For me it didn't. Reduced rear wheel weight by 40lbs when I was at tje tracl and saw no difference in ET. Many runs were done before and after. Just thought you all should know.

 

Weight may not make much of a difference, but going to a thinner width wheel/tire combo does...

 

I was going to swap out my 53lbs 235/45 17 wheel tire combo with 205/65 15 32lbs combo, but the millenia wheels won't fit on the rear. I used Neal's 17" 33lb ssr comps with 235/40's.


Isn't Jime using millenia rims on the rear?? They would not fit for me.

 

Ya I use them on the rear, I think Neal uses them on the front too but I couldn't get the slicks to seal on them.

They do fit on the front too though because I sold them to Jer with my D/R's on them and he ran them a couple of weeks ago when I was at the track so don't know why they wouldn't fit on yours.

 

I use them on the front all the time, but this time I tried to use Neals on the back (the ones with street tires). The wheel would not fit over the metal cap coming out of the hub. The rear does not have the pin going through it like the front axel does. Got me???

Anyhow, the point was that rear wheel rotational mass will not effect acceleration. I have done this same test with the front millenia wheels and gained .25 1/4 mile by shedding 40lbs of wheels and tire weight.

 

You've got hard evidence backing up that rear wheel weight and I can't debate that. I'm gonna run it by my auto-x and roadrace buddies though to see what they think.

http://www.sportcompactcarweb.com/ed..._technobabble/

 

I have heard conflicting things, but it's hard to not believe it when taking 40lbs of the rear wheels did nothing.

 

in the rear weel case the engine doesn't have to rotate them so they don't have any effect on the power of the engine , but u can consider partialy dead weight 100lbs =.1 sec but the wheels are rolling so they are not completely dead weight maybe halfway dead weight so 100lbs =.05 sec so if u shave 40lbs off the weight that's about .02 sec , peace

 

Yes you are correct, I had to do a little corrective surgery with the dremel, I did it a long time ago so almost forgot.

I sure can't disagree with your numbers but the best time I got with my 97 was 15.4. I put the same engine and trans from the 97 into the 95 and did some more lightening, including the sound deadening, crash support etc, plus the wheels, not real extensive but enough. No other performance mods though and the time N/A on the 95 was 14.6 so it had to be weight, how much was the wheels, who knows but weight was the only difference between the two cars because I used the same mods off of the 97.

Sounds a little disheartening when you hear the experts claim that a 1 lb of unsprung weight is equivalent to 8 lbs of sprung weight. By that calculation you should have lost the equivalent of 40 x 8 or 320 lbs.

 

Here are my times. Sorry I could not scan the slips, but Nealoc187 and bejay1 can vouch for me.

full interior and 53lb 17's on the back

60' 2.337
330' 6.251
1/8 9.484
mph 76.61
1000 12.324
1/4 14.706
mph 95.15

Removed passanger seat and 33lb 17's on the back

60' 2.233
330 6.193
1/8 9.430
mph 76.11
1000 12.273
1/4 14.659
mph 94.99



And while I have your attention. I am a bit baffled how my 1/8 mph was at 76mph all day long, which is up from a normal 73 or 74 mph but I did not see any faster times??? We did have a strong headwind, but.....

 

I would assume you have a 2002+ I30. What mods do you have? Good times by the way!

 

96 I30 5-speed. Performance mods are in my sig.

 

actually the engine does have to rotate the rear tires. it's actually like a pulley system. to rotate the rear tires, the car has to move forward. to move forward the front tires need to rotate, so actually the rear tires are linked to the front tires through the motion of the car. imagine putting a big a rope across both tires like tank tracks. the front tires drive the rear tires as well as the front tires. so not only do you lose weight by going to a smaller rear wheel, you lose rotational mass too, which is why reducing wheel weight is very benneficial.

As far as your runs, there are so many factors that could have come into play. most notably traction. looking at your time slips, it seemed like you did hook up a little better on your second run, that's why your et was quicker and top speed was lower. also, it's hard to tell from only 2 runs.

Personally, I have noticed a difference of about .1 seconds by putting my steelies (~36lbs) instead of my 255/40/17's (~66lbs) on the rear. granted this was on different nights, so again, other factors come into play like temperature, wind, etc, but on average, I can consistently get 14.7 with the steelies, but only 14.8 with the rims. one other thing i noticed is that with my car at least, it hooks up a bit faster w/steelies so my usualy R.T.'s of .5-.7 often turn into redlights. gotta adjust for the (seemingly) quicker pick up.

 

Those were just two runs for an example. I had 3 or 4 runs with the heavy wheels and tires and 4 or 5 without them. Same track, same day so I don't see how you could do a better comparison than that.

There difference you got with your steelies is probably do to the reduced friction. 255 is a wide tire, and I doubt your stockers are more than a 215. Different nights is not a good comparison.

