mitsuozboi

iTrader: (18)

Ive heard this many times on here. Allways comes up when people ask about wheels and tires and was just mentioned in a thread in This forum about wheel size in which I posted this but since it does get mentioned a lot and I dont necessarily agree I think mabey some people could learn a few things. Mabey im way wrong but lets discuss.

For those of you that have no clue. This is saying that a smaller/lighter wheel will not have as much "drag" on the car and the car will be able to turn the wheel faster which in turn makes you accelerate faster. A larger/heavier wheel moves the weight farther from the hub which increases the rotational mass making it harder for the car to turn the wheel. Which in turn slows you down.

It makes sense in a way but I dont necessarily agree. If you have a lighter 15" wheel with a tire (lets use stock wheel/tire as example) and then you replace it with a 17" wheel with a tire that would equal the same size as stock. I don't see how it would make any difference.

For one. The bigger tire on the 15" carrys more air which does weigh. Plus the tire is heavier. The 17" wheel is a lil heavier because its bigger BUT, the tire is smaller (lighter) and carrys less air which would make it lighter. Wether its a lighter wheel and a heavier tire or a heaviler wheel/lighter tire. it would be the same. That is why every great sports car or GT car or road racing car uses usually 18's or 19's with a lower profile tire. Yes they have more power then us but the same rules would apply in this case.


PS: This is my thought on it. I may be way off the mark but to me it just doesnt make sense in this case as long as the wheel/tire you use equals the same size. Now if you went from a 17" to a 24" chrome spinner, now thats a whole nother story.

Blacksheepdj

iTrader: (88)

You're moving the majority of the weight away from the center. You can have a 15" wheel and a 17" wheel with the same weight, but the 15" will rotate faster. The closer to the hub the weight is, the easier it is for the car to turn it.

Moving to correct forum...

lancerman360

iTrader: (3)

^what he said.

mitsuozboi

iTrader: (18)

I shoulda put it in this forum. Lazyness on my part. (thanks for moving it)

But the tire on the 15" wheel is heavier then the tire on the 17" wheel. So the wheel/tire combo could weight the same. If I had a scale around id weight my two sets but I dont.

I would see that this would be the case if you had a 15" wheel with a low pro tire on it.

lancerman360

iTrader: (3)

true but the rim part of the wheel is further away from your axle. Try this--- hold a 10 pound weight up to your chest with one hand----- now hold it with your arm streched out in front of you. It takes more energy/power to do so.

mitsuozboi

iTrader: (18)

Ok. I was being a nub and not researching before posting up my crazy comments. I googled rotational mass for a bit and come up with some good info. You guys are right. here some info I found and a right up on the subject that could be helpful for people. We may allready have this info on here. I dunno..

According to what I read. Depending on how the wheel is set-up and how heavy it is as long as the overall diameter doesnt change it "could" remain the same or even be better.

Unsprung Weight - Part 2

By: Eric Albert

Introduction

In the first part of this series, we took a look at the effects of high unsprung weight on suspension and handeling. In this part, we will look at rotating mass. Be careful not to confuse unsprung mass with rotating mass. Reducing both is good, but they are not the same. Let's take a look.

Rotational Inertia (or Momentum)

Rotational inertia is a concept a bit more difficult to deal with than unsprung weight. Inertia can be thought of as why a car wants to keep rolling once moving, or remain in place once stopped (unless you forget to set the parking brake on that hill). I believe the terms momentum and inertia are interchangeable. The term “flywheel effect” also refers to these concepts. In a car, there are a number of rotating masses which require energy to accelerate. Up front, ignoring the internal engine components like the crankshaft, we have to worry about the flywheel, clutch assembly, gears, axles, brake rotors and wheel/tire. Out back its a little simpler (for FWD) with just the brakes and wheel/tire contributing most of the mass.

The more mass an object has, the more energy it takes to accelerate it. To accelerate a rolling object such as a wheel, you must both accelerate its mass plus overcome its rotational inertia. As for braking, you must overcome its rotational inertia plus decelerate its mass. By reducing the weight of the vehicle's rotational mass, lightweight wheels provide more responsive acceleration and braking.

