How to make D15 with locked 50/50 transfercase?

Hi. The new crawler-styled D15 is great but the transfercase seems to be visco-type/Limited Slip. In this pic the rear wheels spin but front dont.
Is it possible to make it locked 50/50 4WD?

Thanks!

 

It is a viscous coupling, I think set in the transmission rather than diff.

 

Is this typical of real-life trucks? I thought a 50/50 transfer case was what a typical 4WD truck would have straight from the factory.

 

Thanks for the response Goosah, it was informative. It looks like I used the wrong terminology. You are correct; I was under the impression that a typical truck (ignoring modern SUVs & other such "AWD" things) would have a transfer case which behaves in the way you described. It would have no center diff at all, just a gearbox allowing range selection and providing the ability to disconnect the front driveshaft from the rest of the drivetrain. (So 1:1 speed being fed to front and rear diffs with variable torque, as you described)

I understand that the current system is a stand-in for a more complex system you'd like to implement. That said, simply FWIW, in my experience I often find that the 4WD stuff unloads through a single wheel. I hadn't payed attention to the past whether it happens mostly with the automatics or also with the manuals.

When I search the text of the files in the /pickup/ directory I only find a single viscousCoupling reference. I'm assuming that as it's currently implemented the viscousCoupling is necessary to keep the engine alive through shocks to the drivetrain. (So it wouldn't be practical for someone to try and increase that value to some arbitrary high number.)

 

Thanks for the insight Goosah, but I'm afraid that I'm going to have to contest that statement that 50/50 designates an open differential.

I apologize in advance for the wall of text.​

In an open diff (for simplicity's sake we will start will axle differentials, as opposed to the transfer case/center diff), as you know, wheel slippage will cause the slipping wheel to get 100% of the available torque causing a single wheel burnout. If you put the car on jack stands and secured one wheel, the other will rotate with 100% of the available torque. Conversely, if you spin one wheel by hand in the same scenario the other will spin in the opposite direction. This is because the pinion and ring gears spin a gear cluster rather than the axles themselves.


Here is an open differential. The ring gear (designated with the Blue arrow) receives the power from the pinion gear and rotates the assembly highlighted in Red (which will also spin at the same speed as the ring gear). The red assembly is affixed to the gears denoted with the Green line. These gears can spin freely about the rod connecting them but, due to the nature of gears, will oppose each other's rotation forcing the Orange gears to rotate and power the wheels. This is describing a situation where no wheel slippage is occurring. The wheels will each turn with 50% of the torque and the green gears will not spin if the car is moving in a straight line.

To explain with the aforementioned jack stand scenarios above with an image (for those who are unfamiliar with differentials), the first scenario with one wheel fixed and the other being powered can be explained by visualizing the left orange gear being held in position. The red assembly will still spin (as the ring gear is being spun by the engine), but since the left orange gear cannot move, the green gears will then spin at twice the speed of the red assembly forcing the right orange gear to rotate with 100% of the torque. In the second scenario where one wheel is being spun manually, the red assembly will NOT spin (providing that the drivetrain offers more resistance than the orange and green gears) and the opposite orange gear will be forced to rotate the opposite direction by the green gears. This is why open differentials have a 100/0 torque split. If one wheel cannot turn, the other will receive 100% of the power.


If you had a limited slip, such as the one pictured below, (note: the picture is of an assembly comparable to the red assembly above with no axle housing and no ring gear. The ring gear would mount on the raised flange with the teeth facing inwards if it were installed) The S shaped spring will resist differences in rotation between the two orange gears thus forcing them to, at the very least, rotate in the same direction. These types of diffs can offer different torque split ratios, but a common one is 70/30. This style of limited slip is the most common in regular 4x4 trucks.


If you repeated the same tests above with a limited slip, the wheels would not turn with one fixed in place, and spinning one by hand would cause the other to spin in the same direction (it would also force the drivetrain to spin).


If you had a spooled differential, such as the one in the picture below, the center gear assembly is replaced with a solid steel rod forcing the wheels to always spin in the same direction as the ring gear with 50% of the torque each, and no possibility of slippage. This is simmilar to a locker, with the exception that a locker will still allow for a wheel to spin faster than the other if it is forced to. An example of this would be cornering. The outside wheel will be mechanically forced to spin faster to accommodate it's larger travel radius, but it will not be assisted by torque while it does so. The other wheel would still only get 50% of the torque. (welding the diff will accomplish the same thing as a locker)

To explain the picture, the red shaft replaces the red assembly and is affixed firmly to both the blue ring gear, and the axle shafts. The red shaft, and therefore the axles, must always rotate with the ring gear and vice versa. There is no possibility of slippage in this configuration, and any differences in rotation will cause mechanical failures in the weakest parts. To quote a friend, "the spool won't break, everything around it will".


Moving on to the transfer case, all of the above explanations can be translated to different real world parts, but the picture below is of a locked 50/50 transfer case which is the most common kind on 4x4 trucks. This particular example is of a chain driven transfer case. If the transfer case or "center diff" is limited slip (they are very rarely open) it would be referred to as an "all wheel drive" transfer case.

The picture explains itself pretty well, but to clarify on operation, the roller chain and connected gear will only rotate if the car is in 4wd. The orange arrow will always power the rear axle, but the sprockets and chains must be manually engaged to provide power to the front axle. When disengaged you would have a 100/0 split (that is, 100% of torque to the rear axle, and 0% to the front), and when engaged you would have a 50/50 split. Once again, there is no slippage when engaged without mechanical failures.

TL;DR 50/50 split is locked, 100/0 is open, and everything in between is limited slip. The D15 can be best compared to an open center diff, or a very weak limited slip (95/5 or something like that). As for the coding behind this, I can only speculate.

This post is only to provide clarification on how differentials work and what makes them what they are for future reference.

 

Wouldn't it be the reverse though? If one wheel is in the air and the other is wedged with an open diff, the wheel in the air will begin to accelerate with power, so it's torque must be >0. The wheel on the ground would not accelerate and thus must have a net torque of 0. Since there is 100% of the torque available to the rear end (ignoring friction and heat loss of course), the split in that scenario would be 100/0.

If the rear end was locked and in the same situation, the wheel on the ground would not accelerate, the wheel in the air would also not accelerate. This will force the engine to die or something to slip/break. Both wheels must turn at the same speed, and since that speed must be 0 with one wheel wedged, the net torque on each wheel must also be zero.

EDIT:
whoops, we're talking apples and oranges. You're right, I'm factoring time into it, and thus talking about power.

 

Yeah you're right. My misunderstanding stems from here-say and I was looking at net torque and power as opposed to static torque and torque from the vehicle alone. Just because it isn't spinning doesn't mean that it isn't receiving torque, just that the force of friction is greater than the torque applied. The one wheel burnout is due to the fact that friction coefficients go down as an object begins to slip.

I'm sure you will agree however with my description of the components themselves with the exception of the torque stuff. I've torn apart axles before, I know how the gearing works.

Feel free to delete this whole conversation if it's cluttering up the thread, or not if you feel like it could help someone else on the straight and narrow.

 

Interesting. Will this be applicable to the transfer case as well or is it more for the axles? I too would like a locked transfer case in the off-road D15, despite the disadvantages on-road. I don't know what opportunities the code currently offers but I suppose you could simulate it like how Auburn lockers work. If you've never seen them before, they're pretty much a limited slip diff that can be electronically tightened to the point that it all but eliminates the possibility of slip. A lot of people like them because they are limited slip while passive and when "locked" they will, in theory, slip rather than break if a failure point is reached.