35" vs 37" - Need some advice!

[Maverick909]

Last chance canyon? I don't have the cojones to drive my JL in the notch until it is paid off. Great job!

yup Last chance a few weeks about. I ended up bailing out just past this and backing out. @Mx5red easied his way through it though!. Ill run it again with out my doors on next go around

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[RubiSc0tt]

Gatekeeper

From my build thread. More pics there. Quick stats:
2.5" Rock Krawler Adventure Series kit w/ Twin tube shocks
37x12.50x17R Milestar Patagonia MT's on 17x9 Vision Nemesis Wheels.
V6, 6 speed, 4.10 gears, dual tops, armor, winch, Tazer, etc.

I daily drive, take family road trips, and go wheeling with this Jeep. This is probably the hardest trail and we do it 1-2 times a year. The rest are slightly easier. I tend to get out 1-2 times a month from April-November, probably. I run with guys who mostly trailer their Jeep or have older TJ's they drive. a few JK's and JL's. All street legal (ish), a few bordering on buggy territory.

Other than what's mentioned above, all axles and driveline is factory. I've got 37k and factory ordered it so I've had it since mile 19. This is my 3rd Jeep though, so I also know my limits and what I can and can't get away with. If i was wheeling heavier and going harder on it, I would probably end up needing trusses and stuff to be safe, but at that point- would probably just save up and go to bolt on Tons.

THAT SAID: If I had to do it all over again from scratch, I would probably just go Rubi XR in either 4xe or V6/Auto, unless a well priced non-SOT 392 came my way, and even then- I would probably start saving for Tons just to be safe. Why? I love my 37's but it just makes more sense and would get me on the trail faster. The difference in most 35's vs. 37's really isn't that much. If it comes with 35's, why bother switching? Mine had the 32.5 or whatever the non-XR Rubi's come with, so I went 37's cause I knew I would be using them and the price difference (at the time) was negligible. I've recalibrated the speedo w/ the Tazer and checked by hand and I can average about 15-17mpg around town, 20ish on the open highway in the summer on road trips.

if you're curious of anything else, I have full stats, pics, and build process breakdown starting a few years back in my build thread. Feel free to DM me as well.

 

[Jtphoto]

Unless you daily drive a lot of hills or plan to add 40s, I would stick to 4.88 for off-roading with 37s on a D44. The ring gear tooth on a D44 5.13 is pretty small and breaking one off-roading is a slow ride home.

Meh I ran 5.38 with 37” Pitbulls on a Jk with zero issues. Still Lots of gear contact but like everything else they have to be set up right and heavy covers put on.

 

[kah.mun.rah]

Meh I ran 5.38 with 37” Pitbulls on a Jk with zero issues. Still Lots of gear contact but like everything else they have to be set up right and heavy covers put on.

This is when I rock crawled with 5.13s and then had to drive it home with a broken tooth. Technically breaking anything comes down to a lot of factors in specific situations. I am sure that breaking a tooth with a 3.45 ratio and 32" tires is possible under the right conditions. The fact of the matter is that the tooth on a 4.88 is bigger than one on a 5.13 and if you are going to put your ring gear through abuse, to err on the side of caution would be to go with the bigger tooth.

 

[Jtphoto]

Interesting photo…. looks like the tops of the teeth sheared off.

 

[SadRobot]

I did 37's and I regret not doing 38's

Same!

 

[Turniipp]

The amount of information and photos are awesome! Thanks for all the insights!

 

[Jtphoto]

This is when I rock crawled with 5.13s and then had to drive it home with a broken tooth. Technically breaking anything comes down to a lot of factors in specific situations. I am sure that breaking a tooth with a 3.45 ratio and 32" tires is possible under the right conditions. The fact of the matter is that the tooth on a 4.88 is bigger than one on a 5.13 and if you are going to put your ring gear through abuse, to err on the side of caution would be to go with the bigger tooth.

Out of curiosity do you remember if you did this in reverse?

So many different perspectives on gear and teeth size. I’m not saying 5.13 or 5.38 are stronger but they can hold their own just fine. Set up and housing support is just more critical.
- larger teeth have deeper set but fewer teeth in contact
- Smaller teeth have more teeth in contact at anyone time.
- One would think if anything that smaller pinion would shed teeth rather then the ring gear
- Proper gear set up is critical
- Housing support is also critical

 

[kah.mun.rah]

Interesting photo…. looks like the tops of the teeth sheared off.

Correct. That is what most of the teeth looked like after diving it home from the trail (100+ miles) after breaking the 1st tooth on the trail.

 

[kah.mun.rah]

Out of curiosity do you remember if you did this in reverse?

Hard to say exactly when the 1st tooth cracked. The differential seized when driving straight but there was a moment before that on the trail where the Jeep was high-centered on a boulder and reverse was used to try and get it off. Recognizing that while in reverse is the weak side of the tooth, that does make sense.

 

[Jtphoto]

Hard to say exactly when the 1st tooth cracked. The differential seized when driving straight but there was a moment before that on the trail where the Jeep was high-centered on a boulder and reverse was used to try and get it off. Recognizing that while in reverse is the weak side of the tooth, that does make sense.

If the diff doesn’t have a heavy enough diff cover on it, Gear Deflection is the biggest concern in reverse. Not only are the teeth weaker on that side but In reverse the gears push away from each other and will walk up the teeth as the housing spreads under the force unti finally you snap the tops of the teeth off. This is very common. This is also a reason hard core wheels do full trusses.
Never thought much about Gear Deflection until about 12 years ago when I started doing my own gears (hard to find someone I trust to do that job). Some shops use a diff spreader to spread open the diff housing in order to easily slide the carrier and preloaded shims packs in. I was surprised to say the least at how easy that housing can be spread by a couple small bolts. That housing is no match for Gears forcing each other apart, hence my stand on heavy, quality diff covers to support the back of the housing to prevent Gear Deflection.