You can't get any better comparison that what I did.

 

He made many more than 2 runs. I don't believe that "better 60' means a slower trap speed" stuff either. I think that theory is a crock personally.

 

Summary of my experiment:

reduced front wheel weight by 40lbs = .25 lower 1/4 mile

reduced rear wheel weight by 40lbs = negateable difference

 

amen brotha that's what i ve been trying to say

 

hmmmmm.... So that would be at least two people now that have done this experiment with no significant difference. But at the same time, saying that the rear wheel weight doesn't matter is impossible because that defies the laws of physics. Big/heavy rims and tires still require more energy to spin up to a certain speed than smaller/lighter rims.

I'll have to think about this some more. There is an explanation for everything.

 

I think I know what's going on. Just need some time to test out the theory.

 

I know you might not want to do this but pull out the back seat to reduce weight. Don't look at it as "cheating". You just have to find your cars combination that's going to work.

 

Exactly.

Very interesting evidence though. Kinda makes me feel better about not getting lighter wheels yet.

D

 

bleh...gonna be all tied up this week for awhile so no time to test theory in cartest.

Here's what I think is going on....

The other half of the Inertia equation that is conveniently omitted is the part about acceleration. It's actually I=mr^2 x A where A is the rate of acceleration. What this says is that the moment of inertia on the wheels is dependent on the rate of acceleration. So the moment of inertia is zero at steady state cruising speeds, but the harder you accelerate, the more they'll resist and hold you back. Same with braking. With big blingin wheels those suckers won't want to stop and will load up the brakes a lot more.

So in the scope of a 1/4 mile race, is the rate of acceleration of the wheels they same. They are not! In a FWD car when you launch you get the front wheels spinning (higher rate of acceleration) whereas the back ones are just along for the ride and not really moving yet. So because the moment of inertia on the front is higher due to the higher acceleration, this would mean that extra weight on the front would hurt you more and heavier rims at the back would hurt you less. Once you have full traction the rate of acceleration between all 4 wheels is equal and all of them are hurting you equally.

It's more or less established that every pound you add per corner will have the effect of about 8 pounds due to the moment of inertia. But that assumes equalized acceleration rates at all four wheels.

So on a 1/4 mile launch...

- Acceleration on Front is > Acceleration on Rear.
- Therefore, moment of inertia is much higher on Front than rear.

Front: 12:1 ratio (maybe) for inertia
Rear: 4:1 ratio (maybe) for inertia

Just pulling numbers out of my **** without having tested it, but this is what is happening and that's how you could model it and think of it. And this would also explain why Mike and Dave got much better results by dropping the weight on the front but did not see any significant differences by dropping the weight on the rears also. Weight on the rear still matters, but just not as much in the scope of a 1/4 mile drag race. A 4:1 ratio on the back is probably a small enough difference that you could almost ignore it. But the 12:1 ratio up front is very big so that's why there are such big difference just by switching the front tires. From a roll acceleration rates are always equal though so it would go back to the 8/8 idea instead of 12/4.

 

Nice theory or is it fact?

Just so happens my auto-x buddies were discussing wheel/tire weights related to solo courses. Much of it applies to the dragstrip.
Here's links for those who can use...

SCC magazine regarding flywheel interia

Dan Cyr's calcuations (SCCA '03 cart champ)

Kevin Steven's response to my direct acceleration question - like the previous post stated

Andy Welter's inertia spreadsheet taking Steve's formula the next step

 

bleh, you have to be a member of the discussion groups (which i'm not) to get to those links. Could you cut and past?

The spreadsheet link works though and is pretty sweet! I was thinking that there HAD to be somebody out there that actually went through all of the calculations to figure this out. I'll have to dig through it a bit later.

 

I already do

 

Ok, here are the cut & pastes from

Sport Compact Car Technobabble: September 2001
The "Technobabble" Study Guide
By Dave Coleman

It has come to my attention recently that between my rambling pseudo-mathematical dissertations, my clumsy stabs at social commentary, and my occasional attempts to squat on unclaimed geometrical phenomena of engineering significance, I have, periodically, put useful information in this space. This fact revealed itself to me through a rash of recent e-mails that, though each unique, seemed all to follow this basic pattern:

"Sport Compact Car is really great, and you aren't so bad either...for a nerd. Anyway, my dog ate my February 1998 issue, so I lost the 'Technobabble' where you explained how to make an oscillation overthruster out of some dryer vent and a used toaster. Your back issues department hangs up on me every time I say 'overthruster,' so I was hoping you could just explain it to me in an e-mail."

This seemed like a perfect excuse to simply re-examine some of the more useful topics on which I have pontificated in the past, thus avoiding the need to actually come up with a new idea this month. Brilliant, no?

