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Different Types of Chassis

Ladder Chassis

AC Cobra’s chassis

This is the earliest kind of chassis. From the earliest cars until the early 60s, nearly all

cars in the world used it as standard. Even in today, most SUVs still employ it. Its

construction, indicated by its name, looks like a ladder – two longitudinal rails

interconnected by several lateral and cross braces. The longitude members are the

main stress member. They deal with the load and also the longitudinal forces caused

by acceleration and braking. The lateral and cross members provide resistance to

lateral forces and further increase torsional rigidity.

Advantage: Well, it has no much advantage in these days … it is easy and

cheap for hand build, that’s all.

Disadvantage: Since it is a 2 dimensional structure, torsional rigidity is very

much lower than other chassis, especially when dealing with

vertical load or bumps.

Who use it ? Most SUVs, classic cars, Lincoln Town Car, Ford Crown

Victoria etc.

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Tubular Space Frame

TVR Tuscan

Lamborghini Countach

As ladder chassis is not strong enough, motor racing engineers developed a 3

dimensional design – Tubular space frame. One of the earliest examples was

the post-war Maserati Tipo 61 “Birdcage” racing car. Tubular space frame

chassis employs dozens of circular-section tubes (some may use squaresection

tubes for easier connection to the body panels, though circular section

provides the maximum strength), position in different directions to provide

mechanical strength against forces from anywhere. These tubes are welded

together and forms a very complex structure, as you can see in the above

pictures.

For higher strength required by high performance sports cars, tubular space

frame chassis usually incorporate a strong structure under both doors (see the

picture of Lamborghini Countach), hence result in unusually high door sill and

difficult access to the cabin.

In the early 50s, Mercedes-Benz created a racing car 300SLR using tubular

space frame. This also brought the world the first tubular space frame road car,

300SL Gullwing. Since the sill dramatically reduced the accessibility of

carbin, Mercedes had to extend the doors to the roof so that created the

“Gullwings”.

Since the mid 60s, many high-end sports cars also adopted tubular space frame

to enhance the rigidity / weight ratio. However, many of them actually used

space frames for the front and rear structure and made the cabin out of

monocoque to cut cost.

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Advantage: Very strong in any direction. (compare with ladder chassis and

monocoque chassis of the same weight)

Disadvantage: Very complex, costly and time consuming to be built.

Impossible for robotised production. Besides, it engages a lot

of space, raise the door sill and result in difficult access to the

cabin.

Who use it ? All Ferrari before the 360M, Lamborghini Diablo, Jaguar

XJ220, Caterham, TVR etc.

Monocoque

Today, 99% cars produced in this planet are made of steel monocoque chassis,

thanks to its low production cost and suitability to robotised production.

Monocoque is a one-piece structure which defines the overall shape of the car.

While ladder, tubular space frame and backbone chassis provides only the

stress members and need to build the body around them, monoque chassis is

already incorporated with the body in a single piece, as you can see in the

above picture showing a Volvo V70.

In fact, the “one-piece” chassis is actually made by welding several pieces

together. The floorpan, which is the largest piece, and other pieces are pressmade

by big stamping machines. They are spot welded together by robot arms

(some even use laser welding) in a stream production line. The whole process

just takes minutes. After that, some accessories like doors, bonnet, boot lid,

side panels and roof are added.

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Monocoque chassis also benefit crash protection. Because it uses a lot of

metal, crumple zone can be built into the structure.

Another advantage is space efficiency. The whole structure is actually an outer

shell, unlike other kinds of chassis, therefore there is no large transmission

tunnel, high door sills, large roll over bar etc. Obviously, this is very attractive

to mass production cars.

There are many disadvantages as well. It’s very heavy, thanks to the amount of

metal used. As the shell is shaped to benefit space efficiency rather than

strength, and the pressed sheet metal is not as strong as metal tubes or

extruded metal, the rigidity-to-weight ratio is also the lowest among all kinds

of chassis bar the ancient ladder chassis. Moreover, as the whole monocoque

is made of steel, unlike some other chassis which combine steel chassis and a

body made of aluminuim or glass-fiber, monocoque is hopelessly heavier than

others.