 

[zouch]

37s fit *kinda'* on my Willys diesel with the "Mopar 2-inch lift". there is the slightest rubbing on the rear fender opening of the Right Rear wheel.
the MPP lift took my Willys up over 4" in the front, and almost 3" in the rear; i would expect that it wouldn't make as much difference to a Rubi that was already starting out higher.
i'm sure it's settled a little in the meantime, but not significantly.
Fenders were updated to the High-Line version (but you already have those) and XR spats, because i dislike 'poke'.

first things first; depending on your rear bumper, your 37" spare may not fit well, if at all. mine barely fit with the factory plastic bumper, but made contact. that (and the 110+lb weight) made me move to a different spare tire carrier and tailgate reinforcement.

i like to keep things (like coolant and lubes in motors and transmissions) spinning close to designed speeds, so regeared. 4.10/4.11s didn't quite get me back down to where things were even again, so i went to 4.56s since they were the next step down i could find from a reputable manufacturer and i'd rather gear a little 'deep' than not deep enough.
since i did a front E-locker and a gear-driven Limited Slip in the rear at the same time, i also upgraded axle shafts to handle the strain; RCVs in the front, and Yukons in the back.

adjustable LCAs were added to try to get the tires to not rub at full lock (it didn't help enough) and to get a little more Negative Caster. UCAs came later, just to match, but i was shocked at how much i could feel the additional stability provided by the UCAs, and how obvious it was that they were doing a better job of keeping things moving on their intended arcs and forcing the shocks to better do their jobs.
steering was all upgraded; Sector Shaft/Track Bar Mount Brace, heavier-duty Tie Rod, Track Bars (front and rear) and a Track Bar Relocation Bracket in the rear, Drag Link, and Anti-Rocks to better balance the front/rear anti-sway bars because i thought the front was too much stiffer than the rear.
ball joints were upgraded to an adjustable a rebuildable sort because having fragile plastic parts in that location just didn't seem smart to me, and it gave me a chance to play with the Camber Bushings.
wheels were eventually changed to some a little wider and further out, and they no longer rub on the CAs. (they rub on the Anti-rocks now.)
i would encourage you to check your knuckles with a magnet in case you haven't already; many of the D44s that came in the front of the Diesels came with cast knuckles. (my '21 Willys did, but it uses the narrower 'Euro' D44.)

an adjustable through-shaft Steering Stabilizer was added, but that was mostly because i was already intent on getting out of the stupid stock location and up out of harms way. it helps a little with feel, i suppose, but i ran for quite some time without it with no problem, so it's not likely doing that much.

arguments could be made that i didn't need to some of these things, or that there are other things that i still need to do; any could be valid in the right use case, but that's where i'm at (for now) and it works pretty dang well for everything i want it to do.

(there are lots of other things i've done to it, but not necessarily related to running a larger tire.)


what would i do differently if i were to do it again?
i might have gone with a wheel that would have gotten me a *tiny* bit less backspacing to get them a*tiny* bit further out off the Anti-Rocks; like the AEV Borahs. i might still; who knows?
and i would have gone straight to 37s instead of making a stop at 35s on the way.

but other than that; it's pretty well sorted for me, and i don't think i'd do much differently.

I have a '21 JLURD and am finally doing a Clayton 2.5" lift and will be putting new rims and tires on it. At one of the shops the tech asked why not throw 37's on it, and claimed not only would they fit without rubbing, but that they wouldn't throw off geometry or add any other issues.

I've been researching like crazy since then, and basically believe the following: 37's would fit and I don't believe it would cause geometry issues or any other major issues, however I've read plenty that state that axles should be trussed and knuckles should be cast iron and hydro-steer installed and so on and so forth.

So, what have you done and what would you do differently?

This is my dream vehicle and I've been saving my pennies to build it correctly, so I'm not as concerned about cost as I am dealing with issues later because I didn't do it right the first time. Any help would be mightily appreciated!

 

[kah.mun.rah]

If the diff doesn’t have a heavy enough diff cover on it, Gear Deflection is the biggest concern in reverse. Not only are the teeth weaker on that side but In reverse the gears push away from each other and will walk up the teeth as the housing spreads under the force unti finally you snap the tops of the teeth off. This is very common. This is also a reason hard core wheels do full trusses.
Never thought much about Gear Deflection until about 12 years ago when I started doing my own gears (hard to find someone I trust to do that job). Some shops use a diff spreader to spread open the diff housing in order to easily slide the carrier and preloaded shims packs in. I was surprised to say the least at how easy that housing can be spread by a couple small bolts. That housing is no match for Gears forcing each other apart, hence my stand on heavy, quality diff covers to support the back of the housing to prevent Gear Deflection.

In my scenario the axle was trussed and it had a TeraFlex HD dif cover.

 

[Jtphoto]

In my scenario the axle was trussed and it had a TeraFlex HD dif cover.

That’s crazy, but it happens. Can you imagine how easy that would have happened if it hadn’t been trussed and covered.
I’ve snapped axles with my 5.38 but never the gears.

 

[kah.mun.rah]

That’s crazy, but it happens. Can you imagine how easy that would have happened if it hadn’t been trussed and covered.
I’ve snapped axles with my 5.38 but never the gears.

It's not a fun day when it happens. I have gone back to stock axle shafts instead of chromoly because in the future I would rather twist or snap an axle shaft than break another ring gear tooth. A lot easier to fix on the trail and less expensive.

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