freestyle

Lightweight Wheels - More Power to the ground and better 1/4 mile times - More Fun (My Calculations)
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I did some calculations on how the wheel weight affects the inertia/momentum on the wheels. Most you probably know about this but never used any calculations to get the effect in numbers. I will basically show you how you can improve your 1/4mile time by at least 0.5sec.

PLEASE READ BELOW

Since I previously owned 2000 VW Golf with 15" stock rims and 17" aftermarket rims, I can present all the calculations and the difference in 1/4mile ET. As we know, Inertia depends on the weight of the wheel and how far that weight is from the center of the wheel and it also depends on the weight of the tire. We know that the weight of wheel is distributed over the rim but the most of the weight is at the edge of wheel. Therefore, here is my assumption:

Assumption: Assume that all the weight is at the edge of wheel and at the edge of tire to make the formulas simpler and since we are dealing with relative values, it will not change our results.

General formula: I = mr^2 = mass * radius * radius
Inertia = Inerita of wheel + Inertia of the tire
Inertia = m1 * (r1^2) + m2 * (r2^2)

I will first explain it on my VW Golf example and then move to Civic 2001. With my VW, I actually lost horsepower to the ground because the aftermarket rims were much heavier. Just read the example below.

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Car: VW Golf 2000
stock wheel: steel 15x6
Mass of stock wheel: m1 = 20.2 lb
Radius of stock wheel: r1 = 7.5"
stock tire: Goodyear 195/65/15
Mass of stock tire: m2 = 20 lb
Radius of stock tire: r2 = 12.5"

Stock Inertia = 20.2 * (7.5^2) + 20 * (12.5^2)
Stock Inertia = 1136 + 3125
Stock Inertia = 4261 ********

New wheel: Enkei CDR9 17x7
Mass of new wheel: m1 = 21.6 lb
Radius of new wheel: r1 = 8.5"
new tire: Sumitomo HTR200 225/45/17
Mass of new tire: m2 = 24 lb
Radius of new tire: r2 = 12.5"

New Inertia = 21.6 * (8.5^2) + 24 * (12.5^2)
New Inertia = 1560 + 3750
New Inertia = 5310 *******

** Conclusion #1: New Inertia is 20% bigger than the Stock Inertia
** Conclusion #2: After testing the car at local drag track, the 1/4 mile time was 0.6sec worse with the bigger and heavier rims and put aside the numbers, the car felt much faster with the lighter stock rims.
** Conclusion #3: Try to get aftermarket wheel and tires that get your total inertia smaller than with stock wheels and tires.
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Lets analyze 2001 Honda Civic with stock 14" wheels and aftermarket Kosei K1 TS wheels which are much lighter. You will see that the straightline performance will be increased noticeably.

Car: 2001 Honda Civic LX
stock wheel: steel 14x5.5
Mass of stock wheel: m1 = 18.6 lb
Radius of stock wheel: r1 = 7"
stock tire: Firestone 185/70/14
Mass of stock tire: m2 = 21 lb
Radius of stock tire: r2 = 12.15"

Stock Inertia = 18.6 * (7.0^2) + 21 * (12.15^2)
Stock Inertia = 911 + 3100
Stock Inertia = 4011 ********

New wheel: Kosei K1 TS 14x6
Mass of new wheel: m1 = 9.3 lb
Radius of new wheel: r1 = 7"
new tire: Yokohama AVID H4 195/65/14
Mass of new tire: m2 = 18 lb
Radius of new tire: r2 = 12.0"

New Inertia = 9.3 * (7.0^2) + 18 * (12.0^2)
New Inertia = 455 + 2592
New Inertia = 3047 *******

** Conclusion #1: New Inertia is 24% LESS than the Stock Inertia

** Conclusion #2: 1/4 mile times should be at least 0.5sec better and the 60 foot times much improved. That's probably better than most bolt-on performance parts. Also, it is not good to have too small inertia because that will give you too much power to the ground and cause too much wheel spin, but that can all be controlled by good sticky tires.

** Conclusion #3: Since these new wheels are lighter (wheels: 9.3lb vs. 18.6lb) (tires: 18lb vs 21lb), then it means that one new wheel/tire is about 12 pounds lighter and since the car has 4 wheels, that equals about 50 pounds. We all know that 50 pounds car-weight reduction means a gain of 0.1sec. Therefore, you improve performance by decreasing the Inertia and also by decreasing the overall weight of the car.