Although monocoque is suitable for mass production by robots, it is nearly impossible

for small-scale production. The setup cost for the tooling is too expensive – big

stamping machines and expensive mouldings. I believe Porsche is the only sports car

specialist has the production volume to afford that.

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Advantage: Cheap for mass production. Inherently good crash protection.

Space efficient.

Disadvantage: Heavy. Impossible for small-volume production.

Who use it ? Nearly all mass production cars, all current Porsche.

ULSAB Monocoque

Enter the 90s, as tougher safety regulations ask for more rigid chassis,

traditional steel monocoque becomes heavier than ever. As a result, car

makers turned to alternative materials to replace steel, most notable is

aluminium. Although there is still no mass production car other than Audi A8

and A2 to completely eliminate steel in chassis construction, more and more

cars use aluminium in body panels like bonnet and boot lid, suspension arms

and mounting sub-frames. Unquestionably, this is not what the steel industry

willing to see.

Therefore, American’s steel manufacturers hired Porsche Engineering Services

to develop a new kind of steel monocoque technology calls Ultra Light Steel

Auto Body (ULSAB). As shown in the picture, basically it has the same

structure as a conventional monocoque. What it differs from its donor is in

minor details – the use of “Hydroform” parts, sandwich steel and laser beam

welding.

Hydroform is a new technique for shaping metal to desired shape, alternative

to pressing. Conventional pressing use a heavy-weight machine to press a

sheet metal into a die, this inevitably creates inhomogenous thickness – the

edges and corners are always thinner than surfaces. To maintain a minimum

thickness there for the benefit of stiffness, car designers have to choose thicker

sheet metal than originally needed. Hydroform technique is very different.

Instead of using sheet metal, it forms thin steel tubes. The steel tube is placed

in a die which defines the desired shape, then fluid of very high pressure will

be pumped into the tube and then expands the latter to the inner surface of the

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die. Since the pressure of fluid is uniformal, thickness of the steel made is also

uniformal. As a result, designers can use the minimum thickness steel to

reduce weight.

Sandwich steel is made from a thermoplastic (polypropylene) core in between

two very thin steel skins. This combination is up to 50 percent lighter

compared with a piece of homogenous steel without a penalty in performance.

Because it shows excellent rigidity, it is applied in areas that call for high

bending stiffness. However, it cannot be used in everywhere because it needs

adhesive bonding or riveting instead of welding.

Compare with conventional monocoque, Porsche Engineering claimed it is

36% lighter yet over 50% stiffer. Although ULSAB was just annouced in early

1998, the new Opel Astra and BMW 3-Series have already used it in some

parts. I believe it will eventually replace conventional monocoque.

Advantage: Stronger and lighter then conventional monocoque without

increasing production cost.

Disadvantage: Still not strong or light enough for the best sports cars.

Who use it ? Opel Astra, BMW 3-series

Backbone Chassis

Kia’s version Lotus Elan Mk II

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Colin Chapman, the founder of Lotus, invented backbone chassis in his

original Elan roadster. After failed in his experiment of glass-fibre

monocoque, Chapman discovered a strong yet cheap chassis which had been

existing for millions of years – backbone.

Backbone chassis is very simple: a strong tubular backbone (usually in

rectangular section) connects the front and rear axle and provides nearly all the

mechnical strength. Inside which there is space for the drive shaft in case of

front-engine, rear-wheel drive layout like the Elan. The whole drivetrain,

engine and suspensions are connected to both ends of the backbone. The body

is built on the backbone, usually made of glass-fibre.

It’s strong enough for smaller sports cars but not up to the job for high-end

ones. In fact, the original De Tomaso Mangusta employed chassis supplied by

Lotus and experienced chassis flex.

