Divorced Transfer Case Doubler w/ single output?

Looking for alternative ways to increase my gear ratio so that I can get more torque and actually crawl my truck. Since nobody really crawls their 2/2.5 gen IFS Tundra there are no bolt on aftermarket TC options out there. So I've been thinking... is there a product out there that is basically a divorced transfer case with a single input and single output?

Please excuse the crude drawings but this is what I'm thinking of:

Tundra transfer case has 1 married input from the transmission and 2 outputs to the front and rear:


Here I'm thinking the doubler could connect yoke to yoke on the input side and a shorter shaft on the output. All I would need is at least a 2:1 ratio in the doubler and that would get me up to 92:1 in low gear. Wouldn't mind a 4:1 either. So does something like this exist? Point me in that direction please. Or is this a pipe dream?

 

Yea, it's often called a crawl box or a doubler. It's just the gear reduction portion, without the 4wd part and goes between the tcase and the transmission. That's usually what "twin sticks" refers to, is one stick which is 2hi, 4hi, 4low, and the second stick is just a hi/low and on the crawl box. So you can run 2hi/single low for instance to get a 2lo, or 4low, or 4 double low, etc. As you said, I've never heard of one on a tundra, usually it's like 4runnera or Tacoma's where they strip a case down and bolt another one behind it.

With that said there's no reason you can't use a normal tcase and just have a front output that doesn't do anything, that's not uncommon. For example put a passenger drop tcase between the tranny and your stock one (Im assuming the tundra is driver drop, forget if that's true ) and then you just have a front output on the first case that isn't hooked up to anything and just spins, and then the rear output obviously goes to your normal tcase.

Have you looked at just running a different tcase entirely that's divorced like an atlas or scs? A normal 2 speed atlas can go to 5:1 iirc and if you go to a 4 speed (which is exactly what we're talking about, they slap another gear reduction unit on the front) then you can have everything up to like 9:1 iirc, plus would have front dig and probably a much stronger unit in general, and wouldn't have to deal with all the electronic bullshit then.

 

Here's some links to stimulate you, these arent for a tundra but you might be able to do a custom adapter. Early Toyota and NP205s are the common tcases for doublers.

https://bustedknuckleoffroad.com/co...ransmission-doublers-planetary-gear-reduction

https://trail-gear.com/trail-creepertm-2-28-crawl-box.html

 

I figured you'd be one of the people answering

I've thought about just using a regular TC but I don't want to drag around all that extra weight, was hoping for something purpose built and less porky.

Thought about the Atlas/SCS as well but the thing is how do I get the front drive shaft around the stock TC without having to go under it and losing more of my breakover angle?

 

I meant replace the stock tcase entirely. With a 5:1 atlas that's nearly double the reduction (assuming the stock case is 2.x). Run a 4 speed or a double on an atlas and then you have way higher reduction and a lot of other perks.

How I see it, any path will be custom work. If you keep the stock tcase now you have to adapt shit to it, all for a case that isn't anything special besides you have it. This is all ignoring the fact you need to adapt to the transmission either directly or run any setup divorced (probably what I would do to keep it simple). If you swap the case to a 205 or an atlas or whatever, you now have a ton of aftermarket support, a stronger case most likely, and more features and capability, plus everything within the tcase ecosystem will be relatively plug and play. You try and just double the stock case and now you're down the road of everything is custom most likely. From where I'm sitting I think you're better off to get rid of the stock case and pick a completely new tcase ecosystem and commit to that, and that will be entirely stand alone from the rest of the truck so besides getting the divorced driveshaft in there you're not fucking with adapter plates and stuff like the stock case will require, and get all the above perks.

It's like the debate of do you build the engine for 1000hp, or just buy a LS and swap it that will do it all day easily? One is a lot less headaches but you have to leave the Toyota parts behind. Though in this case there's nothing uncool about a different tcase like there is with a LS

 

How hard is it to get a custom adapter made? I have been looking at some really nice NP205 cases and your point does make sense that it would have aftermarket support for it

Just thought of something.. do you know if the transmission is the same for a 5.7 2wd vs. a 4wd Tundra? Thinking I could scrap the stock TC, use whatever they use on the 2wd transmission to cap off the transmission, then use a divorced NP205

 

A plate would be easy, albeit not cheap. If you sent a shop the CAD I bet it'd be a $600+ just due to the size and it being a one off part, though anyone with a 3 axis would be able to make it. The real issue is the output of the transmission adapting to the tcase, that may be an off the shelf part or it may be a custom splined part which is suddenly very expensive and relatively speciality machinery to make. I'm not sure what the spline size of the tundra transmission is, you might be able to call advanced adapters and see if they know and/or offer a female. And if you really wanted to get crazy you could take the stock tcase input or even a driveshaft flange for a 2wd and maybe machine that to work TLDR I think it's a lot of work unless someone else has done it.

