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Motorcycle Aerodynamics

Motorcycle aerodynamics is a misunderstood subject and often comes with many misconceptions. Unlike cars, motorcycles are inherently awful at slicing through the air, no matter what shape they are. This is why so little progress has been made to improve their aerodynamic efficiency over the past 130 years. But with exponential advances made in computer simulation and the recent arrival of some wild new solutions on racing motorcycles, could that all be about to change?

Aerodynamics, the study of motion through the atmosphere, has often been called an art. It is not. It is a science the mastery of which requires high level mathematics, careful observation, curiosity, rational deduction, and an open mind; the very founding principles of scientific thinking. Unlike mechanical or electrical engineering, fluid dynamics (the name given to aerodynamics and hydrodynamics, the study of motion through water) can appear mysterious only because they involve almost endless variables and an infinite number of conditions, which is easily enough to cause most people to throw their hands up frustration and call it an “art”.

It’s All Our Fault : The Rider Just Doesn’t Cut It

Motorcycles are inherently un-aerodynamic because of us, the occupants. Unlike a car, plane, or even the hull of a boat, the surfaces of a motorcycle moving through the fluid (in this case air) is interrupted by a human body that is draped rather inelegantly over the top and spills down the sides in a gangly mess of limbs. As a result, air that was previously sailing effortlessly alongside the smooth fairings of the bike come crashing into our wrinkled, flailing bits causing massive air turbulence.

Turbulence is important to understand, because it is the reason why aerodynamics matter on a motorcycle. To most of us, turbulence is just something of an annoyance that forces us into seat belts and to hold our pee during commercial air travel. But the force that causes a 300 ton jet to shudder at near supersonic speeds is the same force that shreds your hearing, wiggles your helmet around and holds you back when riding fast.

That's not spaghetti on his back... this computer simulation of air flow shows violent, turbulent air (in blue) tumbling around behind the rider and sucking at forward motion.
That’s not spaghetti on his back… this computer simulation of air flow shows violent, turbulent air (in blue) separating from the free flowing air (green) and tumbling around behind the rider.

On a bike, once you go over about 30 km/h, air pressure starts to dominate proceedings. Air pressure rises to the cube of speed, meaning that for every kilometer per hour, the corresponding air resistance increases by a multiple of 3,

It doesn’t take long before this quickly adds up to consume all of a motorcycle’s power, so much so that at a mere 100 km/h, 80% of a motorcycle’s energy is spent just overcoming air resistance. While that’s largely the same for any vehicle, including cars or planes, they are mercifully smooth, enclosed shapes. The motorcycle on the other hand, is a collection of random engineered parts and a human body which together form a shape almost perfectly suited to disrupt smooth air flow.

Turbulence is air that behaves like a teenager: it’s listless, high in energy but cannot decide where to go. And like a moody teenager, turbulent air has the amazing talent of agitating everything it touches. When pockets of turbulent air disrupt an object in motion, the air molecules touching that object create friction that pushes against it. Even after it’s gone, it causes a vacuum effect behind you, further draining you of momentum.

Green definitely does not mean "go". That ghostly apparition behind the bike is energy being wasted
Green definitely does not mean “go”. That ghostly apparition behind the bike is energy being wasted

That is aerodynamic drag, and it literally sucks you back. We’ve all heard the rumble and felt the knocks of turbulent flow around our helmets, and probably had the sore necks to remind us about it.

If turbulence is a militant teen then smooth, orderly air flow, called laminar flow, is like a parade of thoroughbred horses. Laminar air moves easy, has modest energy and doesn’t rock your head. It’s happiest when running fast and free. It dislikes sharp corners and loose materials, anything with bumps or rough surfaces that may slow it down. We all hear laminar flow around our heads every time we ride. It’s when the wind noise is a constant drone, like the whistle of a jet engine.

