Downspeeding confusion

Remember these trucks are now built for down speeding. These new engines pull full torque at very low RPM's where your old series 60 operates a much higher RPM for economy and performance. Apples and oranges. We had a 2016 volvo (rental) direct drive I SHIFT and that truck purred up and down the highway at 1250 all day long. If it caught a grade it take it to a 1000 and grab a gear and off it went. I will say I was very impressed with that truck and the MPG's. I agree everyone's area of travel and job are different but these trucks if spec'd right are performing fantastic. I run an 05' cat C-15 18spd at 1350-1400, anything over 1400 it drinks fuel more than usual.

 

Well I have a Volvo at VNL 780 D16 @500 Hp I shift OD transmission with 2.64 rears.lp tires 2050 ftlbs torque. I run at 1150 at 60 MPH in 12 gear. i run out of California so any way I go I am hitting a mountain. I do down shift to keep the rpms up for the hills but the truck seams to be doing just fine. Granted i only have 86K miles on it so far so I will have to see about the premature failure of the drive train components.

 

I fail to see how running an engine at a lower RPM will cause premature wear on the drivetrain parts. Transmission, rears and driveshafts are matched to the torque output of the engine. Running an engine at a lower RPM won't magically cause it to produce more torque than what its programmed to put out. Drivetrain components shouldn't wear out faster. If anything a guy would think they should last longer since they're spinning slower (well the transmission and driveshafts anyways). The benefit of running high gears (like 2.xx or whatever) and a direct drive transmission is that you eliminate parasitic power loss within the transmission. Power goes straight through from input shaft to output shaft. It isn't being transfered over from input shaft to counter shafts to aux drive gear to aux countershafts to output yoke (in the case of a double overdrive transmission).

 

It has been well researched that one of the major issues with downspeeding and taller ratios is harmonic vibrations in the drivetrain that can lead to faster than normal component failure. There have been many articles written about it by Eaton, Meritor, TMC, etc.

 

Did not know that there were harmonic vibrations in the drivetrain caused from lower engine RPM. One would think that it would be no different than simply running slower in top gear.

 

Speed really has little to do with it. It is one thing to run engine at lower RPM, but when you combine that with tall rear ratios, torsional vibrations can be created that eat at u joints and such. It requires very special attention to specs on drivelines. One thing that has been touted is spec'ing higher torque rated drivetrain components than normal. Like 2050 rated components instead of 1750.