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NP231 doubler / crawlbox

psrivats

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We have a trail out here called Red Cone. The highlight is a fairly long, very steep, very low traction descent with a few uneven sections down a narrow ridge. On the way down, you are almost able to stand with full weight on the brake pedal as you fall forward in your seat. It is easy to go into a slide and if you slip too far one way or the other, it's about 2000 feet to the bottom with nothing to stop you.

There are three very strong recommendations that will keep you alive on this section. Idle down the hill in a low gear to keep the speed down. Turn off both lockers to prevent unnecessary loss of traction. And if the Jeep starts to slide, give it a little bit throttle so that you can recover some traction and steer again.

View attachment 302090
View attachment 302091

I've been down this about a dozen times and I have had to drive out of a slide more than a couple times. Really it isn't any different than driving a downhill on an icy road. Go slow, control the speed, and do everything you can to maintain traction. Lockers are very rarely a part of that equation.

I have this on my "must see" list when I visit Colorado. Hopefully I will survive driving down that thing :) Great unaltered photos, btw 😆
 

Mike_H

Off-Camber is scary
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So...Again, back to doublers. The gear reduction "underdrives" if you will. They go in front of the Transfer case. What are the maximum RPMS that you can operate them at? I know one of the big knocks against the 4:1 terraflex kit was over-reving it. Do you have to worry about overrevving an underdrive? how about a T-case behind it?

I've had some conversations with Blaine about something similar. In my case, I'm looking into swapping a 42RLE and rubicrawler, offloading the 241OR and installing a 231 in its place. He's adamant to point out that using the T-case is for that first low is WAY better than using the rubicrawler. I guess the Rubicrawler can over rev, or it overrevs the T-case. Its been a little bit since we've discussed it.

WHen you start thinking about ratios that low though...with low gears in the axles, T-case, and now an underdrive...at what point do you hit critical speed on a driveshaft (the front, specifically. Its pretty long for its diameter). 5 mph? 10 MPH? Currently, with my 241OR, its not unusual for me to travel in 3rd or 4th gear on the trail, at 10 mph with the T-case in low. I'm lazy and don't like stopping to shift it back and forth.
 

NashvilleTJ

I miss the snow...
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No issues with the driveshafts, Mike. The rotating speeds of both vary directly with the speed of the vehicle, regardless of the gearing in the transfer case, the tranny, or any other overdrive or underdrive along the way. The speed issues with things like the teralow are how fast the gears themselves operate. In the case of the teralow it is the planetary which has the problem.
 
Last edited:
OP
LincolnJr

LincolnJr

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Jan 30, 2021
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Harrisburg, PA
So...Again, back to doublers. The gear reduction "underdrives" if you will. They go in front of the Transfer case. What are the maximum RPMS that you can operate them at? I know one of the big knocks against the 4:1 terraflex kit was over-reving it. Do you have to worry about overrevving an underdrive? how about a T-case behind it?

I've had some conversations with Blaine about something similar. In my case, I'm looking into swapping a 42RLE and rubicrawler, offloading the 241OR and installing a 231 in its place. He's adamant to point out that using the T-case is for that first low is WAY better than using the rubicrawler. I guess the Rubicrawler can over rev, or it overrevs the T-case. Its been a little bit since we've discussed it.

WHen you start thinking about ratios that low though...with low gears in the axles, T-case, and now an underdrive...at what point do you hit critical speed on a driveshaft (the front, specifically. Its pretty long for its diameter). 5 mph? 10 MPH? Currently, with my 241OR, its not unusual for me to travel in 3rd or 4th gear on the trail, at 10 mph with the T-case in low. I'm lazy and don't like stopping to shift it back and forth.
Gear ratio comparisons: 3rd gear in low is about the same as 1st in high, and 5th in low is about the same as 2nd in high. I didn't think of that until after the last time I was on the trail and I kept switching in and out of low and my shifter was binding a bit. Wish I had thought of it earlier - that's another nice thing about driving a manual and being able to pick whatever gear you want whenever you want.

These charts obviously don't include final gearing at the diffs, just trans, t-case, and potential 2nd t-case.

Gear ratios.png
 
OP
LincolnJr

LincolnJr

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All that will go away with a $45 hand throttle. Definitely worth it.
On the point of throttle control - I've also seen some people swapping out their throttle return springs with something stronger so the extra resistance keeps you from accidentally going to hard on the gas pedal (or at least that's what I think the reason is). Is that a reasonable modification?
 

Gollywomper

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Corning CA
From what I've seen, people recommend upgrading to the 6-pinion planet in the second t-case, but stock is fine for the first/doubler.
I used a 6 gear planetary from a 241. If you have a u pull it yard you will probably find one laying in the bed of a truck. I took what I needed and only paid for some parts not the whole case. I would recommend taking the front housing with the ring gear home as I found it best to cut the housing to remove the gear.
 
