Build Advice Please (40’s)

[GavinH]

TLDR: Someone tell me why 40’s is a horrible idea lol. If I can run 40’s securely on stock Rubi axle housing with rcv 300m shafts I’ll do it. If a 60/80 swap is strongly recommended I’ll stick to something smaller for now and budget accordingly.

I’m currently on 35’s building up my Jeep to properly run 37’s and 40’s one day. How should I go about the next phase of my build & purchases? Instead of buying things twice I can do it once and go straight to 40’s possibly.

If I go to 37’s from my 35’s I was going to do. 3.5 lift, rcv axle shafts, regear, new diff cover, driveshaft, shock extension brackets until I buy new shocks. Eventually RK long arms as well.

Only thing different i want for 40’s is bigger lift, 60/80 axles (if required) and kmc beadlocks (due to 13.50 width & new lug pattern). If I ran rcv 300m shafts is 40’s realistic while still able to hit the rev limiter with stock Rubi axles and things not break? Don’t always hit those rpms but I’m trying to say it gets wheeled. I’d buy 300m shafts and kmc beadlocks now and go straight to 40’s if it’s realistic. If an axle swap is highly recommended I’ll wait on wheels, shafts, maybe even the regear and get it all at once with new crate axles.

The only reason I see to get new axles is for more gear oil for greater cooling and increased spline count. Educate me on what I don’t know please.

Here’s my current build with plans to start purchasing stuff for big tires:
MC control arms (all 8)
MC TR/DL
MC 2.5 inch springs
MC Rear Track bar
JKS Front Track bar
JKS 2.5 Fox IFP
Yeti track bar/steering brace
Dynatrac BJ
Reid Knuckles
Fox Ats Steering stabilizer
Probably forgetting a couple things

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[Kreepin1]

Sigh... If you even think you want 40's go straight to 35 spline axles. It will be cheaper in the long run. I wasted so much money on my front 44. This was a Chevy Dana 44 under a CJ-7, so not apples to apples but vehicle weights and power are remarkably close.

In my experience you can run 40's on a front 44 for years IF:
1) You run a light tire like an Irok.
2) You don't go any lower than 4.88 gears.
3) You are sensible with the skinny pedal.
4) You are OK with occasional carnage. I lost one carrier, two outer shafts, and one CTM u-joint (this was before RCV was a thing).

I would not run 40's on a rear 44 unless you increase the shaft diameter (spline count). You can bend the shaft with 35's.

Why 4.88 minimum? As gear ratio's get lower the number of teeth on the pinion decreases. Fewer teeth engaging makes them weaker. More importantly, the diameter or the pinion decreases providing less leverage against the ring gear. Stress goes way up. You will notice that 60's have better gearing options. I'm running 5.86 in the CJ-7 now that I've upgraded to a 60 front (I always had a 70 rear).

 

[roaniecowpony]

I think you should look at the troubles that Ted had with his rear D44 in a JLUR on 37s. He's probably the most prolific traveler in his Jeep on this forum. I'm on 37s for a year now with Rubicon D44s, chromoly axles and 4.88s, and have about a dozen BOH trails on this setup. I'm considering a D60 semi-float w/chromoloy axles.

Everyone seems so concerned with weight, but my engineering tells me to be concerned with the leverage a larger tire has. Bending and twisting axles is about moment arm.

You didn't mention anything about ball joints, but I assume you aren't running stock ball joints.

 

[AcesandEights]

Why stop at 40" tires. Seems silly.

"Someone tell me why 40’s is a horrible idea". It looks stupid. It will almost certainly have issues with steering and drivability. It will cost significantly more than leaving it stock.

 

[GavinH]

I think you should look at the troubles that Ted had with his rear D44 in a JLUR on 37s. He's probably the most prolific traveler in his Jeep on this forum. I'm on 37s for a year now with Rubicon D44s, chromoly axles and 4.88s, and have about a dozen BOH trails on this setup. I'm considering a D60 semi-float w/chromoloy axles.

Everyone seems so concerned with weight, but my engineering tells me to be concerned with the leverage a larger tire has. Bending and twisting axles is about moment arm.

You didn't mention anything about ball joints, but I assume you aren't running stock ball joints.

Have dynatracs currently pressed into d44. I’ve read his extensive posts and why he upgraded his, I believe he had gone through several sets of gears or lockers just tearing each one up possibly due to it getting too hot if I remember correctly. I don’t travel or tow like he did but was still very informative and kind of why I’m making this whole post to get a definitive answer on what’s the proper procedure to get yourself to 40’s

 

[GavinH]

Why stop at 40" tires. Seems silly.

