> On 5/24/07, Mike Canfield <[EMAIL PROTECTED]> wrote: > > Actually Phillip, the exact opposite is true with diesel > > engines and manual transmissions. Most failures I have > > encountered occured due to very high temperatures > > generated by large amounts of torque running in high > > gears. There is MUCH more stress on the input and > > mainshaft bearings of a manual tranny under full torque > > in a higher gear. The lower the gear the better for the > > tranny but the worse for everything behind the tranny.
It seems than at Thu, 24 May 2007 15:19:34 -0500, Zoltan wrote: > Is this the reason that automatic tranny cars with > overdrive have the overdrive-off button? (besides for > towing, etc.) Probably not. *smile* The job of the transmission is to trade speed and torque. In a perfect (no friction) transmission, horsepower-in = horsepower-out BUT, in all ratios except 1:1 (top gear in all my W123s), the shaft speed at the input is different than the shaft speed at the output. The speed ratio is the inverse of the torque ratio. So with the output shaft turning 3x slower than the input, it has 3x more torque. This is important because it is torque at the _wheel_ that moves the car. With a transmission that has overdrive (input shaft turns _slower_ than the output shaft), the engine torque is traded for wheel speed. But this means that there is less torque to push the car forward. No big deal if the road is flat, the wind is calm, and the speed is steady. But, if there is a hill, or a need to accelerate, then the wheel-speed-increase/wheel-torque-loss becomes noticeable. This hill and acceleration problem is worse if there is extra weight - 'cause cousin Jimbo in the back seat, or because of the trailer load of spare parts. I have seen a few cars that don't give the driver the control to choose each of the automatic transmission ratios. The inability to directly access 3rd in my W123 comes to mind... I don't know why the overdrive is often a button rather than a detent in the shifter gate. Maybe because it is expected to be operated more often. > Because driving around in too high a gear puts undue > stress on the transmission, yes? The torque on the input is the same regardles of the gear ratio. And the output is subject to _less_ torque in the higher gears (closer to 1:1). But as Mike suggested in a differnt post, torque seems to not be the only factor. > I often think of the example of riding a bicycle in too > high a gear for your speed. This brings awareness of the > improper gearing in the form of straining and pain. The bicycle can be a good way to think about the ratio - but there is a big difference between them. Your leg on the bicycle crank develops maximum torque at near zero RPM and it decreases as RPM increases. An internal combustion engine has zero torque at zero RPM. The torque increases with RPM for a while, then it decreases again. The two are similar in that the transmission is there to match the wheel speed/torque requirements to the limited speed range of the power source. On a bicycle, if the transmission is in a gear that's too high, it's not the transmission that suffers, it's the "engine" - your legs! > Too bad we don't have > pain gauges on cars. Then I could point to it and say to > my wife "SEE?" Yes, that _would_ be nice! -- Philip, now that it's math I'm not guessing - much...
