Axles, Gears, Tires, Decisions

Hey guys, Just looking for some opinions... I've been searching and reading a lot and can't make up my mind on this one... here's my situation...

I'm doing a rebuild on my 99 XJ - it's at around 7.5'' and currently on 35's with 4.56's. stock axles - 8.25 rear, d30 front with upgraded axle shafts and a truetrac.

My rear drive shaft angle needs reduced so I want to put a high pinion in the back, probably a curry 9'' but still undecided. 8.25 has been fine other than that damn low pinion...

So, I want to go up to 37's, I'm gonna go with BFG AT's... I think I could get away with my current 4.56's for a while but would prefer to go lower... If I'm going lower I think 5.13 is the right setup, however that puts me in the position of swapping out the front axle also and I just don't think I can swing the cost right now.

I could go with 4.88's since they are now available for the D30... I'm just not sure they are gonna be enough, I guess it would be about the same as my 35's with 4.56's. also a factor is that I'm going from a mud tire on 15'' steel wheels to an AT on a 17'' aluminum wheels. and I have to wonder, how long is that 30 gonna live with 37's? It's upgraded, but still... perhaps a truss would help it survive for a couple years until I can put a more stout front in... I think I could get away with it for a while if I'm easy on the skinny pedal. I don't think that my new AT/Alum tire wheel combo will induce much more stress than the previous mud/steel combo did, despite the size increase.

I'm trying to rebuild my XJ on a more expedition/roadworthy theme, so I don't wanna go too low on gears anyway... The trails I run don't tend to have a lot of difficult rock sections, mostly woods trails with mud holes, hill climbs, water crossings and snow wheeling.

I'm all ears for opinions and advice - Thanks in advance guys

Anybody ?

My advice?

Stay on 35s. You're opening a whole can of worms.

37" all terrains?

Yeah I prob should stick to 35's, I tell myself that and knew I'd be told that... actually I probably should have stuck to 33's in the first place. I've got room for 37's and have the itch though... that itch is hard to shake ya know...

I can't come up with too many more worms that might come out of the can...? I think most of them already have...

I'm already planning on beefing up the rear... I think the front will be ok until I can afford to swap it...

Yeah I'm thinking BFG AT's, I've run them before and feel that they are the best "all around" tire out there unless you are into primarily very deep mud or rocks. I will have a 2nd set of wheels that I may put some more aggresive tires on, maybe some Iroks or TSL's (35's) for more heavy trail use.

Am I that far off thinking that a 37'' AT/Alum combo is comparable in stress factor to a 35'' Mud/Steel combo?

Actually the 37x12.50-17 BFG AT's are spec'd at 36.3 OD - so they aren't really a true 37... and they don't make a 35 for a 17'' Rim...

Lower it.

Then your itch to get a bigger tire will go away, and you'll gain a lower COG.

MoparManiac said:

My advice?

Stay on 35s. You're opening a whole can of worms.

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x2

The next step your going to try and take is a really big one and you don't have the axles for it and no the 30 WILL NOT SURVIVE on 37's unless you only street it and don't use the 4wd... But the tubes will hate ya unless you truss it and then you can kiss the unit bearings goodbye your brakes will wear very fast and it will handle like a wallowing windbag on the street..

Either get the right axles for it or just stay on 35's trust the opinions of many others who have already made these steps don't repeat our mistakes...

themud

themud said:

x2

The next step your going to try and take is a really big one and you don't have the axles for it and no the 30 WILL NOT SURVIVE on 37's unless you only street it and don't use the 4wd... But the tubes will hate ya unless you truss it and then you can kiss the unit bearings goodbye your brakes will wear very fast and it will handle like a wallowing windbag on the street..

Either get the right axles for it or just stay on 35's trust the opinions of many others who have already made these steps don't repeat our mistakes...

themud

Click to expand...

Well said.

I appreciate the advice guys... I'm definitely torn though... in the back of my mind I know both of you are right and my experience tells me the same thing...

I have this devil on my shoulder that keeps sayin "come on dude, a 36'' AT on a light wheel will be about the same as running the 35'' Muds on steel you have had for several years"

I guess my other consideration is that it's not a DD like it used to be... That gives me a cushion.

