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MOTORCYCLES AERODYNAMICS

Updated on April 29, 2012


Summary

In this work it’s approached the issue of aerodynamics on motorcycles.
It consists of an introduction which is a brief historical perspective of developments in the study of aerodynamics in motorcycles and the most important parameters in studying the same subject.
It is then made a detailed approach of each important topic to understand the aerodynamic phenomena that exist in motorcycles, such as forces, areas and rates.


Introduction

It was only in the late 20s of last century that began the study of aerodynamic motorcycles as a tool to improve their performance and even stability. Hitherto the only concern to improve the performance of a motorcycle centered on the engine. What encouraged the beginning of the aerodynamic development was the attempt to increase the values ​​of world speed records.

Despite several attempts to improve aerodynamics through the introduction of partial fairings, it was only in 1936 that the BMW-R5 equipped with a 500cc engine mounted a full fairing, greatly improving stability at high speed.

Despite the advantages of implementing a full fairing on motorcycles was only in the mid 50s it began to observe the first bikes with full fairings racing world championship speed.
In 1976, BMW again made ​​the difference, launching the first production bike with full fairing (R 100 RS).
In a motorcycle, but in any other vehicle, the shape, the quality of the contact surface and the front area are critical in determining its aerodynamic behavior. However on motorcycles there is another important factor, the size and position of the driver, passenger and luggage.
Given the above factors, the speed at which the motorcycle moves and knowing the force needed to overcome aerodynamic drag is possible to obtain the drag coefficient (Cd or Cx), indicating that the vehicle has a better or worse aerodynamic behavior .
Motorcycles in the value of Cd can vary greatly according to this or not fairing. This value can be enhanced or impaired by the presence of the driver.
In addition they aim to improve the Cd value of the motorcycle, the fairings have other functions:
• driver and passenger protection against external agents (rain, wind, pollution, etc..).
• Provide high-speed stability.
• orient the flow of air towards certain internal parts of the motorcycle.
• Improve the visual appearance of the vehicle.
Important notes:
• This work is considered, sometimes the air is on the move and body immobilized. Although this is not the case, the effects are the same. This change is being adopted to better understood phenomena.

Flow

The air flow through a motorcycle can be analyzed in two distinct areas, the flow on the outside of the motorcycle and an outlet on the inside.
The exterior is caused, obviously, that the vehicle is in motion. However the interior is purposely sent with the main objective to provide the cooling system and the admission of fresh air.
The good or poor outcome of these two components of the flow affect the value of the drag coefficient of the motorcycle.


Inside flow

The concept of interior air Flow does not apply to all motorcycles, because there are some models without fairing in which the engine is exposed. However the vast majority of motorcycles today, especially those with higher performance (sports and tourist) present the need for the existence of that flow.
A major function of the mass of air directed into the moto concerns, so that with the need for cooling the engine (oil cooler and / or radiator coolant) and other elements such as pipes exhaust current rectifiers, etc.
This airflow is directed inwards under the influence of the motorcycle front fairing and front areas of the side fairings, being introduced from behind by the front wheel.
The output is generally carried out by side and in the area of the basement-to-rear wheel, as seen in Figure 2. The latter zone evacuation of hot air, however, has an inconvenience connected with the fact that the airflow generated by rotation of the rear wheel counter its output. To reduce this effect some manufacturers place a fender in the rear wheel, as seen in Figure 3.
There are, sometimes, each air inlet fairing at the rear serving to conduct fresh air for cooling electrical components or exhaust silencers when they are mounted underneath the seat (Figure 2).

In order to increase the volumetric efficiency of the engine some manufacturers exploiting the increase in pressure at the front of the motorcycle when it is in motion to compress the air from the air cleaner housing. See Figure 4.

In the rear of the motorcycle there tend to occur detachment of the boundary layer and the formation of an area of ​​depression. Thus the fact that the exhaust port to place at this site may help to reduce this effect, filling this area with gas at higher pressures than atmospheric.
In terms of this positioning operation of the engine also shows some advantages, because the fact that there is tendency for formation of localized low pressure at the outlet of the exhaust, increases its output speed.
These two reasons have led manufacturers to place the exhaust outlet beneath the rear fairing (Figure 5), to further optimize its effect.

External flow

Next we will present the main elements of the motorcycle that influence the flow outside influences having some positive and others negative effect.
Ideally, the bike was completely covered by an aerodynamic fairing, as the bikes built to beat speed records, but this option has major disadvantages in terms of use making it very impractical.


Front wheel

The front wheel is an element that tends to worsen the aerodynamic performance of the set, not because of its shape but mainly due to this is in rotation, this effect can be seen in figure 6.

