Direct vs. OD setups

Kinda tired of other threads getting hijacked with this baloney. Sorry about that rank. Hopefully you get your answer.

I'm going to try to explain this fully. Please come in with an open mind, there will be a lot of effort here. We're going back to basics here. This is torque flow on a transmission in any gear other than direct:

Code:

            ---- countershaft ----

Input shaft ----               ---- output ----   Rear box ---- driveshaft
            ---- countershaft ----

The torque comes in the input shaft, gets transferred to the counter shafts via gears, then back to the output (or main shaft) via gears. The input shaft to counter shaft gear part is actually quite short, your gear selection occurs between the countershaft and mainshaft. There's many gears along these and to select a gear we actually slide splines into the inside of the gear. The gears are all different sizes, corresponding to different ratios through the transmission.

On any transmission in direct the flow is as follows:

Code:

           ----- countershaft
Input shaft ------------- output --- rear box --- driveshaft
            ----- countershaft

The countershafts are still attached to the input shaft, but they do not take any torque and are disconnected from the output. the torque goes directly from the input to the output. This presents a serious increase in efficiency. The transmission will run cooler, and there will be a lot less power loss through the driveline. The entire rear half of the driveline will spin slower at the same speed with a lower numerical gear ratio. This further adds to efficiency.

"But what about hill starts?"
"But what about pulling hills?"

The trick is in the transmission. A direct drive 10 speed with 2.64 gears will pull exactly the same as an overdrive 10 speed with 3.55 gears. The 3.55 gears are about 34.4% more aggressive than the 2.64 gears, but each individual gear in the direct transmission is also 34.4% more aggressive! So instead of the gearing being in the diffs, the gearing is further up the driveline allowing a more efficient setup.


Drawbacks

Most direct drive setups you can only load up to 80k gross. The Volvo one will allow 110k. Direct drive setups are harder on the driveline because it is spinning slower. If it's spinning slower at the same HP it must have more torque. The other drawback is transmission selection. You can run a direct 10 speed, volvos direct 12 speed i shift, or maybe an 18 speed with its aggressive low gear.

 

This is kind of a good video:

https://www.youtube.com/watch?v=GJzTjaU7aa8

 

I believe they do exist.

 

Not that much really. 600 lbs or so for the 10, and the 13/15/18s are all around 700 lbs.

 

They were from the 80s, I doubt a link exists.