Chassis Looks The Way It Does
good chassis must do several
in every way over
the expected life of the vehicle and beyond. This means
ever break under normal conditions.
- Maintain the suspension mounting
locations so that handling is safe and
consistent under high cornering
and bump loads.
the body panels and other passenger components so that everything
and has a long, reliable
the occupants from external
In the real
few chassis designs
will not meet the criteria of #1. Major structural failures,
kit cars, are rare. (Here's an exception.)
designers, even if they're not
will overbuild naturally. The penalties for being wrong here
great. The trouble is, some think that having a "strong" (no
failures) chassis is enough.
from the July 1999 Machine Design magazine!
stiffness is the basis
of what you feel at the seat
of your pants. It defines how a car handles, body integrity,
overall feel of the car. Chassis
what separates a great car to drive from what
Contrary to some pronouncements,
there is no such thing as a chassis that doesn't flex,
are much stiffer than others. The range of chassis stiffness
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
standards but about as high as you can get in a roadster whose body
sits mostly on top of the chassis.
each have their own strengths
and weaknesses. Every chassis is a compromise between weight,
size, complexity, vehicle intent, and ultimate cost. And even
method, strength and stiffness can vary significantly, depending on the
There is no such thing as the ultimate method of construction
car, because each car presents a different set of problems.
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.
fact, the ratio of stiffness to weight is almost identical to steel, so
aluminum chassis must weigh the same as a steel one to
achieve the same
stiffness. Aluminum has an advantage only
there are very
thin sections where buckling is possible - but that's not generally the
with tubing - only very thin sheet. And even then, aircraft
honeycomb'd aluminum to prevent buckling. In addition, an
aircraft's limitation is not stiffness, but resistance to failure.
The tunnel becomes a
bearing member. This is
a potentially fine design, and if we were building a new car from
we would seriously consider a backbone.
, this is not a new car, it's
it is designed around the original
Ford engines (and we wanted our customers to have several different
choices), the bulk of a compatible structural tunnel was unacceptable,
considering the passenger
was a fairly narrow
one to begin with. A backbone would make it impossible to
look of the original interior and engine compartment.
also create servicing difficulties.
variation to the sheet metal backbone is one that uses small tubes to
the central structure. TVR's Griffith was built like that -
enormous tunnel. The Shelby
added a tubular backbone to the
chassis. It probably
added 50% to the overall stiffness of the car! See below.
Then there is the issue of engine
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.
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
each load-bearing point must be supported in three dimensions.
nearly impossible to build an efficient space frame around the Cobra
The rockers are simply too shallow, and the tunnel shaped incorrectly
make a reasonably triangulated structure.
(shown at the right)
It had rockers 12 inches tall and 10 inches wide and the
hundreds of separate tubes. It was difficult to build and a nightmare
fix. The "space frame" chassis that is currently built for another
simply uses smaller tubes, many carrying bending and torsional loads.
It may look impressive, but functionally it's a bad
Simply more complication without
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
of the diameter, while the weight goes up linearly. The
is - sometimes you're better off with a large tube.
(with a stressed outside skin) is
close to a true monocoque. In the automotive world, it's time
again, but the street car that compromises the least is probably the
Lotus Elite. The design was made possible by the
large fiberglass panels - otherwise the tooling and construction costs
would have been tremendous. In the real world, the interior
are stressed, but many cars have an aerodynamic facade of
- and our ERA GT - have a semi-monocoque chassis. The
panel on our ERA GT is only .045" thick, and most panels are only
Reinforcements are required at the suspension points where
local high loads. With the rockers 10" high x 9" wide, the
is an incredibly stiff structure. But you can't build a
The ladder frame is a shorthand description of a twin-rail
typically made from round or rectangular tubing or channel.
use straight or curved members, connected by two or more crossmembers.
Body mounts are usually integral outriggers from the main
suspension points can be well or poorly integrated into the basic
The original Shelby 289 Cobra used 3" round tubes, a very
design that worked with stiff transverse-leaf
springs for adequate
but primative handling. The 427 was updated to 4" round tubes
the more modern suspension to work properly. Both
very simple to build - and good enough for their time.
Original 289 chassis
modified 289 chassis made
into a tubular
semi-backbone design to correct the extreme flex of the original
You can see how it looks by visiting the site of someone
enough to try to
a Daytona from scratch!
chassis uses 4"
x .125"W structural
tubing in a complex
design meant to take suspension and body loads efficiently, while
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
basic design for
maximum efficiency. There are 4 crossmembers plus an "X"
maximum torsional stiffness. We even box in the "X" for extra
Yes - this chassis is somewhat heavier than most
designs, but it is also by
far the stiffest. A compromise that no ERA owner regrets!
Chassis (FIA similar)
vs. Rectangular frame rails: There
has been a
lot tossed around regarding whose chassis - and what kind of tubing -
"strongest." Factory Five
is numerically the
of round tubes,
but many others have preceded them. We chose to use
in our chassis for several reasons: Under pure vertical bending load,
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
compartment. Not only can you feel a lack of "solidness" with a
chassis. Your variable door gaps will also make latching
and even ocassionally cause paint chipping as the doors meet the main
You can see below that transverse members have
on beam stiffness.
You just add up the individual stiffnesses of the components.
also have an "X" member, acting as an additional longitudinal beam
and as two transverse members. A round tube chassis is
to "X" brace.
little light on Torsional
2% less stiff in torsion than the equivalent round tube, we must
design as a
whole. For each
we create a
system that becomes more like a single large tube spanning the whole
of the chassis- the ultimate in efficiency. We have integrated 7
members along our main rails in such a way that the chassis has much
torsional stiffness than the tubes taken individually. We
on our central "X" member
to make it even stronger.
an ideal unitized
structure is proportional to the square
of the distance of the components from the
the distance and you have four times the overall stiffness. While
automotive considerations eliminate an ideal connection between the
widely spaced tubes that are tied together well work more efficiently
the same tubes on a narrower base. The original 427 Cobras' rails were
20 inches apart. Ours are spaced at 27 inches on center through the
of the chassis, one of the widest
in the industry. And we still are one of the few in the industry that
left room for an undercar exhaust outside the rails.
E.R.A. chassis is
one of the strongest and stiffest of the industry.
And the difference is easy
to feel on the
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