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Old October 25th, 2006, 08:20
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CRASH CRASH is offline
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Rear Axle Strength Comparison

This FAQ is designed to help sort out the plethora of information available on the web regarding the relative strength of axles sourced from various applications.

Disclaimer: I know nothing about wheeling in mud, except that I hate it. I have no idea what works and what doesn't, but if you follow the rules below, you'll have no trouble in the goo.

Disclaimer #2: The below is for XJ's. They run about 5,000-5,300 loaded for a trail run and a weekend of camping. They have 4.0 to 4.7 liter motors, producing 175 to 240 hp at the flywheel.

Rear axles: The weak link in most rear axle setups is the shaft itself. C-clips, tapered bearings or full-float makes no difference in torsional strength, it's all about axle shaft diameter. Lockers add a huge load to shafts, so you'll see two numbers for each axle. That said:

MTS (maximum tire size):

Dana 35: 31's with a locker, 31's without a locker. (This axle doesn't care if it's loaded or empty. It just plain sucks.) Weak housing, thin tubes, and a small 7.5" gear don't add up to much. I've seen them successfully trussed with a 30 spline kit stand up to 33" tire, however.

29 spline Chrysler 8.25: 33's with a locker, 35's without a locker. I just can't trust a 29 spline shaft to 35's with a locker. MAYBE with upgraded shafts. Good stiff housing, though, which contributes greatly to carrier and ring gear strength. upgrade your 27 spline to 29, and you'll gain these benifits.

Dana 44: 35's with a locker, 36's without a locker. You can super-fy this axle with 33 and 35 spline shafts now, but the pinion becomes the weak link. They are a fairly good axle for most duty, but housing rigidity can become an issue with larger tires and high speed romps. Their tubes are not much larger than a 35. You could go 37's with 35 spline shafts if you kept your foot out of the throttle.

Ford 8.8: 35's with a locker, 36's without a locker. Very similar in strength to a 44. Carriers are a weak link when used in extreme environments. New carriers are being introduced to address the weak 2 pinion carrier design. Shafts are beefy at 31 spline, but no diameter upgrades are available. Pinion shaft is very beefy, same as a 9". Weld the tubes all the way around to the pumpkin, or they WILL spin. When used in high side load aplications, like desert romping at speed, cross shaft wear and bending can occur as the shaft slams into the crosspin due to the nature of a C-clip design.

Toyota 8": 35's with a locker, 36's without a locker. I'm always very impressed by Toyota quality, they are worlds ahead in quality control and tolerances. The Toy 8" uses the same shaft size as a Dana 44, 30 spline (different pitch however), but with a bunch more ground clearance.. The case is more rigid, and has a bit more hypoid offset too, which helps in R&P strength. The Toy guys flog this axle pretty hard with 37's, but I have seen enough breaks with that size tire under 22RE 4 cylinder power to make me keep the limits at D-44 levels for a much torquier 4.0.

Ford 9": Special mention for this axle. It is very stout. Shafts are available in 28, 31, 33, 35, 40 and maybe 46 spline. All are not created equal, though. A crappy OEM housing and center section will flex like a wet noodle. Do yourself a favor and get a good Strange nodular iron case and a nice stiff Currie Extra HD housing as a base. Built this way, and with 35 spline shafts, it has no trouble holding 39" tires. The pinion is way low on these, which is why most of us that run the deep rocks shun this axle. The pinion always seems to be spinning on rocks. The new series of high pinion cases made by Currie and TrueHi9 deserve special mention. The Currie unit is truly a Ford reverse spiral 8.8 from the front of an F-150/Expedition. They lack the third pinion support and hypoid offset of a 9" ring and pinion, and thus have limitations when used in the rear (remember, that's running on the coast side of the gear set). 35's is the limit on these, even though you can get it in 35 spline, it's a waste, because the ring and pinion are weaker than a 31 spline shaft. The TrueHi9 folks have addressed this by having Richmond cut a true 9" reverse spiral gearset for them. They have also included extra ring gear deflection support and a nodular housing for extra beef. 35 spline axles plus this case and you're good to go to 39's. This will be my next axle when I finally break my 8.8 (again).

Dana 60: Second to a Tru Hi9, the most desirable axle for a Jeep running 37's or larger. Can accept 30, 35, 40, and 46 spline shafts. Very stout cases and large tubes make this a desirable axle. The pinion is very large, the ring gear is 9.75", and the carrier bearings are huge. This is the axle you want if you want to run 40" tires, larger if you go with a 40 spline shaft. A high pinion unit will accept less than this due to running on the coast side of the ring gear, but still, it would have no trouble with 38's. Easily trimmed and weldable case make this axle fun for shaving and adding link mounts. Be careful with OEM 60's as they often came with 30 spline shafts that are no stronger than Dana 44 shafts. Early units came with even smaller diameter shafts.

Dana 70/Ford 10.25/14 bolt: This is the true one ton series of axles put out by the OEM's. They are fine and dandy, but the parts and gear ratios can be limited, bigger shafts are more difficult to run than a 60, and they tend to have more ring gear strength than is really necessary for a Jeep. The ring gear size comes into play when towing, not really in Jeeping. This makes them more difficult to trim, as the gear fills most of the case. These axles all have 1.5" diameter shafts, (35 and 31 spline due to difference in spline size), and as such, can be run with the same tires as a 60.

Rockwells: A fine axle. Difficult to fit, impossible to re-gear, and quite portly. They can never-the-less handle 44's on a non-steering axle in a rear application. The U-joint version of these axles is most desirable, and fine spline 2" diamter shaft upgrades are now available to help shafts (the weak link) live under high horsepower applications.

I haven't worked on the various portals out there, so I won't speak to them.

Last edited by CRASH; October 25th, 2006 at 08:42.
 


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