Maybe someone who understands the physics of it can explain this to me. I keep reading posts saying that you can't run 40's without upgrading axles to D60's and other expensive mods, and that failing to do so is a recipe for disaster. But I don't see anyone saying anything against hitting a trail with 37" tires on trussed/gusseted D30/D44 axles with a jeep full of people and camping gear. What's the difference?

Of course, to keep my analogy straight I'd have to assume I'd never have people and camping gear in my JK with 40" tires, but you see what I'm getting at.

 

I'm taking a shot in the dark here, but i'd guess it has something to do with sprung vs unsprung weight.

 

Rolling mass, the weight of the tire starting and stopping. The tire spinning then grabbing.

 

It's simple.......both of those are not good at all.

 

And people who crawl beef's up the axles or they break or bend with 37. Or they take it real easy. If you hardcore wheel you will find the weak points.

 

You're somewhat right. The bigger tires are more like a bigger/larger "lever arm". They are much heavier, have larger circumference, and have more grip do to a larger "contact patch". This will make them harder to get going and to stop. The smaller diameter axle shafts are at much more disadvantage than with a smaller tire. It has nothing to do with weight of the vehicle. It comes down how big the "lever arm" is on a particular part. Same goes for a bigger engine with the same size axles. With a small V6 a dana 30 and 44 with stock shafts will be ok. Put a large hemi in there and you'll be snapping axles, ring or pinions, driveshafts, etc much more unless you upgrade to bigger parts. Kind of hard to explain but hopefully you get the point.

I think the OP's question does pertain to the axle housing and outer Cs though. If you're running just 37s or even 35s and you put 3000 extra pounds on the drivetrain you are more likely to bend something (which would be outer Cs and axle housings).

 

Think of a wrench on a nut tightening it by itself will get it nice and tight and you will reach a Max, now take a breaker bar and throw it on there..SNAP something gives...either nut snapped r you stripped out the wrench...

37s are a wrench and 40s are the breaker bar.....axes, ring/pinion are the nut and wrench end...

It has more load on the parts...I'm considering jumping to 39s anywase...I rarely spin and hop, I prefer to air down and let the tire conform to the terrain....

 

Very well stated.

 

Thanks for that (and everyone else for their input). That makes sense, and I'm not disputing those facts, but for this analogy's sake let's compare the two tire sizes and weight types. A 40" tire only has 1.5" more radius (leverage) than a 37" tire. So we're really comparing a 18.5" breaker bar vs a 20" breaker bar. Even if we kept the weight even in both scenarios, that's not a significant increase in leverage or force on those axle shafts or ring/pinion gears.

What I suspect is that vehicle weight, as well as driver habits, are bigger factors in driveline failure than tire size. Great input from everyone though, and I'm open to being convinced either way.

 

You're looking at this way too superficially. It's like going from a 2" water pipe to a 4" waterpipe, It doesn't linearly double in capacity, it increases exponentially. Same thing with the forces on these axles.