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Aerodynamic Considerations

Aerodynamic factors, considered carefully, can improve many aspects of a vehicle. Some key aerodynamic considerations have been summarised here.

[(vi) refers to Road Vehicle Aerodynamic Design by R.H. Barnard]

With an object moving through a fluid, the wake is extremely significant. When considering family vehicles, the nature of the vehicle’s rear, in three dimensions, can make the difference between a low or a high coefficient of drag (Cd).

Improvements at the front can be made by ensuring the ‘front end is made as a smooth, continuous curve originating from the line of the front bumper’. On normal two and three box shapes, drag is often caused by high pressure just upstream of the front windscreen, ‘often with a separation bubble of recirculating air at the base of the screen’. The magnitude of this effect depends upon the windscreen ‘rake angle’. Making the screen more raked (ie. not as upright) ‘tends to reduce the pressure at the base of the screen, and to lower the drag’. However, much of this improvement arrives because a more sloped screen means a softer angle at the top where it meets the roof, keeping flow attached. Similar results can be achieved through a suitably curved roof.

Design in plan as well as profile, is significant. ‘Curving the windscreen in plan view modifies the flow patterns considerably … which reduces the extent and intensity of high pressure.’

The A-post is also an issue: ‘A strong outward cross-flow can occur towards the edges of the windscreen, tending to produce separated vortices around the A-posts.’ These effects can be minimised by smoothing the form of the A-post and increasing the curvature of both the A-post and the screen. Smoothing the transition from the body to door mirror is also significant as it can otherwise be a major source of drag and wind noise.

At the rear of vehicles, the ideal format is a long and gradual slope. As this is not practical, it has been found that ‘raising and/or lengthening the boot generally reduces the drag”.

Results of research state that drag due to rear slope angle will be at its ‘peak at 30º and minimum at around 10º’.

Increasing the curvature of the roofline will also reduce the drag coefficient. Benefits are gained by bringing the roof line down at the front and rear. Simply ‘bulging the roofline up’ however, may cause such an increase in frontal area that any gains may be negated.

In plan view, rounding corners and ‘all forward facing elements’ will reduce drag. Increases in curvature of the entire vehicle in plan will usually decrease drag provided that frontal area is not increased. ‘Tapering the rear in plan view’, usually from the rear wheel arch backwards, ‘can produce a significant reduction in drag’. Under the vehicle, a smooth surface is desirable as it can reduce both vehicle drag and surface friction drag. ‘For a body in moderate proximity to the ground, the ideal shape would have some curvature on the underside.’

In (vi), the author lists the following significant areas for thought when attempting to design a typical car (not a sports car or commercial vehicle):

  • Smooth unbroken contours with favourable pressure gradients as far back as practical should be used.
  • Strongly unfavourable pressure gradients at the rear should be avoided; some taper and rear end rounding should be used.
  • The form should produce negligible lift.

A If a hatchback configuration is required, the backlight angle should not be in the region of 30º, and if a notchback (saloon) is to be used, the effective slope angle (ie. the angle of a direct line between the roof and the highest, most rearward point) should also not be in the region of 30º.

  • The underbody should be as smooth and continuous as possible, and should sweep up slightly at the rear,
  • There should be no sharp angles (except where it is necessary to avoid cross-wind instability).
  • The front end should start at a low stagnation line, and curve up in a continuous line.
  • The front screen should be raked as much as is practical.
  • All body panels should have a minimal gap.
  • Glazing should be flush with the surface as much as possible.
  • All details such as door handles should be smoothly integrated within the contours.
  • Excrescences should be avoided as far as possible; windscreen wipers should park out of the airflow.
  • Minor items such as wheel trims and wing mirrors should be optimised using wind-tunnel testing.
  • The cooling system needs to be designed for low drag.

Although aerodynamic concerns are not as strong in this vehicle as they may be in a sports car, for example, the basic principles outlined here should be observed throughout the design process. Energy efficiency can be improved with low drag and low levels of wind noise improve passenger comfort.

