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
- Thickening the top and bottom flanges by 0.125 inch
- Narrowing the end slot from 0.0625 inch
- Creating a constant taper along the entire length
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.
In describing this analysis, we wish to stress to the reader the extreme importance placed on sound engineering and decision making. The consequences of a component failure on a 3500-lb race car moving at 293 ft/sec are often catastrophic.
DESIGN OBJECTIVE
Reduce the weight of the rear sway bar arms, which support no load in a race car.
DESIGN CHALLENGES
- Keep deflection of the part within acceptable limits
- Make acceptable tradeoff in weight versus strength
- Identify best design before actual field testing
DESIGN TOOLS/SUPPLIERS
DesignSpace FEA software from ANSYS Inc.
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