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By Aravind R Nair
Read time: 12 minutes
In motorcycles, the front and rear of the vehicle are susceptible on having an oscillating motion around its steering axis, irrespective of the extend went into the R&D for balancing the system. These oscillations takes place at a general speed range and its amplitude can decline with the decrease in speed until is completely disappears. It could be a fact in most cases that the more pressure the driver applies on the handle to counteract it, the worse the phenomenon gets controlled. Related to the frequency of oscillation, vehicle speed and involvement of front or rear end of the motorcycle, these oscillations are categorized into three:
Ideally, a motorcycle needs a self-steer property which can aid in contributing automatic stabilization and without becoming too oscillatory under certain running conditions within its designed range.
At very low speeds, Capsize is a root cause of instability in two wheelers due to two non-oscillating modes known to us as roll and steer motion. Typically around 1m/s, its real poles combine to form a complex pole pair associated with weave vibration. This weave mode is generally unstable until 7-8 m/s. Beyond this speed, the two wheeler is considered stable and requires the least amount of steer control from the rider. However, the actual capsize, weave and wobble instabilities majorly depends on the two wheeler classification. Weave frequencies are typically unstable at higher speeds, Wobble frequencies are poorly damped in mid to high range speeds and Capsize frequency instability is non-significant for mid to high range speeds.
A detailed description of the three modes are required to form a basic understanding on dependencies and arrive at good practices to counteract them in an effective manner.
Capsize
Capsize mode is heavily influenced by the mechanical impedance the rider provides i.e., rider action on handle bar that affects the inertia, stiffness and damping. Therefore, the switch from unstable to stable zone is easily shifted.
This mode mainly consists of roll motion combined with lateral displacement with negligible influences from steering and yaw movements. It is highly dependent upon the following factors:
Since, in essence a two-wheeler body can be viewed as an inverted pendulum, Capsize shouldn’t be viewed as a drawback, as this phenomenon aids to control the vehicle to lean and execute curves correctly (free-control condition). So in this consideration, the faster the ability of the vehicle to tilt is preferable during cornering cases.
Wobble
Wobble or “Tank slapper” can be visualized as the independent oscillation of the two wheeler front end rotated around the steering axis, irrespective of the rear end’s motion and roll. Typically, its frequency values ranges from 4 Hz for heavy motorcycles and 10 Hz for light motorcycles.
Wobble frequency is highly dependent on Mechanical trail increase or a decrease in front-frame inertia and is determined mainly by the stiffness and damping properties of the front tire and influences by front fork lateral flex.
This phenomenon is considered only slightly damped in speed ranges from 10-20 m/s, causing huge instability at the speed range.
Weave
Weave is the oscillation of the entire motorcycle body with a major contribution from the rear end. This mode is generated by a combination of two unstable non-oscillating modes namely: Body capsize and Steering capsize. Its natural frequency ranges from 0-4 Hz at high speeds.
This phenomenon is usually unstable at low speeds of upto 7-8 m/s, stable in mid-speeds, but highly uncontrollable in high speeds due to lower damping and practically uncontrollable natural frequency. It is determined by the following factors:
How to prevent them?
Although the preventive measures for these phenomenons seem counter-intuitive as its adjustment parameters may need to be increased or decreased dependent on the modes. A design decision should be made at the initial Rigid Multi-body modelling stage using tests in the following modes:
This decision should be reflected upon the operating ranges, purposes and potential user skill estimations. The model behavior can be clearly encapsulated using a root locus plot of both in-phase and out-of-phase modes at the designed operating ranges of the vehicle. The undesirable mode effects should be moved within the plot, which can be achieved by varying the different vehicle properties.
As a general rule of thumb, for the different vibration modes discussed, an experimentation of 10% variation may be suggested in the specified direction, which can highly influence control of these vibration to a handleable limit:
For Capsize, The stability can be improved by:
Increasing:
Decreasing:
For Wobble, The stability can be improved by:
Increasing:
Decreasing:
It is worth mentioning that an increase in Motorcycle Center of gravity height, caster angle and rear wheel radius gives advantage at lower velocity and adverse in high velocities.
Increase in overall vehicle roll inertia and mechanical trail shows the opposite behavior as advantageous in high speed and adverse in lower speeds.
For Weave, The stability can be improved by:
Increasing:
Decreasing:
The increases in the height of the motorcycle mass center and of the rear wheel radius give a disadvantage at low velocity and an advantage at high velocity. On the contrary, the increase of roll inertia is advantageous at low velocity and proves a disadvantage at high velocity.
The increase of the mechanical trail gives a slight advantage at medium velocities but a disadvantage at high velocity. An increase in the cornering stiffness of the rear tire produces similar behavior.
In real life scenarios
In the actual physical testing of the vehicle, the results may vary slightly in range magnitude and direction of effect progression. This is due to the fact that the model considered in the discussion was a rigid body model.
If we were to consider the rider body flexibility, front fork bending, and rear swing arm torsion and frame stiffness, the ways in which a rider body can control the phenomenon and how the state of motorcycle condition affects the phenomenon can be evident.
In real riding scenarios, counter-weighing the rider upper body, light handle grip, reducing velocity without use of front brakes, localization of rider mass towards the handle and the overall rider mass has a greater effect in instability prevention to prevent a less handleable state of ride.
Vehicle conditions such as front fork play, worn front/back tires, loose bearings, cracked bushings for the rear sub-frames and bend in the chassis components could be a cause to these effects and should be prevented. Weave threshold is designed to be above the vehicle’s top speed, however the threshold can reduce with addition of masses such as luggage or multiple riders.
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