Definitely appreciate the advice though... I'm gonna have to sleep on this one...

I'd like to lower it and still run 35's, but cant seem to go below 7 1/2"... don't really wanna swap out leafs, already have a couple times... I've done about as much trimming as bushwackers will allow. I've def got more room to come down in the front but the back is kinda tight.

I say run the tire and wheels that you want to, BFGs are light tires and some aluminum wheels would even out to be as heavy as most peoples steelies and 33" MTs trust me I change more tires then I would like to admit. Me and another Tech from work was just talking about this, running a light tire and wheel set up and go bigger rather then running a heavy duty MT and a steel wheel, for light duty wheeling it would work nice.

Also till I can save the cash for a front D44 I am running the d30 with the super 30 kit and I am working on some 36" MTRs right now.

keep with the 35s and lower it down to about 5 inches and your will be golden

JRW7072009 said:

I say run the tire and wheels that you want to, BFGs are light tires and some aluminum wheels would even out to be as heavy as most peoples steelies and 33" MTs trust me I change more tires then I would like to admit. Me and another Tech from work was just talking about this, running a light tire and wheel set up and go bigger rather then running a heavy duty MT and a steel wheel, for light duty wheeling it would work nice.

Also till I can save the cash for a front D44 I am running the d30 with the super 30 kit and I am working on some 36" MTRs right now.

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The problem is you equating tire weight to damage potential when what you really need to be looking at is torsional stresses(IE larger diameter tires) they put more stress on the entire drivetrain yes you can snap a joint or shaft by a big heavy tire spinning fast then getting traction...

But with a 37" tire you can snap the same shaft just by loading up the drivetrain trying to climb a little hill the stresses are not the same one is a potential impact and the other is a constant stress...

But meh do what ya want just know that the bigger the tire the more you court disaster on the trail or the street just trying to save you the money and headaches that follow this line of thought...

Stay with the 35's and just lower it a bit if it feels too high..
themud

Going higher with big tires and your axles doesn't make any sense if you want to learn from others mistakes. You asked for advise. It's up to you if you want to benefit from advise given. That being said, I have a 98 XJ that I just bought that has a 8" lift on 36" IROKs. I'm taking that lift off and going down to 5 1/2"s. A tire dealer is coming tomorrow to look at trading me some new 35" or 33" KM2s for the 36" Super Swampers. The end result will be a much nicer XJ that will go anywhere I want to and I won't have to worry about breaking the chromolly axle shafts it has. It will be a rig that I won't be afraid to get more agressive if needed. I have to admit, that I didn't listed to what others were saying and my ZJ and the XJ ended up costing me a lot more than if I had just done the right thing the first time.

Good luck on your build.

OH, I have 488s in the ZJ w/33"KM2 and the XJ. It works good, but the next thing will be a 4:1 TC for better crawling.

my buddy runs 37" mtrs on his dana 30 with 456s and a lockrite and some upgraded shafts... no problems yet and we wheel it but nothing too crazy, but we still actually wheel it and it has held up

Crasher said:

Going higher with big tires and your axles doesn't make any sense if you want to learn from others mistakes. You asked for advise. It's up to you if you want to benefit from advise given. That being said, I have a 98 XJ that I just bought that has a 8" lift on 36" IROKs. I'm taking that lift off and going down to 5 1/2"s. A tire dealer is coming tomorrow to look at trading me some new 35" or 33" KM2s for the 36" Super Swampers. The end result will be a much nicer XJ that will go anywhere I want to and I won't have to worry about breaking the chromolly axle shafts it has. It will be a rig that I won't be afraid to get more agressive if needed. I have to admit, that I didn't listed to what others were saying and my ZJ and the XJ ended up costing me a lot more than if I had just done the right thing the first time.

Good luck on your build.

OH, I have 488s in the ZJ w/33"KM2 and the XJ. It works good, but the next thing will be a 4:1 TC for better crawling.

Click to expand...

I mostly agree with this guy and a few others. Don't go to 37's on a D30, that's crazy. Light wheels/tires or not, the forces on the brakes and gears of the 30 from those big tires is gonna break stuff, no question. Do you really want to have to take it easy every time you go wheeling because you are almost positive you'll break an axle if you hit the gas too hard?

