wheel rotational mass vs. power

[DrLove'sKuDang]

whats the difference in power loss when comparing wheel weight.

i weighed my wheels today and here were the results:

14" steelie w/ 195/60 falken azenis--35lbs
17" sokudo w/ 215/45 kumho ecsta--40lbs

given that i swap the front pair, thats a 10lb difference. me knowing that, i cant rely on my feeling to tell if it makes a noticable difference... cause knowing the difference will make my mind think one is better.

im not worried about cornering, just straight acceleration. how much of a difference will 10lbs make???

[CasperAccord]

that's a physics question

let me preface this reply with two things:
1. this is not a complete answer to your queston.
2. i am not an expert in physics. but i do know something about it, and will double check everything written below.

the short answer is that it has little to do with how much the wheel weighs.

from your subject line is seems like you sorta already realize it, but i'll say it anyway. it is not the mass of the wheel that is important, but where that mass is located relative to the axis of rotation. and that is a good place to start.

now, i know that steelies are quite different from alloys, but for the sake of discussion, lets assume that the weights for both wheels are the same, and that the weight is distributed evenly across the radius of the wheels. so, for example, we have one 15" wheel weighing 30 lbs and one 17" wheel weighing 30 lbs. we know intuitively that the 17" wheel requires more work (on the part of the engine and drive train) to get it moving. so lets go with that.



picture the two wheels. the 17" wheel is two inches bigger than the 15" wheel; so the 17" wheel has a radius that is one inch longer than that of the 15" wheel. so relative to the 15" wheel, the 17" wheel has *some* mass located 1" further from its axis of rotation. How much more mass is a more complicated question, and how much more work is required to move that mass, i don't know how to begin to answer. but i'd guess that the work would be related to the proportion the weight that is contained in that 1". keep in mind that the same unit of weight is harder to move as it moves further from the axis.

but these are not the only variables that go into figuring out how much more work (if any) your engine has to produce to match the acceleration. you also have to consider the tires. with 15" steelies rolling on 195/65R15 tires, you have about seventeen sqr inches of tire (7.6"W + 2 X 4.9"sidewalls). with 17" wheels rolling on 205/50R17 tires, you have only about sixteen sqr inches of tire (8"W + 2 X 4"sidewalls). so in effect, you are replacing the rotational mass of the tire with the rotational mass of the wheel -not exactly, but pretty much.

back to our above example, we would probably expect to have small net loss of rotational mass when switching to 17" wheels and 205/50R17 tires. we are gaining 1" worth of allow mass around the radius, and are losing 1" of rubber mass around the radius. even if rubber and allow weight the same, we make a small net loss of rotational mass -because we're losing mass further from the axis than we're gaining it.

your 15"ers and 17"ers don't weight the same though. going from 30 lbs to 35 lbs is not the bulk of the problem. the real problem is *how much* of the extra 5 lbs is *how far away* from the rotational axis. if the extra 5lbs is on the axis itself, gravy! if it is on the rim, though, bummer. i don't know how you find that sort of information out, but i'm sure if the wheel manufacturer is serious, they can tell you (if you press them hard enough).

also, don't forget that in addition to affecting acceleration, all these variables will also have an affect on braking.

like i said, this is not a complete answer to your question, but it should lead you into some direction on how to go about answering the question. this would be a great reason to take up physics, or to go hit up a physics teacher for some insight.

good luck.

[Fearit22]

Physics major to the rescue heh... You got a lot of the ideas down right. I'll go into a little more detail though. W = force * distance ... Neglecting everything else your engine will have to work harder to turn the larger wheels because you are increasing the circumference.

Another good point that was brought up is where the mass is located. It could get really complicated calculating the inertia depending on how the mass is distributed. If the spokes on the wheels are even then no problem, but if they are the kind that gets smaller towards the outside then you would have to take an integral and I'll just leave it at that. Back to more simple things... Torque = Inertia * angular acceleration. For a solid cylinder, the moment of inertia = 1/2(Mass)(Radius)^2 ... so for the sake of keeping things simple if would be safe to call your wheel a solid cylinder. This shows that mass increases the inertia, but it is more greatly affected by the radius. Since the torque will be the same, increasing the inertia will mean that your angular acceleration will have to go down. That is not the only thing that effects it though. Since Force = Mass * Acceleration, or rearranged, Force / Mass = Acceleration, so when you are increasing the mass and the force is constant, the acceleration again has to go down. Like it was pointed out this is noticed mainly because of the increased tire weight. The tires are going to have a much larger width even though they will be lower profile so it will make most of the difference.

So to answer your question, there is no power loss. There is a decrease in acceleration though. It would be caused not only by the things I listed, but the change in the gearing because of the different size of the wheels. Now whether or not it's worth it because of the traction gained, I'm not really sure about.

[CasperAccord]

right....

good post. i couldn't remember exactly how angular acceleration worked. after reading your post though, i guess it is that simple.

regarding an increased circumference thereby altering the final drive ratio, and the consequent loss of acceleration: i was assuming that circumference would be kept constant (i.e. increasing rim size while decreasing the profile of the tire wall). my reasoning for this is that most of the time when one goes to a different rim size, one chooses a tire that will result in the new circumference being comparable to the previous, mainly for the reason that it keeps your speedometer accurate (for the most part). such a situation would generally hold true when going from 14" or 15" rims up to 17" rims, which i think we can agree is pretty common. the tire sizes i used in my example were congruent with this assumption. i was only trying to address the factors internal to the circumference (if you will) that might contribute to decreased acceleration.

in other words, and in keeping with your example of a solid cylinder, assuming the circumference of the cylinder does not change, i wanted to address the effect of redistributing the mass internal to the cylinder itself. and you're absolutely right about the contour of the spokes, that becomes an integral... but without some data points of course we can't model this.

that said, though, if you do choose a wheel/tire combination that changes the circumference, is a good point that doing so will ultimately be the primary factor in decreased acceleration.

it would be quite possible, though, for larger wheels to actually result in increased acceleration. if the new wheels were significantly lighter, and the circumference of the new wheel/tire combination was similar enough to that of the old.

dude, are we nerds?

[Fearit22]

Yeah, I agree. Most of the time the increased rim size is balanced out by the decreased tire size. That usually stays true about up to 17" like you said. It's really hard to get larger rims that will weigh less with the larger tires. Those kind of rims are usually way out of a budget like mine. I still haven't gotten around to weighing mine with the tires to see how they compare. They are 16" and I think they only weighed 15 pounds a piece. It should probably be pretty close. Nerds ... heh

[cb7-R]

DAMN YOU GUYS ARE SICK!!! Good info fellas. Thanx