Tips: Chassis
The following tips and information focus on how to optimize a race car
chassis, specifically the spaceframe-type chassis. Depending on class
rules, these suggestions may or may not be valid. Always check your
regulations.
General
Spaceframe Chassis Principles
Chassis
Design Tips
General
Spaceframe Chassis Principals
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Spaceframes
The spaceframe chassis is
about as old as the motorsport scene. It's construction consists of
steel or aluminum tubes placed in a triangulated format, to support
the loads from suspension, engine, driver and aerodynamics.
Spaceframes are popular today
in amateur motorsport because of their simplicity. Most everyone who
has access to a level workshop, a saw, measuring tools, and a welder
of some kind can build one.
There are also some inherent
advantages to using spaceframes at the amateur level of motorsport as
well. Spaceframes, unlike the monocoque chassis used in modern Formula
1 or CART, are easily repaired and inspected for damage.
So how does triangulation
work? The diagram below shows a box, with a top, bottom and two sides,
but the box is missing the front and back. The box when pushed, collapses
easily because there is no support in the front or back.
Of course, race cars need
to be supported in order to operate properly, and so we triangulate
the box by bracing it diagonally. This effectively adds the front and
back which were missing, only instead of using panels, we use tubes
to form the brace. See below:
The triangulated
box above imparts strength by stressing the green diagonal in Tension.
Tension is the force trying to pull at both ends of the diagonal. Another
force is called Compression. Compression tries to push at both
ends of the diagonal (Shown above in the horizontal yellow tube). In
a given size and diameter tube or diagonal, compression will always
cause the tube to buckle long before the same force would cause the
tube to pull apart in tension. As an experiment, try pulling on the
ends of a pop can, one end in each hand. Then, try crushing the can
by pushing on both ends. The crushing is much easier, or at least humanly
possible, compared to pulling the can apart.
Spaceframes
are really all about tubes held together in compression and tension
using 3D pyramid-style structures, and diagonally braced tube boxes.
A true spaceframe is capable of holding it's shape, even if the joints
between the tubes were hinges. In practice, a true spaceframe is not
practical, and so many designers "cheat" by using stronger
materials to support the open portions of the structure, such as the
cockpit opening.
In contrast
to spaceframes, the monocoque chassis uses panels, just like the sides
of the box pictured above. Instead of small tubes forming the shape
of a box, an entire panel provides the strength for a given side.
A common shape for 1960s
cars of monocoque construction was the "cigar". The cylindrical
shape helped impart something called Tortional rigidity. Tortional
rigidity is the amount of twist in the chassis accompanying suspension
movement. See the diagram below.
Tortional rigidity applies
to spaceframes too, but because a spaceframe isn't made from continuous
sheet metal or composite panels, the structure is used to approximate
the same result as the difficult to twist "cigar car".
Another reason tortional
rigidity is mentioned here is that it greatly affects the suspension
performance. The suspension itself is designed to allow the wheels/tires
to follow the road's bumps and dips. If the chassis twists when a tire
hits a bump, it acts like part of the suspension, meaning that tuning
the suspension is difficult or impossible. Ideally, the chassis should
be ultra-rigid, and the suspension compliant.
It is important to ensure
that the entire chassis supports the loads expected, and does so with
very little flex.
