Super Reflex: A guide to motorcycle suspension systems

Have you ever wondered how your vehicle compensates for bumps and cracks in the road? Have you ever noticed how vehicles differ in handling depending on for instance if they are a car, truck, or motorcycle? Of course I am talking about suspension systems and more specifically motorcycle suspensions. Among the most important systems of a motorcycle is the suspension. The suspension of a performance motorcycle is important because it keeps the tires on the ground and provides comfort for the rider. There are several types of suspension systems in development and available on the market today. Choosing the right system is critical because it dictates how the motorcycle will steer, brake, accelerate, and respond to different road surfaces. This synthesis will teach and demonstrate basic dynamic suspension geometries, the different types of suspension systems and components available, and highlight the benefits and limitations of each prospective system.

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Before you can fully understand the workings of a suspension system, you need to understand what a suspension is and the various components of a suspension system. According to Merriam- Webster dictionary a suspension is “the system of devices (as springs) supporting the upper part of a vehicle on the axles”. The purpose of any suspension system is to absorb road shock caused by passage of the wheels over irregularities. This is accomplished by springs and dampers that absorb the shock of the wheels and control the rebound rate at which the suspension is pressed back onto the road surface via the springs. A spring is usually a coil of tempered steel that is used to absorb the shock and push the wheels back to the ground. A suspension that consists of just springs will result in a bouncy and less controllable ride. This is why dampers are required in modern day suspensions. A damper is a fluid-filled device, which regulates fluid flow to restrain movement. These two devices are usually combined into a single unit called a shock absorber or coilover unit (where the spring is mounted outside of the damper). The primary function of a shock absorber is to control or damp the movement of the spring. A lack of rebound control will cause a floating or bouncy ride, while too-stiff compression damping can create a lack of control on rough terrain and a harsh ride on the streets. Shocks resist motion by using a piston and valves that are mounted on the end of a shaft and that move through a fluid of specific wight oil. The fluid must pass through holes, valves, and slots in the piston as the shaft is stroked in and out. Resistance is created when the oil is forced through the openings on each cycle. Basically, shock absorbers act as energy transfer devices which remove kinetic energy (movement) from the spring, turn it into thermal energy (heat) and dissipate the heat into the atmosphere.

On a motorcycle suspension system these coilover shock absorber units are mounted not at the axle of the motorcycle, but the swing arm which contains the axle. The basic motorcycle swingarm is a rectangle, with one short side connected to the motorcycle's frame with bearings so that it can pivot (Prentice-Hall, 1984 p 282). This allows the rear wheel travel to be controlled while passing over bumps. Swing arms have been evolving since the advent of the motorbike to keep up with performance standards at each stage of evolution.      

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The first major evolution of the swing arm is the twin shock regular swing arm. This configuration utilizes two parallel control arms that the shock bolts onto mounted above each side of the swing arm. This style of suspension lost popularity due to weight considerations and the availability of newer, stronger materials. Due to its twin control arm design, it became flexible under extreme riding conditions. The only way to make it stronger was to add more metal, which added more unsprung weight, which in turn reduces the efficiency of the suspension.

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In 1969 the mono shock swing arm was first developed by Lucian Tilkens. His mono shock design was later transformed into the well known older style mono shock swing arm. This design was a dramatic improvement over the twin shock design because of the simplicity, weight savings, and improved efficiency it provided. The weight savings was the most significant improvement over the twin shock design. Weight savings was accomplished by eliminating one of the shock absorber units and centralizing the the shock in front of the rear wheel. Because of the shocks relocated position, it has less piston travel than before. Therefore it creates less probability of de-emulsification (the process in which air bubbles rise in the liquid) (Dickson 2007, p187). These small air bubbles can create a froth of dampening fluid in high travel dampers which significantly reduces the dampening ability of the shock unit.

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The modern generation of the monoshock swingarm provides even more weight reduction. The main difference between the new style and the older style is the linkage at the bottom of the shock mount. This allows the shock to lay nearly vertical and provides extra leverage to the coilover unit decreasing the amount of travel, further reducing probability of de-emulsification.

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The latest evolution of the monoshock swingarm is the single-sided swingarm. These are very-strong and very-lightweight swingarms. The advantage of a single-sided system is that the wheel can quickly be taken out and replaced. Single-sided swingarms bear the all the stresses from the rear axle offset to one side, so it needs to also have added torsional stiffness to stop it from twisting under the offset load. As a result, single-sided swingarms are typically a lot larger and have an intricate cross-bracing structure inside them. These swing arms are typically only seen on expensive high performance sportbikes.

The rear suspension is only half of a motorcycles suspension system. The front suspension of a motorcycle is subject to more complex calculation because it is responsible for how the bike will trail in a strait line and steer. The most common form of front suspension for a motorcycle is the telescopic fork. In 1934 Nimbus was the first manufacturer to produce a motorcycle with hydraulically damped telescopic forks (Motorcycle Mechanics Institute, 1984 p277). The forks can be most easily understood as simply large hydraulic shock absorbers with internal coil springs. They allow the front wheel to react to imperfections in the road while isolating the rest of the motorcycle from that motion. Some bikes such as high performance sportbikes use "upside-down" (USD) forks, also known as inverted forks. They are installed inverted compared to typical forks, with the tubes at the bottom and the bodies at the top. This decreases the unsprung weight of the motorcycle and improves its handling.

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The geometry of the front fork suspension is critical to how the motorcycle will handle. Rake and trail are terms often used when discussing motorcycle handling. Trail especially affects how a bike feels, and can determine its stability, steering quickness, and in general, a large portion of the bike's handling characteristics. Rake is defined as the angle of the steering head with respect to a line drawn perpendicular to the ground. A smaller angle, or less rake, is sometimes referred to as being steeper producing sharper turning response. Trail is the horizontal measurement from the front axle to the point at which a line drawn through the steering head intersects the ground (Foale 2008). Sports bikes typically have less rake which means less less trail. Less trail means less stability, which means a quicker-steering bike. This makes these bikes a lot less stable to ride in a straight line, but a lot more nimble in the corners. Conversely, cruisers, choppers and customs, have much more rake. More rake means more trail, which means more stability, but makes the bike harder to turn.

Motorcycle suspension systems vary according to the type of platform they are designed for. The front and rear suspensions vary considerably depending on the type of performance needed out of a specific motorcycle. Engineers designing large bikes such as cruisers build for long road trips are not concerned with weight savings. Because of this heavier suspension platforms are utilized such as the twin shock swing arm. Their front steering geometry also changes to larger rake and trail angles to provide better tracking on the streets. Sport bikes however, are very concerned with weight reduction and sharp steering response. These bikes often utilize the latest swing arm technology to provide weight savings where ever possible. The front suspension geometry of these bikes typically features smaller rake and trail angles to provide better handling at the cost of strait line stability. It is important for engineers to focus on what the motorcycle will need to do, in order to design and use the appropriate technologies accordingly. 

References

Motorcycle Mechanics Institute, (1984) The Complete Guide to Motorcycle Mechanics. Prentice-Hall, Inc.

Wilson, H. (1995). The Encyclopedia of the Motorcycle Limited.

Dickson, J (2007). The Shock Absorber Handbook Wiley.

Foale, T (2008) Motorcycle Handling and Chassis Design: The Art and Science www.tonyfoale.com.