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    • HPF compares the effect of different sized wheels on the dyno

      Well all know lighter wheels are a good thing. But just how big is the difference? Well, as a car makes more power the effect is easier to see. HPF took a Stage 4 and tested it with a wide set of 19's and then the stock 19 inch M3 wheels. The effect is magnified on a more powerful car and at one point in the graph there is an over 60 whp difference between the heavier wheels and the skinnier, lighter stock wheels. Check out the vid on the next page.


      This article was originally published in forum thread: HPF compares the effect of different sized wheels on the dyno started by Sticky View original post
      Comments 35 Comments
      1. black bnr32's Avatar
        black bnr32 -
        its not that difficult guys

        you want to minimize the amount of mass as you get further from the center of the wheel

        more mass further from the center = bad

        unfortunately we can't readily measure the difference between set ups, although total weight is a good indicator
      1. Sticky's Avatar
        Sticky -
        Click here to enlarge Originally Posted by BASELINE Click here to enlarge
        I wonder if the difference is as great because the car here is making such big numbers. If it was a car making 400ish whp, the difference might have been much smaller.
        Definitely, the larger power accentuates the shift.
      1. Sticky's Avatar
        Sticky -
        Click here to enlarge Originally Posted by mazdaspeed6 Click here to enlarge
        tires are pretty heavy too, i wonder what the difference would be from a 305-45-18 to a 305-30-20 would be...
        Pretty big... tirerack lists tire weights which is cool.
      1. DBFIU's Avatar
        DBFIU -
        The power increase has nothing to do with the tire diameter alone.

        When you change rear gear ratios on a car, does it make more power? No, it doesn't, the power output of the engine doesnt change nor does the power to the ground change, only the slope the power curve. Changing tire diameters is like changing rear gear ratio, assuming you neglect weight etc...

        The weight alone; also, does not attribute to the power loss. Physically, what is happening is the bigger wheel has more mass at the extremities of the rim. Meaning, the rim is heavier as you move from the center of the rim to the outside. This is called the moment of inertia.

        You can have two rims that weigh EXACTLY the same on a STATIC scale. And guess what, if one rim is a properly designed rim, it will be as light as possible towards the outside. If the poorly designed rim is heavy near the rim lip (or if you use a heavier tire), it will show you the exact same results as seen in this video because it has a greater moment of inertia despite the rims weighing the same.

        What people dont understand is that its not the weight of the rim, but the weight of the tire and ITS DIAMETER; the COMBINATION of weight distribution from center of rim to radius. A large heavy tire with a lot of circumferential mass will take a lot more torque to rotate than an identical relatively 'heavier' rim with a lighter tire.

        Weight alone is not the answer, diameter AND weight contribute to this phenomena.

        This is how rotating objects work, when something needs to be rotated it requires torque. Just like you can accelerate in a straight line, you can also accelerate in rotation, this is called angular acceleration.

        To get angular acceleration you need torque. The relationship between angular acceleration and torque is

        Angular acceleration = Torque / moment of inertia

        Doesnt that look familiar?

        Sort of like linear acceleration = Force / Mass

        Except for a rotating object we dont consider static mass, but moment of inertia, because static mass only doesnt tell you enough about how much rotational inertia an object has.

        I just needed to clarify some misinformation.
      1. Sticky's Avatar
        Sticky -
        Click here to enlarge Originally Posted by DBFIU Click here to enlarge
        What people dont understand is that its not the weight of the rim, but the weight of the tire and ITS DIAMETER. A large tire with a lot of circumferential mass will take a lot more torque to rotate than an identical rim with a lighter tire.
        Isn't a combination of factors? Where the weight is, weight of the rim, tire, width, diameter, everything, it all plays in.

        Changing the diameter is like changing the final drive, we get it Click here to enlarge
      1. DBFIU's Avatar
        DBFIU -
        Click here to enlarge Originally Posted by Sticky Click here to enlarge
        Isn't a combination of factors? Where the weight is, weight of the rim, tire, width, diameter, everything, it all plays in.

        Changing the diameter is like changing the final drive, we get it Click here to enlarge

        Tire width, diameter, everything, yes etc... Is all summed up into one thing, moment of inertia. At the end of the day, that is what will dictate the power rob caused by the wheels as a rotational mass.

        Weight distribution from center to extremity is what determines moment of inertia, so all those factors are summed up into one number which is proportional to the square of the distance the weight is from the center of rotation.
      1. Shipkiller's Avatar
        Shipkiller -
        As a general rule of thumb, for every pound of unsprung weight you lose, it equates roughly to 10lbs of total weight.
      1. Yukohama's Avatar
        Yukohama -
        pretty interesting vid
      1. mazdaspeed6's Avatar
        mazdaspeed6 -
        Click here to enlarge Originally Posted by Sticky Click here to enlarge
        Pretty big... tirerack lists tire weights which is cool.
        good to know
      1. nickr519's Avatar
        nickr519 -
        since torque (rotational force) is proportional to radius from center and inertia, not just the overall weight but the distribution of the weight from the center of the wheels will make a difference in wtq reading. the more mass towards the center of the wheel the less torque that will be robbed by the rotating assembly (wheel)
      1. nickr519's Avatar
        nickr519 -
        Click here to enlarge Originally Posted by DBFIU Click here to enlarge
        The power increase has nothing to do with the tire diameter alone.

