Application Note 
By Vid Selič, NVH Product Manager, Dewesoft

The wind in the face, the noise, speed, and power - the feeling of freedom. Dewesoft has bought a company chopper bike - just to have some fun. However, after longer rides, some of the guys reported feeling a slight numbing sensation in their fingers. 

Since we have experience in the field of human body vibration, we set out to investigate the levels and volumes of vibration produced by our bike.  We don’t want the health of our teammates to be compromised.

person driving on motorbike

Enjoy the ride - watch our chopper video:

Due to its unbalancing nature, a motorcycle creates enormous vibrations during riding. Riders can be subjected to risks due to the vibrations produced by the engine and road conditions. Vibration energy waves are transferred into the body of the rider, transmitted through the body tissues, organs, and systems causing various effects before it is dampened and dissipated.

The vibration is transmitted to the buttocks and back along the vertical axis via the base and back of the seat while the pedals and steering handle transmits additional vibrations to the feet, hands, and arms of the rider. The vibration levels depend on more factors: the type and age of the bike, engine size, driver body weight, type of seating, type of suspension and road surface factors, etc.

Basic Vibration Concepts

The risk of vibration-induced injury depends on the average daily exposure. An evaluation of the risk involves the intensity and frequency of the vibration, the duration of exposure, and attention to the part of the body that receives the vibration energy.

Magnitude

Vibration magnitude can be expressed in terms of acceleration, velocity, or displacement. All three factors make sense as the human body responds to any of them, depending on the frequency of motion.

The acceleration, displacement, and velocity in the frequency domain are expressed in Hz. In many internationally accepted standards related to the measurement of human vibration, acceleration is the agreed-upon quantity for expressing magnitudes. The RMS value of the vibration magnitude is suitable to represent processes where vibrations are continuous or intermittent rather than shock-like, e.g., a ride on a motorbike.

Frequency

A vibrating object moves back and forth from its normal stationary position. A complete cycle of vibration occurs when the object moves from one extreme position to the other extreme, - and then back again. 

The number of cycles that a vibrating object completes in one second is called frequency. The unit of expressing frequency is Hertz (Hz). One hertz equals one cycle per second. 

Amplitude

The intensity of vibration depends on amplitude. A vibrating object moves to a certain maximum distance on either side of its stationary position. Amplitude is the distance from the stationary position to the extreme position on either side and is measured in meters (m). 

Acceleration

Acceleration is a measure of how quickly speed changes with time. The speed of a vibrating object varies from zero to a maximum during each cycle of vibration. It moves fastest as it passes through its natural stationary position to an extreme position. 

The vibrating object slows down as it approaches the extreme, where it stops and then moves in the opposite direction through the stationary position toward the other extreme. Speed of vibration is expressed in units of meters per second (m/s), or meters per second squared (m/s2).

Human body vibration

Human body vibration (HBV) testing is a standard method to assess the amount of vibration received by the user of different sorts of mechanical equipment such as power tools and construction equipment, or vehicles, and other means of transportation. Our customers often use this solution to certify the suitability of standard operational circumstances for the end-user. Depending on the type of exposure different standards cover different types of vibration such as hand-arm or whole-body.

HBV is defined as the effect of mechanical vibration on the human body. The effect might be on the body as a whole, whole-body vibration (WBV), or on parts of the body, of which the hands and arms - hand-arm vibration (HAV) - are the most important and most frequently affected. In many cases, the vibration of the whole body arises from vehicles, land-based or otherwise, from vibrating floors in buildings, or from big machines, where the operator is seated on the machine. 

Whole-Body Vibration

Whole-body vibration (WBV) is mostly transmitted to the body through seats or floors of vehicles used off-road, such as dumpers, excavators, and agricultural tractors. However, it can also affect drivers of some vehicles used on paved surfaces, such as lift trucks, or motorbikes. 

It is associated mostly with low back pain and is one of the strongest risk factors for low back disorders. It occurs when workers sit or stand on vibrating seats or foot pedals. Prolonged exposure to high levels of WBV causes motion sickness, fatigue, and headaches.

Hand-Arm Vibration

When vibration does not affect the whole body, but just an organ, part, or "segment" of the body, it’s called segmental vibration. The most common type of segmental vibration exposure is hand-arm vibration exposure which affects the hands and arms. 

