October
5, 1998 Design News
BREAKTIME
Readers describe design successes
FEA gives NASCAR driver the edge
Charles "Eddie" Thomas, Owner,
Advanced Component Design Co.
Larry Hedrick Motorsports (Statesville, NC) fields
the #41 Kodiak-sponsored Chevrolet Monte Carlo with
Steve Grissom driving. Team Engineer David Holden, a
graduate of Lehigh University with a Masters in Mechanical
Engineering, is responsible for, among many other things,
developing and refining chassis components.
In the ultra-competitive world of professional auto
racing, the NASCAR Winston Cup Series is considered
one of the most competitive in the world. On any given
race weekend, there are teams capable of winning. With
corporate sponsorship at an all-time high, more teams
are well-funded and well-equipped.
This fact, along with the restrictive nature of the
rules governing the cars and engines, has teams searching
for anything that could be a competitive advantage.
Since a number of the rules preclude the use of many
technologies, gains in performance are made in small
increments. Every single component that makes up a race
car is closely scrutinized.
That's why Team Engineer David Holden focused his attention
on the rear sway bar arms, located at each outboard
end of the rear axle. Consisting of a rectangular bar,
attached at one end to the rear axle housing and at
the other end to a torsion bar, they are used to help
control and fine-tune the handling of the race car.
After testing the arrangement at the race track, Holden
believed that the current geometry of the arm was excessively
heavy for the load it supports. Although all cars in
the Winston Cup Series have a minimum weight requirement,
weight is still critical. It's the placement of that
weight that's important, because it affects the center
of gravity and other dynamic handling characteristics.
In the realm of weight there is "sprung" and
"unsprung" weight, referring to whether or
not a component is supported by one of the suspension
coil springs. Weight of the unsprung variety is a bad
thing, therefore engineers strive to keep it to a minimum.
Since teams are limited to seven track testing sessions
per season, Holden knew it would be beneficial to test
and optimize new designs using software tools. He turned
to our firm, Advanced Component Design (Charlotte, NC)
to assist with the analysis. We use DesignSpace from
ANSYS for all mechanical stress analyses.
First, we established a benchmark. Examination of the
stress distribution confirmed Holden's suspicion: the
relatively heavy component supported no load. He proposed
adding a tapered portion, lightening pockets, and double
shear mounting to the flat bar. That reduced the weight
significantly, but the part had excessive deflection.
To reduce the deflection in the part and arrive at
the best combination of strength versus weight, we subjected
the model to several other design iterations before
arriving at the final geometry. Our final design (see
photos) involved:
Increasing the center web by 0.10 inch
In the end, we increased the safety factor by 17%,
increased deflection by 59%, and reduced the mass by
40%. As in every case, we had to make compromises to
achieve the desired result, but we were able to make
successive iterations in only a few hours of time. Without
using DesignSpace, we would have had to machine a component,
find time to test it, and learn what needed improvement
by trial and error. DesignSpace allows you to get to
the right design before you take a tool to metal.
Design Objective
Reduce the weight of the rear sway bar arms, which
support no load in a race car.
-
Keep deflection of the part within acceptable limits
-
Make acceptable tradeoff in weight versus strength
-
Identify best design before actual field testing
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