TVR’s chassis is adapted from this design – instead of a rigid backbone, it uses

a lattice backbone made of tubular space frames. That’s lighter and stronger

(mainly because the transmission tunnel is wider and higher).

Advantage: Stong enough for smaller sports cars. Easy to be made by hand

thus cheap for low-volume production. Simple structure

benefit cost. The most space-saving other than monocoque

chassis.

Disadvantage: Not strong enough for high-end sports cars. The backbone

does not provide protection against side impact or off-set

crash. Therefore it need other compensation means in the

body. Cost ineffective for mass production.

Who use it ? Lotus Esprit, Elan Mk II, TVR, Marcos.

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Glass-Fiber body

• To many sports cars specialists, glass-fiber is a perfect material. It is

lighter than steel and aluminium, easy to be shaped and rust-proof.

Moreover, the most important is that it is cheap to be produced in small

quantity – it needs only simple tooling and a pair of hands. There are a

few drawbacks, though: 1) Higher tolerence in dimensions leads to

bigger assembly gaps can be seen. This is usually percieved as lower

visual quality compare with steel monocoque. 2) Image problem.

Many people don’t like “plastic cars”.

• Glass-fiber has become a must for British sports car specialists because

it is the only way to make small quantity of cars economically. In

1957, Lotus pioneered Glass-Fiber Monocoque chassis in Elite (see

picture). The whole mechanical stressed structure was made of glassfiber,

which had the advantage of lightweight and rigidity like today’s

carbon-fiber monocoque. Engine, transmission and suspensions were

bolted onto the glass-fiber body. As a result, the whole car weighed as

light as 660 kg.

• However, this radical attempt caused too many problems to Colin

Chapman. Since the connecting points between the glass-fiber body

and suspensions / engine required very small tolerances, which was

difficult for glass-fiber, Lotus actually scrapped many out-ofspecification

body. Others had to be corrected with intensive care. As a

result, every Elite was built in loss. Since then, no any other car tried

this idea again. Today, no matter Lotus, TVR, Marcos, GM’s Corvette /

Camaro / Firebird, Venturi and more, employ glass-fiber in nonstressed

upper body. In other words, they just act as a beautiful

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enclosure and provide aerodynamic efficiency. The stressed chassises

are usually backbone, tubular space-frame, aluminium space-frame or

even monocoque.

Advantage: Lightweight. Cheap to be produced in small quantity. Rustproof.

Disadvantage: Lower visual quality. Unable to act as stressed member.

Who use it ? Lotus, TVR, Marcos, Corvette, Camaro, Firebird …

Carbon-Fiber Monocoque

Carbon Fiber is the most sophisticated material using in aircrafts, spaceships

and racing cars because of its superior rigidity-to-weight ratio. In the early

80s, FIA established Group B racing category, which allowed the use of

virtually any technology available as long as a minimum of 200 road cars are

made. As a result, road cars featuring Carbon-Fiber body panels started to

appear, such as Ferrari 288GTO and Porsche 959.

There are several Carbon-fibers commonly used in motor industry. Kelvar,

which was developed by Du Pont, offers the highest rigidity-to-weight ratio

among them. Because of this, US army’s helmets are made of Kelvar. Kelvar

can also be found in the body panels of many exotic cars, although most of

them simultaneously use other kinds of carbon-fiber in even larger amount.

Production process

Carbon-fiber panels are made by growing carbon-fiber sheets (something look

like textile) in either side of an aluminium foil. The foil, which defines the

shape of the panel, is sticked with several layers of carbon fiber sheets

impregnated with resin, then cooked in a big oven for 3 hours at 120°C and 90

psi pressure. After that, the carbon fiber layers will be melted and form a

uniformal, rigid body panel.

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Carbon-Fiber Panels VS Carbon-Fiber Monocoque Chassis

Porsche 959, employed

carbon-fiber in body

panels only, is

obviously….

