Test time for the Hummer

April 20, 1998

6 Min Read
Test time for the Hummer

Livonia, MI--The terrain was slick and muddy after days of heavy rain. We rounded the corner. Suddenly, the right front wheel of our vehicle dropped into a three-foot-deep puddle. I wasn't concerned. I pressed a little harder on the gas pedal, and the Hummer easily negotiated us out of the crevice. Up and down, we continued through the muddy mogul course.

The Hummer, the civilian version of the military HMMWV (high mobility multipurpose wheeled vehicle), is a popular alternative to sport utilities for those who can afford the hefty price tag. I understand why. Any sport utility would drive itself back to its garage in shame if it saw the terrain this vehicle can negotiate.

Designed for combat situations, AM General Corporation's Humvee is built to handle 36-inch-deep mud holes, 18-inch vertical barriers, 60% grades, and 40% side slopes. In 1992, AM General offered the same performance to the general public. Actor Arnold Schwarzenegger was one of the first customers.

But with the commercialization of the vehicle came a problem. Many drivers are not familiar with the special driving techniques required to go off-road. "You don't need to go fast for off roading, it's all finesse," says Jeffrey Dowell, AM General's director of product assurance. Finesse or no, the company learned that some operators were not maneuvering vehicles through obstacles correctly, causing tremendous stress on vital parts of the driveline.

"The Hummer will go just about anywhere without damage, if operated correctly," says Dowell. "However, in rare situations, inexperienced drivers can compromise performance by putting excessive torque on driveline components."

To assure vehicle quality, AM General set out to identify how transmissions, transfer cases and differentials were reacting in these unique conditions. Equipped with a portable PC-based test system, the Wavebook/512 from IOtech, and a series of wireless sensors from Wireless Data Technologies (Mountian View, CA), Dowell and his engineering team measured the combined effects of engine torque, speed, and controlled brake activation on components during the worst possible operating scenarios.

The test system. The research required simultaneous, multiple-channel measurements during torturous field testing. The WaveBook is a portable PC-based, 1-MHz system designed for high-speed, multi-channel data acquisition. That kind of testing is not an easy job for equipment such as stripchart recorders. "A few years ago, it would have been almost impossible to take measurements from so many points simultaneously," says Dowell.

The data acquisition package included IOtech's WaveView(TM) Out-of-the-Box(TM) software. "This allows engineers to take measurements without the complicated setup and programming often associated with typical data acquisition programs," says Dowell. Via an IOtech WBK20 PC-Card connection, the WaveBook unit transferred data to a Compaq notebook computer equipped with a 150-MHz Pentium processor, 82 MB RAM, and a 2.1 GB Hard Drive. This combination allowed for two full days of measurements to be saved to disk or the PC's hard drive. The system was mounted inside the Hummer's cabin on the center console during field tests.

Using an IOtech WBK10 8-channel analog expansion module and a WBK15 8-channel, multi-purpose isolated signal conditioning module, the WaveBook system acquired data from 16 sensors attached to various Hummer components. Engineers mounted speed sensors to front and rear propshafts and halfshafts. They attached four transducers to brake lines to collect pressure readings. However, measuring torque on front and rear propshafts and all four halfshafts presented a challenge. Because shafts move during testing, traditional wired sensors were not an option. To solve the problem, AM General engineers custom-fit the shafts with non-contact, wireless sensors manufactured by Wireless Data Technologies. These sensors were supported by small 9V, battery-powered transmitters that sent readings to a receiver/signal conditioner mounted in the vehicle's cabin.

Field testing. Once they installed the system, engineers put the vehicle through a host of field applications designed to challenge the transmission, transfer case, and differential. To start, the Hummer made its way through a deep-ditch-crossing exercise where it went through water and mud up to 28 inches deep.

The next challenge: moguls. Like a skier through VW-Bug-sized snow piles, the Hummer worked its way through a bumpy terrain that often limited it to two-wheel ground contact. When the Hummer wasn't negotiating rough terrain, it literally found itself "against the wall" for skid torque testing. With the vehicle nudged against a concrete building, the driver floored the gas pedal until all wheels started to skid. These tests generated more torque than most Hummers will ever have to endure. The WaveBook and notebook PC worked well during these demanding tests, engineers say.

Once they had the data, engineers replicated the test conditions in a lab by mounting the Hummer on a bed-plate and four electric absorption dynamometers, one for each wheel. They controlled loads for each dynamometer individually to replicate exact field conditions. The WaveBook test system was also used in the lab to validate conditions and to further evaluate performance.

Using Snap-Master(TM) analytical software from New Venture, Inc., engineers conducted real-time, post-experiment analysis to make direct correlations between test data and component performance. Engineers learned that transfer cases are subject to the most stress during rare operating conditions. They used this knowledge to make the transfer case "user-proof."

"The ease with which we can do these tests is the beauty," says Dowell. "If this was five years ago, we would still be trying to duplicate the problem. We would have a multichannel strip chart recorder that isn't reliable, spitting paper from one end of the truck to the other. We could never have done the halfshaft tests. Plus the cost would have been 10 times the cost of a Wavebook."

Another advantage, Dowell says, is the compact size and mobility of the portable PC-based DA system. Not long ago, AM General engineers had to rely on bulky, dedicated instruments tied to umbilical cords to acquire measurements.

In addition to size issues, traditional equipment made it extremely difficult for engineers to measure multiple channels concurrently during field tests. Using several dedicated and rack-mounted instruments, tests had to be repeated numerous times to capture readings. Once data was gathered, the measurements were put through exhaustive calculations to evaluate performance. Dowell said that this process was costly and time-consuming. It also limited field testing and compromised accuracy.

"Portable PC-based systems have changed the nature of vehicle field testing dramatically in the past few years," says Dowell. "When we attempted similar tests 18 months ago, our options were limited." Today, he adds, portable PC-based systems enable engineers to record larger amounts of data from a great variety of sources.

The next level of testing with the Hummer will be to develop an instrument to use with the Wavebook to sample temperature and ventilation in and out of vehicle, as well as relative air flow of the system. This is difficult, says Dowell. "We have to determine the amount of fresh air coming in, humidity, moisture in the truck, how much is going out the back." Seems an engineer's job is never done!

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