Overdrive Magazine | Trucking Business News & Owner Operator Info
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and then behave like a standard engaged clutch to save fuel. The TC10's torque converter will multiply the engine's output torque by 1.76, allowing a powerful start while the transmission's gearing provides a 7.4:1 first gear, significantly faster than the equivalent gear in standard 10-speeds. They normally have a first gear in the range of 11:1 if top gear is an overdrive. (The first gear in the overdrive versions of Volvo's I-shift is 11.73:1.) This means the gearbox can have closer steps than other 10-speeds without having more gears. Also, the 8-9 and 9-10 shifts in the TC10 will have narrower steps than the lower gears. The Volvo 12-speed I-shift has consistent steps of about 28 percent, while the Allison's 9-10 step is 17 percent and its 8-9 step is 24 percent. This means that the Allison will cruise in 10th gear at the chosen cruise speed, say 65 mph, but can downshift to 9th without a drop in vehicle speed if the driver demands more power for acceleration or hill climbing with a significant increase in throttle. With such narrow steps, this occurs without revving the engine enough to get the rpm above the sweet spot. The result is that while steady-state, level road cruise will occur at an unusually low rpm, the drivetrain remains quite capable of delivering normal, or even superior, responsiveness. Todd Dygert, Allison's TC10 specialist, estimates the rear axle ratio the transmission will use in over-the- road, linehaul applications at about 2.64:1 with the transmission's 0.86:1 overdrive. The I-shift, Saxman says, uses 2.64-2.69:1 rear axles with its 0.78:1 top gear, depending on whether the truck is a 6x4 or a 4x2 single-drive-axle tractor. To hold cruise speeds pulling hills with an automated drivetrain that incorporates engine downspeeding, the transmission must be able either to downshift as soon as power is demanded, as the Allison does, or earlier than a standard 10-speed would when shifted manually. In the second case, the engine must also deliver full torque down to a lower rpm. Because the I-Shift has 12 speeds, it has narrower steps than a 10-speed, which helps keep the engine in the sweet spot. Saxman says the Volvo XE13's 425-hp rating hides the fact that the engine produces constant torque until it's turning more than 1,500 rpm, behaving like the Volvo engine's maximum rating where the truck cruises and holding that torque down to a lower rpm. So an automated drivetrain with an ultra-fast rear axle is practical because of more frequent downshifts and, best of all, the operator doesn't have to do that shifting. in evolution, there are no shortcuts Greatness takes time. At Great Dane we've had nearly a century to hone our expertise making trailers. To relentlessly test our industry-leading innovations. Not for the sake of change, but because we are ab- solutely driven to continue delivering the world's best trailer. And we've done it again. Our all-new, best-ever line of American-made Reefers, Dry Vans, and Flatbed trailers are as revolutionary as they are evolutionary. Each incorporates the latest industry-best innovations as standard, not optional. Each is re-engineered to deliver the best results for your business. Introducing Great Dane's All-New Line of Revolutionary Trailers: www.greatdanetrailers.com Circle 173 on Reader Service Card or visit overdrive.hotims.com MARCH 2012 OVERDRIVE 39 everest reefers champion dry vans freedom flatbeds