"Someone tell me why 40’s is a horrible idea". It looks stupid. It will almost certainly have issues with steering and drivability. It will cost significantly more than leaving it stock.

I think they look badass, that’s why I’m trying to get everything properly upgraded prior to throwing 40’s on, can’t argue with that one

Forgot full psc steering is also in my future build plans

 

[AcesandEights]

Not going to argue with you. You asked for someone to tell you why it's a horrible idea. It looks stupid, isn't cost-effective, gives you limited options on road, is only marginally better off road, ruins handling, and a whole many more. Do what you want, but if you ask people to tell you why it's a horrible idea you just may get people that tell you it's a horrible idea.

 

[GavinH]

Not going to argue with you. You asked for someone to tell you why it's a horrible idea. It looks stupid, isn't cost-effective, gives you limited options on road, is only marginally better off road, ruins handling, and a whole many more. Do what you want, but if you ask people to tell you why it's a horrible idea you just may get people that tell you it's a horrible idea.

I’m trying to weigh all the options so I do want to know why others find it dumb and I agree definitely not the most ergonomic. It is a genuine question and I suppose comes down to driver skill more than anything but…can 37’s do everything 40’s can?

 

[Zandcwhite]

Ran 38s on our 2019 for years and 26 BOH trails. Currently running 39s on our 2022 with stock shafts. Ran John Bull, Holcomb creek, and gold mountain without issues. Ball joints needed upgraded by 28k miles but it's been great so far. I'll upgrade axle shafts if/ when I break one but I like to push the limits personally. I'm not bouncing it or trying to jump it, it's a $65k rig that still looks new from the sliders up and I want to keep it that way. 40s is pushing the limit, but so long as you wheel smart is say go for it.

 

[stumblinhorse]

Going to 40’s requires everything to upgraded. You can upgrade when stuff breaks, that is a good plan until you are stranded. Or just plan on everything. None of the D44 stuff will hold up. Ball joint, steering, axles driveshaft…. Nothing will hold up.

I would never do 40’s for anything but a trailered buggy.

 

[GavinH]

Going to 40’s requires everything to upgraded. You can upgrade when stuff breaks, that is a good plan until you are stranded. Or just plan on everything. None of the D44 stuff will hold up. Ball joint, steering, axles driveshaft…. Nothing will hold up.

I would never do 40’s for anything but a trailered buggy.

I like to upgrade before it breaks I don’t want that worry in the back of my mind when there’s no cell service. When I bought the jeep my plan was to run 35’s and as I wear through those get all my upgrades for 37’s done so I can buy those next time I need tires. I figured once the new model jeeps come out get one of those as a new daily and make this a trailer queen that’s a crazy badass build, chopping the roof adding a cage maybe even rear steer I’m thinking going all out I think. But when looking at what I’m spending to get 37’s it felt like 40’s are so close other than crate axles

 

[stumblinhorse]

I like to upgrade before it breaks I don’t want that worry in the back of my mind when there’s no cell service. When I bought the jeep my plan was to run 35’s and as I wear through those get all my upgrades for 37’s done so I can buy those next time I need tires. I figured once the new model jeeps come out get one of those as a new daily and make this a trailer queen that’s a crazy badass build, chopping the roof adding a cage maybe even rear steer I’m thinking going all out I think. But when looking at what I’m spending to get 37’s it felt like 40’s are so close other than crate axles

Tons are easy to find in a junk yard. It will save you 20k.

 

[chevymitchell]

Just buy Pain Train. Lol. Free Delivery.

https://www.jlwranglerforums.com/forum/threads/pain-train-72k.121789/

 

[GavinH]

Just buy Pain Train. Lol. Free Delivery.

https://www.jlwranglerforums.com/forum/threads/pain-train-72k.121789/

Wow that’s beautiful and exactly where I’m wanting to go. That’s another future debate I have Coilovers vs Ori’s.

 

[JEEP4U]

If you are serious about 40's........ you will not utilize many of you current build parts. Many of us will start with a JLU sport version......HD axles is a must, if your rig will be heavy. I run D60 35 spline front and D60 40 spline rear, 5.38 gears, with ARB air lockers with min 5.5 inches of lift and long arms....otherwise suspenion travel will be quite limited. Wheel will not be centered in wheel well...... and you will have a lot of bump stop (reduced up travel)..... Also shock lengths are important.

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