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Thread: Unsprung Weight

  1. #1
    Junior Member Father0fm1ne's Avatar
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    Question Unsprung Weight

    I posted this on another forum and got a formula for moment of inertia as my only answer. I dig math as much as the next guy , but can anyone put this in English?

    I have spent my fair share of time searching and searching various forums to learn what I can about unsprng weight and the consequences running large tires. All I have learned is that to heavy can be bad and there may or may not be some formula the translates unsprung weight into sprung weight. What that means in English, I have no idea. Most of what I been able to find online actually pertains to hot rods and drag racing.

    I am looking at three different tire sizes, all KM2's:

    285/70-17 (by my math work out to be a 32.7x11.2x17) weighs 60lbs.
    305/65-17 (32.6x12x17) weighs 67lbs.
    35x12.5x17 weighs 68lbs.

    My current setup is stock. I have 255/75-17 Goodyears SR-A's on stock rims and that setup weighs a total of 60lbs (on my home scale...which should be 38 lbs of tire and 22 pounds of rim if my internet research is correct).

    What are the consequences of going from my stock 60lbs of unsprung weight up to an aftermarket 85lbs (60lbs for the 285/70-17 and 25 for aftermarket rims)? I am concerned about various factors in no real order...handling (both on road and off), gas millage and axle life to name a few. What if I add the extra 8lbs for the 35's? If I am already going up to the 285/70-17's then is the extra 8lbs that big of a jump?

    I understand that the bigger you go to more "sluggish" our fantastic engine will perform. I understand the eventual concern for regearing. That deals with performance. Does that also handle the torque affects of my D30 trying to spin that extra weight around?

    Thanks for your time and any help you can give.
    "People who say it cannot be done should not interrupt those who are doing it."
    2010 Black Rubicon Unlimited
    Stock for now...Still looking at all the different things to do.

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    Default Unsprung Weight

    You need to include the Diff, steering, brakes, suspension, everything from the top of the springs down in your unsprung weight calculations. If you do you will see your tires and wheels are now a smaller percentage of the total unsprung weight, so just a couple of lbs per wheel assembly doesnt make that much of a difference in unsprung weight. Where you will see a difference is in the diameter of the larger tires as the spinning (dynamic) weight now has a longer lever arm that you now has to stop spinning when you hit the brakes or speed up when you accelerate. The thickest part, the most weight, of the tire is in the tread area so the larger the tire the more weight and the further away for the center of the wheel the longer the lever arm. I hope this gives you at least part of the info you are looking for.

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    Default Also

    That spinning large front tire in the air and coming down on solid ground is what will snap your Dana 30 front axle or u jont. So if you are going to wheel then just be careful of the position of the skinny pedal under your right foot.

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    G.I. Grandpa SGM O's Avatar
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    Default

    C130Hilgy is correct, unsprung weight includes axles, wheels, tires, etc. Running larger diameter, heavier wheels and tires will adversly effect gas mileage because it takes more engergy to get them moving and a more aggressive tire tread provides more rolling resistance which will also effect it. Regearing will help, but not return all your performance. So when it comes to mudding or running in deep snow, a lighter wheel will spin up quickers, like on a dragster which will help, but as all ready stated a larger diameter tire on a heavy wheel has more inertia and torque so it is easier to snap axle components if you are not carefull. At some point your brakes will need to be upgraded as well since once you have that larger diameter tire moving it becomes harder to stop it and the more unsprung weight, like upgrading to Dana 60 axles etc to handle the torque load while wheeling mean decreased stopping power ont he street with out bigger brakes.
    I run two sets of tires and wheels. Interco Trxus M/T 35"x12.50"x15" on AR 15"x8" steel wheels that way about 80lbs each. I also have BFG AT/KO 35"x12.50"x15" on alloy 15"x8" wheels which weigh about 56lbs each. On the highway the BFG AT are good for 1.5 mpg improvement over the Trxus M/T. Some of that is unsprung weight and the rest is tire tread pattern.

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