How Much do Rims affect?
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How Much do Rims affect?
How much would 18 inch rims affect a car on the dyno? I have some Chrome 18's and i was just wondering if i dynoed it with smaller lighter rims if the hp would go up?
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Rep Power: 0 i dunno bout dyno stuff, but ive always heard that big rims are heavy, chrome rims are even heavier so it will affect your overall driving speed and acceleration (and mpg too). not sure how it affects hp tho.
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Rep Power: 0 You'll see a difference on the dyno for sure, although I don't know how much. I don't recall how much torque we get at the wheels, but divide that number by 4. That's the torque on one wheel. The torque equals the moment of inertia times the angular acceleration. Thus the angular accleration of the wheel is the torque divided by the moment of inertia. The moment of inertia is given by a constant*mass*radius*radius. Since the wheel and tire aren't a simple geometric object, I don't know what the constant would be, but maybe in the neighborhood of 3/4(?). If the dyno will readout the angular acceleration of the wheels, you can compute the torque and compare that to stock.
Bottom line: Since the angular acceleration depends directly on the masses and the square of the radii of the wheel and tire, you're always going to accelerate faster with wheels of a smaller diameter. Mass (~weight) is also important, but not nearly as much as the diameter. Basically, 15's are ideal because of the smaller radius and inherently lower mass.
edit: oops, don't divide the torque by 4 on the dyno. divide by two.
edit2: you'll find that the torque doesn't change significantly. this is because the torque is coming from the engine itself, and won't change regardless of what wheel/tire combo you pick. Only the angular acceleration will change. As a result, however, you will likely see a significant change in hp, since you're tires are doing more work on the road per unit time.
Bottom line: Since the angular acceleration depends directly on the masses and the square of the radii of the wheel and tire, you're always going to accelerate faster with wheels of a smaller diameter. Mass (~weight) is also important, but not nearly as much as the diameter. Basically, 15's are ideal because of the smaller radius and inherently lower mass.
edit: oops, don't divide the torque by 4 on the dyno. divide by two.
edit2: you'll find that the torque doesn't change significantly. this is because the torque is coming from the engine itself, and won't change regardless of what wheel/tire combo you pick. Only the angular acceleration will change. As a result, however, you will likely see a significant change in hp, since you're tires are doing more work on the road per unit time.
Last edited by mdb_foxmx; Aug 8, 2005 at 01:03 PM.
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Originally Posted by ZapThyCat
If you always drive with the 18's, why would you want to dyno your car with lighter wheels...? So that you can tell other people that your HP is higher than it is?
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Originally Posted by mdb_foxmx
Bottom line: you're always going to accelerate faster with wheels of a smaller diameter. Mass (~weight) is also important, but not nearly as much as the diameter. Basically, 15's are ideal because of the smaller radius and inherently lower mass.
If you don't believe me, check out the tire calculator on the site. Also read the tire forum FAQ's.
I know from personal experience... I started on stock rims, went to GSR rims with very low profile tires and had great acceleration... but my top speed was lower and milage a lot worse. Then I went to heavy Motegi's, 17's, which made my top speed better, but acceleration was a lot worse, both because they were heavier and also because of the higher diameter of my wheels + tires.
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Rep Power: 0 It also has everything to do with the shape and mass distribution of the wheel/tire. When you have a 15" wheel + tire, most of the mass is closer to the center than it would be with a 17" wheel + tire. So, when you talk about the radii being the same, you're only half right. The actual distance from the center of the wheel to the edge of the tire might be the same, but because the wheel + tire is not a simple geometric object, like a cylinder or a hoop, you can't take that radius measurement as is. A 17" wheel and tire looks more like a hoop (depending on the actual shape of the wheel), and a 15" wheel and tire looks like a smaller hoop, or even closer to a thicker hoop with a shorter inner radius. What this means is that a) the scalar by which the mass and radius are multiplied when computing the moment of inertia will be smaller for the smaller rim/tire and larger for the larger rim/tire (and that's negating the effect of mass on the whole thing, which will surely be less for a 15 than a 17), and b) the actual radius of the wheel/tire will not be simply the distance from the center of the wheel to the edge of the tire, as you really want the distance from the center of the wheel to the center of mass of each 'section' of the wheel and tire that lies along a straight radial line. (Actually, b) is a kind of generalization...it might be more prudent to compare the wheels and tires to cylinders instead of hoops...in either case, it is a difficult comparison because mass isn't uniformly distributed).
