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NVH Diagnostics Case Study

NVH complaints used to be handled mostly under warranty, but vehicle owner’s expectations, tires with lower profiles, decreasing tire quality, and lighter vehicles are bringing more of these issues into independent shops.

For years OEMs used dedicated and expensive tools for NVH diagnostics, while the aftermarket depended upon stethoscopes and a variety of listening devices. But now Pico has an option for the aftermarket that may give you the justification to make NVH diagnostics a standard offering.

The case study referenced below is from Matthew Shanahan, Assistant Professor, College of DuPage.  Its a simple problem about steering wheel vibration at highway speeds on a 2004 Mitsubishi Gallant.

Which part of the car is causing the steering wheel to vibrate at highway speeds? 

Since everything that rotates on the car rotates at a certain frequency, the Pico NVH tool can be used to measure that frequency and determine which part of the car is making the vibration.  Vibrations can be classified into four categories:

1. Tire speed related vibrations (T1, T2, or T3):
- Usually appears at 50 mph and above
- Occur at the same vehicle speed regardless of engine RPM.
-Cannot be detected with the vehicle stopped.

2. Propshaft speed related vibrations (P1, P2, or P3):
- Usually appears at 25-50 mph and usually gets worse at higher vehicle speeds.
- Occur at the same vehicle speed regardless of engine RPM.
- Cannot be detected with the vehicle stopped.

3. Engine speed related vibrations (E1, E2, or E4):
- Usually occur at the same engine RPM regardless of vehicle speed.
- Can usually be detected with the vehicle stopped, sometimes only under load.

4. Other systems such as brakes, clutch, Failed Bearings, Failed Gears, etc.

 

NVH Setup

To analyze the vibrations, the Pico NVH software needs details about the vehicle. The setup wizard prompts you to fill in the blanks and then uses this information to determine the frequencies associated with the different types of vibrations.

 

Connection to Vehicle

Matt placed the accelerometer on the seat rail since most vibrations can be found with the accelerometer mounted on the seat rail where the driver sits. The accelerometer measures vibrations on the X, Y and Z axis.

 

The Analysis

Matt took the car for a drive and recorded the results.  Back at the shop, he replayed the recording, and in the lower section of the screen shown below, selected a window of time (blue section) that corresponded to when he felt the vibration (15 secs) which occurred at 60 mph.

NOTE: By default, the NVH software records vibrations caused by Tires (T1, T2, T3), Engine (E1, E2, E4) and Propshaft (P1, P2) and plots values for each axis: Blue (X), Red (Y), Green (Z).  Since the readings for Engine and Propshaft were negligble (less than 2.mg), Matt eliminated them from the view to show only the values for Tires (T1, T2, T3).

The frequency for each vibration is listed in a table along with the amplitude for each axis (X,Y,Z).

 

Luckily, the colorful bar chart is much easier to comprehend. Each axis (X,Y,Z) on T1 has an amplitude above 10mg, while the amplitudes on T2 and T3 are well below 3mg.

 

According to the bar graph what is the source of the steering wheel vibration at highway speed? T1.

Clicking the T1 label below the bar graph opens the comprehensive help file which provides details about the causes for each type of vibration.

 

If the bar graph view does not provide enough details, you can switch views to see the trace for each axis and to overlay cursors that display frequency values and mg values:

 

Zoom in, just like the PicoScope software, the NVH recording can be zoomed, and time and frequency can be changed. 

 

Go 3D to look at vibration frequency trends over time:


 

The PicoScope NVH software does a great job of quickly identifying issues with tires, engines, engine accessories, drive shafts, and other components. The software can also be used with a J-2534 device that will give the tool engine RPM and vehicle speed data. 

As you can see below, when it comes to recording and analyzing NVH data, the Pico software is capable of keeping it simple with bar graphs or getting excrutiatingly detailed with 3D plotting on the most complex or even the simplest of jobs such as this case study.

To learn more about NVH diagnostics please attend the Automechanka event in Chicago on July 26th-29th, 2017 where Matt Shanahan will present a variety of case studies of simple and complex vibration issues. No NVH or DSO experience is necessary for this class.

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Or send us your questions.

 

3-axis and 4-axis NVH kits are now available with plastic cases or foam trays.

 

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