The best way to minimize the problems with the front wheel would be putting a fender involving the largest possible amount of its front surface and direct the runoff to the sides and front of the motorcycle. However, this solution has severe drawbacks: high winds with instability and poor cooling of both the front disc brakes and the motor (situated in the rear wheel). So the manufacturer has to find a compromise.


Side, front and rear farings

In the first motorcycles with some sort of fairing, it was usually mounted (optional) protection for the chest and shoulders of the pilot. These elements were intended to protect the driver and often worsened the aerodynamic characteristics, and as an aggravating factor hindering control of the motorcycle. This is due to their assembly being made ​​into elements of the front axle, in particular elements of the steering or suspension. This accessory is still widely used, as shown in Figure 8, devoid of motorcycle fairings.

Currently the majority of medium-sized motorcycles and high performance have at least a front fairing, which is applied to elements fixed to the frame. However, the most common is the use of integral fairing.
This type fairing may be divided into two types, taking into account the main purpose for which they were constructed:
1. Cowlings with the basic purpose of protecting the driver and sometimes passenger external elements being used in large tourist bikes.
2. Fairings in order to maximize the value of Cd, being employed in sports and motorcycles where the rider to have some protection, will have to adopt a position of "attack".

Despite these two distinct types of fairings, its configuration is basically identical, with the main elements:
1. Fairing (see A in Figure 9) it supports the majority of forces arising from the displacement and guiding the air flow to the sides and certain internal organs, as previously described. In terms of protection for the driver that the cowling is most important. Emphasizing the importance of the windshield, responsible for the diversion of airflow from the shoulders and head. Existing including glass front, mounted on motorcycles tourism, which have regulation to adapt to the taste and stature of the pilot.
2. And lower side fairings (see B in Figure 9) are mainly intended to guide the flow in the lateral and bottom zone (shielding of the pilot flow and reducing the formation of recirculation zones). The sides also act as areas of hot air output from the engine and radiator. We can also observe small air inlets at the sides in order to more cooling zones of the rear engine (clutch, for example).
3. Rear cowling, is intended to guide a transient, flow of air from the side but especially the legs, arms and back of the pilot. Thus avoid trying again, the formation of harmful recirculation zones in the rear of the motorcycle.


Prejudicial flow accessories

In general, any element that is an geometric accident affects the flow and most of the time also increases the frontal area of the bike.
As an example of these elements have (see Figure 10): the mirrors (Cd increased in the order of 0.025), the change of direction signals (blinkers), exhaust silencers etc..
In addition to these elements that accompany the bike home, there are others who applied have the same effect as accessories. One example is the side bags and rear (top case).

The bike builders tend to decrease the size of these elements, giving them more aerodynamic shapes and even change its position. We have an example that even the placement of the front turn signals on the front of the mirrors and the exhaust muffler positioned underneath the rear fairings.


Front motorcycle area

The front area of a motorcycle, like any other vehicle, corresponds to the projection of its front view. However, in the case of motorcycles it is necessary to enter into account also the discovery frontal area of the driver. As shown in Figure 11.

Thus, to analyze the aerodynamics of a motorcycle is necessary to determine the frontal area of the motorcycle further frontal visible area of the pilot when the motorcycle is positioned opposite.
However, this second component may vary widely, depending on the driver is higher or lower, more or less bulky. Even keeping the same driver to the frontal area can vary dramatically, depending on whether it adopts a position of "attack" (Figure 12 - A) or a more "tourist" (Figure 12 - B).

When adopted the first position the frontal area of the pilot reduces visible enough, logically reducing the frontal area of the set.
In position "tourist" the frontal area increases as the torso of the driver is in a more vertical position. Also, the arms and legs are positioned furthest from the vehicle, causing an increase in the front area of the assembly.


Drag force

For a vehicle being set in motion and to this motion keeps is necessary winning some forces. These forces can be grouped into two types: mechanical forces (tires and transmissions) and aerodynamic forces.
The aerodynamic force at low speed has little influence on the vehicle, however as the speed increases it increases exponentially and eventually becomes the most important force to win to keep the vehicle moving.
In figure 13 it is possible to observe the sample forces the evolution of resistance to movement through the power consumption required for the win.

The aerodynamic force, commonly known as "drag" (D) is calculable by the equation [1], but these forces, when designing a new vehicle, are usually measured using the experimental process using a wind tunnel.
It is employed a scale model with sensors that detect the force applied by the displacement of air masses. After collecting the data are certain values ​​that corresponded to the prototype in full size.