You are exactly right about top speed (and therefore, fuel economy), however. This makes sense...our engines can only rotate at some maximum rpm before exploding, and the same is true of our transmissions. This means that our wheels can only have some maximum angular speed. Now, if you choose a wheel + tire with a larger total radius, then yes, your linear top speed will be greater, because for each rotation of the wheel, you cover a greater distance than you would with a rim/tire with a smaller overall radius. However, it would seem that your argument was that the difference in radius between larger rims + tires is negligible compared to smaller rims + tires.
Of course, I base all of this on no 'hands on' experience. This is just simplified applied physics. As such, do not take all of this as the final word. I will check out the forum posts and faq to see what others are thinking. Thanks Zap.
You are exactly right about top speed (and therefore, fuel economy), however. This makes sense...our engines can only rotate at some maximum rpm before exploding, and the same is true of our transmissions. This means that our wheels can only have some maximum angular speed. Now, if you choose a wheel + tire with a larger total radius, then yes, your linear top speed will be greater, because for each rotation of the wheel, you cover a greater distance than you would with a rim/tire with a smaller overall radius. However, it would seem that your argument was that the difference in radius between larger rims + tires is negligible compared to smaller rims + tires.
Of course, I base all of this on no 'hands on' experience. This is just simplified applied physics. As such, do not take all of this as the final word. I will check out the forum posts and faq to see what others are thinking. Thanks Zap.
Last edited by mdb_foxmx; Aug 8, 2005 at 08:47 AM.
Originally Posted by mdb_foxmx
You'll see a difference on the dyno for sure, although I don't know how much. I don't recall how much torque we get at the wheels, but divide that number by 4. That's the torque on one wheel. The torque equals the moment of inertia times the angular acceleration. Thus the angular accleration of the wheel is the torque divided by the moment of inertia. The moment of inertia is given by a constant*mass*radius*radius. Since the wheel and tire aren't a simple geometric object, I don't know what the constant would be, but maybe in the neighborhood of 3/4(?). If the dyno will readout the angular acceleration of the wheels, you can compute the torque and compare that to stock.
Bottom line: Since the angular acceleration depends directly on the masses and the square of the radii of the wheel and tire, you're always going to accelerate faster with wheels of a smaller diameter. Mass (~weight) is also important, but not nearly as much as the diameter. Basically, 15's are ideal because of the smaller radius and inherently lower mass.
edit: oops, don't divide the torque by 4 on the dyno. divide by two.
Bottom line: Since the angular acceleration depends directly on the masses and the square of the radii of the wheel and tire, you're always going to accelerate faster with wheels of a smaller diameter. Mass (~weight) is also important, but not nearly as much as the diameter. Basically, 15's are ideal because of the smaller radius and inherently lower mass.
edit: oops, don't divide the torque by 4 on the dyno. divide by two.
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Originally Posted by D5cIvIc
why can't you just say that "YES" it does affect it and leave out all of the other CRAP
I actually prefer to read his expliantion, rather then see just a "yes". it proves he's not just regurgitating second hand info or talking out his *** like most people on forums.
any way, From personal experiance... There is a Huge difference in the way the car performs and reacts when I swap between my Street wheels/tires (215/45-17) to my Race wheels/Tires (205/50-15). the car is just so much more responsive with the 15's. it makes me cry when I have to swap back to the 17's.... though alot of this deals with the tire's compound differences too.
i went with 16s and sticky low pros on my car. Made a noticable difference in the low end. Felt a little quicker than my crappy and heavy *** stock wheels.
There was a test done a while back on a sentra, where just by replacing the wheels/tires, it shed a bit off the 1/4 mile. If you look around on the net you'll find wut i'm talking about.
There was a test done a while back on a sentra, where just by replacing the wheels/tires, it shed a bit off the 1/4 mile. If you look around on the net you'll find wut i'm talking about.
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D5: It's hardly crap. This stuff was formulated by some of the greatest minds in the history of mankind, including Newton himself. It actually depresses me a little to know that even though the physics described above is well over 200 years old, I still find it difficult to comprehend sometimes. Actually, the fact that these formulae still exist in their original form is truly remarkable.
Anyway, back to the purpose of the thread...people, post your experiences. Maybe someone can tell him a definitive difference between 18's and 15's on the dyno...
D5: It's hardly crap. This stuff was formulated by some of the greatest minds in the history of mankind, including Newton himself. It actually depresses me a little to know that even though the physics described above is well over 200 years old, I still find it difficult to comprehend sometimes. Actually, the fact that these formulae still exist in their original form is truly remarkable.
Anyway, back to the purpose of the thread...people, post your experiences. Maybe someone can tell him a definitive difference between 18's and 15's on the dyno...
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Rep Power: 0 it also depends on what type of dyno. Where I go (went before my car blew) they take the wheels off, and connect them to some rod things. They measure hp/tq. Its not one that you put the wheels onto, and it rolls. I dont know what they are called, lol.
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