Hand-arm vibration (HAV) is vibration transmitted from hand-operated power tools, such as road breakers or jackhammers, or as in this case, a chopper handlebar, into the hands and arms of users. 

HAV can affect the nerves, blood vessels, muscles, and joints of the hands and arms leading to a painful and disabling condition with tingling and numbness in the fingers, reduced grip strength and sense of touch, and affecting the blood circulation - vibration-induced white finger (VWF). If detected early, this disease is curable. If not, it can cause permanent disability in the use of the hands. 

Frequency of vibration and its effects on the human body

Frequency of vibration Types of effect
Below 1 Hz Motion sickness
3.5 to 6 Hz Alerting effect
4 to 10Hz Chest and abdomen pain
Around 5 Hz Degrades manual action
7 to 20 Hz Communication problems
8 to 10 Hz Backache
10 to 20 Hz Intestine and bladder pain
10 to 30 Hz Degrades manual and visual control
10 to 90 Hz Degrades visual actions

Table 1. Effects of vibration frequencies on the human body according to Brammer A.J., & Pitts P.M. (1) and Bridger, R.S. (2).

Issue and Application

Well, coming back to our motorbike and the safety of our colleagues. We wanted to gain and share a better insight into measuring vibrations produced by the bike during a normal ride - and the effects on the rider.

Research and studies have been made to evaluate the effect of human overexposure to vibration, especially in the working environment. Human body vibration testing is the standard method to assess the exposure of vibration induced on the user of different sorts of mechanical equipment such as power tools and construction equipment, vehicles, or other means of transportation. 

Regulations

Employers in a range of industries need to accurately evaluate vibration exposure levels and check them against the limits described by regulations.

In the European Union, the Vibration Directive (Directive 2002/44/EC) sets the minimum requirements for controlling the risks from both work-related hand-arm and whole-body vibration. The directive specifies vibration dose values (VDV) - action values, above which it requires employers to control the vibration risks, and limit values, above which people are not be exposed. 

Exposure limit values and action values

For hand-arm vibrations these values are:

  • A daily exposure action value of 2.5 m/s2
  • A daily exposure limit value of 5 m/s2

For whole-body vibrations these values are:

  • A daily exposure action value of 0.5 m/s2
  • A daily exposure limit value of 1.15 m/s2

Standards

International standards have been established to define best practices for measuring and evaluating human exposure to vibration, including measurement instrumentation. Different standards cover different types of vibration depending on the type of exposure such as hand-arm vibration and whole-body vibrations.

ISO 5349 is the accepted standard on how to measure human exposure to hand-transmitted vibration while ISO 2631-1 and ISO 2631-5 covers whole-body vibration. ISO 8041 is the standard that equipment used to measure vibration should meet.

The Dewesoft human and whole-body vibration solution support the measurement and calculation of the whole-body and hand-arm vibration according to all relevant international standards such as:

This standard covers instrumentation as well as general and specific methods to measure and evaluate vibration effects such as multiple shocks or the risk of vascular disorders. Our solution is most often used by customers to certify the suitability of standard operational circumstances for end-users in work-related situations.

Though both regulations and standards are defined concerning workplace machines and vehicles, the instrumentation, methods, and risk values are equally relevant when evaluating the vibrational impact of a joy ride on a customized chopper.

Measurement Setup

Since the impacts and vibrations of the motorbike can vary in intensity - small and big shocks as well as vibrations - we used SIRIUS-6xACC-2xSTG data acquisition system with DualCoreADC technology.

This device covers a 160dB dynamic range making sure no data are clipped or missed out. Both hand-arm measurements and whole-body measurements were performed with triaxial accelerometers - it is common to use 50 g sensors and special adapters.

SIRIUS DAQ unit with the battery pack mounted on the motorbike

Fig.1. The SIRIUS DAQ unit with the battery pack is mounted on the bike behind the driver.

The Data Acquisition System

  • 8-channel SIRIUS 6xACC 2xSTG data acquisition system (data logger)
  • B&K seat-pad with 3-axial accelerometer
  • Dytran 3-axial accelerometer
  • DS-IMU1 GNSS/IMU device
  • GNSS antenna
  • Battery pack (DS-BP2i)
  • USB webcam camera
  • Laptop with DewesoftX software, including the human vibration module

The Equipment Mounting

  • The seat pad sensor on the driver’s seat for whole-body vibration
  • The tri-axial accelerometer on the steering wheel/handlebar for hand-arm vibration
  • The IMU for roll, pitch, jaw, speed, GPS for location, and the SIRIUS data logger with the battery pack mounted behind the driver
  • The laptop computer was fitted into the driver’s backpack to acquire data in DewesoftX

Tri-axial accellerometer mounted on the handlebar

Fig 2. The tri-axial accelerometer is mounted on the handlebar for hand-arm vibration.

