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What is the biggest change with the new Meritor drive axle Volvo is offering? Meritor's new 14X Tandem Drive Axle offers a ratio that makes direct drive a great choice for fuel economy.
The new Meritor® 14XTM Tandem Drive Axle, now available for order from Volvo, has brought several new refinements to Meritor's axle program. They include a more robust, 20% larger inter-axle differential to handle today's high-torque engines, as well as standard AmboidTM gearing to reduce vibration while improving ride quality. But the biggest change is a new ratio which makes direct drive more attractive. "Direct drive" is a gear in the transmission that simply locks the input shaft to the output shaft. It's as if you left all the gears out of the transmission and replaced the mainshaft with a pipe. There is no loading at the gear teeth interfaces and friction losses are reduced. On the other hand, "overdrive" gears are those where the teeth counts cause the output shaft to spin faster than the input shaft. All the other gears ratios, where the output shaft spins slower than the input, are "underdrive." Every time the power flow leaves the mainshaft and travels through gear teeth, it costs about 1.5% in efficiency. That is why a transmission cooler is often required for overdrive transmissions. Direct drive transmissions offers a definite fuel economy advantage of about 1.5%, but in the past even the lowest numerical rear axle ratio available, in combination with the 1:1 ratio of direct drive, often failed to provide a low enough cruising rpm at the speeds trucks travel at today. However, when many truckers lowered their cruise speed from 68 to 62 miles per hour, there was a significant uptick in vehicles with direct drive transmissions using a 2.64:1 rear axle ratio, which was the lowest ratio available until now. Meritor's new 14X drive axle offers an axle ratio as low as 2.47:1. What
effect can that have on performance? Let's compare a truck using Volvo's overdrive I-Shift transmission with 0.78:1 transmission ratio, together with 3.21:1 rear axle and 512 rev/mile tires. At 65 mph, the engine is turning at 1389 rpm. On the other hand, make that a direct drive I-Shift transmission with the new 2.47 ratio, and engine turns only 1370 rpm at 65 mph an ideal speed to maximize the fuel efficiency of Volvo's EPA 2010-certified 13-liter engine. Compare direct drive to overdrive, and on-road performance is almost identical. However, there is another difference that will certainly benefit your pocket: a direct drive transmission will deliver about 0.1 mpg better fuel economy. This is due to its higher efficiency when it's not running the transmission gears under load. So with Meritor's new 14X drive axle offering, direct drive transmissions have even greater potential help you deliver freight while you deliver fuel savings.
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Editorial Director: Rob Simpson Art Director: Lyndsey Kone
Contributors: Jim McNamara Dewey Mooring Ed Saxman
Frank Bio Chuck VanDyke Chad Parker Allison Long
PO BOX 26115, Greensboro, NC 27402 | (336) 393-2000 | fax (336) 393-2197 | www.volvotrucks.us.com
Driver's Digest is published monthly by Volvo Trucks North America. It is produced expressly for and provided at no charge to drivers, fleet managers, owners, and other professionals in the commercial trucking industry. Contents may not be reproduced by any means, in whole or in part, without prior written consent of the publisher. © 2009, Volvo Group North America, Inc. All rights reserved. Printed in USA. Publication No. PV835-905-0904
DRIVER'S DIGEST 14
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