Andrew Halonen

Click here to see this story as it appears in the March-April 2018 edition of Metal Casting Design & Purchasing

With the increasing CAFÉ 2025 fuel economy targets, automobile manufacturers are using many methods to improve fuel economy. Engine downsizing, turbo systems, electrification, aerodynamics and lightweighting are among the most effective methods. In 2017, there was much discussion about electrification: will it gain market acceptance and if so, how will it affect the material selection? Logically, range-limited vehicles will benefit most from lightweight materials, yet because car manufacturers are adding expensive batteries and related operating systems, they need to carefully manage costs.  On the materials front, each issue of an automotive magazine includes color-coded images representing different materials that make up the “multi-material vehicle.”

Most of the time, this is a vehicle body structure, commonly referred as the body-in-white, yet it often includes the closures, the doors, hood and trunk. The body and closures cover a significant portion of the weight of a light-duty vehicle, on the order of 30% of the whole vehicle. 

What about underneath the body? Which materials are used?  Which processes are used?  To find out, a study looked at 89 vehicles over 16 brands and includes five performance cars, 24 cars, 38 cross-overs/SUVs, 15 pickups, four minivans and three work vans (Table 1).

The wheel-end and suspension components are roughly 20% of the total vehicle weight, yet these components and systems get little attention in the automotive press and across the material consortiums promoting their respective materials and process innovations. These components include wheels, brake calipers, brake discs, steering knuckles and suspension arms and links (Fig. 1).

Wheels

Most wheels on light duty vehicles are cast aluminum. Years ago, steel dominated the wheel market. The first cast aluminum wheels were a weight-add, meaning they were heavier than steel. You can find steel versus aluminum weight studies dated back to 2009, yet regardless of weight, the vehicle designers desire the look of aluminum wheels. 

The casting process is versatile, providing an open palette of wheel designs. In the car study, 22 of the 24 cars had aluminum wheels. The Chevy Spark had steel wheels and unsurprisingly, the other car with steel was a Toyota Camry hybrid. While it might be surprising that a hybrid had steel wheels, this OEM selects mostly ferrous materials.

In all of the vehicles across all segments, nine of 10 wheels are aluminum.  Industry insiders say the overall aluminum wheel content is around 75% because most spare tires are steel. That is, until OEMs replace that spare tire and the jack with a can of “fix-a-flat,” saving about 30 lbs. and reducing the vehicle cost.   

Brake Discs and Drums

Typically, brake discs are made of cast iron. The automotive market has introduced new technologies in recent years–carbon disc, two-piece discs, coated discs to name a few, yet none are used on the study cars. All 24 cars had iron discs (aka rotors) in the front axle, where 60-70% of the brake energy is applied. Three of the 24 cars had brake drums on the rear. 

The impetus of the study was to determine the brake caliper material. It began after spending many years working on alternative brake disc technology and hitting roadblocks in the marketplace. Though OEMs are committed to using iron discs, would they at least address the unsprung weight by using aluminum in the calipers?
The lighter the vehicle, the larger the impact of weight savings. Wheel-end components carry additional benefit as unsprung mass, though the unsprung mass is most appreciated in performance cars where the entire system is optimized.  Despite the mass impact and unsprung mass impact, and the fact that most front axles on front-engine sedans are challenged by weight, the study found that the heavier cast iron material dominates on the front (Fig. 2).  
On the rear, where there is a reduced braking load, the disc can be smaller in diameter and thinner, with no air convection venting required. With the large gap between the disc and the wheel, it is easy for the OEM brake designer to fit an aluminum caliper, and it can be a late program decision. Conversely, because the front brake disc is larger, the decision to package a thicker aluminum caliper must be made early in the vehicle development program. Figure 3 shows a mix of brake caliper offerings.

Three major factors are considered in brake caliper material selection: cost, performance and space. Gray iron is the lowest cost, and its higher modulus (stiffness) enables a thin bridge section, the region of the caliper extending across the brake disc.  A thin bridge makes it easier to package.  However, iron also has a higher density.

