BMW 320D M Sport xDrive Touring | Cabin vibration at motorway speeds

As a technician, there are a number of customer complaints that you know are going to be a real challenge, one of which is cabin vibration. Come to think of it, there is a number of customers who are going to be a challenge but that’s a discussion for another time.

Complaints of cabin vibration are subjective and depend on a number of human factors, such as tolerance levels, expectation and misinterpretation.

Tackling complaints of Noise-Vibration-Harshness (NVH) is no different from how we approach any diagnostic challenge. We use a process that will deliver results, which we can apply to make an informed decision on how to proceed to be able to make a conclusion. Note, however, that the conclusion may not be a fix but that it can still be satisfactory to all parties involved!

If I could give one tip to anyone faced with such a challenge, keep notes and capture/save objective data.

The following case study is one such example.

Customer’s description

The customer reports that vibration can be felt through the vehicle at approximately 75 mph. The symptom has been present for over 2.5 years, and they have had numerous attempts to resolve the issue.

Technical description

Verifying the customer complaint is an essential step in the diagnostic process and with complaints of vibration, it is absolutely paramount. Given that vibrations are subjective (a matter of opinion), a tip here is to road test the vehicle with the customer and ask him/her to indicate when they experience their concern. Typically, at this stage of the diagnosis, we are gathering vital pieces of information without applying any test equipment. However, I would like to break from tradition at this point in the process.

If we are lucky enough to have the chance to do a road test with the customer, I advise we have the NVH kit connected to the vehicle during this verification process! You can find information on how to connect and collect data in this case study in our NVH video series.

When our customer indicates that they can feel the vibration, we are prepared to capture an objective measurement that we can save for review, and more importantly, to obtain evidence. This may sound a little odd but one man’s vibration is another man’s harshness (once again, a matter of opinion).

Our initial NVH capture (with the customer present), highlighted a first-order front and rear tyre speed/frequency vibration (FT1 and RT1). The vertical axis was the largest offender (red bars within the bar graph). We have also captured a second-order engine vibration (E2), which we can discount while the vehicle develops the power to maintain the road speed. You can find more information on engine order vibrations in this forum thread.

On this occasion, I agree with the customer. An objectionable vibration was clearly evident through the floor pan and driver’s seat cushion, but I could not sense anything via the steering other than brake judder when applying the brake (a point worth adding to your notes).

Diagnosis

With the customer complaint verified, we confirmed the Vehicle’s ID and Specification.

Confirmation of vehicle specification is of the utmost importance when it comes to vibration complaints!

There is often an understandable temptation for customers to modify their vehicle with fashionable accessories that may add a “nice cosmetic touch” but lack the fundamental quality control and engineering that was intended for the vehicle at the point of the design. I am referring here to wheels and tyres. You could argue that nothing changes the look of a vehicle more than wheels and tyres (and I would agree) but I would also argue nothing transforms a vehicle’s handling, performance, drivability and noise/vibration levels than inferior wheels and tyres.

The customer interview highlighted that a number of wheel balance procedures, wheel alignments, tyre replacements (Inc. wheel straightening) had all been actioned over the complaint period, which spanned 2+ years. I think it’s worth mentioning here once again how vital the customer interview has become the diagnostic process. Quite often, customers don’t have time to be questioned, they want answers, and this is why we need to drill down with “targeted” open questions such as:

1. How long has the problem been evident?

This is a great question for customers who are new to their vehicle as they are in a transitional period from a vehicle they knew to a new vehicle. If the customer has only been using the vehicle for less than two months, I would ask about their previous vehicle. If they have moved from an SUV to a Sports Convertible their complaint about vibration/ harshness, while justified, is a vehicle characteristic.

We cannot repair or improve upon characteristic vehicle behaviour!  If we try, we are openly admitting to our customer there is a fault and it’s a long road back home if we choose this path.

2. When did you first notice the problem?

I like this question as it places a timestamp when the vibration became objectionable. It also leads on from question 1 above if the reply is “ever since I purchased the car”. It may also lead you to historical vehicle events if the customer replies “ever since the accident repair” or “since my last service”. We fondly refer to these answers as “the ever since syndrome”.

3. Has any work been carried out to the vehicle recently?

This question has the potential to guide you to areas of the vehicle you may not have considered if the customer is prepared to surrender this information. But be careful here, a customer may not consider a puncture repair as work carried out, mistakenly believing a puncture repair is a simple process not worthy of a mention. I cannot think of a single repair that could be more of a game-changer when it comes to vibration than a puncture repair!

4. When do you experience the problem?

Here is our opportunity to ask additional specific questions during the customer’s reply. For example: “The problem only occurs on the motorway”, where we can add “at what speed?” They may answer “60 mph”, which leads to our follow up question: “Can you still feel the problem at 70 mph?”, etc. This feels more like an engaging conversation than a question and answers session. If we look back, we have asked six questions in total and gained a huge amount of key information.

