Trail Testing the 40 inch Tires

[BootLeg JK]

I've been thinking about going big, 40"+. Obviously going that big will require a lot of change to my suspension, axles and gearing. Been watching a lot of whatever I can find for 'inspiration'

Todays find:

 

[smritte]

Depending on how hard you drive will determine what you need to replace. My buddy destroyed a Dynatrac Dana 44 with 40's. The biggest issue is axles, then balljoints. Bearing wear will be accelerated even if you don't drive like me. Moderate bouncing will bend D44 housings. This makes axle gussets a must. Pinion gear is an issue once you hit a certain point.
If you have a D44 and plan to keep it, run nothing but RCV axles. Find upgrade ball joints also. I cant remember who made the last set I got but I think it was Poly Performance. jeep really cheaps out on their ball joints. Be happy you don't have a TJ Rubicon. OMG are those bad

Dana 44 ring gear is 8.5 and the Dana 60 ring gear is 9.75. The issue isnt the ring gear but the pinion. The lower the gears, the smaller the pinon. In my opinion, the load from a 40" tire is pushing the strength on a 5.13 and is happy with a 4.88.
I found 5.38 to work pretty good with 40's. The 60 will run 5.38's all day and work well.
Spline count on both, JK dana 44 is 32 max (commonly its 25-27) with Dana 60 commonly having 30 spline but can go to 35 spline. I believe the 35 spline is 25% stronger than 30. Also, just because a vehicle comes with a larger spline axle doesn't mean its good steel. Ive seen people break 30 spline axles where they shouldn't have because the factory didn't use a good steel. They use the size as a selling point but cheap out on materials. This is very common across the board.
If you have a 44JK axle, RCV and you never have to think of it again. Ring and pinion is where you need to watch out and upgrade the ball joints.

I keep kicking around 40's on my Cruiser. I already have 30 spline 300M RCV axles and 9.5 ring gear. Toyota already gusseted the housing but the guys I talk to say I need to add some more. Ball joints in mine are diffrent and almost indestructible. Chrome molly tie rods and panhard's once you start bending those.

You'll see people running them on stock vehicles and just dealing with the extended wear and possible breakage. Odds are these guys sell the vehicle within three years and never see the stress wear. My mind set is, if I build it, I don't want to worry during a four day trip over the Dusy trail. 40's will get you places others cant go, they look cool as hell but way more stress on everything. I personally think you should go for it.

 

[Martin Cunningham]

That's a big tire, heavy. I think anything past a 35 or even 37 is just asking for it. Perhaps I don't get it, but for my needs, I'm happy with just that.