I don't know anything about 2wd tundras, I ASSUME the transmission is identical and just has a different tail housing? That would certainly be the way to go if that's true. If that's not the case you again are paving ground by getting a flange onto the tranny output and finding something with the right splines to mate to it.

And just thinking out loud here: there might be issues if the output shaft on the trans is totally different. Unless the 2wd driveshaft is different from a 4wd and has a slip yoke on the trans side, then there would have to be some way to retain a flange onto the splines. For instance the tcase output flange bolts to some splines, but I doubt the trans output has threads on it so if the 2wd output used a flange then there's probably a different output shaft. Now that might not be an issue with a divorced shaft, maybe the trans side can just slide. Maybe you put a flange on the trans output and the divorced shaft bolts to the flange, and the flange can just move axially (and then you don't need a slip on the shaft itself). I don't know, I'm just spit balling things to think about and rambling

 

In your opinion, what would be the minimum thickness for an adapter made out of 6061 aluminum? The more I research, the more I'm leaning to just making my own adapter to marry a doubler and retain the OEM TC.

 

I don't know, the thickness will be defined by the features and depths of everything around it. I think the adapter for my 2JZ was .625 or .750, and seemed to be driven by the shear pin height--but that is assuming everything else meshes correctly so is really a lower threshold. If you need to space the tcase away from the transmission due to the output on the trans being too long or something, you could easily have a couple inch plate (effectively a big tube).

6061 would be cheap, 7075 may not be much more and is considerably stronger (though to be fair the plate is entirely in compression with some hertzian contact stressed at the shear pins, so that shear pin stress is really the driver for material selection). The adapter on my engine is just A36 mild steel, which is about as strong as 6061 but much stiffer (doubt that really has any effect in this case though). And steel is not cheap, and takes longer to machine. Honestly you might be able to get a plate laser cut which would be by far the cheapest I bet, since the plate is probably a 2D part effectively. You could also stack multiple plates to create a 2.5D stack, food for thought.

 

On the transmission side the adapter can be a single plate as the output shaft is recessed into the bell housing, which would be fine, because the stupid output shaft is 22 spline and would need an adapter to go to 23 spline, which will add the needed length.


The TC side however is just the opposite, where the input shaft protrudes, which, as you stated, will require layering the adapters AND the output shaft of the doubler is 23 spline, which will also need an adapter to go down to 22 spline.

 

What about cutting the front output off of the stock tcase, and running a divorced 4wd case behind it? Then there's no plates or weird splines, just use the stock output flange to a u joint/flange input on the divorced case and cut the 4wd portion off the stock case so it's not in the way. Weld or make a sealing plate where the front output area used to be.

 

Is there a possibility that you can find a low cut/crawler set of gears for the current transfer case? Something like a lot of the 80 and 100 series land cruiser folks are doing? https://www.marlincrawler.com/transfer-case/line-ups/single/land-cruiser-crawler-gears

These guys had some interesting stuff for crawling.https://northwestfab.com/products/landcruiser-blackbox

 

Yes. You're talking about an Atlas divorced transfer case. They used to offer the divorced option in the 4 speed case, but I don't see it on their website today.

https://www.advanceadapters.com/atlas-transfer-case-builder-trail-series-4

 

I’m currently focused on the NWF Blackbox at the moment. That’s the doubler I was referencing earlier with the 23 spline input and output.

 

I’ve heard stories of the Atlas not being that strong. At this point in my research I figure squeezing a doubler between the transmission and TC will get me the torque I need and still use the oem TC. That way I can retain the oem electronics to get in/out of 4wd/Hi/Lo and just install a single stick for the doubler If the TC blows up then I’d go with an NP205. Going for the low hanging fruit.