I have read a lot on here that the D30 can not handle tires much larger than 35". Can someone explain why this is the case? Is it the weight, the circumference of the tire, a combination of both, or something else completely?
-thanks

 

Most of the time it is people being stupid. Spinning tires that catch = breakage, also if your tires are wedged or stuck, don't try to force it.

 

Well, to make it as absolutely short and non-physics geeky as possible, it comes down to torque, which is rotational force. Rotational force is applied to the axle from both the Jeep side and the ground side (every force applied has an equal and opposite reaction, thank you Newton). So, you have the Jeep driving the axle around, and now you have the tyres doing their job, creating traction...

If you look at all the physics equations blah blah blah there are a number of factors which are variable when it comes to the force applied to the axles. On the one side, you have the Jeep, which for those of us who didn't get a Hemi swap for Christmas, is not variable (at least not outside of "normal" parameters) and on the other you have your tyres. The amount of force that they can exert back on the axle is dictated by:

1) The distance from the point of rotation to the point of force (radius of tyre/wheel combo)
2) The weight and speed of the tyre/wheel combo (angular momentum)
3) The amount of traction, because if the tire just spins, it isn't generating a force back on the axle.
4) Lastly, and most importantly..... the angular acceleration, because Force = Mass x Acceleration (angular because it's rotational force, confusing yeah)

Taking all these things into account, here's the simple answer in all of it... larger tyres with a bigger radius, that weigh more, and get more traction increase the likelihood that you will snap an axle IF you are one who likes to spin the tyres and gas your way out of situations where you could suddenly gain traction. THAT is what breaks stuff. It all comes down to sudden changes in 'acceleration' which really means, if you're spinning and suddenly gain traction, the amount of force increases exponentially (literally).

 

A D30 is actually a reasonably tough axle. Its small ring gear, about 7", is part of the liability, but, overall, its pretty sturdy. Axles are rated based up the weight that they can carry, and details like the strength of the case and tubes, the spider gears, ring gears, etc, all come together to create the axles overall toughness.

As with any axle...its ultimate job is holding up the truck, and twisting the tires. ALL axles can be broken, and larger tires act as larger lever arms on the axle's internal components.

If a diff case is weak, and the leverage can distort the case, that can misalign teeth/meshing, and lead to failure, etc. Same with a tube warping, a carrier bearing getting over loaded, etc.

If the case is strong, but another internal component is the weak link in the chain...then THAT'S what tends to break, etc.

All axle will be more likely to break if you are spinning the tires, and one catches...so half the diff is going at full speed, and the other half crashes to a stop...the force of that crash is capable of breaking some serious materials.

All front axles, as they have to STEER, as well as propel the truck, operate at more stressful angle for the drivetrain. That increases the odds of a front break down, etc.

Combine al of these factors, the amount of leverage applied by the tires and their radius, etc, and it works out that a D30 can handle 35's, maybe 37's if you're easy on the go go juice, etc. Really, similar to what you'd ask the D44 to do.

If you throw an axle under the front that has a larger ring gear, etc...like a Dana 44 and its ~ 8.5" ring gear, a Ford 9" with its, yep, 9" Ring gear, or a D60 with its 9.75" ring gear, etc....the diff tends to hang down lower and lower...a problem for clearance...and big $ to make so they are higher and more out of the way, etc.

The D30 was therefore a compromise between strength, cost, and clearance...and good for 35's as mentioned.

 

Hey, that was a great response.

"good answer" ...(Back to school)

 

yup that about somes it up

I had 160 000 miles on my tj before my d35 gave out but all is rebuildable and you just make it stronger

 

Ok, Then does it make more sense to run a locker or not in a D30. would the locker help distribute torque to both or make sure it broke by not letting something give. ??
sorry if this is a hiijack but ive been wondering how to prolong my front axle and wheel it too.

 

I put over 80K miles on my YJ D-30 w/ 35's and a lock-rite lunch box locker in front. The only thing I ever broke was an outer axle shaft U-Joint. Considering what I put that Jeep through, 2 outer shaft u-joints in 80K miles, with significant off road usage, is just routine maintenance
Don't forget we have a high pinion D-30 that rides on the coast side of the gear. This translates to much less gear strain. And many of our D-30 parts are the same as D-44's. I've heard it said a front axle HP D-30 is nearly as strong as a front axle LP D-44
It's big foot on the skinny pedal, OR not letting off the gas when a lifted tire makes contact with the ground, that will break things. I've seen plent of front D-44's requiring field repairs

 

OK, here is a very simple explanation of why larger tires break things.

Think torque wrenches. Think what happens when you use a longer bar for a torque wrench. The longer the bar the easier it is to break a bolt. (ot the more force you can easily apply)

When you use larger tires the radius of the tire is longer, when it catches on a rock or whatever it instantly transmits all the force you the HP to the axle. Once tires exceed a certain radius it can break axles very easily.

Then add the weight factor of the wheel and tire to it all and KA-BOOM!

Now, would you like to sell me those humongous wheels and tires cheap? lol