7.5" lift is huge, lower that thing down some. You can run 35's on a 4.5" lift if you trimmed, plus it'll probably look a lot better and drive much better.

Just remember, you are telling us you don't have the money to upgrade your front axle to a D44 or something better than the 30. Are you gonna have money to replace axles all the time?


Oh, and I saw a guy this past weekend break a locked 8.8 with 33's. You better have some dough to spend on yours to take on 37's.

A 36" tire has a greater lever arm than a 35" tire and therefore will put more stress on the axle regardless of weight.

Jeeppilot said:

Actually the 37x12.50-17 BFG AT's are spec'd at 36.3 OD - so they aren't really a true 37... and they don't make a 35 for a 17'' Rim...

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BFG's like a lot of tires run small. The 37 really isn't 37 inches tall and the 35 isn't really 35 inches tall. You're still going almost 2 inches more in tire size.

And a 315/70/17 (stock Hummer H2) is a 34.5 inch tall tire for a 17 inch rim.

If you're running BWs and have trimmed for them, you can fit 35s under 5".



Just have to bumpstop accordingly.

Well Guys... I've been thinking it over and feel it's certainly in my best interests to go with the
general consensus of the members here- So I'm gonna take my medicine, quit being bull headed
and irrational and stick to 35's.

I know weight has a part somewhere in the stress equation, but you guys are absolutely right...
a longer arm makes more leverage and that's what accounts for more of that equation than weight.

I'm still gonna put the 9'' in the rear, can't go wrong with that mod, and I'm still gonna ditch the
15'' steelies and swap em out for 17'' AL wheels, but I'll stick to 35's for a while...

I have been thinking pretty hard about lowering it a bit, I already did once, when I first did the
build back in 02 I got a little lift happy and then came to my senses. I guess I've been avoiding
going any lower just to keep all the flex I have now, so I never really considered going lower and
adding bump stops... After thinking about it though I think the trade off in flex would be well
worth the gains in stability from a lower CG, not to mention improved drive shaft angles,
steering, less drag... etc etc etc...

My main plan of going with AT's was to get away with running a bigger tire and to help out with
the road manners. Now I'm thinking though, if I'm gonna lower it, and stick with a 35 i don't wanna
give up an aggressive tread also. I have had BFG Mud KM's since I built it and am a big proponent
of BFGoodrich... I think they are great tires and the KM's lived up to my expectations. I'm not sure
about these new KM2's - Looks shouldn't matter so much but I really don't like the look of the tread
pattern as much as the originals.

How do these KM2's compare to the originals as far as road manners and trail ability?

I'm pretty BFG loyal, but I gotta admit the Toyo M/T's have caught my attention since I saw
them on a JK at a gas station... They "look" like pretty good tires... does anyone have any
experience with these? how do they stack up?

Now as far as suspension goes - My current set up is 6" BOR leaf's with a 1.5" shackle - ACOS
up front with a 4.5" RE Coil. If I remember correctly ACOS minimum lift is 1.5" so I can get
down to 6" in the front pretty easily. The back though, looks like I'll be forced to swap out springs.
I'm not sure if it's wise to run 6" leafs on stock shackles? That would certainly get me down to 6"
in the back, but seems like I've read in the past that bigger leaf packs should be run with a longer
shackle. What do you guys think? so it looks like I'm setting six inches as a goal, well as long as it
can be done without changing out springs... otherwise I guess I'll be looking at some 4.5'' leafs
with 1'' shackles, and maybe 3'' or 3.5'' coils with my ACOS set in the middle somewhere... That
front setup would probably limit flex enough that I wouldn't even need longer bumpstops due to
the shorter coil...

I think I've rambled enough... And again I def appreciate all the input from everyone... I definitely
want to get this rebuild dialed in correctly.

I would get smaller leafs and stick with the bigger shackle. When I forst bought my jeep it had stock leafs with add a leaf and 2" blocks for a total of 4.5" lift. It dident flex good at all. So i got someused 4.5" leafs with stock shackles, they flexed better than what I had but I was still lacking flex. So i got some used 3.4" Rustys leafs and a 1.5" teraflex shackle, and they flex great. %100 better than both setups. Go with smaller leafs and bigger shackle.