86 Electric Vehicle Chassis and Body Design

8.6.1 Body/chassis requirements

This section is intended to give guidance on the design of chassis for electric vehicles. Chassis design should be carried out in conjunction with other texts on chassis design, not to mention computer packages that specialise in this area. Nevertheless a basic understanding of what the chassis should do and other parameters related to electric vehicle chassis is needed.

In the early cars chassis and bodies were separate items. The chassis gave the basic strength of the vehicle while the body and glazing acted as a cocoon to keep the passengers and luggage protected from the outside elements.

In recent times the body and chassis have been combined as a monocoque so that every part, including the glazing, adds to the strength and stiffness, resulting in a much lighter vehicle. Either monocoques or separate chassis/body units are an acceptable basis for design. Despite the popularity of monocoques, several modern electric vehicles use a separate chassis, most notably the advanced new General Motors Hy-wire fuel cell vehicle, which will be discussed in more detail later.

It is worth pausing to think precisely what the chassis/body does; ideally a chassis/body should fill the following criteria. It should:

• not vibrate, particularly at frequencies and harmonics of rotating parts;

• be resistant to impact;

• crumple evenly in an accident, minimising forces on driver/passengers;

• be strong enough to fix components to easily;

• be impact and roll resistant;

• be aesthetically pleasing;

Chassis/body design requires optimisation of conflicting requirements such as cost and strength, or performance and energy efficiency. There are important differences when designing electric vehicles compared to their IC equivalents. For example, extra weight is not so important with an internal combustion vehicle, where a little more power can be cheaply added to compensate for a slightly heavier chassis. The same is true for aerodynamic drag, where a slight increase in drag can be similarly compensated for. Savings in weight as well as increases in efficiency contribute directly to the size of the batteries and these are both heavy and expensive.

It must also be borne in mind that most internal combustion engine vehicles are quantity produced, whereas at the moment, and probably for the immediate future, small scale production of electric vehicles is likely. This in itself will tend to result in the use of materials such as reinforced plastics, where there is potential scope for more perfect aerodynamic shapes and weight saving.

8.6.2 Body/chassis layout

There is plenty of scope for designers of electric vehicles to experiment with different layouts to optimise their creation. To start with there is no need for a bonnet housing and engine. In addition, batteries can be placed virtually anywhere along the bottom (for stability) of the vehicle and motors and gearing can be, if required, integrated with the wheel hub assemblies.

Most batteries can be varied in size. Height can be traded against length and width, and most batteries (not all) can be split up so that they can be located under seats and anywhere else required, all of which can help to use every available space and to reduce the vehicle frontal area. Batteries can also be arranged to ensure that the vehicle is perfectly balanced around the centre of gravity, giving good handling characteristics.

A picture of an interesting experimental drive system assembly is shown in Figure 8.11, consisting of one driven wheel, with batteries and controller all built into the unit. The scope for using such a device on a range of interesting vehicle layouts is considerable. It could be incorporated, for example, to drive the rear wheel in a tricycle arrangement. Interestingly one of the most popular three-wheel electric vehicles is the Twike illustrated in Figure 8.12. Based on the previous argument the vehicle layout could be interpreted

Figure 8.11 This power module comprises motor, controller, battery and one driven wheel in a neat unit that could be built into a wide range of vehicle designs
Figure 8.12 The famous Swiss electric Twike

as being the 'wrong way round'; the body is like a tear drop going backwards. However, as it is a low-speed commuter vehicle based on bicycle components the aerodynamic shape is not as important as those of a high-speed vehicle. The two rear wheels with the passengers sitting side by side give stability.

The layout for an electric van also has considerable scope for new ideas. Electric motors and gearing can again, if required, be incorporated into the wheel hub assemblies, avoiding space requirements for motors, gearing and transmission. Batteries such as lead acid, NiCad or NiMH can be spread as a thin layer over the base of the vehicle leaving a large flat floored area above, an essential requirement for vans.

8.6.3 Body/chassis strength, rigidity and crash resistance

The days have long past, thankfully, when stress engineers regarded aircraft as hollow cylinders with beams stuck out of the side and cars as something simpler. Modern predictions of chassis body behaviour and virtually every aspect of car design rely ultimately on complex computer packages. Nevertheless a basic understanding of the behaviour of beams and hollow cylinders does give an insight into body chassis design.