Aluminum is a lower density, and to overcome the reduced modulus (stiffness), its bridge section must be made thicker. A 9.5mm scratch-free spacer was placed between the caliper and the wheel, and it was recorded whether the spacer fit.  On the cars, front and rear, all iron calipers had at least the 9.5mm gap between the caliper and the wheel.

The gap between the caliper and the wheel on two vehicles can be compared in Figure 4.

Knuckles

Knuckles are a critical safety component. If the knuckle fails catastrophically, the vehicle would be out of control.  
To ensure the capability for the knuckle to bend before it breaks, OEM specifications require a minimum of 8% elongation, with some pushing for 10% elongation. In iron, a common choice is 65-45-12 ductile iron (12% elongation). In aluminum, all the cars that had aluminum knuckles were castings, presumably A356-T6, and likely produced in gravity or tilt pour permanent mold, VRC/PRC, squeeze casting, COBAPRESS, or counter pressure casting.

Across the 24 cars, cast aluminum outnumbered cast iron front knuckles 15-9.
On the rear, the suspension design varies by vehicle weight. The lowest curb weight cars tend to use a compound crank rear suspension, nearly identical to a torsion beam or twist beam suspension.  The compound crank suspension is often a steel weldment, yet the Dodge Dart has an aluminum casting.

The remainder of the cars had traditional cast knuckles: 14 were aluminum and seven were iron.
Caliper and knuckle materials and processes are basically limited to cast aluminum or cast iron, with the few exceptions on rear knuckles.  This all changes in the suspension, where there is ample opportunity for castings and ample competition by other materials and manufacturing methods.

Suspensions

There are six different ways to produce a lower control arm on a small sedan.  And, seven different ways were found to produce the rear upper control arms.

Looking at a subset of six vehicles with curb weights ranging from 2,246-3,969 lbs. (1,018-1,800 kg), the study found forged steel, welded steel, stamped steel, forged aluminum, cast aluminum, and a unique product on a rear LCA, extruded aluminum with a cover, formed via a process called Extruform. One of the six vehicles is a plug-in hybrid electric vehicle. It has steel suspension arms, a mix of aluminum and iron on the brake calipers and steering knuckles, and it has a cast aluminum subframe.

Most of these cars fall beneath a gross vehicle weight of 4,700 lbs. (2,132 kg), which is around the cutoff between a front suspension using a MacPherson strut (no upper control arm), and a double wishbone (upper and lower control arms), as is used on heavier vehicles.  

On the upper control arms, we found seven different material and process combinations were present with the addition of ductile iron, which was not found on the lower control arms. Across 24 car models, the upper and lower suspension data is found in Table 2.

Lightweighting and the Future

From the data above, were 2017 model year cars emphasizing lightweighting? 

For the brake calipers, designers have said that an aluminum caliper, even though it needs to be beefed up to overcome reduced modulus, will deliver a conservative five lbs. (2.3 kg) of weight reduction per axle. Clearly, eight of the nine OEM vehicles in the car study have not valued this weight over the additional cost of cast iron calipers. 

Conclusions

What conclusions can be drawn from this multi-material make-up of suspension and braking components?   
There are plenty of options for control arms, varying by price, performance, material, process and weight.
With exception to the premium vehicles, there is considerable opportunity for lightweighting on already proven applications. An example is the brake caliper that represents 5 lbs. (2,267g) of weight reduction available on the front axle, yet most vehicles were equipped with cast iron calipers.

As the industry draws closer to the CAFÉ 2025 regulations, will a large change in the component make-up occur? Perhaps the six lower control arm materials and designs will be reduced to fewer options, or maybe other vehicle lightweighting efforts will allow for more options like aluminum vacuum diecasting or magnesium castings or forgings.

For the performance cars and EV’s today that are already using lightweight materials for suspension, steering and brakes, it is fair to wonder what is their next move.  Will it be an innovation in wheels?  In the brake system?  The subframe/K-frame and engine cradle are likely targets because they are bolt-on components, and they are being studied now.