        When you change rear gear ratios on a car, does it make more power? No, it doesn't, the power output of the engine doesnt change nor does the power to the ground change, only the slope the power curve. Changing tire diameters is like changing rear gear ratio, assuming you neglect weight etc...

        The weight alone; also, does not attribute to the power loss. Physically, what is happening is the bigger wheel has more mass at the extremities of the rim. Meaning, the rim is heavier as you move from the center of the rim to the outside. This is called the moment of inertia.

        You can have two rims that weigh EXACTLY the same on a STATIC scale. And guess what, if one rim is a properly designed rim, it will be as light as possible towards the outside. If the poorly designed rim is heavy near the rim lip (or if you use a heavier tire), it will show you the exact same results as seen in this video because it has a greater moment of inertia despite the rims weighing the same.

        What people dont understand is that its not the weight of the rim, but the weight of the tire and ITS DIAMETER; the COMBINATION of weight distribution from center of rim to radius. A large heavy tire with a lot of circumferential mass will take a lot more torque to rotate than an identical relatively 'heavier' rim with a lighter tire.

        Weight alone is not the answer, diameter AND weight contribute to this phenomena.

        This is how rotating objects work, when something needs to be rotated it requires torque. Just like you can accelerate in a straight line, you can also accelerate in rotation, this is called angular acceleration.

        To get angular acceleration you need torque. The relationship between angular acceleration and torque is

        Angular acceleration = Torque / moment of inertia

        Doesnt that look familiar?

        Sort of like linear acceleration = Force / Mass

        Except for a rotating object we dont consider static mass, but moment of inertia, because static mass only doesnt tell you enough about how much rotational inertia an object has.

        I just needed to clarify some misinformation.
        oops beat me to it. high school physics for the motha frickin win Click here to enlarge
      1. Remonster's Avatar
        Remonster -
        I don't see where you guys are getting confused...

        The Z4M gained 20mm in rear tire size, so the weight was by far the bigger issue. In HPF's case, they added 90mm (they're 255 stock, right?) which also increased the height of the tire a lot more (in HPF's words 3/4" to 1") so in their case both weight and overall diameter were significantly changed.
      1. Sticky's Avatar
        Sticky -
        Click here to enlarge Originally Posted by Remonster Click here to enlarge
        I don't see where you guys are getting confused...

        The Z4M gained 20mm in rear tire size, so the weight was by far the bigger issue. In HPF's case, they added 90mm (they're 255 stock, right?) which also increased the height of the tire a lot more (in HPF's words 3/4" to 1") so in their case both weight and overall diameter were significantly changed.
        That's right, I do not think anyone is getting confused necessarily but just expounded on the various factors that contribute.
      1. Rysandro's Avatar
        Rysandro -
        Nice! My 18" Advan wheels weigh about 18-19 lbs and my 265/35 rear tires weigh 25lbs according to tirerack. That's 43-44 lbs compared to stock wheels that weight 29 lbs and run flats that weigh like 30 lbs.
      1. PEI330Ci's Avatar
        PEI330Ci -
        Click here to enlarge Originally Posted by DBFIU Click here to enlarge
        The power increase has nothing to do with the tire diameter alone.

        When you change rear gear ratios on a car, does it make more power? No, it doesn't, the power output of the engine doesnt change nor does the power to the ground change, only the slope the power curve. Changing tire diameters is like changing rear gear ratio, assuming you neglect weight etc...

        The weight alone; also, does not attribute to the power loss. Physically, what is happening is the bigger wheel has more mass at the extremities of the rim. Meaning, the rim is heavier as you move from the center of the rim to the outside. This is called the moment of inertia.

        You can have two rims that weigh EXACTLY the same on a STATIC scale. And guess what, if one rim is a properly designed rim, it will be as light as possible towards the outside. If the poorly designed rim is heavy near the rim lip (or if you use a heavier tire), it will show you the exact same results as seen in this video because it has a greater moment of inertia despite the rims weighing the same.

        What people dont understand is that its not the weight of the rim, but the weight of the tire and ITS DIAMETER; the COMBINATION of weight distribution from center of rim to radius. A large heavy tire with a lot of circumferential mass will take a lot more torque to rotate than an identical relatively 'heavier' rim with a lighter tire.

        Weight alone is not the answer, diameter AND weight contribute to this phenomena.

        This is how rotating objects work, when something needs to be rotated it requires torque. Just like you can accelerate in a straight line, you can also accelerate in rotation, this is called angular acceleration.

        To get angular acceleration you need torque. The relationship between angular acceleration and torque is

        Angular acceleration = Torque / moment of inertia

        Doesnt that look familiar?

        Sort of like linear acceleration = Force / Mass

        Except for a rotating object we dont consider static mass, but moment of inertia, because static mass only doesnt tell you enough about how much rotational inertia an object has.

        I just needed to clarify some misinformation.
        Great post...I was just about to post something along those lines.

        The dyno doesn't care how tall a tire is......you could run a 26" tall tire, and a 33" tall tire, and as long as the rotational inertia was the same, the dyno would read the same....or almost. (Frictional losses from greater dyno roller speed with taller tires affects things a little bit too)

        One final factor is that taller tires mean less RPM of the rim, meaning less inertia to overcome to a certain road speed. (This is not RPM dependent, it is road speed dependent.)