Sprung weight vs unsprung weight.

Discussion in 'Mopar Racers Forum' started by inkjunkie, Sep 12, 2017.

  1. inkjunkie

    inkjunkie Well-Known Member

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    Correct me if I am wrong....in order for weight to be unsprung it needs to be below the suspension? Have more questions but want to see if I am right with this thought......
     
  2. RustyRatRod

    RustyRatRod Just another dumbass. FABO Gold Member Technical Editor

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    Yup, essentially that's right. Spring weight is exactly what it implies. Weight that's heldup by whatever type suspension you have. Leafs, coils, bags, magic beans, whatever.

    Unsprung weight is that which is not supported by the suspension. Axles, wheels, tires, brakes, and anything else not supported by suspension.
     
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    • Killer6

      Killer6 Well-Known Member

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      Basically, correct, it can be tricky nailing down exactly how much of what component, for ex. the ball-joint end of the upper a-arm incl. the upper b-joint is unsprung, but
      the UCA bushings and those ends of the UCA are sprung.......etc., etc.,.................
       
    • autoxcuda

      autoxcuda Well-Known Member

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      Upper, lower and strut arm are 1/2 sprung and 1/2 unsprung.

      Same for leaf springs and driveshaft
       
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      • MRGTX

        MRGTX Well-Known Member

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        I think you guys covered it but the reason why this concept is so important should be known by everyone interested in the performance of their car...if not, go find out. You'll be glad you looked it up!
         
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        • 66jim

          66jim Well-Known Member

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          There is also rotational weight that is wheels tires disks calipers, that's the lead boots your wearing.
           
        • jos51700

          jos51700 Well-Known Member

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          My understanding is that the springs themselves count as sprung weight?
           
        • dukeboy_318

          dukeboy_318 Get off my lawn!!! FABO Gold Member

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          yes they do. If you can get a hold of one of Ron Sutton's books/writeups, i think he covers it very well.
           
        • inkjunkie

          inkjunkie Well-Known Member

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          This was my next question.
          I remember reading about this in a chassis book a long time ago.
          Next part of this question...is unsprung weight seen the same as sprung weight? Perhaps that is not the best way to put it. In the book I was reading the author was emphasizing how losing 10 pounds of unsprung weight is the equivalent of losing 100 pounds of sprung weight performance wise. Didn't make much sense to me back then...still doesn't now. Might have actually seen it in a wheel catalog...or perhaps it was an old Wilwood catalog. Sounded like a bit of a sales pitch but...
          Anybody know if this is actually factual or just some sort of claim to try and get folks to spend their hard earned $$ on a certain brand of products?
           
        • dukeboy_318

          dukeboy_318 Get off my lawn!!! FABO Gold Member

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          yes and no. this is where is gets complicated. Unsprung weight and sprung weight, while independent of each other, can, in certain aspects, affect each other. For drag racing,most of the unsprung weight isnt a huge deal. For road racing, unsprung weight because very important. not only from a spring selection but for shock selection and adjustment as well as wheel control, suspension travel, roll center and center of gravity. That being said, unsprung weight changes can alter a cars performance just as sprung weight can. However, there is NO 1:10 rule or 1:5 or anything. Unsprung weight changes will not affect a car's acceleration or top speed any different or at any different ratio than sprung weight changes.
           
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          • MRGTX

            MRGTX Well-Known Member

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            Unsprung mass' affects on acceleration would mostly come into play when the unsprung mass in question is located in the rotating portions of the system. So the rotational inertia of the wheels, tires, brakes all play a major role in how the car accelerates...these things all require work (energy) to get spinning.
             
          • jos51700

            jos51700 Well-Known Member

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            I don't agree with the idea of parts being x percentage sprung and unsprung, that sounds suspiciously like engineering approximation for calculations.

            Think of unsprung weight as any mass that has movement controlled by a spring. This is suspension, valves, etc. And the assumption is that the body of the car is not moving in reaction to things.

            High unsprung weight scenario: you hit a bump. The tire leaves the road. It's going to take longer for a given spring to get that heavy tire back down to the road and get you back in control of whatever it was that tire was doing.

            The other side is, you hit that bump, it takes more energy to move that tire to react to that bump. If the spring isn't absorbing that energy (because it's too stiff for the given impact), it's being transmitted to your car (aka you) as a bump or jolt. Soft spring to absorb energy and give nice ride doesn't control the wheel properly. Stiff spring to control doesn't ride well. Reduce the unsprung mass, and the two springs become closer to being the same spring.

            ROTATIONAL mass is typically the one where x pounds rotational is worth y pounds non rotational.
            This is because the energy needed to accelerate the weight in rotation is reduced, giving an increase in acceleration equal to the acceleration gained by losing a certain amount of fixed weight in the car.
            I want to say one pound of rotational weight is worth four in the car. Lose ten pounds off each tire and it's like you dropped forty out of the car.

            This is a two-for, less weight overall AND less rotational weight to spin up. All the above are factors of total overall vehicle weight. Ten pounds on a three Hundy pound sport bike is instantly noticeable. on a three ton tank, not so much.

            There is probably some way to relate sprung and unsprung mass equivalents, but I don't know what it is. these pay off in acceleration, braking, cornerning, anywhere traction is the action.
             
            Last edited: Sep 13, 2017
          • mbaird

            mbaird mbaird FABO Gold Member

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            I think you meant to say 160 lbs dropped from car....

            Our Porsches , Mercedes , Jags and The Acura NSX can be ordered with Ceramic Composite Brakes for around 10k.
            I always thought it was a waste of money for the street because no one brakes hard enough to require them on the street.
            Then I picked up one of the rotors to an NSX ! WOW! they are light !
             
          • jos51700

            jos51700 Well-Known Member

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            Shit, man, in high school they said we'd never use math in the real world
             
          • mbaird

            mbaird mbaird FABO Gold Member

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            A 13" rotor was under 5 lbs! That includes the hub...
             
          • ir3333

            ir3333 Well-Known Member

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            unsprung weight has a dramatic effect on ride and handling...keep it at a minimum.
            Hit your buddy in the shoulder with your closed fist...(not too hard) now using the
            same amount of force hit him again but this time with a roll of silver dollars in your fist.
             
          • Killer6

            Killer6 Well-Known Member

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            That is a rule of thumb approximation which is usually "close enough", however it could only be 100% correct if the mass was identically divided on both halves, and the
            components were perfectly level on the horizontal plane..........................
             
          • autoxcuda

            autoxcuda Well-Known Member

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            Yes, that is an approximation/rule of thumb.