| Why
Our
Chassis Looks The Way It Does |
Any
good chassis must do several
things:
- Be
structurally sound
in every way over
the expected life of the vehicle and beyond. This means
nothing will
ever break under normal conditions.
- Maintain the suspension mounting
locations so that handling is safe and
consistent under high cornering
and bump loads.
- Support
the body panels and other passenger components so that everything
feels solid
and has a long, reliable
life..
- Protect
the occupants from external
intrusion.
In the real
world,
few chassis designs
will not meet the criteria of #1. Major structural failures,
even in
kit cars, are rare. (Here's an exception.)
Most kit
designers, even if they're not
engineers,
will overbuild naturally. The penalties for being wrong here
are too
great. The trouble is, some think that having a "strong" (no
structural
failures) chassis is enough.
It
isn't.
Read
this article
from the July 1999 Machine Design magazine!
Structural
stiffness is the basis
of what you feel at the seat
of your pants. It defines how a car handles, body integrity,
and the
overall feel of the car. Chassis
stiffness
is
what separates a great car to drive from what
is merely
OK.
Contrary to some pronouncements,
there is no such thing as a chassis that doesn't flex,
but some
are much stiffer than others. The range of chassis stiffness
has
varied greatly over the years from about 500 lbft/degree in a Morgan to
more than 20,000 lbft/deg in a modern race car. The ERA 427's
chassis runs about 3500 lbft/degree. Not high by current
sedan
standards but about as high as you can get in a roadster whose body
sits mostly on top of the chassis.
Different
basic chassis
designs
each have their own strengths
and weaknesses. Every chassis is a compromise between weight,
component
size, complexity, vehicle intent, and ultimate cost. And even
within a
basic design
method, strength and stiffness can vary significantly, depending on the
details.
There is no such thing as the ultimate method of construction
for every
car, because each car presents a different set of problems.
Below,
I have summarized the characteristics of some chassis alternatives.
Remember, though, that detail execution is
as important as the basic design, if not more!
Some think an aluminum
chassis is the path to the lightest
design, but this is
not necessarily true. Aluminum is more flexible than steel.
In
fact, the ratio of stiffness to weight is almost identical to steel, so
an
aluminum chassis must weigh the same as a steel one to
achieve the same
stiffness. Aluminum has an advantage only
where
there are very
thin sections where buckling is possible - but that's not generally the
case
with tubing - only very thin sheet. And even then, aircraft
use
honeycomb'd aluminum to prevent buckling. In addition, an
aircraft's limitation is not stiffness, but resistance to failure.
|
| Backbone:
The tunnel becomes a
primary load
bearing member. This is
a potentially fine design, and if we were building a new car from
scratch,
we would seriously consider a backbone.
But
, this is not a new car, it's
a replica
of a
classic! Because
it is designed around the original
Ford engines (and we wanted our customers to have several different
transmission
choices), the bulk of a compatible structural tunnel was unacceptable,
especially
considering the passenger
compartment
was a fairly narrow
one to begin with. A backbone would make it impossible to
maintain the
look of the original interior and engine compartment.
It would
also create servicing difficulties. |
 |
A
variation to the sheet metal backbone is one that uses small tubes to
create
the central structure. TVR's Griffith was built like that -
with an
enormous tunnel. The Shelby
Daytona Coupe
added a tubular backbone to the
original 289
chassis. It probably
added 50% to the overall stiffness of the car! See below.
Then there is the issue of engine
compartment
esthetics. With our rectangular tube
chassis, we can duplicate
the round-tube X (with
the 427SC) or
the spring tower
(with the 289FIA) at
the front of the engine to maintain visual accuracy.
|
Space
frame: A true space frame
has small tubes that are only
in tension or compression - and has no bending or twisting loads in
those tubes. That means
that
each load-bearing point must be supported in three dimensions.
It is
nearly impossible to build an efficient space frame around the Cobra
body.
The rockers are simply too shallow, and the tunnel shaped incorrectly
to
make a reasonably triangulated structure.
Remember the
300SLR Mercedes?
(shown at the right)
It had rockers 12 inches tall and 10 inches wide and the
chassis used
hundreds of separate tubes. It was difficult to build and a nightmare
to
fix. The "space frame" chassis that is currently built for another
replica
simply uses smaller tubes, many carrying bending and torsional loads.
It may look impressive, but functionally it's a bad
compromise.