We captured the RPMs by using the motorcycle RPM sensor (24-2 gear tooth) and speed directly from the gearbox using existing motorcycle wiring and applying a formula to account for belt ratio and tire diameter. 

We also connected the DS-IMU1 device to be able to capture the GPS location of the motorbike and acquire a roll-pitch-yaw - this way we were easily able to correlate different driving conditions - RPM, lateral acceleration, etc. - to the vibrations.

DS-IMU1

Fig 3. DS-IMU1

We connected the accelerometers to the SIRIUS DAQ slice. The GPS antenna was connected to the IMU device to ensure a strong GPS signal. The USB camera was connected to the SIRIUS to document the visual aspect of the measurement - to control for shock spikes due to bumps and holes in the road. The SIRIUS was connected to the battery pack for the power supply.

In an accompanying vehicle, we also set up a remote desktop computer to enable monitoring of the measurement during the ride - all in all the setup functions as an advanced human vibration meter.

measurement in DewesoftX software of hand-arm and whole-body vibration on motorcycle

Fig. 4. Measurement in DewesoftX of hand-arm and whole-body vibration on a motorcycle.

Measurements

Our measurement was set for 1 hour. Testing was done on the open public road, and we were driving on both city roads - 50km/h, traffic lights - and on the highway - non-interrupted drive, 90-100 km/h. Overall, the quality of the road was good, with just a couple of stretches having bumps or smaller holes in the road surface.

Hand-arm Vibration Measurement

Using Dewesoft HBV software and measurement setup we were able to calculate the hand-arm vibration according to ISO 5349. We evaluated the results against the exposure guidelines the standard defines.

The probability of a person developing symptoms of the hand-arm vibration syndrome depends on the individual’s susceptibility, any pre-existing diseases and conditions, and the task-related, environmental, and personal factors such as age, the direction of vibration, coupling force, posture, and many more.

According to ISO 5349, the daily level of vibration exposure can be calculated to an 8-hour reference period of vibration, A(8), which is the 8-hour energy-equivalent frequency-weighted acceleration in meters per second squared (m/s2).

Studies suggest that symptoms of the hand-arm vibration syndrome are rare in persons exposed with an A(8) of less than 2 m/s2 at a surface in contact with the hand, and are unreported for A(8) values of less than 1 m/s2.

Before we performed the exposure calculations and evaluated them against the specified limits, we wanted to observe the frequency content of measured vibration for peak values and frequencies at which they occur. 

Analyzing the hand-arm data, we measured prominent peaks in vibrations in the X and Y direction at 63Hz - plus some smaller vibrations at 10 Hz - and peak vibrations in the Z direction at 125 Hz. From this, we can see that most of the vibration occurs in the lower frequency range.

vibration data transformed into the frequency domain and displayed in octave bands

 Fig 5. Vibration data transformed into the frequency domain and displayed in octave bands.

calculation of the hand-arm exposure value

Fig 6. Calculation of the hand-arm exposure value.

We used the Math function in DewesoftX to compose the formula to calculate the exposure level A(8) from the data measured - this could tell us the vibration level the driver would experience if driving for 8 hours straight. 

If we compare our results for hand-arm exposure value = 0,813 m/s2 against the probability of developing symptoms of the hand-arm vibration syndrome, which states that symptoms are unreported for levels of A(8) below 1m/s2, we can conclude that riding the motorcycle for 8 hours straight does not pose a risk to the rider.

In addition, the permitted vibration exposure level, the Exposure Limit Value (ELV), for a working day is, in most countries, for hand-arm vibration set at 5 m/s2 A(8), which is well above the measured exposure in our test ride. From this, we can conclude that riding a bike like ours for professional purposes and the entire duration of the working day would not exceed the permitted ELV.

Besides the ISO 5349 standard, hand-arm vibration can be evaluated according to ISO 18570 which is dedicated to the evaluation of the risk of vascular disorders. This standard uses a slightly different weighting, Wp, from which the Wp weighted daily vibration exposure Ap(8) may be obtained. 

calculating HBV

Fig 7. The Ap(8) is automatically calculated when selecting ISO 18570 in the software Channel setup of the HBV module - results are readily available in Measure mode. 
 