Slicing Through the Air Like a Wet Sack of Potatoes : A No-Win Situation

Motorcycle designers have spent a lot of time worrying about your head, making sure that the evils of turbulence stay as far away from your helmet as possible. Wind screens are shaped, punctured and vented so as to direct the air past your noodle as smoothly and pressure-free as practicable.

These staged publicity photos show lovely parallel lines of laminar air flowing nearly over the rider
These staged publicity photos show lovely parallel lines of laminar air flowing nearly over and around the motorcyclist.

But the rest of your body is not much of a consideration. It may come as a shock to most motorcyclists, particularly those used to reading boastful claims in motorcycle advertising to the contrary, but 99% of motorcycle OEMs don’t give a damn about aerodynamic efficiency. The reason? It’s such a lost fight that it isn’t worth the effort.

The average passenger car on the road today has a drag coefficient (the measure of air resistance) of about 0.3 (a lower number being better). Even a large, blocky car like a full size SUV or my ’87 Westfalia camper rate around 0.4. A racing motorcycle with a smooth, full fairing and professional rider in a full tuck? 0.6. In other words, those 5’9” jockeys crunched up behind the windscreen of a tiny Moto3 grand prix bike create more drag in the air than a Chevrolet Suburban.

This actual photo of a wind tunnel test shows how violently laminar air flow unravels even on a racing motorcycle
This actual photo of a wind tunnel test shows how violently laminar air flow unravels after the rider, even on a purpose-built racing motorcycle.

This is just the nature of the beast, and no amount of finesse in the wind tunnel can fix that. The designer of a motorcycle can do a lot to control air flow and minimize aerodynamic resistance at the front, around the front wheel, radiator, engine and in front of the rider, but once the fairings and bodywork ends, its turbulence city.

It’s not just the sticking out bits like elbows, head, feet and knees that break up the lamina air flow, it’s the sudden disappearance of a smooth guided path. Once air crosses the trailing edge of a motorcycle’s fairing into the open space between the fairing and rider, laminar flow disintegrates. The air pocket behind the wind screen is roiling mess of eddies and currents that are highly turbulent. Slow moving, but full of odd and unpredictable pressures, they slam into the high-speed, laminar flow rushing past. That collision creates a vacuum effect, sucking the turbulent air into the laminar flow, agitating the collision still more.

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The resulting turbulence then squeaks out in pulses wherever it can: under an armpit, over a shoulder, along a an invisible vortex of high-speed air trailing dozens of meters behind the motorcycle, like the foamy currents of a fast-moving river after a large rock.

Behind the rider air is in pure chaos. The rear wheel is churning up air like a blender, sucking it in on one side and blowing it out another, while the hot exhaust gasses are blasted out backwards, adding much heat energy to the random mix. No matter what shapes exist behind the rider’s back, the air has been chopped up and spit out like a felled tree through a wood chipper.

Motorcycle Aerodynamics 101 : It’s Not What You Think It Is

Sharp, pointy, and brown : motorcycle aerodynamics in the 1970's
Sharp, pointy, and brown : motorcycle aerodynamics in the 1970’s

In most production motorcycle projects, maintaining rider comfort is the aerodynamic goal. Fuel burn being managed by a useful combination of gearing, engine torque and consumer indifference, the aerodynamicists can concentrate their efforts on reducing wind noise, buffeting, and using laminar flow to suck heat away from the engine and exhaust so as to not cook the occupants.

Heat management is a major aerodynamic challenge. Even in this era of low-cost supercomputing and extremely precise computational fluid dynamics (CFD) simulation software, the sheer number of possible circumstances cannot always be taken into account prior to releasing a new model into the marketplace.

The first generation Yamaha FJR1300 tourer was notorious for sending searing hot radiator exhaust up between the frame and fuel tank, which made for pleasant heating in cool weather and terrible discomfort in warmer climes. It was a flaw not detected in testing (done in northern Europe and Japan in autumn), but later fixed with the addition of one internal plastic duct. On the original Ducati Multistrada, the under seat mufflers caused the plastic passenger seat unit to melt due to insufficient air flow when the passenger’s legs were present.