Let us look at a hollow cylinder as shown in Figure 8.13, subjected to both bending and torsion. Bending would be caused by the weight of the vehicle, particularly when coming down after driving over a bump and the torsion from cornering. The weight of the vehicle will cause stresses to mount in the tube and will also cause it to deflect. The torsion will likewise result in shear stresses and will cause the tube to twist.

Assuming an even weight distribution, the maximum bending stress a (N.mm-2) will be given by the formula:

wL4r0

where w is the uniform weight/length (N.mm 1), L is the length (mm), r0 is the radius (mm) and I is the second moment of area (mm4). I will be given by:

And the maximum deflection S (mm) in the middle of the beam will be given by

5wL4

384£7 v where E is Young's modulus (N.mm-2). Similarly, the shear stress in the cylinder wall q (N.mm-2) is given by:

where T is the applied torque (N.mm) and J is the polar second moment of area (mm4), which will be given by:

The angle of twist 9 (radians) is given by:

where G is the rigidity modulus (N.mm-2).

Certain clear conclusions can be drawn from these equations. To minimise stress due to both bending and torsion both I and J must be kept as large as possible. For a given mass of material, the further it is spread from the centre of the tube, the larger will be both I and J, thus reducing stresses, deflection and twist.

For example, consider a solid cylinder of 200 mm diameter. I and J will be n(100)4/4 = 25 000 000n and n(100)4/2 = 50000000n mm4, respectively. The same material can be spread around the circumference of a tube of 1000 mm diameter 10.1 mm thick. This would have values for I and J of 1.23 x 109n and 2.45 x 109n, respectively, an increase of 49 times. The deflections and twists will also be much less for the tube, less by a factor of nearly 50 in fact, as can be seen from equations (8.14) and (8.17).

This remains true until the material buckles, which can be predicted by modern finite element packages. Buckling can be minimised by using two layers of the material, with foam in the middle effectively creating a sandwich, hence sandwich materials. Alternatively two thin sheets of aluminium can be joined by a thin aluminium honeycomb; both of these techniques are widely used in the aircraft industry.

To keep both deflection due to bending and twist due to torque as low as possible it is necessary to use materials which are as rigid as possible, i.e. having high E and G values in addition to optimising the design to keep I and J as large as possible.

Due consideration must be given to material rigidity as well as strength. For example an infinitely strong rubber would be useless as it would deform and twist far too much. Similarly a rigid but weak material would be useless.

Steel, being relatively cheap, as well as rigid, is a traditional choice for manufacturing car bodies and chassis, but it is not necessarily a good choice for electric vehicles. Firstly it has a low strength to weight ratio, resulting in a relatively heavy structure. Secondly the manufacturing cost is low when mass produced, but relatively expensive for small number production, which may be the initial option for electric vehicles.

Materials such as aluminium, and modern composites have much better strength to weight ratios than steels, and are both widely used in the aircraft and racing car industries. A list of some potential materials is shown in Table 8.1 (Kemp 2002).

It can be seen in the table that carbon fibre has the best strength to mass (a/p) as well as the best rigidity to mass (E/p), nearly 6 times that of the other materials, which interestingly have almost identical rigidity/mass. This accounts for its widespread use in the aerospace and racing industries. A carbon fibre formula 1 chassis/body can have a mass as low as 35 kg. GRP has the next best strength to mass ratio, 3.5 times that of aluminium which is next.

Before a decision is made on what the appropriate body and chassis materials are, the behavior of a car in a crash must be considered. In a crash situation a car body and/or chassis will absorb energy. If the car is designed so that the energy is absorbed in a controlled manner the forces on the driver and passengers can be minimised. It is therefore normal to design cars with energy-absorbing crumple zones. There is national and international legislation to define a crash situation that cannot be ignored. In the late 1960s one large motor manufacturer had to strip out a brand new production line as the cars they were producing did not comply with crash legislation.

Both metals and composites absorb energy on impact, metals through plastic deformation and composites through fragmenting. The behaviour of metals in a crash can now be predicted accurately using large finite element packages, whereas it is much harder to predict the behaviour of composites. This means that if a metal structure is used the

Table 8.1 Comparison of material properties

Material

Density

Fracture

Young's

Strength

Rigidity to

P

stress

Modulus

to mass

mass

(kg/m3)

a (MPa)

E (GPa)

a /p

E/p

Mild steel

7850

465

207

0.059

0.026

Stainless steel, FSM 1

7855

980

185

0.125

0.024

Aluminium alloy (DTD

2810

500

71

0.178

0.025

5050B)

Magnesium alloy (AX 31)

1.780

185

45

0.104

0.025

(DTD 742)

Carbon fibre reinforced

1500

1050

189

0.70

0.126

plastic, 58%

unidirectional fibres by

volume in epoxy resin

Glass reinforced plastic

2000

1240

48.2

0.62

0.024

(GRP), 80% uniaxial

glass by weight in

polyester resin

car can be designed to deform in the optimum manner to meet legislation; this prediction would be much harder with composites.