Aerodynamics

Image courtesy & © Ford Motor Company

Aerodynamics is a highly refined science that vies for position with other key vehicle design considerations such as styling and ergonomics. It’s importance with respect to the operating efficiencies of a vehicle is undisputed but manufacturers must steer a balanced path between the push an pull of the many other aspects of a car necessary to sell it to the consumer.

Aerodynamics started life as much as an art as a science. Early experiments used fish as the inspiration. Their sleek form was considered important to facilitate fast movement, but the precise details were not yet understood and developments were largely based on trial and error. It was as a result of this approach that the ‘teardrop’ form was conceived.

Aerodynamic Considerations

A summary of key principles and basic rules to follow in order to improve aerodynamic efficiency when designing a vehicle.

Aerodynamics - pressure sensitive paint Ford GT Aerodynamics Wind Tunnel Testing
Pressure Sensitive Paint Aerodynamic work on the 1960s Ford GT programme Wind tunnel testing for modern vehicles

Automotive Design

Welcome to the design section of Car Design Online. This area includes information on most of the pre-production elements of automotive design. These include the creative elements of vehicle development, such as sketching and modelling, as well as other important considerations, namely aerodynamics and ergonomics.

Aerodynamics

In this section we take a look at the science of aerodynamics and how manufacturers use CAD and wind-tunnel testing to perfect new vehicles.

Ergonomics

An increasingly important aspect of automotive design, this section on ergonomics and anthropometrics looks at design for human form and behaviour.

Modelling

Whether virtual or actual, realising a design in 3D has always been a critical element of vehicle development. From clay to CAD, we look at the different methods and practices.

Sketching

From a designer’s initial idea to fully resolved renderings; we take a look at how ideas are realised in 2D.

Pressure Sensitive Paint

With ever increasing demands for refinement in vehicle design, new methods are developed to provide a greater level of detailed information. Special pressure sensitive paint is now used in the wind tunnel to graphically show levels of air pressure on a vehicle.

The Process

Two different images are obtained, one at normal room air pressure (wind-off) and a second in which the wind tunnel is running (wind-on) at a desired test speed. These differences in color, from wind-off to wind-on, are used to calculate surface pressure.

A bank of blue lights illuminate a 2002 Ford Thunderbird that has pressure-sensitive paint applied on the driver’s side window. The car and lights are in a wind tunnel at Ford Motor Company’s Dearborn Proving Ground. Ford researchers have developed a computerized, pressure-sensitive paint technique that measures airflow over cars, shaving weeks off current testing methods. A digital camera near the blue lights captures this information and feeds it into a computer, which displays the varying pressure as dramatically different colours on a monitor.

The images obtained from tests in the wind tunnel are captured on computer. They can then be used to study air flow patterns across a vehicle, highlighting areas of possible refinement or improvement. Additionally, actual data from a production ready model can be compared with pre-production computer predictions which can in turn help improve the accuracy of the early design stages.

Mazda’s Renesis Rotary (Wankel) Engine

The rotary engine was conceived by Felix Wankel in Germany in 1926 with the first functional prototype not actually running until 1957 – this was largely due to the Second World War and the fragmented post-war Germany.

Mazda’s Renesis rotary engine can be seen here with the basic stages of rotation illustrated.

 

Mercedes NECAR Hydrogen Fuel Cell Car

Mercedes-Benz (DaimlerChrysler) believe that fuel cell vehicles offer the best options for sustainable vehicle propulsion. Since the 1990s, DaimlerChrysler and its affiliated companies have developed and demonstrated hydrogen and fuel cell technology for automotive applications. Researchers and engineers have been working toward practical implementations of this technology since the early nineties. DaimlerChrysler presented its first fuel cell concept study for the NECAR series in 1994. Since then, 20 different vehicle prototypes with fuel cell drives have been developed and tested. The vehicles range from the Mercedes-Benz A-Class and the Jeep Commander to the NEBUS.