…. Inferior to McLaren

F1’s carbon-fiber

monocoque. This structure

not only supports the

engine / drive train and

suspensions, it also serves

as a very rigid survival

cell.

Exotic car makers like to tell you their cars employ carbon-fiber in

construction. This sounds very advanced, but you must ask one more question

- where is the carbon-fiber used? Body panels or Chassis?

Most so-called “supercars” use carbon-fiber in body panels only, such as

Porsche 959, Ferrari 288GTO, Ferrari F40 and even lately, the Porsche 911

GT1. Since body panels do nothing to provide mechanical strength, the use of

carbon fiber over aluminium can barely save weight. The stress member

remains to be the chassis, which is usually in heavier and weaker steel tubular

frame.

What really sophisticated is carbon-fiber monocoque chassis, which had only

ever appeared in McLaren F1, Bugatti EB110SS (not EB110GT) and Ferrari

F50. It provides superior rigidity yet optimise weight. No other chassis could

be better.

Carbon Fiber Monocoque made its debut in 1981 with McLaren’s MP4/1

Formula One racing car, designed by John Barnard. No wonder McLaren F1 is

the first road car to feature it.

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Car Body Chassis

Ferrari 288GTO

(1985)

carbon fiber panels steel tubular space frame

Porsche 959 (1987) carbon fiber panels steel monocoque

Ferrari F40 (1988)

carbon fiber panels +

doors

steel tubular space frame

McLaren F1 (1993) carbon fiber panels carbon fiber monocoque

Ferrari F50 (1996)

carbon fiber panels +

doors

carbon fiber monocoque

Lamborghini Diablo

SV (1998)

mostly aluminium panels,

with carbon fiber bonnet

+ engine lid

steel tubular space frame

Lamborghini Diablo

GT (1999)

mostly carbon fiber

panels + aluminium doors

steel tubular space frame

Engine act as stressed member – Ferrari F50

Unlike McLaren F1, Ferrari F50’s

rear suspensions are directly bonded

to the engine / gearbox assembly.

This means the engine becomes the

stressed member which supports the

load from rear axle. Then, the whole

engine / gearbox / rear suspensions

structure is bonded into the carbon

fiber chassis through light alloy. This

is a first for a road car. Advantage: lighter still. Disadvantage: engine’s

vibration directly transfers to the body and cockpit.

In 1963, a revolutionary chassis structure appeared in Formula One, that is, the

championship-winning Lotus 25. Once again, that was innovated by Colin

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Chapman. Chapman used the engine / gearbox as mounting points for rear

suspensions in order to reduce the width of his car as well as to reduce weight.

In particular, reduced width led to lower aerodynamic drag. Of course, the

engine / chassis must be made stiffer to cope with the additional stressed from

rear axle. Today, F1 cars still use this basic structure.

Characteristics of carbon-fiber monocoque:

Advantage: The lightest and stiffest chassis.

Disadvantage: By far the most expensive.

Who use it ? McLaren F1, Bugatti EB110SS, Ferrari F50.

Aluminum Space Frame

Audi ASF

Audi A8 is the first mass production car featuring Aluminium Space Frame

chassis. Developed in conjunction with US aluminium maker Alcoa, ASF is

intended to replace conventional steel monocoque mainly for the benefit of

lightness. Audi claimed A8’s ASF is 40% lighter yet 40% stiffer than

contemporary steel monocoque. This enable the 4WD-equipped A8 to be

lighter than BMW 740i.

ASF consists of extruded aluminum sections, vacuum die cast components and

aluminum sheets of different thicknesses. They all are made of high-strength

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aluminium alloy. At the highly stressed corners and joints, extruded sections

are connected by complex aluminum die casting (nodes). Besides, new

fastening methods were developed to join the body parts together. It’s quite

complex and production cost is far higher than steel monocoque.