** Conclusion #4: Many of you will say that 14" wheel on Civic don't look good and the tires are too fat. Well it all depends what you like. For example, you can even go with lightweight 15x7 Kosei K1 TS wheels and 205/55/15 tires and that will give you around 10% better inertia (about 0.3sec in 1/4mile). Or if you want to go with lightweight 17x7 Kosei K1 TS wheels and 205/45/17 tires, then you might keep the inertia the same as stock, but you might improve the handling with low profile tires.
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*** FINAL CONCLUSION: If you are going straight-line performance, then it would mean that you need to get smaller and lighter rims; however, if you are going road racing, then probably a compromise of light wheels and lower profile tires (not too low) is the best bet. Also, lighter wheels also improve handling besides straight-line performance.

Many of you should ask yourself a question if you want to go BLING BLING with heavy 19" rims, or you want go with smaller and lighter wheels, or maybe a compromise of both.

NOTE: Do not take this for granted. It is my opinion backed up with some physics calculations.

runner1738

iTrader: (2)

damn now your making me want to do my math before i decide on new wheels and tires, lol, but i guess it would be a good idea to see how much it will change

Civic_Racer9

iTrader: (9)

dude how long did it take u to type this down?

good one tho

runner1738

iTrader: (2)

why is r2 the wheel plus tire? and not just the difference?

nindoo

iTrader: (1)

You read my mind with this post. I also wanted 14s and those Kosei's are really light.

I had 14 in steel wheels on for the winter I took them off and put back my 15 inch alloy wheels. The 15 inch felt more slugish now there is the math to proove it.

freestyle

runner1738, to answer your question.

Inertia is measured from the center of rotation. The center of the tire is not a the edge of the wheel so you cannot take difference. You have to take the full radius of the tire and it is 12.15" for stock tire and 12" for the example of new tire.

runner1738

iTrader: (2)

thanks did you actually go out and measure it or did you just go off the tire sizes? cause there are 2 wheels im considering but now i guess ill do the math to see how bad it will hinder or help

teampointless

iTrader: (10)

Welcome to the world of rotational mass... old news to anyone with common sense, some brains and a calculator.

Redline04

iTrader: (3)

Yeah I was planning on getting a set of those Kosei's up front for my DR's and eventually mount some slicks on them. They should be good for .1-.2 in the 1/4 for me over the stock 14's. I plugged my 12.5lbs 17's (street wheels) into your formula and they are about 500??(units) over the stock 4011, so I'm pretty close but probally losing .1-.2 in the 1/4 and some acceleration on the street. Cool stuff.

freestyle

If you want to find the exact diameter or radius of the tires you have, just go on tirerack.com and choose the tire the same size and click on specs and it will tell you how much it weighs and its diameter.

freestyle

Anybody with ligthweight wheels (i.e.Kosei wheels) and first hand experience

runner1738

iTrader: (2)

does anyone have the specs from the stock 15s to the 16 slipstreams i know the weight but dont know the mass of the tire or radius, but considering hte stock 15 weighs 19 to 20 and the slipstream weighs is 13.5 , keeping even the same 19-20 tire weight i think the moment of internia would be less if not just the same, but i dont have numbers to prove it

Blainestang

Not a bad write-up. The biggest problem is the way in which you determine the ET difference you will get w/ smaller/lighter wheels. It it based solely on how much better YOUR car did at the track on, I'm assuming, 2 different days. Not only could weather make a HUGE difference in times, but driving or tires could account for a lot of that gain. I totally believe that smaller/lighter wheels can make a difference, but the "my car did .6 better w/ the smaller/lighter wheel" does not cut it for estimating someone else's gain. Now, I am certainly not discounting this write-up or anything, just sharing my concern w/ estimating other's gains. Also, effective gear ratio becomes more aggressive with a smaller overall diameter as well, which positively affects acceleration. So, the difference that you saw was a result of:

1. Lower rotational inertia
2. Lower overall weight
3. Better tires
4. Effectively more aggressive gear ratio

also it is likely that...

5. Driving
6. Weather

had an effect, whether positive or negative, on your new times.

Again, good write up, I just wouldn't necessarily say others WILL see those kinds of gains. Truth is, they could see more, but they could also see less, there are just way too many factors.

04civicvp

When I had my Jetta, I went from the stock 14" alloys to lightweight 15" Enkei Racing rims and noticed a drastic improvement. In the winter I would run 14" steelies and the car really felt sluggish compared to the Enkeis.

gsrchad

the size not only effects the weights but the gearing. there are a lot more factors in there than just tire weight change.

and 50lbs will not gain you .1 seconds. its based on hp, weight and what your trapping. i loosing 100lbs will make a lot smaller difference than say a stock 02 cav.

freestyle

Blainestang, to answer your question,
I did both tests on the same day and the tires on stock 15s were not that good either. So the conditions were the same which means that the only different factor is the rotoational mass.

