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Discussion Starter · #1 ·
Talking with a vendor earlier today about a relatively stock Unlimited build to use 37 or 38 tires (probably Milestar Patagonia M/T).

On the JK his comments were:


  • A JK would require a 3.5" lift with new inner fenders, a high fender, and <4" backspace wheels.


  • The Rubicon D44 in front wasn't very strong and had issues even with trusses and slaves. He recommended getting a Sport or Sahara and the Dynatec Trail Leader
    package which includes a ProRock 44 axles assembly with 35-spine air locker, gears, axles, and axles, gears, and locker for the rear OE D44. $6500. I'm guessing that would need 5.38 gears.(https://www.dynatrac.com/axle-assem...ts-for-jeep-jk/trail-leader-axle-package.html)


  • The Rubicon JL's D44 can be used as is with a differential skidplate. The short 4.71 first gear makes the 4.1 standard final drive ratio fine with the tall tires. A 2.5" lift could be used with the OE Rubicon fenders.

Do you agree with those assessments? If not, what differences do you have? What other ones would you add?

Thanks,

David
 

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First you are not going to run 37/38 inch tires with a stock JL. 35's maybe but if you off road at all you will still want a lift.

With a JK flat fenders and a 2.5 inch lift and you can run 37's. 3.5 inch backspacing is ideal but people are doing it with 4.5.

A D44 that is trussed, sleeved and gusseted is at least as strong as a ProRock 44 if not stronger. The only thing the ProRock or any aftermarkets axle has is more built in caster. 5.13 or 5.38 gear are fine if running 37/38.

The first gear in the JL is not that different than the manual first gear in the JK and it is a dog with 35's and 4.10's. The JL would not be any better, forget trying to run 37's.

Now the things not talked about. Running 37's requires a whole lot more upgrades other than bigger axle. You have to upgrade the steering unless you are build a mall crawler. JK or JL it does not make a difference.

I don't think it much matters what you starting point is you will spend about the same amount of money to do it right on either vehicle.
 
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I would do a cost comparison between an aftermarket D44 and modifying a stock D44. After regearing. axle shafts, and adding all the welded components (skip the sleeves, they are not a major contributor to increased strength) that can be quite a bit of money depending on if you do it yourself or not. Money that does nothing to increase resale value if you decide to ever go D60s. An aftermarket D44 is going to cost more than the modified stock D44 route, but it’s also a different look (OEM and sexy vs. Frankenstein). Some money can be recovered from selling the used stock D44, $1K minimum, $1.5K if the axle is low mileage. I would stick with 5.13s for 37s.
 

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Jeep promotes the JL Rubicon fenders fitting 35’s without a lift.

Yes, the NSG370’s first gear (4.46) is close to the 850RE’s (4.71), but the automatic has the advantage of a torque converter.

I would also consider bigger brakes with that size tire.
 

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Of course he is going to recommend a $6500 package lol- he is a vendor. Vendors sell stuff.

I run 37s. Stock D44 fully trussed. 5.13 gearing. For 5-1/2 years. On the tough rock crawling trails. Drove home from every single one of them. One of the few daily drivers that has tackled what is actually a rock buggy trail. The key is respect the skinny pedal and only use it when absolutely necessary, and just enough to move.
 

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Oh and I have 2.5 inch lift with trimmed factory fenders.
 

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All good advice. I've been running my combo in my signature for 30k miles so far and have driven from Miami <---> Moab, and DC <---> Vermont without issues.

I'm on stock rubicon axles with 4.88 gears and C-gussets on the front knuckles. Bent the rear axle flanges, so they got revolution shafts. Holding up so far.

3/3.5" lift (3" springs, .5" spacers on passenger side to level) (3" evo plush ride springs, teraflex bump stops, braided brake lines, Core 4x4 front lower control arms, teraflex adjustable track bars, artec welded rear trac bar bracket). Love the lift.

Cut stock fenders, cut some pinch weld metal out of rear fender area for clearance

Big brake kit made a big difference

Hydraulic ram steering if you want to be able turn the wheels in the rocks- not needed for the mall

My ATX wheels are 4" b.s. and it just works with 13.5" tires. They kiss the king shock bodies and lower control arms at full lock.

Plan for an upgraded front driveshaft.
I went though the OEM ball joints pretty quick, now I'm on dynatrac pro-steers
 

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First you are not going to run 37/38 inch tires with a stock JL. 35's maybe but if you off road at all you will still want a lift.

With a JK flat fenders and a 2.5 inch lift and you can run 37's. 3.5 inch backspacing is ideal but people are doing it with 4.5.

A D44 that is trussed, sleeved and gusseted is at least as strong as a ProRock 44 if not stronger. The only thing the ProRock or any aftermarkets axle has is more built in caster. 5.13 or 5.38 gear are fine if running 37/38.

The first gear in the JL is not that different than the manual first gear in the JK and it is a dog with 35's and 4.10's. The JL would not be any better, forget trying to run 37's.

Now the things not talked about. Running 37's requires a whole lot more upgrades other than bigger axle. You have to upgrade the steering unless you are build a mall crawler. JK or JL it does not make a difference.

I don't think it much matters what you starting point is you will spend about the same amount of money to do it right on either vehicle.
That statement is sketchy.....I'll agree it gets close to the same strength...but not stronger....
 

