What better place to traction-test a 6x2 drive axle system than Michigan's Upper Pennisula in February?
The 6x2 concept isn't new, but it's not popular on our side of the Atlantic Ocean. Meritor aims to change that with its new SmarTandem single-drive-axle tandem configuration.
Meritor estimates using one drive axle instead of two will im prove fuel economy by as much as 2%, but we've heard user testimonials claiming as much as half a mile per gallon.
The single axle eliminates two of the three gear sets found in a twin-screw tandem (inter-axle differential gearing, drop gears and a second ring-and-pinion set), and the newly designed ring-and-pin-ion gears offer a significant reduction in gear-mesh friction, Meritor claims. The SmartTandem also weighs about 400 pounds less than a similar twin-screw setup.
The increased tire wear often seen on the drive axle of a 6x2 is typical of any single-axle drive setup, as only two wheels are driving versus four wheels on a twin-screw. The cost of the increased wear can be partially offset by the lower cost of the tag axle tires. Trailer tires, which cost less than drive tires, are suitable at that position.
Additionally, the electronic controls of the SmarTandem micro-manage load distribution across the four wheel positions, all but eliminating uneven wear caused by weight imbalance.
That leaves one outstanding concern: traction. And Meritor may have put that to rest as well.
In mid-February, I spent a morning at the Smithers Winter Test facility in Brimley, Mich., evaluating SmarTandem's handling on snow and ice and mixed surfaces. As any driver will tell you, icy surfaces make life more interesting that it needs to be. I found Meritor's new traction technology performed as well as any cross-lockable 6x4 setup — better in some cases, and way better than a twin-screw setup with only inter-axle diff locks.
On different surfaces, the differential locks automatically, sending 100% of the available torque to the non-slipping wheel.
The trouble with traction
As Bridgestone's Guy Walenga noted in a recent tire column I penned about low-rolling-resistance tires, “Traction is a funny thing: We know when we don't have enough of it, but can we quantify how much we need or how much is enough? More might be better and less might be worse, but we don't know what's ideal.”
So it is with a single- versus a twin-screw drive-axle grouping. Normally, all four wheels in a 6x4 share the tractive duties on good surfaces. Likewise for a single drive axle. On less than ideal surfaces, if one of the four wheels in a 6x4 loses traction, the vehicle won't move until the inter-axle diff is locked. However, if the truck is on a split surface, i.e., ice on one side and pavement on the other, locking the IAD won't help. There, you need a cross-lock differential — and it's the same with a single drive axle.
Meritor's SmarTandem levels out that little playing field.
The SmarTandem actually improves the single-screw's tractive properties in a couple of ways, and it virtually eliminates the possibility of a driver wrecking a differential by engaging the locks while a wheel is spinning.
“The SmarTandem is an electronic control system as well as an axle system,” explains Charlie Allen, general manager of Meritor's Customer Technical Support. “It monitors wheel speed and wheel slip to determine wheel slippage. With that information, the system brings several levels of intervention to the traction problem.”
First, Meritor's SmarTandem controller system will automatically shift the load from the tag to the drive axle to bias the load for the greatest tractive effort. Allen says the weight transfer stays within the mechanical rating of the drive axle, and within the bridge formula, so it's not running overloaded.
With the second level of intervention, the electronic controls sense wheel slippage off the ABS exciter ring, and automatically engage the cross-locks on the differential to get torque out to the non-slipping wheel.
“If shifting the load to the driving axle isn't enough, the system can automatically lock the differential so 100% of the available torque can be applied to one wheel end,” Allen says.
As an additional proactive function, the driver can manually lock the diffs and shift the weight onto the drive before wheel slippage occurs. The system automatically disengages at 25 mph or with a 90-degree turn of the steering wheel, so there's no possibility of forgetting to disengage the system.
To protect itself from damage, the diff lock uses a face-clutch mechanism rather than a full spline lock. The clutch allows for safe, smooth engagement even if a wheel is already spinning on ice, but the system will not engage the cross-lock if there is a great difference in wheel speed.
The face-clutch versus splined coupling (as found on the FuelLite's 160-series axle) makes the difference, Allen says.
“With the 160 series you have to be almost stopped to lock the diffs,” he explains. “SmarTandem has a face clutch that can be engaged with a greater speed differential between the wheel-ends. The control system manages the engagement and lockup almost immediately upon sensing slippage, so it's very proactive, as opposed to a driver's reaction, which would normally be after-the-fact and only at very slow speeds.”
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