The ISO 18570 standard sets guidelines for exposure levels and the onset of symptoms of vibration white finger (VWF) when exposing hands to vibration. Here, the daily exposure threshold is given in Ep,d, which relation to Ap(8) is expressed in this formula:

calculation

Where T0 is the reference time of 8 h (28 800 s).

The research and analysis of Brammer and Pitts [1] also permit an estimation of the least daily vibration exposure, Ep,d, at which symptoms of VWF may be expected to occur. The threshold for the onset and continuing development of VWF is in the range for Ep,d, of 1150 m/s1,5 to 1750 m/s1,5.

To evaluate the daily exposure against the threshold range according to Brammer and Pitts, we used Math to calculate Ep,d from the Ap(8) data as output.

calculation of ep

Fig 8. Calculation of Ep,d from the Ap(8) data as output in the Dewesoft HBV module.

From the obtained result we can conclude that even when riding the bike for 8 hours, the daily exposure is well below the proposed exposure threshold - riding our chopper does not represent any significant risk for developing symptoms of a vibration white finger syndrome.

Whole-body Measurement

Regarding the spectral data of the whole-body measurement, we measured peak vibrations in all three directions X, Y, and Z at 63Hz, and minor vibrations in the X and Z direction at 10Hz.

results of whole-body vibration measurement

Fig. 9. Results of whole-body vibration measurement in all three directions X, Y, and Z up to 1000 Hz.

We wanted to assess the comfort during the ride according to ISO 2631-1. The standard states that for some environments it is possible to evaluate human comfort using the frequency-weighted RMS acceleration value of a representative period. It then has to be weighted with Wk  - the frequency weighting for the z-direction, the vertical recumbent direction.

The Dewesoft Human Body Vibration module implements weightings as per ISO 2631-1 in the calculations, when selecting the whole body method. In ISO 2631-1, the overall vibration value defined as aw is calculated by selecting RMS as an output along with the Vector Sum output option. 

Dewesoft human body vibration module

Fig 10. Dewesoft Human Body Vibration module - selecting the whole body method weightings as per ISO 2631-1 are implemented in the calculations.

Annex A of ISO 2631 a Guide to the effects of vibration on comfort and perception. This specifies a WBV with a time-averaged, frequency-weighted, single-axis vibration acceleration (aw) of less than 0.315m/s2 to be comfortable, while levels between 0.315m/s2 and 2.5m/s2 are found to be uncomfortable and a level greater than 2.5m/s2 extremely uncomfortable.

Reactions to whole-body vibration
by overall vibration values

Less than 0.315 m/s2 Not uncomfortable
0.315 to 0.63 m/s2 A little uncomfortable
0.5 to 1 m/s2 Fairly uncomfortable
0.8 to 1.6 m/s2 Uncomfortable
1.25 to 2.5 m/s2 Very uncomfortable
Greater than 2 m/s2 Extremely uncomfortable

Table 2: Guide to the effects of vibration on comfort and perception limits from ISO 2631.

For whole-body vibration, using the frequency-weighted RMS acceleration we measured an aw value of 0,47 m/s2 - just above the defined comfort level. In conclusion, a ride on the bike is a little uncomfortable, closing in on fairly uncomfortable. However, a custom-built chopper that’s still not that bad! 

Result of whole-body vibration using the frequency-weighted RMS acceleration

Fig. 11. Result of whole-body vibration using the frequency-weighted RMS acceleration.

For evaluation of potential health risks of whole-body vibration, ISO 2631-1 establishes health guidance caution zones. 

Health guidance caution zones

Fig. 12. Health guidance caution zones according to ISO 2631-1.

For exposures below the zone, health effects have not been documented or objectively observed. Within the zone, caution for potential health risks is indicated and above the zone health risks are likely. This recommendation is mainly based on exposures in the range of 4 to 8 hours. 

ISO standards for vibration exposure
and its effects on the health of driver/rider
Exposure duration in hours ISO 2631 – Average RMS meters acceleration limits in m/s2
Likely health risk Caution zone
4 0.63 1.20
8 0.82 0.48

Table 3.  Limit levels for 4 and 8-hour exposure stated in ISO 2631-1.