Both carbon fibre and aluminium are considerably more expensive than steel. However, by using these materials not only is the car lighter, but for a given range a considerable amount of expensive batteries are saved, which must be accounted for in the overall costing of the vehicle.

8.6.4 Designing for stability

As well as being rigid and crash-resistant, it is clearly important that a vehicle design should also be stable. For maximum stability, wheels should be located at the vehicle extremities and the centre of gravity should be kept as low as possible. This is one area where the weight of the batteries can be beneficial, as they can be laid along the bottom of the vehicle, making it extremely stable. During one visit to look at an electric van manufacturer the author was challenged to try to turn it over while driving round roundabouts. Perhaps regrettably, he declined the offer, but it did give an indication of the manufacturer's confidence in the stability of their product. The Duke of Edinburgh drove the same model of vehicle for a while; it is not known if he received the same challenge!

8.6.5 Suspension for electric vehicles

Suspension has the purpose of keeping all of the wheels evenly on the ground, reducing the effects of bumps and ensuring passenger comfort. Suspension on an electric vehicle should, from the energy efficiency viewpoint, be fairly hard. As with tyres pumped to a high pressure, the energy loss is reduced but the ride tends to be less comfortable.

Other than this, the suspension design for electric vehicles will not be different to that for regular vehicles of similar size.

8.6.6 Examples of chassis used in modern battery and hybrid electric vehicles

Some electric vehicles are simply adapted from an internal combustion engine vehicle and use an existing vehicle chassis/body. This has obvious advantages in as much as the whole vehicle is available and simply has to be adapted, which is obviously a cheaper option than designing a whole new vehicle. Although these vehicles often have an adequate range and performance, better results are obtained if the body chassis is purpose built.

Many of the more recent electric vehicles use aluminium for the main structure despite the lower strength/mass of aluminium compared to carbon fibre composites. The vehicle panels are often made from composites.

Some examples are shown in Figures 8.14 and 8.15. The first shows the Twike, a simple elegant design using a tubular aluminium chassis. (The complete vehicle is shown in Figure 8.12.) The aluminium body of the Honda Insight, together with some views of the whole vehicle, is shown in Figure 8.15. The vehicle panels are often made from composites. Note that front crumple zones are a feature of both designs.

8.6.7 Chassis used in modern fuel cell electric vehicles

Chassis bodies for fuel cell vehicles need to house the hydrogen fuel tanks or a hydrogen generator, the fuel cells, the motor and radiators for getting rid of excess heat. Fuel cells can be used in conventional vehicle chassis units, and some examples have been seen

Figure Eight Chassis
Figure 8.14 Twike chassis
Figure 8.15 Aluminium body from the Honda Insight (Reproduced by kind permission of the Honda Motor Co. Ltd.)
Figure 8.16 General motors Hy-wire chassis (Reproduced by kind permission of General Motors Corp.)

in earlier chapters, for example Figures 1.14 and 1.15. However, General Motors have taken the view that a modern power source required a totally new approach to the design. Their fuel cell vehicle the Hy-wire uses a 'skateboard' chassis illustrated in Figure 8.16.

This chassis unit is based on a simple aluminium ladder frame with front and rear crush zones. The chassis contains hydrogen tanks, drive-by-wire system controls for the steering, cabin heaters, radiators for dispensing with excess heat from the fuel cells and the air management system. The electric motors are built into the wheels. The whole unit is elegant and compact and allows a range of bodies to be attached to the chassis unit, the steering and controls being connected electrically. This allows the chassis to be used as the basis for a range of vehicles, from family saloons to sports cars.

Hybrid Cars The Whole Truth Revealed

Hybrid Cars The Whole Truth Revealed

Hybrid Cars! Man! Is that a HOT topic right now! There are some good reasons why hybrids are so hot. If you’ve pulled your present car or SUV or truck up next to a gas pumpand inserted the nozzle, you know exactly what I mean! I written this book to give you some basic information on some things<br />you may have been wondering about.

Get My Free Ebook


Post a comment