Simply more complication without
improvement. |
Mercedes
300SLR
Consider - the bending
stiffness of a tube increases the by the square of the diameter of the
(equal-wall-thickness) tube, and the torsional stiffness by
the cube
of the diameter, while the weight goes up linearly. The
bottom line
is - sometimes you're better off with a large tube.
|
|
1958
Lotus Elite
|
Monocoque:
An
airplane
(with a stressed outside skin) is
close to a true monocoque. In the automotive world, it's time
to compromise
again, but the street car that compromises the least is probably the
1958
Lotus Elite. The design was made possible by the
use of
large fiberglass panels - otherwise the tooling and construction costs
would have been tremendous. In the real world, the interior
panels
are stressed, but many cars have an aerodynamic facade of
'glass
or aluminum.
The original
GT40
- and our ERA GT - have a semi-monocoque chassis. The
heaviest
(steel) main
panel on our ERA GT is only .045" thick, and most panels are only
.032"!
Reinforcements are required at the suspension points where
there are
local high loads. With the rockers 10" high x 9" wide, the
net result
is an incredibly stiff structure. But you can't build a
classic roadster
like this.
ERA
GT Chassis
|
| "Ladder"
frame:
The ladder frame is a shorthand description of a twin-rail
chassis,
typically made from round or rectangular tubing or channel.
It can
use straight or curved members, connected by two or more crossmembers.
Body mounts are usually integral outriggers from the main
rails, and
suspension points can be well or poorly integrated into the basic
design.
The original Shelby 289 Cobra used 3" round tubes, a very
flexible
design that worked with stiff transverse-leaf
springs for adequate
but primative handling. The 427 was updated to 4" round tubes
to allow
the more modern suspension to work properly. Both
chassis were
very simple to build - and good enough for their time. |
An
Original 289 chassis
|
|
|
The Shelby
Daytona used
a
modified 289 chassis made
into a tubular
semi-backbone design to correct the extreme flex of the original
design.
You can see how it looks by visiting the site of someone
ambitious
enough to try to
build
a Daytona from scratch!
|
Daytona
Chassis
|
|
| The ERA
chassis uses 4"
x 3"
x .125"W structural
tubing in a complex
design meant to take suspension and body loads efficiently, while
maintaining
the original look from outside and in the engine compartment,
simutaneously allowing easy service and assembly.
Roll bar, body and door mounting points are built
into the
basic design for
maximum efficiency. There are 4 crossmembers plus an "X"
member for
maximum torsional stiffness. We even box in the "X" for extra
strength!
Yes - this chassis is somewhat heavier than most
ladder
designs, but it is also by
far the stiffest. A compromise that no ERA owner regrets!
|
ERA
Chassis (FIA similar)
|
| Round
vs. Rectangular frame rails: There
has been a
lot tossed around regarding whose chassis - and what kind of tubing -
is
"strongest." Factory Five
is numerically the
biggest exponent
of round tubes,
but many others have preceded them. We chose to use
rectangular tubing
in our chassis for several reasons: Under pure vertical bending load,
4"
x 3" rectangular tubing is about 37%
stiffer than an equal wall thickness 4" round
tube. This is especially
important because a roadster doesn't have a roof to stiffen the
passenger
compartment. Not only can you feel a lack of "solidness" with a
flexible
chassis. Your variable door gaps will also make latching
unstable -
and even ocassionally cause paint chipping as the doors meet the main
body!
You can see below that transverse members have
little effect
on beam stiffness.
You just add up the individual stiffnesses of the components.
We
also have an "X" member, acting as an additional longitudinal beam
reinforcement
and as two transverse members. A round tube chassis is
extremely difficult
to "X" brace.
|
 |
A
little light on Torsional
Stiffness
Even
though an
individual
rectangular tube
is about
2% less stiff in torsion than the equivalent round tube, we must
consider
the chassis
design as a
whole. For each
transverse
tie-in
we create a
system that becomes more like a single large tube spanning the whole
width
of the chassis- the ultimate in efficiency. We have integrated 7
transverse
members along our main rails in such a way that the chassis has much
more
torsional stiffness than the tubes taken individually. We
even put
extra braces
on our central "X" member
to make it even stronger. |
| The stiffness
of
an ideal unitized
structure is proportional to the square
of the distance of the components from the
centerline. Double
the distance and you have four times the overall stiffness. While
practical
automotive considerations eliminate an ideal connection between the
rails,
widely spaced tubes that are tied together well work more efficiently
than
the same tubes on a narrower base. The original 427 Cobras' rails were
only
20 inches apart. Ours are spaced at 27 inches on center through the
middle
of the chassis, one of the widest
spacing
in the industry. And we still are one of the few in the industry that
have
left room for an undercar exhaust outside the rails. |
|
THE
BOTTOM
LINE:
The
E.R.A. chassis is
one of the strongest and stiffest of the industry.
And the difference is easy
to feel on the
road!
|
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