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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.
