When we think of the key parameters that contribute towards enhancing a two-wheeler’s performance and efficiency, we often tend to think of increasing the engine’s power output and/or reducing the vehicle’s weight. Those two factors are critical, of course, but another very important factor – especially when it comes to bigger, faster two-wheelers – is aerodynamics. As speeds increase, the vast majority of the engine’s power is spent in overcoming wind resistance, and with improved aerodynamics, OEMs benefit from higher performance levels combined with improved fuel economy. Here, we take a closer look at how motorcycle aerodynamics work, and the key factors that affect its efficacy.
HOW IT WORKS
In the context of powered two-wheelers (PTWs), aerodynamics is the science (and art!) of designing the vehicle in a way that minimises wind resistance at higher speeds. This is usually accomplished by adding a full or a partial fairing that optimises air flow around the vehicle and its mechanical components, thereby reducing drag. However, in addition to achieving reduction in drag, designers must also pay attention to cosmetics – the vehicle must also look good – as well as ergonomics, so that it remains easy and comfortable to operate.
Most motorcycles, but especially ‘naked’ bikes (which do not have any sort of fairing), have pretty poor aerodynamics since the engine, all components and of course the rider himself are all fully exposed and hence create ‘drag.’ Scooters, which have full bodywork that covers the engine and other mechanicals, can have better aerodynamics but these often do not travel fast enough for aerodynamics to become a very big factor in their overall performance. Also, unlike cars (which can have a fixed coefficient of drag), two-wheelers have another factor that affect their aerodynamics – the rider. How the rider sits on a two-wheeler, his height, weight, posture and the way that posture keeps changing during acceleration, braking, cornering and other manoeuvres, means that a two-wheeler’s aerodynamics keep changing all the time.
If you thought that’s challenging enough for two-wheeler design engineers, wait, there’s more. A motorcycle’s bodywork/fairing (key elements that dictate how its aerodynamics work) must not only reduce drag and look good doing it, it must also not negatively impact the vehicle’s handling. Wind pressure, and how it acts on a two-wheeler’s fairing and bodywork, can affect stability and must be designed in a way so that it can deal with wind turbulence without causing the rider to lose control of the machine.
The rules of physics dictate that air pressure rises to the cube of speed. So, for every 1 kph, the corresponding air resistance increases by a multiple of three. What that means is, at 100 kph, around 80 % of a motorcycle engine’s power output goes towards overcoming air resistance. Hence, while designing a PTW and figuring out its aerodynamic properties, a key objective for engineers is to reduce drag – the obstructive force generated because of the difference in air pressure between the front and rear of the vehicle. Drag acts on the frontal area of a two-wheeler (usually, the larger the frontal area, the greater the drag), slowing it down. Also, drag increases at an exponential rate as speeds increase, so the faster you go, the harder the engine has to work to overcome drag.
When air comes into contact with a two-wheeler and its rider, at and near the surface, air speed becomes the same as that of the moving man and machine. However, friction pulls adjacent layers of air (in addition to the thin surface that’s in contact with the moving bike) along for the ride, and that’s where the problems begin. The difference in air speeds keeps increasing as we move outwards from the surface layer that’s in contact with the moving motorcycle (with the outer layers of air moving at a much slower pace), and the faster the PTW goes, the bigger those differences in air speeds become, increasing drag exponentially and building up what is referred to as the ‘velocity gradient.’ As the velocity gradient increases, air flow becomes more unruly and turbulence increases.
The designers’ objective is create a shape that reduces turbulence as much as possible, ensuring smooth air flow, allowing the vehicle to ‘slip through’ air with as little disruption as possible. While designing a PTW’s bodywork, engineers try to keep the separation point (the point where the layer or air that’s in contact with the moving vehicle detaches from the bodywork) as close as possible to the rearmost point on the vehicle, and with the smallest cross-section possible, so as to minimise wake. Think about it for a minute and you’ll realise why the classic ‘teardrop’ works best for aerodynamics, though it’s not particularly well suited to most two-wheeler applications – the Suzuki Hayabusa is just about the closest that any production motorcycle has gotten to that shape, which is perhaps why it has a top speed of more than 300 kph.