The Audi A2 employed the second generation of ASF technology, which

involves larger but fewer frames, hence fewer nodes and requires fewer

welding. Laser welding is also extensively used in the bonding. All these

helped reducing the production cost to the extent that the cheap A2 can afford

it.

Advantage: Lighter than steel monocoque. As space efficient as it.

Disadvantage: Still expensive for mass production

Who use it ? Audi

Lotus Elise

Elise’s revolutionary chassis is made

of extruded aluminium sections

joined by glue and rivets. New

technology can make the extruded

parts curvy, as seen in the side

members. This allow large part to be

made in single piece, thus save

bonding and weight.

To Lotus and other low-volume sports car makers, Audi’s ASF technology is

actually infeasible because it requires big pressing machines. But there is an

alternative: extruding. Extrusion dies are very cheap, yet they can make

extruded aluminium in any thickness. The question is: how to bond the

extruded parts together to form a rigid chassis ?

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Renault Sport Spider bonds them by spot welding, while Lotus Elise uses glue

and rivet to do so. Comparing their specification and you will know how

superior the Elise is:

Renault Sport Spider Lotus Elise

Weight of chassis 80 kg 65 kg

Torsional stiffness 10,000 Nm/degree 11,000 Nm/degree

Thickness of extrusion 3 mm 1.5 mm

Lotus’s technology was originated by its supplier, Hydro Aluminium of

Denmark. Hydro discovered that aluminium extrusion can be bonded by

epoxy resin (glue) if it is adequately prepared by a special chemical in the

bonding surface. Surprisingly, glue can bond the sections together strongly

and reliably. Most important, the aluminium extruded sections can be made

much thinner than traditional welding technique. Why ? because welded joints

are weak, so the thickness of material should be increased throughout a

member just to make a joint strong enough. Therefore Elise’s chassis could be

lighter yet stiffer.

Glue can be clearly seen during

production.

Unquestionably, Lotus Elise’s aluminium chassis is a revolution. I expect to

see more British specialty cars to go this way.

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Advantage: Cheap for low-volume production. Offers the highest rigidityto-

weight ratio besides carbon fiber monocoque.

Disadvantage: Not very space efficient; High door sill.

Who use it ? Lotus Elise, forthcoming Lotus M250, Opel Speedster

One-Box design

It is widely believed that one-box design offers the biggest interior space for a

given external dimensions. However, I always doubt its effectiveness.

Compare with conventional two-box hatchback, one box car frees up the space

in front of the driver by pushing the windscreen forward. Nevertheless, as

shown in the above drawing, such additional room (grey area) does not really

contribute to driver’s comfort. It just create a “freer” feel to the driver.

Because the windscreen is pushed forward, visibility is actually deteriorated,

as shown in the drawing. The driver even cannot see the front end of his car,

thus made arise some problems for parking.

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Cab-forward design

Push the front-wheels towards the corners, shorten the

engine compartment, move the windshield forward so

that its base rests near the front wheels, this is the socalled

“Cab-foward” design. Chrysler tells us Cabfoward

design frees up the room for front passengers….

…. this is right when compare with long-nose traditional

American cars ….

….but when compare with any standard European cars,

Chrysler’s cabin seems to be not so Forward.

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Sandwich Structure – Mercedes A-Class

_ This structure is called “Sandwich” because the horizontal-orientated engine is

placed above the floorpan but under the cabin. As a result, the cabin is raised

by a massive 200 mm and so is the roof. What is the advantage of such

structure? Firstly, because of the disappearance of the front engine

compartment, it made the car more compact than any other cars but

simultaneously offers class-leading cabin space (actually runs close to C

class).

_ Secondly, it provides exceptional crash-protection. Under crash, the engine

will be pushed underneath the cabin instead of pushed towards the driver’s legs

as conventional cars. Therefore A class will pass any foreseeable crash test in

the future. Thirdly, due to the inherent advantage in crash-protection, no

additional crash structure is needed, thus a lot of weight is saved.