From the measured aw8 value of 0,47m/s2 is below the caution zone form which we can conclude that by riding the bike for 8 hours any health effects are not likely to occur.

Besides checking the measured results in regards to comfort, perception, and the ISO 2631 health guidance caution zones for vibration exposure, we even took another specified limit into account when evaluating the data: the ELV. The Exposure Limit Value (ELV) is the permitted vibration exposure level for an 8-hour working day. In most countries, for whole-body vibration, the ELV is defined as 1.15 m/s2 A(8). 

As during our measurement, we only acquired slightly more than one hour of data, the calculated Overall Vibration Value had to be corrected to cover the 8 hours. To do so, we used Dewesoft Math to define a simple formula for the calculation of aw8.

A simple formula is defined in Dewesoft Math for the calculation of aw8

Fig. 13. A simple formula is defined in Dewesoft Math for the calculation of aw8.

Our result was a maximum value of 0,639 m/s2 - well below the limit. This means that our custom bike could be used on a normal working day without concerns of exceeding the ELV. 

So, for joy rides and picking up pizza, the Dewesoft custom chopper would be a safe choice. 

 Dewesoft team during the evaluation of the bike ride measurement data

Fig. 14. The Dewesoft team during the evaluation of the bike ride measurement data.

Conclusion

Our bike ride vibration measurements demonstrated that riding the bike is safe, but you still have to be cautious and not overdo it. The riding style greatly influences the vibration exposure - during our test we respected road rules at all times and rode the bike on mostly smooth roads.

When we evaluated the hand-arm vibration exposure according to ISO 5349 the obtained average vibration dose value for 8-hours was below the level limit. The measured A(8) against the Exposure Limit Value (ELV) was ok too. 

The obtained daily exposure Ep,d was well below the daily exposure threshold set by ISO 18570 for the onset and development of vibration white finger syndrome. Riding the motorcycle for 8 hours straight does not pose a risk to the rider’s hands and arms.

As for the whole-body vibration of the ride, the overall vibration value according to Annex A of ISO 2631 is deemed a little uncomfortable. This is not the best possible result, but taking into account the relatively heavy modifications that were performed on the bike itself, the result is quite good. The NVH team was unanimous in the conclusion that having a little uncomfortable ride is an acceptable price to pay for riding the custom monster we have measured. 

Against the health guidance caution zones established in ISO 2631-1 an 8-hour ride on the bike is unlikely to pose any threat to the rider’s health. And against the workday limit, which in most countries is 1.15 m/s2 A(8) the motorcycle is a suitable means of transportation for the normal 8-hour workday.

Riding our custom bike on an 8-hour daily basis would not be perfectly comfortable but most importantly, not likely to pose any serious health risk at all. 

We can enjoy the feeling of freedom - and still, be safe. 

Sources

  • Brammer A.J., & Pitts P.M.: Frequency weighting for vibration-induced white finger compatible with exposure-response models. Ind. Health. 2012, 50 pp. 397–411
  • Bridger, R.S.: Introduction to Ergonomics, McGraw-Hill international editions, 318-409 (1995)
  • ISO18570: Mechanical vibration - Measurement and evaluation of human exposure to hand transmitted vibration - Supplementary method for assessing the risk of vascular disorders (2017).
  • ISO 2631: Mechanical vibration and shock — Evaluation of human exposure to whole-body vibration — Part 1: General requirements (2018), Part 2: Vibration in buildings (1 Hz to 80 Hz) (2003), Part 3: Evaluation of exposure to whole-body z-axis vertical vibration in the frequency range 0,1 to 0,63 Hz (1985), Part 4: Guidelines for the evaluation of the effects of vibration and rotational motion on passenger and crew comfort in fixed-guideway. transport systems (2001), Part 5: Method for evaluation of vibration containing multiple shocks (2018).
  • ISO 5349 Mechanical vibration - Measurement and evaluation of human exposure to hand-transmitted vibration - Part 1: General requirements, Part 2: Practical guidance for measurement at the workplace, Part 3: Supplementary description of special signal forms (2001).
  • ISO 8041 Human response to vibration - Measuring instrumentation - Part 1: General purpose vibration meters (2017), Part 2: Personal vibration exposure meter (2021).
  • Sanders, M.S., & McCormick, E.J.: Human factors in engineering and design. Mcgraw-Hill Book Company, 1993. (p 627-634).