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Unread 08-04-2009, 03:32 PM   #1
amonostereo
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Lower Backspacing=Weaker Axles???

I have been debating Cragar Soft 8's or maybe just a nice Wheel Spacer Kit, and they got me thinking physics (never a good thing ).


Lets say you have a 93-95 ZJ (Dana 35 rear, stock)
Lets also say you keep the same tire size.

You still have the same force distribution from the road on your tires (since your ZJ still weighs the same). But now that force is applied 1.25" to 2" further out than it was before. Doesn't sound like much, but when you consider that the nearest downward (vertical) force on the axle (wheel bearing) is only 8" or so away....

What is it? Torque=Force*Radius? (90deg angle, so the cross product is just multiplied)

... so an increase in the radius of 25% (8" to 10") increases the torque by 25%.


This means that the axle is about 25% more likely to snap when experiencing a high load.

Right???


Should I be worried about a higher propensity for my 15 yr old Dana 35 axles to snap? What about shearing (torsion)? I think the equation for torsion is similar, yeilding a similar increase, but that gets into other (more complicated) stuff...



let the debate begin!

or maybe just polite conversation... either is fine with me.....




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Unread 08-04-2009, 04:49 PM   #2
BlackShuck
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From what I've read, the new backspacing might stress your ball joints more. But I don't think I've ever read anything about snapping an axle because of adding spacers or soft 8's.
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Unread 08-04-2009, 05:16 PM   #3
Bort62
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You're half correct.

You increase the lever arm when you go to a longer track width. However, the lever you are worried about here is between the center line of the tire (assuming the force is evenly distributed) and the wheel bearing.

This distance is not large to start with, and adding 2" to it certainly substantially increases the bending moment in the axle shaft at that end.

However, that isn't really a concern area. How many axles sheer off from a radial load? With the ability of a relatively soft tire to absorb the energy of any radial impact, the dynamic radial loads on the axle are inconsequential.

The concern with axle failure is torsional, and while lengthening the axle shaft will make it less torsionally stiff, it doesn't effect it's strength.

Putting spacers on also subjects the axle housing to a larger moment, but again - the loads there just aren't that large to begin with.
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Unread 08-04-2009, 09:03 PM   #4
BigDaveZJ
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Dude, it's a Dana 35. It'll break anyways. Don't worry about it.
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Unread 08-04-2009, 11:47 PM   #5
ZJTechie
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Bort62 is correct, but to that I have to add the sheer number of jeepers that use wheels like this all the time with wheel spacings amounting to the 2" or more if increased stance on each side. If an axle is going to break, it will be due to the shearing stresses caused by the torque going through the shaft as the tire rotates, not by the bending moment caused by the smaller backspacing. Wheeling with larger tires will cause that larger torque, but simply having a wider stance with those soft 8's isn't going to kill your axles with the bending moment, provided you don't go obscene by getting a 2" backspacing.
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