In Allentown, Pa., there’s a three-quarter-mile stretch of road that makes the most pockmarked portion of the Trans Canada Highway seem like a gentle suburban thoroughfare. It’s designed that way–part of the Mack Trucks proving grounds, 65 acres of heavy grades and bumpy roads. This is where Mack pushes vehicles to the breaking point and beyond in an effort to simulate conditions customers experience in the real world. In addition to the three-quarter-mile track, there’s a 1,200-foot-long skid pad with enough width for a 500-foot radius curve for testing ABS and vehicle stability. There are grades that range up to 20 per cent for brake-hold and startability tests. The real fun comes at the durability-testing areas that surround the track. One is a 1.3-mile loop studded with the sort of ruts and rocky surfaces you’d find in logging areas or oil fields, the other a half-mile “off-road” track complete with boulder fields, washouts, and drainage ditches.

The idea, explains my host, Glenn S. Hinderliter, chief engineer in charge of durability at Mack’s Vehicle Development Laboratory (VDL), is to stress a truck in a manner consistent with a given application–everything from standard highway conditions to severe vocational situations. In developing the facility, Mack used data collected over thousands of miles of actual road surface in North America, then created testing areas to replicate those conditions.
The challenge, of course, is to quickly stress the truck in a manner that’s consistent with normal use over time. Some tests happen in a laboratory, where engineers can apply realistic loads to a part or system but run it through thousands of cycles over a short period of time. Or apply greater-than-realistic loads over a smaller number of cycles.

During my rare glimpse inside Mack’s vehicle development lab, I saw the proverbial slam-the-door-a-million-times test, a roll-up-the-window test, and one where a fully fitted cab is mounted on a short chunk of frame, lifted from the rear, and dropped. It falls only a foot or so, but it hits hard. The idea is to ensure the cab doesn’t fall apart or that the interior panels don’t rip loose.

One test I witnessed was a steering lever being flexed a fraction of an inch off centre–right to left–by a pair of hydraulic rams. Somewhere upstream of the test bed, somebody estimated that over the expected life of the vehicle the steering lever would be cycled X times, with X pounds of pressure applied by the power steering pump. The test reflects that scenario.

Literally every load-bearing component or system on a truck undergoes such a test. If the life-cycle estimates are off, a part may end up over-engineered and therefore too expensive or too heavy, or under-engineered and not robust enough to survive in the real world.
That’s why in the lab and at the track, every parameter is carefully monitored and very repeatable, Hinderliter says.

Everything from the height of the stones embedded in the concrete to the depth and the distance between the ruts on the washboard road is calculated to approximate real-world conditions, but slightly exaggerated.

Through a calculated number of cycles over the different obstacles, he says his team can simulate, say, four years of regular service in just a few weeks.

Hinderliter and his people devise test cycles with different exposure ratios to certain obstacles that simulate conditions in different chassis categories.

For example, a highway truck might spend 90 per cent of its test-track time on-highway and 10 per cent off-road, while a severe-service chassis might do 25 per cent highway, 50 per cent off-road, and 25 per cent on the unpaved loop.

If Hinderliter’s team concludes that 4,000 test track miles (not the actual number) is equivalent to the expected life of the truck, they’ll run the truck around the course enough times to clock 4,000 miles.

But it’s not always complete trucks that need proving on the test track.

Often, components like torque rods are fitted with dozens of stress sensors and mounted on a truck for a trip or 50 around the course. The sensors measure the stress values and record them for collection by the engineers who use the results in their computer modeling. This helps reduce design and testing time by giving engineers real-world data to work with.
There’s a lot hanging on how well Hinderliter’s people do their jobs, he says.

Build too light and test too tough and you’ll have a product that appears to fail prematurely. Build tough and test light and you’ll have a truck that appears bulletproof.

It’s a delicate balancing act. Warranty and service depend on accurate measurements of life expectancy, and when you warranty a product, you had better be sure that it will live up to expectations. Otherwise, it’ll cost you a fortune a few years down the road.