Why use GPS to do Brake Testing?

Vehicle and tyre manufacturers expend considerable resources in the development of tyres and braking systems in order to reduce braking distances, speed up response times and improve stability.

Tyre manufacturers carry out ABS brake tests to benchmark the tyre against other compounds and constructions.

The improvements in braking systems and tyres areas are getting smaller each year, so it has become increasingly important to measure braking distances very accurately and repeatably.

Nowadays it is a requirement that the measurement system can stay accurate to less than 2cm to be useful in determining differences in performance.

How are brake tests performed?

Brake testing is usually done under carefully controlled conditions at a test track. The tyres and brakes are first warmed up in a series of standard manoeuvres, then the vehicle is accelerated to a speed above the desired starting point and the transmission is placed into neutral. Once the target speed has been reached, the brake pedal is activated as fast and hard as possible (sometimes this is done using a mechanical brake robot to achieve a consistent result at exactly the right speed). These tests are repeated a number of times and the results are averaged.

What types of braking tests are there?

There are many different types of brake test, some are specifically for braking systems and components, such as the ECE13H regulation, R90 regulation and ABS development and some are for tyre testing.

A common test for evaluating the performance of braking systems is the trigger activated test to a standstill. A pressure switch is added either to the face of the brake pedal, or into the hydraulic system. This is to detect when the brake pedal has been pressed and signals the start of the test. The test stops when the vehicle comes to a rest and the braking distance is the primary measurement used in such a test. Because this test starts from the moment the pedal is pressed, it measures both the response of the braking system, as well as the performance of the tyre, brake pads, discs, and other components.

During this test the deviation of the car from straight ahead can also be measured. This gives an idea of the vehicle's lateral movement under heavy braking.

Graph of a trigger to zero brake test

The second most popular is the brake test between two different speeds. These speeds are usually picked in order to eliminate the response time of the braking system, which is during the more linear part of the deceleration period. These tests are useful for tyre testing, as it produces very consistent braking distances.

Other brake tests include ‘split mu’ testing where the left hand side of the vehicle is running on a different surface to the right hand side to determine how well the braking system copes and the stability of the vehicle. Another test is the brake and turn where the car is not braking in a straight line, again to determine the stability during such a manoeuvre.

Graph of a speed to speed brake test

What are the different methods for measuring braking distance?

There are a number of different methods for measuring braking distance:

• Fifth wheel
• External radar
• Microwave sensor
• Optical sensor
• GPS

Fifth wheels were the original way of measuring braking distance, but these are difficult to fit, easy to damage and suffer from a number of mechanical limitations which affect accuracy, such as skipping and bouncing.

Radar is used by law enforcement agencies because it is very accurate, but you can only measure the speed from a stationary point directly in front of the target vehicle.

Microwave sensors use a similar method, but are used on the vehicle. These suffer from quite a high measurement noise, and are very sensitive to the height they are mounted above the road. Surface water and snow also affect the readings.

Optical sensors have proved a popular replacement to the fifth wheel in the past due to their size and accuracy but they take a long time to fit to the vehicle, struggle to work consistently in wet or snowy conditions, and can be easily knocked off during testing.

GPS has now become the most popular way of measuring braking distances due to the very high accuracy, size and ease of fitment, flexibility to carry out many different tests, and ruggedness. The only drawback with GPS is that it requires a clean line of sight to the sky in order to work, which is generally not a problem at a test track.

What are the challenges in measuring braking distance?

Measuring braking distance using a brake trigger is not as easy as it first appears. The most crucial part of this test is measuring the exact point at which the brake pedal is pressed, and starting the distance measurement from this point. It is the most important area on which to concentrate because this is the fastest point at which the vehicle is travelling. For example, if the vehicle is travelling at 100km/h, then every second it covers ~28 metres, so a 1000th of a second error will lead to nearly 3cm of error.

A very effective way to deal with this is to use the GPS engine to scan the brake trigger, and mark the point at which it happens relative to GPS time. As GPS time is based on the atomic clocks from multiple satellites, this time source is extremely accurate. This allows the Racelogic VBOX3i, for example, to measure the braking distance to within +/- 1.8cm.

VBOX GPS data loggers:
The industry standard for brake testing

The VBOX range of high accuracy GPS data loggers are standard equipment for almost every automotive manufacturer, tyre manufacturer, and vehicle testing company in the world.

This is due to their high accuracy, flexibility, and ease of fitting (they can be moved from vehicle to vehicle in minutes).

Full product details about our VBOX range are available on the right.

Please contact us with specifics of your testing applications, and one of our technical sales team will be pleased to advise you.

The VBOX3i 100Hz measures braking distance to within +/- 1.8cm.