How do big tires stress axles? - Page 2 - Off-Road Forums & Discussion Groups
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post #11 of (permalink) Old 01-07-2002, 11:07 PM
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Re: How do big tires stress axles?

Pi/rim size x weight of tire = _____ . Then 3.092638 x tire dia. x rim width.Multiply this by the coefficient of drag of the tire @ normal air pressure you will be running. Subtract the weight of the drums or disks, but add in the weight of the valve stem and lug nuts. Then you will still have no idea, just like me!! [img]/wwwthreads_images/icons/crazy.gif[/img]

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post #12 of (permalink) Old 01-08-2002, 07:11 AM
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Size does matter!

Size does matter!
It's all about torque, as everyone has mentioned. The equation for measuring torque ( called a Moment arm in physics) is simply = (r) x (F)
where (r) is the perpendicular distance from the axis to the point at which the force is applied , the radius of the tire in our case. The (F) is the force measured in pounds. This is the tough one to measure, it has to do with the weight of the vehicle, friction with the ground, etc.... someone else can take it from there!

[img]/wwwthreads_images/icons/cool.gif[/img] Big Ed
<font color=red>'88 YJ, 4" susp,3" body,35's,283 Chevy V8,TH350,4.11's,D30,D35c</font color=red>
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post #13 of (permalink) Old 01-08-2002, 08:42 AM
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Re: Size does matter!

Fella's the weight is a factor, but if you really want to kill an axle fast, put on a rim with minimal backspacing (pokes the tire a long way out of the fender well, like the idiot lowrider trucks). When I was designing semi-truck trailers we had a customer who was consistently breaking rear axles. After a lot of cajoling we finally discovered that he had taken the duals off of the rear axle and was running a single tire in the outside position. This in effect put a very large moment on the axle. We instructed him to get a rim with more backspacing, to reduce the moment arm, or put the tires to the inside position. He changed the tires, and we never heard of another broken axle. This type of loading is why full float axles are better than and semi floating axle. In a full float situation the weight of the truck acting vertically is carried by the spindle and the axle housing, and only rotational loads have to be transmitted by the axle. In a semi-float situation the axle shaft is forced to do double duty, carrying both the weight and the rotational load. This combined loading situation increases the stress dramatically on the axle, and putting the center of the tire farther from the flange of the axle only serves to amplify an already marginal situation. In the case of the D30, the larger diameter will be worse, because of the fact that you have a much larger lever arm to break the u-joints. Most of the front axles on our Jeeps are full-floating so only the rotational load is carried by the axle shaft. The 35's will take more force to stop and get moving, the polar moment (resistence to rotation, or stopping rotation) is more a function of diameter than weight, to a point, i.e. it takes less force to spin a flywheel that is smaller diameter, even if it weighs the same as a larger diameter diameter flywheel. Even though your 33's weigh more, the weight is probably in the wheel and not the tire, so the 35's would still be worse due to the larger diameter.

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post #14 of (permalink) Old 01-08-2002, 09:04 AM
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Re: How do big tires stress axles?

Taller tires also change your overall gear ratio. Lower gears are needed to offset the taller tires to keep in the torque curve area.

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post #15 of (permalink) Old 01-08-2002, 10:12 AM
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Re: How do big tires stress axles?

A lot of it has to be traction and how you wheel it, in addition to the leverage of a taller tire. If it were only leverage, 35's are 25% taller than 28's. It's hard to believe that only 25% more leverage crosses some line over which breakage is inevitable. It's all the factors added up with that leverage that stresses axles more. As mentioned previously, locker for instance will potentially double to quadruple the torque applied to a wheel. Imagine a front wheel on a rock and the other 3 wheels on a slippery surface. 100% of the torque generated by the drivetrain can go to that one wheel. All that said, got to have a locker or two, and what's off roading without the true clearance generated by taller tires? That's why were are always buzzing about axle swaps.


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post #16 of (permalink) Old 01-08-2002, 11:24 AM
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Re: Size does matter!

yep v8jeep has got the math going now.
PHYSICS is a wonderful thing.
newton's second law can be put into a form suitable for describing the rotational motion of a rigid object about a fixed axis.

TORQUE is the culprit raised by the increased radius and weight or mass of the larger tire.
the formulas are a little tough to explain and how they are arrived and me not being a physicist, i can't, but my physics book covers it quite well.

lockers thrown in and how hard you wheel, type of terrain........
the list never ends.

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