Forum Index
Jeep Horizons HomeLinksJeep Horizons Tech Write-upsJeep newsLand useContact

 Watched TopicsWatched Topics   FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 

Relative axle strengths and gearing choices


 
Post new topic   This topic is locked: you cannot edit posts or make replies.     Forum Index -> Drivetrain
View previous topic :: View next topic  
Author Message
ThePhantum
Pissed-Off Admin
Pissed-Off Admin


Joined: 23 Jan 2004
Location: I knew it...I'm surrounded by Assholes!

PostPosted: Sat Jan 05, 2008 5:30 pm    Post subject: Relative axle strengths and gearing choices Reply with quote

***Please note that this writeup is based on my experience. What follows, therefore are my recommendations based on my experience. Anyone using these recommendations does so at their own risk. You may link to this writeup, but you must obtain my permission to re-post it elsewhere.***


This writeup is a work in progress. In an effort to make it easy to reference, it has been locked. If there is something that is technically incorrect or you would like to see further information added, PM me.


***********************************

The following is meant to be a general guide for gearing choices and relative axle strengths on late model Jeeps. It is based on a 4.0L that is a daily driver. It is also therefore based on the axles that came in late model Jeeps as well as some popular upgrade choices. Much of this is based on my experience, but some information was also referenced from many sites, including NAXJA and Pirate4x4. Not all axles that came in all Jeeps are covered, nor are more of the hardcore swaps that can be done. This is therefore meant for the individual that has stock or close to stock YJ/XJ/TJ/ZJ and wants to know what their options are to build up their rig to 33's or 35's for weekend warrior activities. If you are looking for info on swapping in a D60 and running 40's, go to Pirate....otherwise read on.


Gearing
While there are general rules for gearing choices, much of it is dependent on the motor/tranny combo. For example, the choice in gearing will be different for a 4.0L auto then it will for a 2.5L stick. There are is also a subjective factor: someone playing in mud bogs in Louisiana will most likely want a different setup than someone playing on the rocks at Moab. One point of confusion that should be pointed out is that as you go numerically higher, you are getting lower gears. For the duration of this writeup, references to going lower or higher will indicate the gearing itself, not the number.

The following list is what I would recommend for a daily driver that likes to crawl rocks. It is therefore based on a balance between being drivable on the street and capable on the trail. The recommendations are based on a 4 speed auto or a 5 speed stick. If your rig is carted around on a trailer or if you have a 6 speed manual, consider going to an even lower gear:
    30'/31's: 4.10 for auto, 3.73's for stick
    32's/33's: 4.56 for auto, 4.11's for stick
    35's: 4.88's


Having said that, you should always do the calculation for yourself and tailor the gears to your particular situation. There are LOTS of gear calculators out there, a google search will reveal them. I will just say that the formula they all use is a derivative of the following:

rpm = mph x gear ratio x 336 / tire diameter

Open Carrier vs. Locker
If your current gearing is 3.55 or higher and you plan to regear to 3.73's or lower, you will need a new carrier. This is the best time to consider a full case locker (instead of another open carrier). It will set you back a bit more, but in my opinion, why spend $500 in parts then another $400-$800 in labor for the regear and NOT lock it? If at some point in the future you do decide to go with a full case locker, it's going to cost you the labor all over again for the gear setup.

Axle strengths/comparisons/limitations
Front axles
Unless you have a Rubicon, you've got a D30 front axle:
    Dana 30 - 27 spline, 1.16" diameter shafts, 7.13" ring gear

Although there's a lot of different variations, there's 2 basic versions of this axle: a low pinion with standard cut gears and a high pinion with reverse cut gears. The high pinion is just a little bit stronger. Why you ask? The biggest weak link in the D30 (and the second weakest link in rear axles) is the housing itself. When torque is applied to the pinion shaft, it tries to push the ring gear away from itself. This causes the pinion to ride up the ring gear teeth, which flexes the carrier and the housing unless they are strong enough to counteract it. In a nutshell, the high pinion is less prone to housing flex than the low pinion. That is not to say that the high pinion is immune...it's not. But under the same loads, the high pinion will hold out just a little bit longer.

Earlier versions of this axle (found mainly in YJ's and older XJ's) have a vacuum disconnect system as well.

