Hey guys,

I've been thinking about the stresses due to larger tires. What I'm trying to understand is what stresses are applied to the drive train due to having larger tires.

Are the increased stresses due to the added weight from the tires?

Is it due to the increased leverage of the tires on the axle?

In my mind, I still can't seem to pinpoint where the extra strain is coming from and where the points of failure would be.

Thanks for your insights!

Micah

 

I'm thinking more about axle and axleshaft strength than anything else. It seems that the added stress from the weight of the tires is no more important than the increased weight of the overall vehicle. Is that true?

Also, if you move from a 31" to 37" tire, you effectively gain 3 inches of leverage. That would translate to a 20% gain. That's a decent amount, but it doesn't necessarily seem like it would warrant any issues by itself.

I still feel like there's a factor not being taken into account.

 

Torque on a drive shaft is effectively translation of moments. A moment is a force at some distance from a point of rotation, multiplied by that distance. I'm gunna pull random numbers here to give you an idea, but say you tires are outputting 100 lb of force. At a 31" tire, that's 1550 lb/in on the axle. Depending on your ring gear dia (assume 8") thats about 388 lb or force on the teeth.

Now go up to a 37" tire. If you have the same tire output force, then your axle load is about 1850 ft/lb. Translate that to the same dia ring gear, and thats about 463 lb on the teeth.

Now this is all assuming same forces applied, etc. but what I'm getting at is just by changing the tire dia, you drastically boost the forces on everything in the drive line, even with re-gearing. On top of all of this you also have to take into account the larger moment of inertia of the wheel, or its tendency to rotate, which will also increase as size increases.

I think this is what you meant by 20% increase, but 20% is not all that small, and when you wheel hard, you also are introducing impulse/impact forces, which can be twice as strong and happen in a fraction of a second.

 

Thanks very much for your detailed explanation. I think I'm beginning to understand. One of my initial thoughts was that if I get a lightweight 37" tire such as the Yokohama MTs which weigh less than most 35s, I would be able to save a lot of the strain on the axles. The more I thought about it though, the less it made sense. It seems the only time the mass of the wheel matters is when it is spinning free; at all other times, it's only translating the mass of the vehicle. The combination of the increase in torque and overall weight of the vehicle are what make the largest difference. It seems the weight of the tires is actually relatively unimportant as far as strain on the axle is concerned.

 

Well yes the weight of the vehicle is important, but don't discount the tires. I'm not gunna go into detail, but if you want to know that effect the mass has look at the mass moment of inertia. (http://en.wikipedia.org/wiki/Moment_of_inertia). This property will change based on size and weight.

Engineering set aside, look at what others are saying. There are people destroying D44's with 35's. The reason people go to a stronger axle/driveline is to avoid ever having the issue of breaking on the trail, rather than have the possibility looming over their head every time they wheel.

If you really want to get detailed drive line stresses, do an online search into engineering mechanics: moments and moment of inertia.

Hope all this has answered your questions.

 

x2 what jkred said... but allot depends on how you use you're Jeep also... meaning,,, if all you do is street and light trails, you probably won't run into any problems with the extra strain on things..
Its when you go out and really wheel hard, i.e.... mud, deep snow, rock crawling, steep hills etc. that the added stesses and all that will come into play.....
The stock axles and drive shafts are pretty stout.. they will take allot of abuse..
I say, use what you got, wheel it and if and when it breaks, then spend the $$$ for the upgrades...

 

I think a lot of it has to do with driving style. Friends of mine in the local club I'm in have been running 37's on locked D35s and 30s for years with no problems. If you are smart and nice to the skinny pedal a lot of problems are avoided

 

Agreed, pedal mashing will just dent your wallet