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NP231 doubler / crawlbox

chimmike

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Better gas mileage? I understand the other two benefits, but that part is counter intuitive to me because the rpm would be higher. But if it will be better, great!

if you're cruising too low in the rpm, you're not in optimal power band and lugging the engine. There's a sweet spot at cruise load and rpm where the engine is producing optimal power at cruise and load...and your gearing + tires + overdrive is below that.
 

Gollywomper

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How do I find your build thread?
 
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LincolnJr

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if you're cruising too low in the rpm, you're not in optimal power band and lugging the engine. There's a sweet spot at cruise load and rpm where the engine is producing optimal power at cruise and load...and your gearing + tires + overdrive is below that.

Ok, so according to this chart, I'm less than ideal with 3.73 and 33s. To get back to stock (LJ came with 30s originally) I would need to be at 4.11 gears, but that's hardly worth doing for the cost and effort involved. "Best mix" would be 4.27 gears, but that also seems like only a small gain.

Someone suggested 4.88 gears "where I should be"... I would appreciate the 30% improvement in final gear ratio at low speed on the trail, but that would also increase my highway rpms into like 4cyl range (70mph in 6th gear on 4.88s would be 2924rpm)... would the 4.0 be ok with that? If so, then I'm all for it. I'll start with regearing and see how much lower I want to go on the trail after that.

regear chart.png
 

jjvw

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Ok, so according to this chart, I'm less than ideal with 3.73 and 33s. To get back to stock (LJ came with 30s originally) I would need to be at 4.11 gears, but that's hardly worth doing for the cost and effort involved. "Best mix" would be 4.27 gears, but that also seems like only a small gain.

Someone suggested 4.88 gears "where I should be"... I would appreciate the 30% improvement in final gear ratio at low speed on the trail, but that would also increase my highway rpms into like 4cyl range (70mph in 6th gear on 4.88s would be 2924rpm)... would the 4.0 be ok with that? If so, then I'm all for it. I'll start with regearing and see how much lower I want to go on the trail after that.

View attachment 302041
Does that chart include your transmission's .84 OD? You should gear for the final drive gear.
 
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LincolnJr

LincolnJr

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Does that chart include your transmission's .84 OD? You should gear for the final drive gear.
The chart is for 1:1 (5th gear in my case). Right now in 6th gear (0.84 overdrive) I can do 70 at 2235 rpm. That would go up to 2924 with 4.88 gears. What's a safe max rpm for continuous highway usage? I don't drive it a ton on the highway, but I don't want to lose the ability to do so.
 

freedom_in_4low

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The chart is for 1:1 (5th gear in my case). Right now in 6th gear (0.84 overdrive) I can do 70 at 2235 rpm. That would go up to 2924 with 4.88 gears. What's a safe max rpm for continuous highway usage? I don't drive it a ton on the highway, but I don't want to lose the ability to do so.

Your torque peak is in that 2800-3200 range, which makes it a nice speed to cruise at. With a 6 speed and 33s I'd go with 4.56.

I've had the following combos in TJs:
6speed/30/3.73
6speed/33/3.73 - tolerable in Oklahoma, not so great in Colorado
6speed/32/4.88 - too low
6speed/35/4.88 - perfect

5speed/31/3.73
5speed/32/3.73 - miserable
5speed/32/4.56 - perfect
5speed/32/4.88 - crawled better, higher rpm on the highway than 4.56 but still ok. It was great in Colorado. Wouldn't have wanted it in OK with our new 80mph speed limits and not enough mountains to need the extra torque.
 
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SvtLdr

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I learned early on by getting myself in a super sketchy situation to always power down hills. I was engine braking on a loose dirt descent and the back end started sliding. I could have sworn I was about to lay it on the side.
In this situation, a rear locker helps keep the rear axle tracking with the front. Just an observation.
 

pagrey

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I've gone down entire descents just in neutral and riding the brake, but that's not ideal for control or for the brakes.
Off road that's a great technique unless you are going down a massive grade and are getting the brakes warm. As far as control 4wd keeps all the wheels spinning just like engine braking.
 

pagrey

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Tracking as in pushing straight ahead, no matter where the steering is pointed?
With an open diff one rear wheel can spin almost twice as fast as the other causing it to attempt to catch up to the front two wheels that probably have traction. The locker keeps that from happening.
 