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In racing, the main objective of aerodynamic refinement until this year has been heat management. It may seem an unlikely place to find speed, but when Yamaha made slight changes to the size and shape of the radiator outlet on the 2011 M1 MotoGP design, they reduced coolant temperatures by 17 degrees. Lowering the temperature of a racing engine can translate into more horsepower, increased engine reliability or reducing the amount of coolant you carry, which means shaving kilograms of weight. It all adds up.

Take It To The Track : Relegated to the Dustbin of Time

By far the oldest use of aerodynamics in competition motorcycles was the fully enclosed motorcycle body that existed from just before the second world war and lasted until the late 1950’s. Epitomized by models like the NSU Sportmax and Moto Guzzi 500 V8, the fully covered front wheel treatment suggests amazing reduction in drag and increased speed for the same horsepower.

"She may not look like much, kid, but she's got it where it counts." 1957 Moto Guzzi 500 V8
“She may not look like much, kid, but she’s got it where it counts.” 1957 Moto Guzzi 500 V8

Named dustbin fairings by the British (who has a waste bin for dust? Editor ‘Arris, can you answer this…? Yes, England is very dusty – ‘Arris) were a brief diversion in the evolution of motorcycle aerodynamics. First experimented with in the prewar years by ambitious Nazi government backed manufacturers in Germany like NSU, Zündapp and BMW, they crafted smooth, bullet-shaped bodies over conventional motorcycles and discovered, unsurprisingly, that this made them much faster without the need for more power.

The experiment continued into the post-war era with NSU, Giilera and Moto Guzzi leading the charge. Forbidden by the victorious Allies to manufacture aircraft, Germany and Italy’s aerospace engineers turned to making motorcycles, applying their expertise in aluminum construction and aerodynamics to boast some of the most slippery race bikes every conceived. Guzzi possessed a wind tunnel on site in their factory, taking full advantage to minimize drag.

OldGilera3

Which they did. In a straight line.

The point must be emphasized because the very thing that made dustbin fairings so efficient at cheating the wind head-on also made them dangerously unstable in other conditions. This was because they place the center of air pressure (CoP) ahead of the centre of gravity, which introduces a strong yaw, or left/right turning force on the motorcycle.

Besides reducing turbulence and protecting laminar flow, the third pillar of safe motorcycle aerodynamics is controlling the center of pressure (CoP). CoP is vital to maintaining control, as anyone who has ever carried a large flat object on a windy day can attest. When the wind blows toward you, all is well. When the wind gusts from an angle, or broadside, not only is the person pushed sideways but is pivoted to a side as well. If the flat object is mostly behind you, your body acts like an anchor as you go forward, reducing the amount of effort you need to keep going straight. If more of the flat object is ahead of you, as in the dustbin fairing motorcycle, the steering effect is much greater.

Dustbin fairings were banned by the racing authorities on this basis in 1958, and fully enclosed street motorcycles have similarly remained mostly just a curiosity. With ever increasing power and speed potential, the repercussions of losing lateral stability in a sudden gust of wind are too great to risk.

PJG_6436 STONER on way to pole
Holes drilled into the fairings reduce the lateral pressure exerted by strong side winds at Philip Island in Australia.

Even conventionally bodied, full fairing race bikes can experience this phenomenon. The circuit at Philip Island in Australia is renowned for strong winds coming off the nearby ocean, which create lateral air pressures significant enough that racers reported fighting too hard to turn in, as well as unacceptable levels of instability on the straights.

Teams drilled holes into the largest surfaces of the fairings to reduce lateral surface area. The results are bikes that gain some drag in straight line travel (the holes act like vortex generators) but are much more manageable in cross winds and during roll transitions in corners. With motorcycle aerodynamics at least, it is a zero sum game. You may find gains in one area but have to pay for them elsewhere.

