Both axles use the same R-Series spindle with common hubs, bearings and seals for easier parts management.
But does it work?
Over the course of two decades and 2 million miles, I've found myself stuck more than once. A common winter hazard is warm tires parked on hard-packed snow. They melt themselves into cups in the ice that might as well be wheel chocks. That actually happened to the engineering crew that brought the truck from Troy, Mich, up to Smithers for our test drive. They had parked it the night before on a hard-packed surface, and the next morning found it sunk almost an inch into the ice.
The driver, Mark Kleckner, a development and test engineer in Meritor's Advanced Engineering & Electronics department, engaged the system and rocked the truck gently back and forth a few times and out she came.
Test No. 1: Pass
I drove the truck about 20 miles from the hotel to the track on some northern Michigan two-lane roads and noted no shortage of traction, even in sharp turns where the grade of the two intersecting roads was uneven.
At the track, we ran the truck over several different surfaces, from glare ice to hard-packed snow and even loose and broken icy snow. We also ran it over some mixed coefficient-of-friction pads with hard snow and dry pavement, ice and dry pavement, and ice and hard snow.
Tests No. 2, 3, 4: Pass
Because the truck was an engineering test truck, it was equipped with gauges to monitor air pressure and blinking lights to indicate system status that a stock truck wouldn't have. I had the benefit of “seeing” the system perform as well as feeling it. The transfer of weight from back to front was very fast. It took between five and eight seconds for the pressure in the suspension to go from about 50 psi in each axle to about 75/25. And it returned to normal just as quickly. I never heard any grinding sounds when the cross-locks were engaging, contrary to my previous experience with locking inter-axle diffs.
I have never had to use cross-locking diffs before, so I can't provide a 6x4 versus 6x2 perspective, but this truck had very little difficulty launching on any surface. Naturally, with both wheels on glare ice there was a bit of slippage even with the differential locked, but the traction control gently applied a little brake to the wheels to minimize the spinning.
Test No. 5: Pass
After a few hours on the track, I concluded that about the only situation where the SmarTandem might be at a disadvantage compared to a 6x4 setup would be on dramatically uneven ground, where the drive axle was left, literally, dangling in the air. If you were you to back the tractor up onto a curb, it would take a few seconds for the pressure in the tag axle to bleed off and the pressure in the drive axle to push the wheels onto the pavement. I suppose the height of the curb would be a factor in the effectiveness of the system, but what's a driver doing backing over a curb to begin with?
Test No. 6: Conditional pass
Allen is quick to point out that SmarTandem is designed for on-highway operation, which implies a minimum amount of time spent off-roading. How often your trucks wind up on really uneven terrain would be something to consider when specing such a system.
I'm sure some driver will find a way to get stuck with the SmartTandem, but in my short time on some really dicey surfaces, I don't think there's any situation where a full 6x4 could do any better than this 6x2. And I'll even say that the SmartTandem would do a better job in some situations, one being its ability to protect itself. Allen says it will not do anything beyond its design envelope, and as soon as the truck reaches 25 mph, or the steering wheel is turned beyond one-quarter turn from straight ahead, the system completely disengages.
Final verdict: Pass