Before continuing with our informal analysis here, I want to point out something very important about rotational inertia. We’ve all seen the ice skating move where the skater starts spinning. She pulls her arms in and speeds up, then extends them again and slows down. Why is this? Well, the further a mass is from the center of rotation, the faster it must travel for a given angular speed (how many degrees of an arc it traverses per time unit). The faster anything moves, the more energy it has, so when the arms are pulled in, conservation of energy says that the rotation rate must increase due to equal energy being applied to the same mass over a smaller diameter. Applying this to wheels and tires, which have most of their mass spread as far as possible from the rotation center, I think you’ll agree that it naturally takes more energy to accelerate them. Example: Take a two identical masses, but one is a solid disk of diameter D, the other is a ring of diameter 2D. The ring will require more force to accelerate it (in a rotational manner). Therefore a heavier rim with a smaller diameter could have less rotational mass than a lighter rim of a larger size, and accelerate faster with the same force applied.

The effect of rotating mass can be calculated using Moment of Inertia (MOI). MoI is related to not only the mass of the rotating object, but the distribution of that mass around the rotational center. The further from the center, the higher the MoI. The higher the MoI, the more torque required to accelerate the object. The higher the acceleration, the higher the torque required.

Because of this, the weight of rotating mass such as wheels and tires on a car have a bigger effect on acceleration than static weight such as on the chassis on a car. When purchasing new wheels and tires for a performance car, it can be useful to compare the effects of different wheel and tire combinations. This is especially true when considering upgrading to larger wheels or tires on a car.

The use of light-weight alloys in wheels reduces rotational mass. This means that less energy will be required to accelerate the wheel. Given that each pound of rotational mass lost provides an equivalent performance gain as a 10 pound reduction in vehicle weight, the benefits of light alloy wheels on vehicle performance cannot be overlooked.
For example:
***A reduction in the weight of the rim/tire assembly of 5lbs x 4 (all around the car) is equivalent to a 200lb weight reduction in vehicle weight (thats worth 0.200 in the 1/4 mile)***

So What's the Point?

The point of this discussion is as follows: There is a great deal of rotational mass to deal with in a car and tires and wheels may only make up half of it. Estimates for weight (o.k. for comparison since they’re all in the same gravity field, therefore the mass would be a similar ratio)
Front: Rear:
Wheel/tire: 30-35 lbs each 30-35 lbs each
Flywheel: 15-20 lbs
Clutch: 15 lbs
Halfshafts: 7-10 lbs each
Gears: 5-7 lbs
Rotors: 3-5 lbs 3-5 lbs
Misc: 3-5 lbs 3-5 lbs
------------------------------------------------------------------
Total: 115-148 lbs 76-90 lbs

So a couple pounds here and there on wheels and tires will make a difference, but that difference is magnified because that weight is placed further from the axis of rotation than any other mentioned (remember the ice skater). All these masses must be accelerated, so any reduction is a good thing. Now you know why we always say don't get those 18" rims for your civic. Not only are the heavier, they have a larger overall diameter. Even with lower profile tires, most plus sizing leaves us with a slightly larger wheel.

lancerman360

iTrader: (3)

good find.

So you going to 16's now?

mitsuozboi

iTrader: (18)

***A reduction in the weight of the rim/tire assembly of 5lbs x 4 (all around the car) is equivalent to a 200lb weight reduction in vehicle weight (thats worth 0.200 in the 1/4 mile)***

This statement here makes me wish I had some Rotas or Rays for sure... Not ditchin my wheels now. Mabey later on in the Lancers future.

lancerman360

iTrader: (3)

I was just jokin. My 18's are stayin.

mitsuozboi

iTrader: (18)

^^

haha. Im not. The Lancer needs all the help it can get! next year for autox'ing ill deffinitely get some R compounds for my stock OZ's tho enstead of racing my enkeis. I dont street race (although I do some spirited driving from time to time) so wheels for looks are fine for everyday. Autox is another story tho.