She Blinded Me With Science

The conventional thinking of most people regarding aerodynamics is to simply cover everything on the motorcycle in round, flowing bodywork. This is regressive thinking that only treats the areas that can be influenced: everything before the rider. While grand prix and production street bikes have bloated fairings to shroud occupants, raised huge humps in the tail unit, blended mirrors, and flushed in turn signals, the results lower the drag coefficient only slightly.

The secret to significant improvement lies in air management around the rider. Since covering the rider up is not a possibility, then the next best thing is to deploy aerodynamic devices to manipulate the air flow into behaving as though the rider wasn’t there.

2016-motogp-tech-2

2016 will go down as the year that aerodynamic advances finally came to motorcycle racing, because of their sudden popularity in MotoGP. Since the beginning of the season, Yamaha, Honda and Suzuki have followed long time aero champion Ducati in the adoption of winglets and vortex generators (VGs) as speed weaponry. The free test after the Spanish grand prix revealed lots of new aerodynamic devices from many brands, including strakes, turbulators, and pressure ducts.

These devices, borrowed from a century of aerospace experience, deliberately cause small but controlled turbulent channels that, although adding a small amount of drag, extend an invisible air wall over the rider’s body that the laminar air flow can continue to follow smoothly. In effect, they create an air-fairing.

Those tiny vertical tabs deliberately cause a long, high energy air vortex that helps the rest of the clean air flow stay laminar long past the wing.
Those tiny vertical tabs deliberately cause a long, high energy air vortex that helps the rest of the clean air flow stay laminar long past the wing.

The use of such devices is not new. Ducati has experimented with vortex generators since 2007. Aprilia developed VG technology for the first RSV Mille and deployed it successfully in world superbike racing as far back as 1999. Gilera’s ill-fated 250cc grand prix machine featured strakes (an air fence placed in the rear wheel area) in the early 1990’s. All were exhaustively developed in wind tunnels and with universities, and all contributed to performance.

The Numbers Game : Cost Versus Gain

The advanced, active and passive aerodynamic devices seen in MotoGP and in past years are about to disappear again. The governing body for motorcycle racing has deemed that winglets, VG s and other such devices are to be banned, in the name of saving money and preserving competition. It is the right decision.

The uppermost winglets are there to generate downforce (or negative lift), the lower black triangles are vortex generators to extend laminar air flow. Image : asphaltandrubber.com
The uppermost winglets are there to generate downforce (or negative lift), the lower black triangles are vortex generators to extend laminar air flow. Image : .asphaltandrubber.com

Aerodynamics is a science that requires unspeakable budgets to study properly. By far the most expensive line item on any vehicle development program is wind tunnel time. The tunnel itself may be rented from any number of national laboratories or university research centers, but the technicians and engineers required to operate it, gather and then analyze the data is are hard to find and in high demand.

Then there is the engineering needed to leverage the findings into practical solutions that can be executed. And then there is the testing, in different places, with different temperatures, on different surfaces. When aerodynamics is what separates the winners from the rest, then only the biggest and richest manufacturers will be able to compete. This is what has happened in formula one.

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Motorcycles would certainly benefit from real advances in aerodynamic research, but it’s just not needed or wanted by the buying public. The best-selling categories of motorcycle are cruisers and naked street bikes, because people the world over like the way they look and their functionality is good enough.

The motorcycle is a reflection of the rider’s personality, and very few of us are desperately seeking efficiency. Until consumers demand a ten fold increase in energy efficiency from their bikes, don’t expect the aerodynamic advances seen elsewhere to appear on your next motorcycle.

21 thoughts on “Motorcycle Aerodynamics”

  1. The force known as drag is a product of the drag coefficient times the cross sectional area of the face of the vehicle and rider. A suburban may have a lower drag coefficient than a bike but it has much larger frontal area and therefore more total drag force to overcome. The drag coefficient is just a rating of the slipperyness of a shape but the size of the shape is needed to calculate the net overall drag in units of force.

  2. The ST1100 I had at one time was terrible in side winds and also blurred my vision from turbulence from the windscreen. The ST1300 I have now does neither but throws tons of hot air on my lower body. Progress

  3. Force is pressure (function of velocity^2, air density) * cross sectional area.
    So motorcycle small frontal area reduces force compared to cars; even with poor Cd.