The other difference and potential weak point in D30's are the axle shaft ujoints. There's the 1/4 ton 260 joint which has a 1.063 diameter cap, and the 1/2 ton 297/760 joint which has a 1.188 diameter cap. In the case of the 297/760, the larger cap allows a larger trunnion (the part the cap rides on), hence it's greater strength.

Now, the ears that hold the ujoints in the stock OEM axle shafts are fairly soft. So when a ujoint fails in a stock shaft, the ears usually end up with stress fractures...or worst case they deform beyond recognition. If you plan to lock your D30 and play on the rocks, upgrade to alloy shafts.

If you are going to go bigger then 33's with lockers, start thinking about going to at least a 44 and ditch the D30. Otherwise, you will chew through unit bearings, the brakes will be totally inadequate and you'll probably start having issues with the ball joints. In addition, with gears lower then 4.11, the ring and pinion will be begging for sweet mercy on the highway. Finally, the lowest gears you can stuff in a D30 are 4.88's.



Rear axles
The biggest weak link in most rear axles is the axle shaft diameter. Period. People will talk about c-clip vs. non c-clip, full-float vs. semi float...it's all BS...they do nothing for torsional strength. Lockers add huge loads to shafts and those loads will try and twist up and/or snap the shaft. Period.

Having said all that, the following rear axles are listed from weaker to stronger. Included are the relevant specifications, the maximum tire size I would run (both locked and open), as well as the strengths and weaknesses of each:
    Dana 35 non (c-clip & non)- 27 spline, 1.18" diameter shafts, 7.58" ring gear, 2.62" axle tube
    31's with a locker or 32's without a locker. It has skinny tubes, thin shafts, a small ring gear and a weak housing. You can upgrade to a 30 spline kit and truss it to run 33's, but you might as well dump that money into a stronger axle. Polishing a turd just leaves you with a shiny turd.

    Chrysler 8.25 (XJ) 97-01 models - 29 spline, 1.21" diameter shafts, 8.25" ring gear, 3" axle tube
    33's with a locker or 35's without a locker. Good housing, strong tubes, decent shafts and a good sized ring gear. Upgrade the shafts and truss it to run 35's locked. If you have an XJ with the 27 spline version (91-96)...swap it out for the 29 spline.

    Dana 44 - 30 spline, 1.31" diameter shafts, 8.5" ring gear, 2.75" axle tube
    35's with a locker or 36's without a locker. A fairly good all around axle, good spline engagement, decent shafts, nice sized ring gear...but it's tubes are only 0.13" thicker then a D35. Plus, on larger tires housing rigidity starts becoming an issue. If you are light on the throttle, truss it, upgrade the axle shafts and shore up the housing to run 37's locked

    Ford 8.8 (out of '95+ Explorers) - 31 spline, 1.32" diameter shafts, 8.8" ring gear, 3.25" axle tube

    35's with a locker or 36's without a locker. Similar to a D44 in terms of strength. Beefy shafts, strong housing, nice sized ring gear. The carrier is a weak point though...mainly due to the pinion shaft being so strong. However there are carriers available that fix the weak design. Also, the tubes need to be welded to the housing or they will spin.

(It should be noted that t he Ford 8.8 is not direct bolt in...the spring perches need to be moved, regardless of what Jeep it's going under. However, it is a popular choice which is why it is listed here.)

***********************************
This writeup is a work in progress. In an effort to make it easy to reference, it has been locked. If there is something that is technically incorrect or you would like to see further information added, PM me.

***Please note that this writeup is based on my experience. What follows, therefore are my recommendations based on my experience. Anyone using these recommendations does so at their own risk. You may link to this writeup, but you must obtain my permission to re-post it elsewhere.***
_________________

97 XJ Sport with a bunch of stuff
Quote:
Never argue with idiots, they drag you down to their level and beat you with experience
Back to top
View user's profile Send private message AIM Address Yahoo Messenger MSN Messenger
Display posts from previous:   
Post new topic   This topic is locked: you cannot edit posts or make replies.     Forum Index -> Drivetrain All times are GMT - 8 Hours
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Visit links4jeeps.com for write-ups, forums and clubs




Jeep®, Wrangler, Cherokee, Liberty and Grand Cherokee are copyrighted and trademarked to Chrysler LLC.
jeephorizons.com is not in any way associated with or endorsed by the Chrysler LLC.
All other content is copyright Jeep Horizons 2004-2008.
Tech write-ups Links Discussion board Land use News Site Map Terms of Use

Powered by phpBB © 2001, 2005 phpBB Group