D =V * (ρ * V² * Cd * S) / 2 [1]

which:
- V - vehicle speed [km / h];
- Ρ - fluid density [kg / m²];
- Cd - drag coefficient;
- S - frontal area [m²].

Having regard to Figure 13 and the equation [1] can understand that a motorcycle with an efficient aerodynamic and with a relatively low engine power may have a relatively good performance at high speeds and / or a more favorable fuel consumption.
In other words, if we can reduce the coefficient of drag (improving the design of the motorcycle fairing) and frontal area, we will have a lower total force of resistance. In this case remains the same available power and to reduce drag, which is a speed increase and a decrease in consumption.


Drag coefficient

The drag coefficient (CD) is a dimensionless value that defines the performance of a solid body traveling in a fluid. Cd indicates the shape of a body is furthest from or closest the form of a body leak.
The value of this coefficient is influenced by two factors:
1. Quantity and intensity of the air recirculation zones defined by the shape of the body;
2. Friction between the surface of the motorcycle and the air, which is a function of quality surface finish.
The first aspect is what most influences the value of Cd, because the conventional motorcycle has a form called a little body fairing. That is, there are many separations between the vehicle and the air layer existing at the surface, this layer which is known under the name of the boundary layer.

By observing Figure 14 is easy to understand that the more continuous the smoother body and variations in form, is the most favorable value Cd of the body. In fact, more streamlined shape for a land vehicle is the same as a drop of water has to slide a smooth surface. If you look carefully the first fairing motorcycles that were thought to beat speed records had a very similar, as well as the first racing bikes with fairing (figure 1 and 15).

As mentioned above, when a bit body has a streamlined shape there is formation of recirculation zones of air, which causes, in some places pressures lower than the front. These pressure differences are contrary to the motion and consequently a greater force is required to overcome aerodynamic drag.

Again, as in the determination of the front area, the size and position of the pilot influence the value Cd of the set (motorcycle and rider). An example of this is shown in Figure 16, which in this example demonstrated a negative influence on the coefficient of drag. In this type of bike the driver positions the body in a posture very upright, caused the departure of the boundary layer in the area of the back.
However, sometimes the existence of the driver causes a more continuous flow, as can be seen in figure 17. In this case the presence of the motorcycle driver attenuating the effect of decoupling the bed close to the boundary surface of the assembly pilot/machine, thus reducing the possibility of recirculating air.

However, a motorcycle with the respective driver has relatively high values ​​of Cd, when compared with other vehicles. For sports bikes values ​​are around 0.30 to 0.60 and that a car values ​​are around 0.35.
Nevertheless motorcycles have an advantage, the reduced front aria. So if we compare the S.Cd factor (S - projected frontal area), we found that the bikes are clearly at an advantage


Supporteffect

The shape of the fairing of the motorcycle generally cause an effect of support, ie, an effect similar to that found in the wings of an airplane. This same effect can be observed in figure 19, where it is visible mark of the difference between the front wheel 80 and 200 km / h.

This effect may lead to instability, therefore, one would think that when they built the fairing for a motorcycle they were designed to bring about an opposite effect (negative support). This would, in displacement, the vehicle tends to be pushed against the ground. However, this would increase the value of Cd, increasing the resistance to displacement and degraded performance at high speeds.
Thus, this issue must be resolved through a compromise, so as not to jeopardize the control of the motorcycle and without affecting too much the value of Cd The way to control the effect of lift is done by the manufacturers, through design the front fairing and / or placing of reverse wings (contrary to the effect of the wings of an airplane) in side fairings, as shown in Figure 20.


Contact surface quality

Another important factor that influences the value Cd is the surface quality of the vehicle. In this case, it is important to the driver's clothing. The clothing size and material which is manufactured have an effect very important in the amount of Cd, since the body of the conductor has a large contact area with the flow. This takes the more important factor when it comes to a motorcycle fairings without in that situation the contact area between the pilot and the air is significantly higher.


Conclusion

The aerodynamic study on motorcycles is not very different from the study for other land vehicles, however there is a very important component in other vehicles is not taken into account. This component is, of course, of the motorcycle and in some cases also the passenger.
In thinking about bikes think in agility, she being mainly due to low weight, but it is also important to have an aerodynamic evolved, especially at high speeds.

References

[1] Gaetano Cocco, Aprilia, “Motorcycling and Technology”

[2] Arias Paz, “Motocicletas”

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    • profile image

      Martin 2 years ago

      In a lot of sources the equation [1] is noted as

      F = (ρ * V² * Cd * S) / 2 ; In this context equation noted here seems invalid

      D =V * (ρ * V² * Cd * S) / 2 ; Please, explain if I'm wrong.

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