So, theoretically, the most aerodynamic motorcycle would be one that’s shaped liked a teardrop. However, that shape simply doesn’t work in the real world. To overcome that, designers have tried various work-around methods, including all-enveloping ‘dustbin’ fairings that were tried in the 1950s. Back then, OEMs like Gilera, Moto Guzzi and Norton were using all-enveloping fairings, which earned the dubious nickname of ‘dustbin fairings’ due to their bulbous shapes, on their racebikes. While these may or may not have provided some benefits in terms of aerodynamic efficiency, they also made bikes unstable at high speeds and were consequently banned by the FIM in 1957. All-enveloping bodywork has still been used on sportsbikes in the 1980s and the 1990s (for example, on the Ducati Paso and the Honda CBR series), and the current Suzuki Hayabusa and Kawasaki ZX-14R use it to this day, but all these bikes a much leaner, slimmer form than the dustbin fairings of yore.
At the other end of the spectrum was the revolutionary Britten V1000 of the early-1990s, which despite its minimalist design and hardly any bodywork to speak of, proved to be faster than any other superbike of its time. Indeed, while all-enveloping bodywork continues to be useful for two- and three-wheeled streamliners used for land speed record attempts, that’s perhaps because those vehicles only have to travel in a straight line at extremely high speeds, and their riders do not have to deal with bends or corners.
Indeed, covering a motorcycle in flowing bodywork is only part of the equation and while this does lower the coefficient of drag, there’s still the rider himself, who disrupts all the fancy airflow envisioned by the aerodynamics engineers. Hence, managing airflow over and around the rider may be the key to achieving ideal aerodynamics on a PTW. MotoGP engineers, who work at the very cutting edge of aerodynamics technology, may have some answers. In recent years, some MotoGP bikes have used winglets or air-blades mounted on their fairings to create downforce for added grip and stability, along with strategically placed strakes and ducts to manage air pressure and high-speed turbulence. (The winglets were banned in MotoGP at the end of 2016, due to their potential to cause harm to other riders in the event of a crash, but have since made a comeback in a modified form.)
These winglets/air-blades are aerodynamic aids that come from the world of aerospace engineering and are designed to cause small amounts of controlled turbulence, which ultimately helps create a virtual ‘wall’ of air around the rider and smoothing air flow over the entire moving vehicle. These winglets, also referred to as ‘vortex generators,’ have been used by various OEMs for the last ten years, though their use has become more commonplace only over the last 2-3 years, as engineers’ understanding of how these work – and how they affect a motorcycle’s high-speed handling – has improved.
COMPUTATIONAL FLUID DYNAMICS
The study of aerodynamics is complex, with airflow evaluation requiring dedicated wind tunnels and massive, powerful fans, which are expensive to build and run. Plus, repeated wind tunnel testing, followed by incremental vehicle modification every time, is not only expensive but also time consuming. However, with the advent of powerful computers and CAD programs, things have become a bit easier for engineers, who can now use computational fluid dynamics (CFD) for aerodynamics design. With CFD, using data from CAD drawings, the air surrounding a moving PTW can be split into millions of small virtual cells, and engineers can study parameters like airflow velocity, pressure and turbulence etc. for each cell, ultimately allowing designers to understand how the entire vehicle will behave at various speeds.
As with finite element analysis (FEA) that’s used for PTW chassis design, CFD is also complex technology that requires powerful computers and sophisticated software to run, but compared with using full-size scale models and a wind tunnel, CFD has the potential to make aerodynamics design at least a bit simpler and faster for OEMs and/or their design partners. However, it remains to be seen what direction this technology will take in the near future, especially in the context of PTWs, given that dealing with rider movement and position – and its effect on the bike’s aerodynamics – remains a challenge. Reducing turbulence/drag, and increasing smooth airflow (also referred to as ‘laminar flow’) is, on a motorcycle, dependent on so many variables, including the rigidity of the chassis, suspension movement, rider movement, cornering angles and so on, that full mathematical analysis of all variables remains a complex engineering issue and many answers still need to be worked out.
Over the next few years, new electric powerplants, which will ultimately replace the IC engine on many PTWs, will bring further disruption to motorcycle design. Electric motors and controllers can inherently be simpler and more compact than IC engines, with fewer moving parts. However, electric bikes will also need big, heavy battery packs and their placement within the bike’s frame and bodywork will create its own set of challenges, also in terms of aerodynamics, which engineers will have to solve.
Another major factor that may affect PTW aerodynamics may be safety. With increasingly stringent safety requirements, OEMs may need to pay attention not just to style, ergonomics and high-speed stability, but also how elements of a motorcycle’s bodywork – the fairing and windshield, for example – behave in the event of a crash. Newer materials, the need for vehicle lightweighting, increasing engine output and the never-ending quest for higher speeds will, in the foreseeable future, continue to push the evolution of two-wheeler aerodynamics.
TEXT: Sameer Kumar