The basic inspection confirmed no visible signs of damage to suspension components, alloy wheels or tyres. On the subject of tyres: In a perfect world, all the installed tyres should be the same brand/speciation with near-identical construction date (stamped on the tire wall), “approved” for the vehicle and on recommended rims. In reality, this is near impossible, however, it should not be overlooked. Working with multiple vehicle manufacturers has opened my eyes to the importance of “approved tyres”. In this case study, the tyres were Star-rated, which ensures they are BMW approved for performance, deformation and Radial Force Variation (RFV).

RFV is often either overlooked or not even considered when diagnosing cabin vibration. However, it has proven to be the main cause of tyre speed related vibration orders (T1, T2 and T3).  Luckily, the Hunter “Road Force Balancer” has the ability to measure rim and tyre run-out (under load), presenting the operator with the optimal tyre to rim phase match, as well as the preferred wheel/tyre orientation on the vehicle.

The following video describes how this is achieved and I hope it can help cure nuisance vibrations that conventional wheel balancing techniques fail to resolve.

Prior to diving in, it is paramount to take a step back and check for Technical Bulletins (Recalls & Campaigns etc.). None were relevant, and so based on the vehicle history and symptoms we could move on to possible causes.

Possible causes

• Wheel/tyre imbalance
• Excessive tyre RFV
• Deformed Wheel/rim
• Incorrectly mounted tyre
• Brake disc/hub imbalance
• Wheel centre to hub alignment
• Driveshaft deformation, run-out or imbalance
• Excessive wear in suspension components resulting in the transmission of inherent vibration

The action plan

The action plan was predominately governed by accessibility, probability and cost.

1. Zoning of the vehicle to narrow down offending vibration
2. Confirmation of wheel balance
3. Wheel and tyre run-out measurements
4. RFV measurement

To recap:

• There is a cabin vibration at motorway speeds.
• The NVH software has captured a first-order tyre speed related vibration (vertical axis).
• The peak vibration occurs at 83 mph.
• The replacement tyres that have been installed are approved for the vehicle (Star-rated).
• Numerous wheel balance procedures have been carried out on the vehicle to deal with the problem.  

One question that is often raised when we have identified a vibration is: “How do we find out which corner of the vehicle is responsible for the vibration?” In other words, how can we zone in on the vibration?

For those lucky enough to own an NVH advanced kit (with a 4-channel PicoScope), we have the distinct advantage of placing an accelerometer at each corner of the vehicle to measure our offending vibration (first-order tyre vibration in the vertical [Y] axis). The following capture highlights the vertical vibration levels captured by each accelerometer placed on this vehicle.

Note how the red bar graph indicates the highest level of vibration which corresponds to the LH front chassis in close proximity to the road wheel. However, I think we can agree that all FT1 and RT1 bars indicate a high level of vibration in close proximity to all road wheels. The keen-eyed amongst us may also have recognised that, while the NVH software has separated the front and rear tyre vibration levels of this BMW, the values are almost identical in terms of amplitude and frequency (18.6 Hz).

This is because the overall diameters of the wheel and tyre assemblies are near identical even though the tyre sizes differ. With near-identical diameters, the front and rear road wheels will rotate at near-identical frequencies. Our NVH software, however, will still list front and rear tyre frequencies separately even with such minuscule differences.

Front Tyre 225/40R19 = 66.26 cm

Rear Tyre 255/35R19 = 66.11 cm

Diameter difference = 66.26 – 66.11 = 0.15 cm

At this stage of the diagnosis, the application of 4 x accelerometers failed to yield a conclusion as to the offending zone of the vehicle. Or did it?

What could cause such high levels of first-order tyre speed related vibrations at all four corners of the vehicle? What about four imbalanced wheels or deformed alloy wheels?

Given that the road wheels had been balanced numerous times (and we had no access to an RFV balancer), the alloy wheel run out was the next step in the diagnosis. Again, governed by accessibility and probability.

Results

Based on the test results captured, I measured each alloy wheel, tyre radial and lateral run-out with an inexpensive tried and trusted dial gauge. (A tool that has never seen such action since I started working with NVH).

The following videos describe the measurement procedure and the offending results. The vehicle required immediate attention to be restored to driveability levels that were deemed acceptable for all parties involved.

VIDEO 1: Original alloy radial run-out

VIDEO 2: Original alloy lateral run-out

VIDEO 3: Original tyre radial run-out

As we can see from the LH front-wheel, which I chose to measure in the videos above, we identified considerable lateral and radial run-out of approximately 0.7 mm and 1.45 mm respectively. This would most certainly affect the radial run-out of the tyre as a direct consequence. I would like to add here that trying to find alloy wheel run-out specifications from an assortment of vehicle manufacturers has been challenging. A number of values have been discovered relating to a total run-out of 0.1 mm, while others refer to the relationship between the hub centre and rim assembly. None, however, have specified the maximum run-out in terms of wheel span/rotation.

As you can see in VIDEO 1 above, we captured 1.45 mm of radial run-out across a very short wheel rotation (approximately 45°). If we were to exaggerate this for descriptive purposes, this could be compared to a “V” indent in our desired perfect circle. A deformation like this would generate a first-order tyre speed/frequency related vibration (FT1 and RT1) one disturbance per rotation of the wheel.

