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Engineering A Serious Chassis

When you’re developing a chassis for a vehicle that weighs 63,300 lb. and that not only has to travel on land and water, but must deal with people shooting at it, you do it very, very seriously.

Product development for the U.S. military isn’t what it is in the auto industry. Consider this: General Dynamics Land Systems (www.gdls.com) obtained the development contract for the U.S. Marine Corps’ Expeditionary Fighting Vehicle (EFV) in 2000. The Marines had decided in 1994 that it was going to need a replacement for the Amphibious Assault Vehicle, which debuted in 1972. But that’s another story. The EFV was originally scheduled for deployment in 2010. That date has been pushed back until 2015 due, in part, to things like steering problems resulting after water testing, and the fact that the EFV prototypes are requiring more maintenance than had been originally anticipated. In addition to which, Congress has asked General Dynamics to improve the vehicle’s operational readiness as regards the improvised explosive devices (IEDs) that have been proving to be so deadly in Iraq. The per-unit cost of an EFV is expected to be $22.3 million, not the original $12.3 million.

All of which is to say that given the time frame and the near doubling of the cost, one might imagine that there’s nothing that auto engineers can learn from the EFV program. Which is not necessarily the case.

The Parameters. Consider these specs: 

 

• Length:
• Width:
• Height:
• Weight:
• Seating capacity:

• Speed (land):
• Range (land):
• Speed (water):
• Range (water):
34.6 ft
14.6 ft
9.2 ft
63,300 lb.
20 (17 of whom are
fully armed Marines)
45 mph
400 miles
20 knots
65 miles

 

Unless you are a Marine, this is certainly not your daily driver.

According to Darrell Duszka, engineering and technical director for the EFV project at General Dynamics Land Systems, the considerable mass of the vehicle would have been significantly greater had the engineers failed to develop a chassis that is both light-weight, yet strong enough to withstand the pounding waves of the ocean and the varying terrain of the battlefield, not to mention various ballistics. Designed using a space frame structure on the upper half of the vehicle—the first time a space frame has been used on a heavy-tactical vehicle—along with a monolithic structure on the lower section, the EFV chassis is constructed from high-strength aluminum alloy 2519 encased in ¼ in. thick armor using friction stir welding.

Welding With Friction. The welding process uses the friction of the rotating tool to heat and stir together the materials being joined. The materials aren’t brought to their melting points but to the temperature at which they develop a plastic-like condition to enable stirring together. The result: reduced material property changes due to heat transfer and welds with greater strength and more ductility than traditional gas metal arc welding. “The use of friction stir welding allowed us to more accurately join different aluminum alloys together, including some lithium-based alloys,” says Duszka. The welding process also provided improved ballistic test results, along with reduced cycle times—up to 400% for 1-in. thick plates compared to the original 2-pass process—and material distortion. The military expects the use of friction stir to provide improved return on investment results within 5 years, with a savings upwards of $25.8 million, according to the Office of Naval Research. “We think this is where we have found the most transferable technology for use outside the defense industry,” says Duszka.

The EFV rides on aluminum, double pin, rubber brushed tracks with replaceable pads—the tracks alone add 2,688 lb. per side—within which there are seven wheels. The EFV’s suspension is a retractable hydropneumatic design with 18 in. of jounce and 5.6 in. of rebound. “We had to look at all of the technologies that would allow the vehicle to go high-speed on the water and we knew that in order to achieve that we would have to develop a new suspension that would fully retract in the water,” Duszka says. Being able to retract the suspension provides a smoother surface for improved hydroplaning, which results in improved operating efficiency with less applied power. A retractable front bow provides added performance during water maneuvers and helps to control stress on the chassis itself. Managing the various operating environments on a single chassis structure required a number of modifications along the way. “We have had a couple of areas where we have had to go back and reinforce the structure. We thinned some of the material out where we could save additional weight, but we had to make changes to the final drive attachment and add some material back where the water jets are attached. We also had to make some adjustments to the welding in certain areas.”

In case you’re wondering, the EFV’s primary weapon system will be a Bushmaster MK44 Mod 1 30-mm cannon with 215 rounds at the ready and another 180 available on-board. Additional firepower comes from the 7.62-mm M240 machine gun, with 600 rounds at the ready and an additional 800 on board.

 

How’s This for Power?

When traveling in the water, the EFV is powered by two 23-in. diameter Honeywell (www.honeywell.com) water jet propulsion systems located at the port and starboard in the aft of the vehicle, providing more than 12,000 lbs. of maximum thrust apiece. Once on land, the EFV is powered by a water-cooled turbocharged MTU (www.detroitdiesel.com) 12-cylinder diesel engine (MT883) producing 850 hp @ 2,600 rpm and 2,626 lb.-ft. of torque @ 1,700 rpm mated to a 6-speed Allison (www.allisontransmission.com) X4560 transmission, all of which is managed by a Packard Electronics (www.delphi.com) control unit.