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That statement is sketchy.....I'll agree it gets close to the same strength...but not stronger....
Here is the analysis:

Originally Posted by Jantz Engineering View Post
Hot dang the Finite element analysis is even better than my estimations.

Stock tube bending any direction .. 1.13
Stock with sleeves,,,,,",,,,,,,,,",,,,,,,, 1.66
3" with 1/2" wall,,,,,,",,,,,,,,,",,,,,,,,, 3.185
With box truss upward bending ,,,,,,4.77
With box truss backwards bending,,,3.95

That's almost 4 times stronger in backward bending, and almost 5 times stronger in upward bending than the stock housing, and quite a bit stronger than a 3" tube housings. The resistance to twisting is about 4 times stronger as well 2.26 stock vs. 8.72 trussed.
 

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.....(skip the sleeves, they are not a major contributor to increased strength).....
You are correct that they don't add much strength to bending forces but that is not the reason to sleeve. The reason to is to protect the tube from impacts.
 

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Can’t speak to the JL, but the recommendation re the JK is a good one. I would stay with 5.13 gears though, as the pinion gear on the 5.38s is very small. With 37s or 38s you’ll also want to consider a big brake kit. If you actually wheel it, consider a hydro assist steering kit. You’ll also want new, double cardan drive shafts. And upgraded ball joints. And a drag link flip kit. The list goes on and on if you want to do it right.
 

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I will foolishly wade into this discussion, which highlights the tendency of the forum experts to provide grim warning’s of dire consequences when they don’t necessarily have real world experience.
I have had multiple JKURs with 3 inch lift and 35 inch tires.
I have numerous friends who moved the JKUR up a notch to 37’s.
I have a JLUR with the Mopar 2 inch lift and 37‘s.
So at least I can provide some insight as to both models.
A great many naysayers said a JK with 3 inch lift and 35 inch tires would have issue after issue. I never did, with generally all stock parts, including drivetrain, and the naysayers I think have by and large been proven greatly wrong as to 35s. From experience with my fellow wheelers, Moving a JK up to 37‘s, or 38s as you suggest, simply moves virtually every part on the JK to the limit of or beyond its capabilities without major retrofitting. I would suggest youbelieve every word of naysaying thrown out there concerning the risks of 37‘s or 38 on a JK without significant other expenditures.
As for the JL, mechanically I have made no changes from stock other than the lift and slapping on 37 mile star Patagonia‘s on stock wheels. Plus I added plenty of weight in terms of skid plates, winch, metal bumpers.
I have had zero issues, I wheel Offroad constantly, and I have absolutely zero complaints about the acceleration, I find pickup to be excellent and a big improvement over the JK.
Bottom line, speaking from experience with both, moving a JK to 37 or 38 is a major rework. Slapping 37‘s on a JLUR is plug-and-play and be happy with minimal expense. As for 38s on a JL, I looked into it and it seemed that simply crossed the line in several regards concerning lift and configuration, remember 37’s come in 12.5 but 38s in 13.5.
Alternatively, slap some 35’s on a JKUR with a 2.5 inch lift and you can’t go wrong.
 

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From shops that weld trusses.

I work in aerospace as an engineer and I’m constantly seeing FEA results from programs that I’m working on. Show me the report of exactly what was done in the analysis and assumptions. I am sure that a truss will improve the bending moment in one direction and not as much in the other. - the question is the amount.
 

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Yah I ran 37” Pitbulls on a D30 for a year before going to a D44 front. Lockers, 5.38 gears, no issues other then I broke the cheap Crown axles I chose to put in. Once the RCVs went in no problem. Traded that Jeep in with 312,000klm ..
37” tires fitment on a JKU also depends on the tire you choose. Some 37s are no bigger then true 35”, others like the Pitbull are 37”.



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Yah I ran 37” Pitbulls on a D30 for a year before going to a D44 front. Lockers, 5.38 gears, no issues other then I broke the cheap Crown axles I chose to put in. Once the RCVs went in no problem. Traded that Jeep in with 312,000klm ..
37” tires fitment on a JKU also depends on the tire you choose. Some 37s are no bigger then true 35”, others like the Pitbull are 37”.



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There's always someone who says they ran a big tire on a Dana 30 and claims it's just fine. Fortunately most people know better.

Sent from my SM-G955U using Tapatalk
 

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There's always someone who says they ran a big tire on a Dana 30 and claims it's just fine. Fortunately most people know better.

Sent from my SM-G955U using Tapatalk


Completely depends on what you're doing.
If you're crawling on the rocks with 37's and a d30 then good luck to you, but most of us in the Great White North buy jeeps not for rocks but for the snow.... which is with us for 6 to 7 months of the year. Less friction, less wear/tear... less need for a D44
 

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From shops that weld trusses.

I work in aerospace as an engineer and I’m constantly seeing FEA results from programs that I’m working on. Show me the report of exactly what was done in the analysis and assumptions. I am sure that a truss will improve the bending moment in one direction and not as much in the other. - the question is the amount.
The other issue with trusses is skill and experience of the person installing it. Things can go awry if it’s not done correctly. For example, warping of the tube as the material cools.
 
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