Further measurements confirmed a similar story for all the alloy wheels (including the alleged straightened wheels). We informed the customer.

After much deliberation and trawling of the internet, we found four used alloy wheels/tyres that were fully original and not refurbished. After some persuasion, the seller of the used wheels/tyres gave us permission to visit and measure the run-out prior to purchase to ensure their authenticity. A word to the wise, this was not the first set of second hand/refurbished alloy wheels and tyres I had measured! The word “refurbished” does concern me based on my experience of measuring such wheels. While they look great, those I measured were more triangular than round! That is not to say that all refurbished alloy wheels are inferior, I just want those who commit to purchasing them to have their wits about them.

The good news here is that these second-hand wheels/tyres were confirmed as the original. The maximum radial run-out of all rims was approximately 0.3 mm (measured when mounted to a wheel balancer), and the included Star-rated tyres had never been removed from the wheels since new. (It does not get any better when playing the second-hand game.)

The wheels and tyres were then installed on the customer’s vehicle and we also measured the lateral and radial run-out prior to road testing.

VIDEO 4: Replacement tyre radial run-out

VIDEO 5: Replacement alloy radial run-out

VIDEO 6: Replacement alloy lateral run-out

As you can see in the videos above, we most certainly struck gold with these replacement wheel/tyre combinations. Ironically, prior to purchase, I had measured radial run out of the rims on a wheel balancer and obtained 0.3 mm, whereas when they were mounted to the vehicle, we achieved 0.15 mm. This suggested that there was a run-out in the wheel balancer assembly!

Confirmation of repair

The proof is most certainly in the pudding, and confirmation of repair is often a procedure that is overlooked or sacrificed due to time constraints. Post-fix measurements provide further objective evidence that protects all parties involved and ensures confidence during the hand-over to the customer. Never has it been more important to qualify any repair we carry out than with complaints of noise or vibration. They are a matter of opinion and without scientific measurements, we are “up against it!” When carrying out post-fix confirmation, Make sure that all the variables are kept to a minimum; be sure to match the road/engine speeds between pre- and post-fix measurements and, where possible, measure along the same stretch of road with the vehicle driving modes set accordingly. These may include steering, suspension damping, transmission and stability modes.

I think we can agree, based on the video footage of the wheels/tyres and the data above, that we improved the cabin vibration level we attributed to first-order tyre vibrations (T1). While I did not capture 4 x accelerometers at the key chassis locations post-fix, if we use the peak vertical axis measurement at the start of this study (18.3 mg) and compare with our post-fix values above (3.99 mg) we have improved the vertical cabin vibration level at 83 mph by over 75%.

All Parts Fitted

Alloy Wheel and Tyre combination:

BMW 19“403 M Sport Alloy Wheels (2 Front 8J x19 & 2 Rear x 8.5J x 19)

4 x Bridgestone Potenza S001 RFT (2 Front 225/40R19 & 2 Rear 255/35R19)

Additional comments

I mentioned earlier in this case study that I could only feel brake judder through the steering wheel, and not inherent cabin vibration at the sweet spot of 83 mph. Looking at the data below we have captured a tyre speed/frequency second-order vibration (T2) which equates to two disturbances per revolution of the wheel when braking from high speed. While this was not our customer's original complaint, I felt it necessary to report these findings as the vibration level is identical to the original concern (approximately 18 mg)! By doing this we can avoid customer confusion and misinterpretation post-fix while protecting the integrity of our repair process.

To conclude:

No vehicle manufacturer produces an intentionally “bad car”. All vehicles are engineered to perform as intended and this includes wheel/tyre choice.

Working with multiple vehicle manufacturers, I have grown to accept the importance of “approved tyres”, especially for those vehicles that are finely tuned for performance. Such vehicles are designed with wheels and tyres at the forefront to ensure “feedback” to the driver under all operating conditions.

This includes not only speed rating and rigidity but “shape” and RFV from the highway to the countryside in all weather conditions! This is a lot to ask from a tyre and something the manufacturer has no control over (in terms of replacement) once the vehicle has been purchased. The temptation to install non-approved tyres is understandably high but does come at a cost! The majority of problematic vibration cases I have visited have proved to be wheel and tyre related and I would go so far as to say that they are the greatest offender.

Tyre manufacturers ensure that the cream of their produce achieves approval status for vehicle manufacturers (VM) and Motorsport. Thereafter, tyres that fall outside specified stringent tyre approval requirements are distributed accordingly. While these tyres are more than sufficient for their intended use, without VM approval we risk introducing variables, and diagnosis demands that all variables are kept to a minimum.

Wide section alloy wheels (some spanning over 22”) are vulnerable on roads today and the potential for deformation cannot be ignored given the forces involved as the vehicle hits a pothole. Remember the inner rim has no support other than the transfer of energy to the outer wheel spokes in the alloy assembly!

Thank you again for taking the time to follow me in this study, I sincerely hope it helps you on your diagnostic journey.