The fixed number of 0.6sec is really for my old 2000 Golf, and having in mind that Golf and Civic are about in the same class in terms of HP, Torque and the weight, so my numbers are just an educated estimate. If you want me to give you a good educated guess for ET times for Mustangs or Corvette, I cannot because there HP to weight ratio is much different and the weight gets overcome by HP much faster in muscle cars than in our 4cyl cars.

runner1738, to answer your question. You can get the mass of tires on tirerack. Just choose the tire and click on the specs and you will see the diameter and weight and some other specs.

Thanks for your comments.

S2000man01

iTrader: (1)

the "only" different factor?

ok then what tire psi was on the first tire? and the 2nd? and the handling characteristics? i see they weren't even the same brand of tire, so i gaurantee one is going to grip differently than the other while launching, as well as react to being cold or warm depending on track conditions. did you chemically analyze the chemical composition of each tire to be certain they were made of the same materials, which would affect launch characteristics? did you measure the exact amount of tread depth left on each tire to determine if one was more worn than the other? 6/32 of an inch makes a much bigger difference than 2/32 of an inch. and i gaurantee you can't tell the difference between the two just by looking. did you launch at the same rpm for each tire? I could go on and on.

same day or not, i'm willing to bet you overlooked all of these other things.

you'd have to go back, do it again, using the same exact brand and model of tire, the same exact psi in each tire, as well as measure the surface operating temperature of each tire before running, as well as cover everything else I mentioned.

and EVEN THEN a 17inch tire with a 3.8 inch sidewall will NOT launch like a 15 inch tire with 5.0 inch sidewall. the way they flex during launch has little to do with rotational mass and more to do with sidewall depth and stiffness.

not trying to **** in your pool, but there's far too much you overlooked, not to mention what you cannot control or equal out.

gsrchad

^.... very well put

gsrchad

not to mention since you were also driving both cares im willing to bet you didnt launch them the same. for human error......
when you shifted, how long you feathered.... how hard you shifted...

freestyle

S2000man01,

You are right, the world is full of variables but sometimes we have to draw a line where to put a limit and I drew a line at a specific point and I stress again, my ET numbers were estimates. If you want to include some formulas for tire pressure and other stuff and give me results how it affects the behaviour of a car, go ahead; I would like to read it.

All I am saying, take two brand new Civics from the dealer, one for me and one for you. I will put the lightweight 14" rims on mine and race you 10 times. Majority of races should be won by me assuming that my driving skills are the same as yours.

Regards,

Freestyle.

scansel912

iTrader: (4)

The lesson from this post was not to take literally that you will knock of at least .5 seconds of your 1/4 mile time. It's just his experiment and his results that he knocked off .5 seconds off his personal time. The purpose of his initial post was to back that lighter, smaller rims does effect your acceleration and ET with actual physics calculations. I for one lost over .7 seconds by switching from heavy *** 17" Motegi's to 15" rota slips. Be that it was just better driving or launch skill, the wheels and tire size and weight definately made a difference. +1 for Freestyle for doing the calculations.

Blainestang

This is true, and no one would argue that. The car with the wheels w/ lower rotational inertia SHOULD win, but we are just saying that there are far too many factors to even estimate, based on your experience alone, HOW much the light-wheeled civic would win by. We are agreeing with you but saying you took it a smidge too far by estimating how much faster someone else's car would be with the lighter wheels. Again, good write-up, however.

S2000man01

iTrader: (1)

Yes, your initial post seemed like it was trying to say you could calculate what gains would be had by simply calculating rotational mass. You even said in later posts that you said the only difference was rotational mass.

So at no point did you really make it sound like you were just trying to say less rotational mass should make you faster. Which is something you think would be common knowledge, but who knows.

Your initial post and post thereafter are much different than your current tone of "All I am saying, take two brand new Civics from the dealer, one for me and one for you. I will put the lightweight 14" rims on mine and race you 10 times. Majority of races should be won by me assuming that my driving skills are the same as yours."

zachgeyer

iTrader: (3)

dam thats so work

freestyle

bump

doubledeuce2k2

iTrader: (20)

...on your head from being dropped as a child.
why is this back up for discussion?

S2000man01

iTrader: (1)

bump.

er no, wait.