Measuring Rotional Speed Patterns in Motorcycle Engines

A crankshaft may experience irregular rotational speed patterns caused by a combustion engine’s discrete firing process. These excite components of the driveshaft, which causes rotational vibrations that, in turn, may lead to undesirable noise and vibration. The engine and driveshaft are not acoustically encased on average motorcycles. Therefore it's very important to seek and analyze a reduction of undesirable noises and vibrations.

Rotational vibration data can be acquired from motorcycles while running on a roller dynamometer. It's preferable to use Polytec’s RLV-5500 Rotational Laser Vibrometer especially when the motorcycle engines are high revving (>10,000 cycles/min). 

The design aim of contemporary, high-performance motorcycle engines is to achieve peak efficiency from a lightweight build. Simultaneously the customer desire smooth power delivery and comfortable attributes from the driveshaft. This means a drive that is clear of resonances and does not jerk. Additionally, the stress caused by irregular rotational speed patterns, hard load and gear changes, and misuse require exact calibration of the individual components in the vibrating driveshaft.

With the use of dynamic simulation, the spring and damping elements of the driveshaft (Fig. 1) are adapted to suit the type of vehicle – from giving a sporty to a comfortable ride. However, as many things can affect the performance of a motorcycle, it is important to analyze and test the driveshaft in detail. This is achieved by taking measurements and correlating results with real test drives which are especially vital for determining subjectively perceptible and irritating vibrations and knocks. The vibration behavior of the total driveshaft can be studied in depth with the help of special rotational vibration measuring setups.

Schematic of a motorbike’s driveshaft.

Figure 1: Schematic of a motorbike’s driveshaft.

Experimental Setup

There are multiple advantages to analyzing the vehicle’s drive vibrations on an acoustic roller dynamometer.  With the exception of rotational vibrations, the acoustic radiation can be studied at the same time. The vehicle setup on the dynamometer reflects realistic conditions very accurately and offers the ideal improvement of reproducible driving conditions for the variety of loads and RPM ranges.

The measurement objects that can be acquired by the Rotational Laser Vibrometer are special measurement adapters, which can resist rotation and are attached on to the respective end of the crank and gear shaft, which lead out of the crank housing.  (Fig. 2). To achieve this, the vibrometer is set up at an appropriate distance (the operating range of the RLV-5500 is approx. 200 mm) and is aligned so that both laser beams emitted hit the measuring shaft at right angles and are aligned with the direction of rotation (Fig. 3 and 4).

RLV-5500 Rotational Vibrometer.

Figure 2: RLV-5500 Rotational Vibrometer.

Measurement of torsional vibrations on the crank shaft using the RLV-5500.

Figure 3: Measurement of torsional vibrations on the crankshaft using the RLV-5500.

Measurement points at the crank shaft and at the gear shaft.

Figure 4: Measurement points at the crankshaft and gear shaft.

Applying a diffusely reflecting self-adhesive film improves the measurement signal-to-noise and therefore leads to a reduction in signal variations. The Doppler effect forms the basis of the measurement principle of laser vibrometry. This measurement methodology is extremely successful, can work without making contact and is, therefore, non-reactive (zero mass loading), which allows it to be used independent of the material properties and temperature. With the recently released RLV-5500 Rotational Laser Vibrometer, it is simple to make high-resolution rotational measurements on high-revving motorcycle engines with a maxmum RPM of well over 10,000/min.


Measurements made on the prototypes, in general, agreed very well with simulation results. However, within the structure of comprehensive analyses and test drives, there were special driving conditions, which, under certain circumstances, were felt by drivers as uncomfortable vibrations in the vehicle. Such effects are often overlooked during construction and simulation and can only be picked up and classified by selective study of the rotational vibration in the driveshaft (Fig. 5 to 7). With the use of carefully calibrated variations of the spring package for the clutch torsional vibration damper, maximizing the potential of the rubber packages of the rear wheel jerk damper, taking precautions to cap the play in the driveshaft, selectively influencing and remedying these resonance vibrations then became possible.

First irregular rotational speed pattern vs. RPM.

Figure 5: First irregular rotational speed pattern vs. RPM.

First irregular rotational speed pattern vs. load.

Figure 6: First irregular rotational speed pattern vs. load.

Campbell diagram of a run-up measurement.

Figure 7: Campbell diagram of a run-up measurement.

Spring package for the clutch torsional vibration damper.

Figure 8: Spring package for the clutch torsional vibration damper.

Conclusions and Outlook

Comfort expectations of modern motorcycles and their powertrains are continuing to rise.  Ensuring that the driveshaft of future motorcycles is sporty and dynamic, yet comfortable, is technically challenging. The described measurement methods for acquiring rotational vibrations are extremely vital and form a comprehensive process for growing knowledge of complex vibration systems and to achieve the design objective.

This information has been sourced, reviewed and adapted from materials provided by Polytec.

For more information on this source, please visit Polytec.


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