Early Motorcycles were developed after the inventions of bicycles and engines as an attempt to combine the two. In 1770, the first steam powered tricycle was manufactured by Nicolas Joseph Cugnot (3) in France, however presented numerous safety and reliability problems, meaning they would never be mass produced and an alternative source of power was required. In 1885, German inventors Gottlieb Daimler and Wilhelm Maybach were the first to produce a motorcycle with a gasoline internal combustion engine, patented as ‘The Daimler Reitwagen’ (figure 1). The first mass production motorcycle by Hildebrand and Wolfmueller began in 1894 (figure 2). When roads and technology began to improve in the 50s and 60s, Motorcycles became affordable and a favourable mode of transport. Today, there are many uses of motorcycles from sport racing, touring and cruisers each with different comfort and handling experiences.
The first telescopic fork incorporating a front suspension damping mechanism was in 1908 by Scott motorcycles (figure 3), introducing the first coil and spring suspension damping mechanism inside the front fork tubes (1). The first hydraulically damped telescopic fork was introduced by BMW on their R12 (figure 4) and R17 models in 1935 (5)(18). Initially the hydraulic dynamic damping technology was used only in racing vehicles, but eventually made its way into off road races, it is still the most popular form of fork damping used today. The demand for the ability to adapt to stimuli including changes in weight and speed to meet stability control criteria led to manufacturers producing alternative front suspension on different bikes. These alternative systems aim to improve various aspects of damping, stability, road holding and control, however during analysis of these systems over time shows that performance of the traditional telescopic fork often outweighs benefits gained from alternative solutions.
Improvement of the front suspension system of a motorcycle over time is to enhance one of the two key functions of the system, firstly to isolate the rider from bumps in the road, and secondly to provide comfort and safety to the rider and passengers. The design of the front fork is the most recognizable design of a modern motorcycle, clearly obtained from a bicycle structure stemming from the steering hub and positioned on each side of the front wheel. As emphasised by Noce (4) in his works that ‘the desired motorcycle design is to maximise frame stiffness whilst minimizing weight’ he suggests that structural stiffness is an important characteristic of not only the motorcycle but also the front suspension. A high structural stiffness gives a quick response to rider input and the optimum performance is a combination of medium lateral stiffness and high torsional stiffness.
This literature review will begin with an overview of the workings of a front suspension system and its characteristic parameters and how these relate to the performance of the front suspension during braking applications. Section three will provide an overview of existing and alternative front suspension systems outlining key design features, characteristics and performance advantages of each respectively. Following this the performance of alternative double-wishbone suspension designs are compared from simulation results. Section five discusses how the suspension is one of the most important components for improving comfort, road holding, and stability and different setups maximise different characteristics. Finally, this report will review advanced semi active suspension technologies used in motorcycles, and how magnetorheological fluids are being used to improve the response and handling of top of the range vehicles.
Suspension systems have developed over time to enhance rider comfort and prolong the lifetime of a vehicle. When a rider accelerates, or activates the brakes, the front suspension supports a large portion of the weight, thus a demand for a damping energy absorption mechanism is required. Due to the range of variation in riding styles, road surfaces and conditions, all affecting the load experienced by the suspension, different types of motorcycles and riders will require alternate setups of the front suspension and geometry.
A suspension system is comprised of springs, dampers, sprung and unsprung masses, and tyres. The choice of characteristics for each of these parameters determines the overall performance of the suspension system and therefore the road handling of the vehicle. Foale (6) models a simple motorcycle suspension as represented in figure 5, separated into the front and rear sprung and unsprung masses respectively.
Spring performance parameters include: ‘spring rate’, the stiffness or additional force required to compress the spring a given distance (N/mm), ‘pre-load’, the compression of the spring during mounting, and ‘sag’, the compression distance with a static load of the bike and rider.
Dampers are a key element of the design that act as energy absorbers to prevent the suspension from oscillating uncontrollably. Without this loss of energy to heat, the energy would be transferred between stored and kinetic in opposing directions and result in a repeated bouncing of the rider. Classical dampers used friction however modern damping systems use hydraulics for viscous damping.
The total mass of the motorcycle model is split into the ‘sprung mass’, anything supported by the suspension, and the ‘unsprung mass’ everything else which is not supported by the suspension. The unsprung mass included the wheels, tyres and brakes and is simply sprung mass subtracted from the total mass. The unsprung mass does not affect springing since it is not supported by the suspension.
Humans are more sensitive to different disturbance frequencies, the most comfortable being disturbances in the vertical direction of 1 to 1.5 cycles per second (6) . Whereas a racing motorcycle spring mass system has a natural frequency of 2-2.6Hz (7). The undamped suspension frequency is calculated using the basic mass, spring damper model.
The speed and nature of the road determines the range of frequencies imposed on the suspension. Horizontal disturbances are unpleasant to a rider and play an important role in the design and direction of suspension damping. As mentioned above, the suspension is included to maximise rider comfort and road handling however the two can often oppose each other. Comfort is achieved by minimising the force transmitted to the sprung components, achieved by increasing the unsprung mass. On the other hand, road handling is increased by a high sprung to unsprung mass ratio thus a lower unsprung mass. This is an example of one of the many challenges faced by designers when finding the optimum mass for road holding and comfort.