In 2001, DaimlerChrysler presented the “Natrium” (the Latin word for sodium), which demonstrated an innovative and unconventional method for storing hydrogen: on board a minivan, hydrogen was generated directly from a white salt – sodium borohydride.

In May 2002, the NECAR 5, running on methanol travelled 3000 miles across the US to prove the technology to the World at large. Practical use came in 2004 when the first F-cell, based on the Mercedes-Benz A-Class began use under normal conditions around the world. Additionally, bus and commercial vehicle fuel cell vehicles are in operation in many countries.

The A-Class lent itself well to use as a fuel cell vehicle. It is believed that the A-Class was originally intended to house electric propulsion and was hence designed with space below the passengers. This space is utilised in the NECAR for the fuel cell system components – especially energy storage in the form of hydrogen tanks and batteries.

Battery technology continues to develop and improve leading to increasing efficiencies and space savings. Future generations of fuel cell vehicles are likely to use smaller battery packs, making packaging simpler. However, hydrogen is already stored in compressed form and offers very little room for space saving in the future. Instead, improvements in the fuel cell process will be the key to reducing the need for fuel and hence the need for fuel storage.
Images courtesy & © DaimlerChrysler

Vehicle Intelligence

Basic computer systems have been used in cars for several years. Most recently, they have been used to offer more advanced on-board information systems. Central processors have for some time been used to control engine management – ensuring smooth running, good fuel efficiency and performance. They have, however, performed relatively simple tasks and are not safety critical.

More advanced use of information technology in cars will lead to an increase in user-orientated systems. Systems will continue to be developed to control and manage the mechanical elements of a vehicle – such as engine, suspension and braking – but the most noticeable changes will take place inside.

Displays & Interfaces

One area of vehicle intelligence that has already raised its head above the parapet is that of user interfaces. It is now possible to display all the information a driver or occupant may want without the use of three-dimensional analogue displays. Above and beyond that, it is now possible to cater the information that is displayed to the particular preferences of a user and to further vary that according to changing conditions – such as traffic, weather, time of day etc. It is envisaged that information relating to specific traffic conditions could be relayed to a driver in motion. Such systems would not only indicate congestion problems but perhaps warn of large numbers of pedestrians, accident black spots, approaching emergency vehicles – even opportunities for refuelling should the tank be empty.

Importantly for designers, the manipulation of displays and interfaces will allow the use of new aesthetic concepts, the simplification of dashboard forms and the ability to incorporate a large number of controls into small areas.
Design considerations

  • Controls can take the form of touch sensitive displays, allowing greater use of smooth surfaces.
  • Using multi-mode and menu systems, digital displays can eliminate the need for dashboard space for every control.
  • Using technologies such as E-Ink, it will be possible to place displays and controls over entire surfaces; this offers opportunities for 2D graphic aesthetics (like computer screensavers) to be developed as well as the creation of large interactive work surfaces.

Environmental Interaction

Many location based information systems are already available and use satellite positioning coupled with pre-stored data about certain locations as well as live traffic information. With the increase of wireless technology, and ultimately its ubiquitous presence, it will be possible for a vehicle to simply collect (and pass on) information as it moves through different areas.

Vehicle Safety

A high-speed camera is used to capture visual information during crash tests. Image courtesy & © Volvo Car Corporation

Safety is an increasingly significant element of vehicle design. The earliest automotive legislation related to safety and still accounts for the bulk of all vehicle regulation.

Today, there are two main forces driving improvements in vehicle safety. The first is the consumer, the car buyer who wants a vehicle that will protect them and their family in the event of an accident. The second is the legislature, the organisation responsible for laws and regulations; they not only want improvements in safety for occupants but also for third parties such as pedestrians and other road users.

