Category Archives: Coast Down testing

Integrated Inertial Measurements

It is largely accepted that the most accurate and cost effective way to carry out vehicle dynamics testing and development is with the use of GPS equipment. In some situations, however, it pays to also use inertial measurements to either augment the available data, or to overcome problems encountered in certain environments – such as a test track that isn’t in an entirely open area, or one which suffers from an obstructed view to open skies in some areas.


We’ve been supplying IMU integration with the VBOX3i data logger for a few years, but now we’ve got a new Inertial Measurement Unit which has a much higher performance due to tighter GPS synchronisation, better calibration and a higher sample rate.

Integrated Speed Measurements

Firstly, the new IMU04 has improved levels of velocity smoothing to cope with interruptions to the GPS signal, thanks to a new real time Kalman Filter which blends the inertial data with that of the GPS system. Although most test tracks are located in open areas, bridges over the test track and overhanging trees can lead to signal degradation– this is where the integration with the IMU signals ensures that GPS dropouts are eliminated.

In this example you can see that the GPS velocity trace (blue) drops outs underneath a bridge. The IMU integrated data (red trace) gives a much cleaner and un-interrupted signal.

For instance, carrying out manoeuvres such as a coast-down test (the results of which are critical to a manufacturer from a homologation perspective, as they form a significant proportion of their calculations of fuel consumption data released to the public) can be completed on a course that takes the vehicle under a bridge, which would not have been possible using GPS alone. IMU integration ensures a consistent velocity log is maintained during the momentary loss of satellite signal lock.

Indeed, there are some facilities that are not ideal places in which to conduct other procedures such as high-dynamic brake stops – they may be tree-lined, but are still used because they’re conveniently located. Integrated Inertial and GPS measurements allow for this convenience to be fully exploited because it can provide the same level of accuracy normally only possible from testing in an entirely open arena.

Interrupted GPS signal. The blue trace is GPS Position only; the red trace is the corrected position data obtained via integration of the accelerometer data from the IMU and GPS signals.

This is an extract from an article originally featured in Automotive Testing Technology – for the full content click here.


Why Is Coast Down Testing So Important?

A few years ago I had an illuminating chat with an engineer from a UK-based manufacturer (they make very nice cars, that’s all I can say about who he worked for) who described their coast down practices. One thing I learned was that going somewhere hot, like Arizona, nets better results than they could ever hope for if they did it in the UK – for one thing the consistency in temperature makes life easier. Plus they like going to Arizona.


I was reminded again recently about the importance of coast down testing. We’ve been selling VBOXs to the industry for years specifically for this purpose, but recent conversations with the guys at the sharp end have made me realise just what a big deal it is.

Why is it so important? It’s those emissions regs again (for which the EC type approval document is… long). For the manufacturers, making cars with low emissions values is vital these days – not just from a legislative, but also from a sales and marketing perspective.

Official emissions testing is carried out on a dyno where there is no wind resistance. To compensate, an extra load is placed on the engine via the dyno’s rollers – the amount of which is derived from the coast down results previously generated at the test facility. No wonder they want to get these numbers in a conducive environment.


The coast down values must have a statistical accuracy that falls within a 2% range, so consistency is essential. We supply specific software to abide by the regulations laid out in EC70/220 and it’s good to know that wherever they’re testing, the engineers can rely on it.

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