VW Golf R mystery: part TWO

The Golf R from the previous issue has returned, bringing with it a reason for Frank to discuss diagnosing and solving NVH issues

Fig. 1

Published:  12 February, 2020

With the challenges of current vehicle engine technology, lack of access, and potential cost over value, the need for an accurate and reassuring diagnosis is vital. The technician should not allow cost or client pressure to influence the diagnosis or repair process. The ownership of the vehicle, fault, condition and repair cost is entirely the owner’s responsibility. Prior to any work, it essential that a legally enforceable agreed contract be in place.

This introduction may seem a little heavy, however it’s very likely that without a contract you may accumulate large labour cost in stripping engines to establish internal faults, then are refused agreement to complete repairs.

The answer, as is often the case, is new test techniques, training and continuing investment in technology. I recommend two options. First, a quality endoscope. Second, the Pico NVH kit.

Endoscope quality is governed by the number of optical fibres, a bright light at the boom tip the ability to articulate the mirror in multiple directions.

NVH monitoring
The next option and the focus of my topic is noise and vibration monitoring. So, let’s begin with the basics. Noise measured in decibels can be detected by changes in air pressure by three tiny bones in our ear, hammer, anvil, and stirrup. The frequency range is limited to around 20-22khz.

Vibration is the transmission of mass energy, measured with the unit gravity. It can be detected by sight, touch, or sound. There are three essential elements to vibration; 1) the source 2) the transfer path 3) the respondent.

A simple example may help; A tyre has an out of balance mass. The source, the energy, is transmitted through the road spring, shock absorber and the vehicle body. The transfer path, the dash panel, is vibrating, making it the respondent.

It is quite common to focus on the respondent instead of fixing the cause. You will have all seen mysterious weights attached to drive shafts and gear boxes. This simply transfers the frequency to a less intrusive value.

The motor vehicle is a series of mechanical systems in constant conflict. If we can identify the various frequencies across the entire operating range, we can identify the actual causes and predict potential critical failure non intrusively. Put simply we can see through metal, perhaps I should say the scope can.

Now for some simple rules. A heavy mass will always have a lower frequency than a light mass. For example, road wheel vibration, and exhaust resonance. The amplitude of vibration is affected by the transfer path, for example a light body panel, and engine block.
The distance from the source will affect frequency. Damping systems will reduce or arrest and cancel mass vibration, for example road wheel balance, dual mass flywheels, sound deadening body panels.

The next task is to separate the major vehicle components, engine rotation frequency, transmission frequency, and road wheel frequency. To achieve this, we enter specific vehicle data into the set-up wizard. Engine frequency is collected via a serial link. rotation speed divided by 60 = frequency in hz.

We can separate any frequency between 20-22khz and with a little maths relate the vibration to ancillaries, bearings, or normal generic background vibration signatures. For example, my Seat Cupra has adjustable suspension damping which totally transforms the ambient driver experience.

Environmental influences
This brings me to the next important consideration; environmental influences, the road surface, and driving style. To detect mass vibration, one or more three-dimensional accelerometers are attached to the vehicle, the location and attitude is crucial. A microphone may be added for ambient sound analysis, from this data the software will provide a choice of display options.

Now let’s look at that highly modified VW Golf R track car. The car is well known by us as we prepared and maintain it for a very proactive track day enthusiast. You know it too, as it was the subject of my article in the November issue of Aftermarket.

Producing well over 500bhp, the car boasts fully adjustable race suspension, a roll cage and Kevlar seats with trim removed. The Golf developed a severe vibration following a recent track event. The engine idles normally with no obvious problems, no noises and good oil pressure. Depressing the clutch and selecting gears has no effect.

Let’s review the images:

Fig.1 Normal trace at idle
E1 represents the crankshaft frequency, E2 represents the combustion events both values are exceptionally low at 6mg or less, there is a little ambient vibration at 50/60/70hz. The blue trace represents engine speed. The lower plot is the complete data buffer. All is good.

Fig.2 Trace at elevated speed
Note engine speed at approx. 2586rpm 42.1hz, vibration at crank E1 is still very low however at around E2 there is a broad band contact representing a mass displacement between 55 and 90hz.

This represents the failure of the dual mass flywheel secondary mass. Also note the mass displacement is shared across all three dimensions. There is also a broad band vibration at 120hz. This represents an increase in ambient vibration through the vehicle body, remember high frequency equals low mass.

So, the fault is a faulty dual mass flywheel. Total diagnostic time around 15 minutes. Solid evidence for the customer. Interestingly, with the flywheel removed it was not possible to determine the nature of the fault.

Fig. 2

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