With sales of premium brands increasing as value brands decline, safety is often a large part of the premium product. In addition to Volvo, most brands now market their safety credentials to the consumer; not least Renault who have invested substantial resources in achieving high EuroNCAP ratings for the reasons outlined above. Safety is a core component of vehicle design.

EuroNCAP Rear Collision Tests

Principles

Some basic concepts in safety design for vehicles.

Passenger Safety

For decades, vehicle safety has centred around the occupant(s). This section takes a look at the significant aspects of passenger safety.

Pedestrian Safety

A relatively new field, with much yet to be discovered. This section covers the new design approaches to dealing with pedestrian-vehicle impacts.

Technologies

Simple and complex products and systems work together to make up the full portfolio of safety elements on a car. In this section we explore the technologies that are key to safety improvements from crash

Resources

SAVE-U – Sensors and system Architecture for VulnerablE road Users

Safety Principles

There are two main routes to improving vehicle safety. Firstly, there is prevention – keeping people, objects and vehicles away from each other and out of harm’s way. This is achieved by combining many hundreds of factors such as driver education, design of pedestrian crossings and requirements for vehicle performance and maintenance. It is this approach that brought about much of the earlier vehicle legislation that addresses lighting, turning indicators and basic demands on components such as windscreens, mirrors and tyres.

As traffic volumes increased, so did the rate of accident and injury. This lead to further requirements and laws for the design of vehicles as well as a rethink (in most countries) of speed limits and road networks. It begun the second stage of safety design – passenger or passive safety.

Nils Bohlin of Volvo invented the modern seat belt in 1959. This was the three point seat belt and made such a difference to crash safety that it was included as a basic requirement to install belts in cars in some of the earliest European legislation – although compulsory use came much later. In effect, this was the first in a long line of developments from Volvo to improve passenger safety; an aspect of design that most other manufacturers cared little for until the 1990s.

Nowadays, safety is considered in many more ways than ever before – from the structural performance of a vehicle in impact to the ability of a driver to see clearly past an A- or B-post. Increasingly, pedestrian impact is also being considered.
Visibility

Preventive safety is about designing a vehicle that can be easily seen by other road users, a vehicle that is easy to see out of and a vehicle that presents a driver with all the information they require and no more. Good visibility is key to identifying problems quickly and making the correct decision in good time. Poor visibility due to weather leads to dramatic increases in the rates and severity of road accidents.
Energy Transfer and Absorption

Reactive safety is about minimising damage and injury once an accident becomes unavoidable; this means designing structures and devices that absorb the energy of impact rather than transfer it to a person or object in a dangerous and uncontrollable way.
Vehicle Control and Handling

ABS, or anti-locking brakes, are an example of control assistance that aids the safe performance of a vehicle. This and other systems such as traction and stability control can enable safer driving by compensating for limits in human ability. They make a substantial difference when a vehicle is being used to its maximum but can lead to a reliance or complacency by drivers which can in turn negate the safety benefits. Manufacturers recognise that there is a point at which safety features make a driver feel so at ease that their driving deteriorates and becomes more dangerous.

Pedestrian Safety

Designing for Pedestrians in Impact

In direct response to proposed and actual EU legislation, manufacturers are trying to stop pedestrians impacting with hard-points at the front of vehicles. The principle responses are to either raise the bonnet to a stance that better absorbs energy, or to use airbags to cushion against these hard-points. Although these approaches offer a way to maintain existing styling traits, they are unlikely to be as simple or effective as more dramatic changes in vehicle front design.

In 2000, 28% of UK road fatalities were pedestrians. Key improvements seem to revolve around giving the right amount of support, in the right areas, to a pedestrian in impact. It is suggested that bumpers have a deeper profile or a support structure below the surface to reduce “pitching of the leg-form and bending of the knee joint”. ‘Foam plastics’ could be used to absorb the energy of the impact as they possess good ‘recovery characteristics’ to reduce permanent damage to the vehicle in “low-speed car-to-car collisions”.

