Long before President Obama announced the goal of achieving 54.5mpg by 2025, fuel economy has been a major concern of automakers. Reducing vehicle weight is one way to improve fuel efficiency. Automotive engineers routinely struggle to remove a few pounds of weight from a design. The use of lightweight materials is key to these efforts. Recent articles in Design News have covered such topics as aluminum metal matrix composite brake rotors, hybrid aluminum wheels with a polyurethane foam filling and a plastic outer shell, and carbon fiber composite structural components, among others.
Most of these developments have the disadvantage of adding significant cost. It’s rare to find a way to reduce weight that is cost-neutral, and rarer still to find one that actually reduces cost.
In a paper published in the January issue of the SAE International Journal of Materials and Manufacturing, and presented last year at the SAE Small Engine Technology Conference in Madison, Wis., Gouki Yotsuya and Ryo Yamauchi describe Suzuki’s successful efforts to do just that.
Suzuki developed an extruded aluminum lower control arm that weighs 50 percent less than a welded steel design, while costing roughly the same. It weighs 30 percent less than an aluminum die-cast control arm, and costs 22 percent less. The new extruded control arm is currently being used on the Suzuki Kizashi midsized sedan.
Yotsuya and Yamauchi’s paper is interesting not only because it describes a relatively new use for aluminum extrusions, but also because it provides a window into Suzuki's design process.
Suzuki already had experience using aluminum extrusions for motorcycle swing arms. For swing arms, extruded sections are typically welded to a cast cross-member. For the lower control arm, Suzuki decided to go with a single extrusion, eliminating the need for welding or multiple pieces.
In order to do this, a heat-treated extruded profile was cut to the desired length, then bent into the desired shape using a press. Lengthwise slots were cut on both ends of the extrusion (the bushing end and on the steering knuckle end), and on the top (for the coil spring). The material on the bushing end was then pressed (“shrunk”) to the desired width. Finally, holes were drilled for the steering knuckle and the bushing.
This process made it possible to form the control arm from a single extrusion, but the open section on the bushing end had a low torsional rigidity. Pressing in the bushing increased the stiffness. However, cyclic corrosion tests showed galvanic corrosion between the aluminum extrusion and the steel outer shell of the bushing. To avoid this problem, Suzuki engineers decided to make the outer shell of the bushing out of aluminum.
This eliminated the galvanic couple, but pressing aluminum into aluminum led to a new problem: galling. Reducing the interference fit to prevent galling did not leave enough press force to retain the bushing. The Suzuki engineers attempted to improve the retention by expanding the ends of the outer shell after installation, pressing on them to clinch the bushing in place. Unfortunately, the shell buckled before the ends expanded enough to provide an adequate clinch.
In order to solve this problem, the Suzuki engineers developed a special punch with four raised, curved features. Instead of pressing on the entire outside diameter of the bushing, the new punch pressed on the outer shell of the bushing into a four-leaf clover shape. This clinched the bushing into place without buckling the outer shell.
Normal manufacturing variation in the properties of the extruded aluminum was another challenge. If the material was too hard, it would crack in the forming process. If the material was too soft, it would break in endurance testing. Suzuki developed a special chemistry and heat treatment for the aluminum to avoid these extremes.
The result of all of these efforts was a savings in both cost and weight. Suzuki’s success in achieving both goals simultaneously is noteworthy. The behind-the-scenes look into the development process, as seen in Yotsuya and Yamauchi’s paper, provides valuable lessons. Their example may encourage other engineers to explore the potential of aluminum extrusions.
What an amazing story. Too often, the best material innovations cost far too much and therefore never see the light of day. It seems hard to believe, but most automakers fight for pennies -- because by the time they build a million of one part, those pennies add up. It's startling to see a part that weighs 30% less at equal cost. Dave, any idea how many pounds are saved here?
@Charles: Actually, it weighs 50% less at an equal cost, compared to a welded steel part. The 30% figure is in comparison to an aluminum die casting, but the die casting is more expensive.
Unfortunately, the paper doesn't say the amount of weight saved, just the percentage reduction. The big deal, which I should have mentioned in the article, is that this is unsprung weight. Reductions in unsprung weight mean better ride quality in addition to better fuel economy.
By the way, if you want to know what the part looks like, it's part 16 in this diagram.
It's unfortunate that Suzuki made the decision to stop selling cars in the U.S., because the Kizashi is a pretty neat car. However, the authors indicated that this technology may find its way into Suzuki's ATVs and other vehicles.
I wonder what the business implications of this are. Are any aspects of this technology patentable? Could Suzuki be seeing some licensing revenue on the horizon? With the 54.5-mpg mandate coming, I'm sure a lot of automakers would be interested.
@Charles: Yes, there is a Japanese patent (2010-254255) that covers this invention.
Interestingly enough, there is a U.S. patent (7,850,182), assigned to Hyundai, that covers something pretty similar, except that the extrusion has a double wall, and is formed in a different way. Hyundai presented their work at the SAE World Congress. I'm not sure whether this is currently being used on any Hyundai vehicles or not.
Hyundai had a lot of problems with corrosion on steel control arms a few years ago, so their interest in aluminum control arms is understandable. (Of course, aluminum is not immune to corrosion, either, as the Suzuki engineers found out!)
"fuel economy has been a major concern of automakers. Reducing vehicle weight is one way to improve fuel efficiency."
Dave, there is no doubt that mileage of automobiles is a major concern, especially when crude oil prices are rising day by day. For these automobile companies has to tune the engine performance for a better mileage either by reducing the curb weight or increasing the engine performance.
I keep a vehicle until it is ready to go to the scrap yard. The down side of this is now we have a rubber bushing that will age and fail, that requires the replacement of an entire control arm $$$. This proabaly will be a OEM only part, in 10 years not available. Over the life of the vehicle the fuel saved will not cover the cost of the repair.
@ragtoplvr: That's a good point. The crimping method used to lock the bushing in place would make it difficult to replace just the bushing; it would be hard to get it out, and you'd need a special tool to install a new one. (Assuming you can even get just the bushing by itself, rather than the entire lower control arm assembly). This part was clearly designed for manufacturing, not for service.
You're definitely right about OEM pricing, too. The MSRP for the lower control arm assembly is $357, although you can buy it online for under $300. For comparison, you can get a (non-OEM) rear lower control arm for a Ford Focus for around $35 - $50 online. And given that Suzuki isn't selling cars in the U.S. anymore, you'll be lucky to find one at any price ten years from now.
I agree that this is a good story. To me it is surprising that most articles focus on weight reduction. While I agree that weight hurts mileage, aerodynamics play a much bigger role in efficiency (even at low speeds).
Another MAJOR area of improvement would be updating the infrastructure. I know research is being done in this area, but it feels slow coming. Countries like Belarus already have digital signs posted on the roads instead of speed limits showing the speed you should be traveling to avoid stopping at a red light. Rest of the world has roundabouts instead of 4 way stops which allows traffic to flow without stopping.
As a bicyclist, I am constantly reminded how much energy it takes to ride in constant start-stop traffic and it makes me try to coast as much as possible to avoid stopping. Or how much more effort it takes to ride upright vs tucked position even at 10 or 15mph.
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