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SvtLdr

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On the point of throttle control - I've also seen some people swapping out their throttle return springs with something stronger so the extra resistance keeps you from accidentally going to hard on the gas pedal (or at least that's what I think the reason is). Is that a reasonable modification?
I wouldn't think that mod to be recommended. There's a reason our Jeeps came with that return spring. There are better ways to train yourself to control the throttle while driving off road.
 
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NashvilleTJ

I miss the snow...
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I wouldn't think mod to be recommended. There's a reason our Jeeps came with that return spring. There are better ways to train yourself to control the throttle while driving off road.
Or, you could wheel like me and never take your foot off the throttle…




What? ;)
 

Apparition

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That being said. I wouldn’t run around in 5th gear in low. Use the gears appropriately.
I accidentally did, thought I was in high but must have bumped one of the boxes into low and took it down the highway. Revved the hell out of it wondering why it didn't downshift. Nothing sploded, yet.
 
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mrblaine

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Quail Valley, CA
I'm watching this thread as an affordable 3-speed tcase would be tits. Most of the time the 231's 2.7:1 is perfect and a 4:1 would be too low, but I would like lower on the rocks. So far I get by working the clutch but that isn't ideal. I suppose for the price of a double I can buy a lot of clutches... and advil
There's a bunch of retarded shit in the world. Up near the top of that list is 3 speed t-cases in a TJ.
 
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mrblaine

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Quail Valley, CA
A Dana 300 flipped with 4-1 would be great for most folks. And the ability to clock it to what ever position isn’t a bad way to either. At 12” long would solve a lot of rear drive line worries as well. If someone didn’t need the doubler gear choices.
We already can't keep the driveshaft clear of the trans oil pan with the Atlas and the pair of stock t-cases. The 300 has a closer center to center of the front and rear outputs which makes that much worse.
 
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mrblaine

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Quail Valley, CA
We have a trail out here called Red Cone. The highlight is a fairly long, very steep, very low traction descent with a few uneven sections down a narrow ridge. On the way down, you are almost able to stand with full weight on the brake pedal as you fall forward in your seat. It is easy to go into a slide and if you slip too far one way or the other, it's about 2000 feet to the bottom with nothing to stop you.

There are three very strong recommendations that will keep you alive on this section. Idle down the hill in a low gear to keep the speed down. Turn off both lockers to prevent unnecessary loss of traction. And if the Jeep starts to slide, give it a little bit throttle so that you can recover some traction and steer again.

View attachment 302090
View attachment 302091

I've been down this about a dozen times and I have had to drive out of a slide more than a couple times. Really it isn't any different than driving a downhill on an icy road. Go slow, control the speed, and do everything you can to maintain traction. Lockers are very rarely a part of that equation.
I've had to reach down and shift the auto to second and drive several times going down with engine braking and feeling back end start to slide out and come around on me. Once was with Gerald's wife in the passenger seat because he didn't want to scare her with his driving.
 

Gollywomper

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Corning CA
We already can't keep the driveshaft clear of the trans oil pan with the Atlas and the pair of stock t-cases. The 300 has a closer center to center of the front and rear outputs which makes that much worse.
Very good point here I didn’t consider. The 300 or similar boxes probably isn’t a good choice for the autos.
 

mrblaine

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Quail Valley, CA
I was looking into the bohemoth stuff. They were just bought recently by an investment company who seems to have the capital the rebuild the brand. I’m not sure either on the size differences. It’s good to have some options as the Dana 300 units are getting harder to find.
The nice thing with the DEO flip kit is the atlas shifters which fit the stock console nicely with minimal work. And it takes care of all the flipped 300 leaking issues. My reference to the price was that a flipped 300 can be done for $2k with the 4-1 gears.
Those aluminum cases are nice though.
Have you seen the Midnight Metalworks stuff yet?
 

mrblaine

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Joined
Nov 20, 2015
Messages
22,991
Location
Quail Valley, CA
So...Again, back to doublers. The gear reduction "underdrives" if you will. They go in front of the Transfer case. What are the maximum RPMS that you can operate them at? I know one of the big knocks against the 4:1 terraflex kit was over-reving it. Do you have to worry about overrevving an underdrive? how about a T-case behind it?

I've had some conversations with Blaine about something similar. In my case, I'm looking into swapping a 42RLE and rubicrawler, offloading the 241OR and installing a 231 in its place. He's adamant to point out that using the T-case is for that first low is WAY better than using the rubicrawler. I guess the Rubicrawler can over rev, or it overrevs the T-case. Its been a little bit since we've discussed it.

WHen you start thinking about ratios that low though...with low gears in the axles, T-case, and now an underdrive...at what point do you hit critical speed on a driveshaft (the front, specifically. Its pretty long for its diameter). 5 mph? 10 MPH? Currently, with my 241OR, its not unusual for me to travel in 3rd or 4th gear on the trail, at 10 mph with the T-case in low. I'm lazy and don't like stopping to shift it back and forth.
It over-revs the t-case bearings and burns them out if you use the Rubi crawler first.