At the leading edge of the bonnet it is desirable to reduce the stiffness of the structure and avoid the location of catches and other fixings close to the surface. Bonnet reinforcing structure and panel seams add to the number of risk areas for impact. Statistics by the (UK) Transport Research Laboratory predict design improvements could prevent 8% of all pedestrian fatalities and 21% of serious injuries. The UK Department of the Environment, Transport & the Regions (DETR) is more optimistic, believing up to 20% of pedestrian fatalities could be prevented within 8 years.

Several key changes to design can be considered as a means to improve pedestrian impact performance:

  • Bumper foam needs to be 20-40mm thicker than on current vehicles and may need to be bigger in the vertical direction.
  • “A low level foam-covered beam is needed to reduce rotation of the knee joint. This could be disguised under a spoiler-style skin..”
  • Lights should be kept below the upper leg crush zone or designed to deform in a controlled way.
  • Under bonnet clearance should be at least 75mm, with special consideration paid to major features such as shock absorber mounts. Some suggestions have been made that double-wishbone suspension may be an alternative – this depends on the packaging in this area.

There is some difference of opinion on bonnet leading-edge height. Some sources state that anything above 650mm in height is undesirable where other point out that “making the hood edge height higher is effective in lowering the vehicle-head collision speed”. It is noted though, that “if the edge of the hood is too high, it might be dangerous for children because their heads might be directly hit by the front of the car”. They chose 800mm as a suitable height as it is lower than the head of a 3 year old child. There is no defining conclusion on the subject of leading edge height; it makes more sense to look at reducing hard points, improving controlled plastic deformation to absorb energy and stiffening lower bumper structure to minimise leg injury.

In tests on bonnet structure, it was concluded that steel, backed with a ‘soft foam elastic material’ performed better than any other metal-based structure. No solely polymer structures were tested. Traditional bonnet design involves dangerous points of reinforcement and its performance in impact is very difficult to predict or control.

Modifying existing methods of manufacture to improve pedestrian impact performance may not be the ideal direction to take. It should be noted that bonnet clearance needs are different for children and adults, that clam-shell bonnets are preferred, that simply raising everything for greater clearance over componentry will increase drag and thus fuel consumption. Existing vehicle structures cannot produce uniform responses to impact and some common practices – such as the use of MacPherson strut suspension – are almost incompatible with long-term improvements in this field.

Headlight design may also need to change. The front of the headlight could become part of the passive safety system, where the lens will be collapsible and packaging requirements will alter as the lighting unit is moved back from the likely point of impact.

Bonnets

Looking specifically at the bonnet area, user intervention in the engine bay area is constantly decreasing. In fact, with current levels of reliability, most users need access to the engine bay only to replenish items like the screenwash. Given that the bonnet is simply a reinforced sheet metal lid on most vehicles, why not separate access to the engine from that of the replenishable fluids? Access to these items could be tidied away to a more convenient place. This would allow the bonnet to be replaced by simpler, stiffer structure that could save weight or be used more efficiently in dissipating the energy of an impact.

With the bonnet replaced by a stiffer structure, it may then be possible to create a more efficient body using fewer and lighter materials. The result would be a vehicle that weighs less, requires less energy to propel and impacts with decreased momentum; ideal characteristics for a safety- and environmentally-conscious vehicle. If access from above is not required for most major engine bay components, it is then feasible to more densely package them, moving all major hardpoints even further from areas of pedestrian impact as well as reducing the vehicle’s footprint.

Bumpers

Research into bumper development used ‘special energy absorbing elements’ made of PolyPropylene under a PolyPropylene skin to achieve a balance in impact performance across the bonnet leading edge, bumper and spoiler area. Although an ideal vehicle front “is not completely achieved by choosing special material properties only”, the only firm suggestion relating to styling is that features creating high local stiffnesses should be avoided.

Useful Links

Australia: Road fatalities among older pedestrians

Road Deaths: EU Comparison, UK Office for National Statistics

NHTSA Research and Development