    1. Alan G

      Not sure what you are saying…

      Small frontal area has a significant positive effect on reducing overall drag coefficient by reducing friction drag, but the main problem with motorcycles as stated is the breakup of shape, edges, and uncontrolled pockets of low pressure in and around the wheels and rider that add so much “parasite” drag. Even a poorly designed closed shape, like a blocky car or SUV have a much neater wake for air after it passes by than a racing motorcycle.

      1. Here’s an example that shows the importance of frontal area. My 2002 Saab convertible has 200hp and does 155mph. A new R1 has 200hp and does over 180. Frontal area.

    2. I was going to say the same but I realized that CdA is not the same as Cd.
      CdA is an automotive coefficient that corresponds to Cd x Area.
      from wikipedia: “The product of drag coefficient and area – drag area – is represented as CdA (or CxA), a multiplication of the Cd value by the area.”

  4. There used to be (and maybe still is) a motorcycle racing website where, among other things, the guy insisted that positioning the exhaust outlets in a particular way created significantly less drag and decreased lap times. These were the same people who advocated full-throttle, fully loaded seating in of piston rings. Any thoughts on how directing the exhaust could tune the aeordynamics?

    1. Yeah.

      See “Red Bull Formula One Team, Rule Changes Due to Adrian Newey’s Exhaust Blown Diffusers”.

      Really?

        1. Sorry. For some reason I assumed you were an F1 fan,
          and I was bemused about one not knowing this.

          Again, my blindness has led to asinine behaviour.

          Apologies, sir.

          1. I loved F1 until the end of the turbo era. Having said that, I was working for Piaggio in Italy in 1999 when Eddie Irvine almost won the title and that was pretty amazing

    2. “These were the same people who advocated full-throttle, fully loaded seating in of piston rings.”

      That got me thinking of the articles in the 80’s in “Hot Rod” and “Car Craft”
      magazines. At the time, the engine builders of the day all espoused this
      technique. Mind you, they were drag-racing enthusiasts and the benefits and
      obvious drawbacks are plain to see. To them, the engine is a tool to be
      re-built every 1/4 mile lap. I seem to see their philosophy as rather pragmatic,
      and not unlike that of Formula One builders of the day (sorry; do not have
      much knowledge in today’s secretive F1 world).

      The thing these folk had over us, was talent, skill, parts, money and wherewithal. Would any of us do this to our new Suzuki/Chevy?

      As one may be aware, my area of ‘know my gd job’ is electrical, so
      please take all of this with a large boulder of salt.

  5. Michael, any thought on how wheel design is influenced as well ?
    The experiments in motorcycling (and bicycling) have indicated that disc wheels influence aerodynamics at the expense of stability.
    Is there any future in examining this ?

    1. The realm of CFD goes as deep as you want, so yes there is a lot of work being done on wheels but not in the direction you might think.

      Disc wheels just suck the same way dustbin fairings suck. On an indoor velodrome they work great, outside they cause lots of instability and negative gyroscopic effects.

      Like modern formula one style air management, the future of aero wheels is subtle devices on the tire sideway and rim profile that manipulate the airflow to bypass the huge churning blender of spokes, brakes and hubs.

      http://www.roadandtrack.com/new-cars/car-technology/news/a27052/yokohama-puts-spoilers-on-tire-sidewalls-for-better-aerodynamics/

      http://triathlon.competitor.com/2011/08/gear-tech/ask-aaron-the-aerodynamics-of-wide-rims_36923

      http://www.zipp.com/technologies/aerodynamics/ablc.php

      1. I like the idea of dimples on the wheels and spoilers on the tire sidewalls, but motorcycles experience significantly greater lean angles than bicycles and are far more subject to side forces than cars.
        This might limit such applications.

        Not to mention that keeping them clean would be a b*tch !!! 🙂

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