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jjvw

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With an open diff one rear wheel can spin almost twice as fast as the other causing it to attempt to catch up to the front two wheels that probably have traction. The locker keeps that from happening.
That would require the spinning tire to have more traction than the not-spinning tire.
 

JMT

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Looking for info/experience about building a t-case doubler.

06 LJ with 6spd/231/3.73 on 33s. I’d like to have lower gearing options for better off-road control that won’t effect on-road behavior.

  • as far as I know, my NP231 can’t be regeared to 4:1 or anything like that (Teralow 4:1 is no longer a thing, and probably for good reason since they apparently blew up a lot)
  • swapping t-cases for a NP241 might be an option, but they’re expen$$ive and it’s not a straight plug in
  • regearing the diffs is also expensive and/or time consuming, and I don’t think going to 4.10 would be enough change to justify it, and going to 4.56 would change highway driving (higher RPM)
  • Rubicrawlers can only be mated to 42RLEs

So that leaves me with the intriguing option of building a doubler… but also some questions:
  • most search results for doublers/crawl boxes/etc are from 10 years ago or so, which seemed like a red flag – is there a catastrophic reason not many people go this route? Or just that most people don’t have the extra wheelbase to extend the drivetrain and/or are satisfied with regearing or other options?
  • I’ve found a few options:
    • NWF has their BlackBox, which is as complete of a setup as you can buy off the shelf and the most expensive (but still cheaper than buying an Atlas or even most used 241s!) ~$1700
    • NWF also has their EcoBox, which is several hundred $ cheaper, you just have to supply all of the guts from a donor 231 and have access to a press to install the ring gear. ~$1000
    • Froehlich Suspension makes an adapter plate, which requires cutting up a donor 231 (removing the front driveshaft yoke) and welding the adapter in place of the back half. $475 + donor 231 + probably paying a shop to do the welding

All doubler options would require fabrication of a mount to help support the longer drivetrain, SYE and new driveshafts (longer front, shorter rear), and finagling double t-case shifters. Also, I would upgrade my NP231 with the 6-pinion planetary gear and wider chain.

So… what else can/should I consider? Has anyone built a doubler with an adapter, or know where such a build is documented?
The TeraLow was solid. They continued to make them till 2018. The vast majority didn’t blow up.

Re-gearing is more important than the TCase issue. Higher rpm’s aren’t a negative.

You can sort out the TCase after that. I realize the frustration with the 231 and crawl ratio. Mine was solved with the TeraLow.

If I started all over I’d start with a TJ that came with a 32rh tranny and drop an Atlas in. Lock both ends, tuck it as high as possible, and re-gear.
 

pagrey

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That would require the spinning tire to have more traction than the not-spinning tire.
Imagine a Jeep sitting on a hill facing down, one rear tire has traction and the rear axle is locked. The driveshaft is fixed, now the rear axle is unlocked, what happens as the locker is disengaged?
 

srimes

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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
 

srimes

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Imagine a Jeep sitting on a hill facing down, one rear tire has traction and the rear axle is locked. The driveshaft is fixed, now the rear axle is unlocked, what happens as the locker is disengaged?

I don't understand what you're saying. The driveshaft is fixed?

I do know that launching (2wd) with a locker/limited-slip will fishtail way more than an open diff. Once a tire is spinning it easily goes sideways.
 

Apparition

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That’s my alternate setup. It really doesn’t matter either way to me. I like the 32rh ratios better, but I like the late model 42rle TJ’s better, so it’s a toss up. 😜
The downside to the Rubicrawler is lack of front dig. But my Atlas cost way more than my Rubicrawler.
 
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pagrey

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I don't understand what you're saying. The driveshaft is fixed?
It's an easy analogy to help understand what happens when engine braking. If you start to consider everything moving it makes everything difficult to understand. If you picture the engine braking enough to stop the Jeep entirely it's easy to see when the rear diff unlocks the wheel with traction rolls down the hill as the opposite wheel without traction spins backwards. When everything is moving down the hill with the driveshaft spinning that really simple concept becomes difficult to see.