Gary C

iTrader: (2)

Actually, its more like 120lbs.

For every lb off a wheel/tyre is 6 lbs off the car...

So 10 lb less wheel/tire combo times 4 is 40 times 6= 240lbs sprung weight(like taking your fat buddy out of the passenger seat)

x838nwy

• There isn't a formular to precisely calculate the 'equivalent weight' based on the weight of the wheels alone.
• The whole thing about the rotatonal inertia is that it's not exclusively related to the total mass. For example, you can have two wheels that weigh the same while having different rotational inertia.
• For the same total mass and overall size to have different inertia, one could have most of its mass concentrated about the centre of rotation (lowe rotational inertia) and one has its mass concentrated about the circumferrence. The two will have very different inertias.
• Racing cars normally use large wheels due to the brake rotors. The larger the rotors, the better the cooling. The disadvantage is the requirement for larger wheels.

The way to workout the rotational inertia is to say divide the wheel into tiny little bits and add the mass of each bit multiplied by the distance from the hub axis (or any axis in which you are interested) squared. So it really depends on the shape also.

Anothing about wheel/tyre combo is that the larger the wheel (assuming we're keeping the rolling radius the same) the more square the tyre shoulders will be and therefore more rigid. Up to a certain point, this will be perceived as more responsive to steering inputs.

However, the squarer shoulders tend to be less progressive at breakaway, when the tyres are running out of grip in the corner. Additionally, and perhaps more pronounced, is that the smaller wheel taller tyres combo will have more tyre damping which improves ride quality.

There is a slight twist in this. If you have a wheel/tyre combo that's larger than you stated with there is possibility of a gain in performance.

Power put out be the engine goes into (overcoming drag) + (increasing tranlational kinetic enegy of the car) + (increasing the rotational energy of the moving parts). This is fairly general but serves the purpose here. Now the effect of the rotational inertia is multiplied by the rotational speed squared. With a larger wheel/tyre diameter you rotate more slowly for a given speed and therefore it has a similar effect as reducing the inertia of the wheel/tyre.

For example, if you go from a combination with say 25" overall diamete to a 26" then you'd rotate (25/26) times slower and the multiplier to to the rotational inertia would be (25/26)^2 = 0.925 so you've effectively reduced the effect of the rotational inertia by ~7.5%. So you are effectively 'allowed' and increase of inertia by a little bit and the effect, inertia-wise, would be the same because of the increased rolling radius.

airlinevomitbag

iTrader: (1)

Yeah but they are made of unobtanium and weigh less than my d!ck and I'm not John Holmes.

rhyzin

iTrader: (1)

muahahaha... that movie always puts a smile on my face...

Elvis

iTrader: (2)

what ever the science is i can testify personally about lighter wheels, though i dont have any hard numbers to prove it the difference is there.
my voxx 17's with nankang ns-1's = 45.5lbs each 45.5 x 4 = 182lbs total rotational mass
my enkie 15's rpf1's w/falken 512's = 28lbs each 28 x 4 = 112lbs total rotational mass
thats a 17.5 lb difference 17.5 x 4 = 70lbs total reduction in rotational mass.

all i know is when i switched to the lighter wheels the difference in performance was phenomanal. greater than any bolt on i have so far. and better since not only acceleration was greatly increased but the braking is 10x better as well.
now granted the fact that the wheels were more than a grand, more than any single bolt on mod, but the 17's were almost a grand to. if only i knew then what i know now i would have paid more attention to weight.
as some one already said. the lancer needs all the help it can get. things like lighter wheels make a huge difference on a little 100 hp car. probally not such a big difference on a 400 hp drift car though. but i rather be faster with my 15's than look like a chump with a loud slow car rockin some wagon wheel lookin 18s. its all about performance for me man.
any body wanna buy some 17's with tires. only had em on one summer. plenty of tread left.