessing braking efficiency can be considered in turn (see the other sections in this Glossary).
The measurement of stopping distances as a means of testing brakes is inconvenient and subject to considerable inaccuracy unless specialised equipment is used. The first requirement is to determine the position at which brakes were applied and the second is to measure the distance from there to the appropriate point on the vehicle once it has stopped.
To interpret the measurement obtained, the initial speed of the vehicle must be known accurately and the test surface should be level. The most common means of meeting the first of the above requirements is to fit a device which fires a pellet of chalk downwards onto the road surface, measurement is then made with a tape.
This method gives an average of the performance throughout the stop. Attempts on the part of the driver to apply the brakes on passing a given landmark or on a signal from an observer are subject to considerable error.
As with the measurement of stopping distance, the measurement of stopping time by ordinary means can only give an indication of the average deceleration.
With a test speed of 40 mph (64 km/h), the stopping time at 50% efficiency will only be 3.7 sec; and an inaccuracy of only 0.2% sec in measuring leads to an error of +/- 5.4% in the stopping time recorded.
From 20 mph, the corresponding error will be +/- 11%.