Finding the Ideal Shift Point
Here's a hint: In almost any high-performance car, shifting at the redline will offer the best acceleration. Of course, it's that "almost," that rare exception, which keeps you up at night thinking about your quarter mile times, isn't it? The goal when deciding where to shift for maximum acceleration is simply to have the most force accelerating you forward at any given moment. Accelerative force comes directly from engine torque and gearing, so to figure this out, you need to know the torque curve of your engine and you need to know your gear ratios.

OK, armed with this information, your goal is to make sure that for every moment during your quarter-mile blast to glory, the sum of your engine's torque output at that moment and the gear you are in is higher than it would be in any other gear. Let's say you are at 6500 rpm in second gear in a stock Focus, for example. Our Project Focus made about 80 lb-ft of torque at 6500 rpm, and second gear is 2.136:1, so the torque at the output shaft is about 171 lb-ft. To check torque in the next gear you have to see what rpm a third gear shift would get you. For that, you use this little formula:

Plugging in the third gear ratio of 1.448:1, you get about 4400 rpm. Back to the torque curve (our most recent stock Focus torque curve was printed in "Project Focus, Part I" in the May '01 issue) we see about 120 lb-ft of torque, which ends up being 173 lb-ft after it goes through a 1.448:1 gear ratio. Since the torque is about the same, the 2-3 shift should be at about 6500 rpm. If the torque in third had been significantly higher, you would want to try this whole exercise again with a lower shift point. If it were lower, you could hang onto some more revs in second.

Running through this whole exercise over and over again until you find the right point for every gear change can be a little tedious. If you prefer to play with spreadsheets, and you can get the torque data every 100 rpm or so, you can simply plot torque at the outptut shaft vs. speed for all of the gears. Anywhere the line for one gear crosses the line for the next, that's your shift point. If the lines never cross, take it to the redline.

Rotational Inertia
The rotational inertia topic was so big, it took me two months (June and August '99) to get it sorted out. Here's the deal: There is this rule of thumb among racers that adding weight to something that rotates is far more detrimental to performance than if you add it to the body of the car. This is absolutely true, and by bumbling through some physics, and after slipping and falling on a radian, I managed to get a few formulas figured out that could tell you just how much worse.

Any moving object has kinetic energy, as does an object sitting in place and rotating. An object that is both rotating and moving (like a rolling wheel, for example), has kinetic energy from both, meaning that accelerating or decelerating that rolling object will take more power than one that is just sliding along. How much more power is the question.

The answer, it turns out, depends on how the weight is distributed on the wheel. An extra pound on the tread of a rolling tire has as much kinetic energy as 2 lbs on the floor of the car. As you move toward the center of the wheel, the rotational effect drops until, at the center, a pound is just a pound. The formula I derived to determine the exact relationship between weight on a wheel and weight in the car isn't worth repeating here for one simple reason. It requires that you know the moment of inertia of the wheel, and measuring that is virtually impossible. What you need to know is that changing to tires that are 1 lb heavier will effectively add 8 lbs to the car (four tires, remember) and that adding a pound to the wheels will effectively add somewhere around 6 lbs to the car.

That only considers acceleration and braking; handling is dramatically affected by unsprung weight as well, but no simple formula is going to tell you how big the effect is.

The one useful formula that did come out of my diatribe on inertia was the effect of a light flywheel. Because the flywheel's rotational kinetic energy is directly linked to the car's motion (through the gearbox), you can also calculate the effect of a lighter flywheel. Here's the formula: Next month, we will tell you all about the aluminum FocusSport flywheel we just put in our Project Focus. Until then, know this: The flywheel is 12.5 lbs lighter than stock and has a radius of 5.5-inches; the Focus' first gear is 3.667:1, the final drive is 3.82:1, and the tire radius is 12.1 inches. What's it all mean? In first gear, the car feels more than 250 lbs lighter!

The HydraCharger (thought I'd post this too since an I30 started the thread)
Way back in the April '99 issue, I introduced a prototype supercharger that Garrett was working on called the HydraCharger. By mating a turbocharger's compressor to a tiny little turbine driven by high-pressure hydraulic fluid, this little device promised the efficiency of a turbocharger with the response of a supercharger and tunability and packaging benefits all its own. Over the next few months I was besieged with e-mails and phone calls wanting to know more. Unfortunately, I had told all I knew.

Well, now I know more. Apparently, Dave Kapich, a consultant for Garrett, designed the HydraCharger (which will probably go by a different name in the future), at the time.

Garrett ultimately dropped the program, and the rights to the design reverted to him. Kapich Engineering is finishing the development work as we speak, concentrating mainly on boring stuff like long-haul truck engines, but there is one glimmer of performance hope. A two-stage hydraulic pump and a prototype of Kapich's compressor are sitting under the hood of an Infiniti I30 at Jim Wolf Technology. The car is supposed to be drivable by the time this magazine hits the newsstands.

Dan Cyr's calcuations (SCCA '03 cart champ)
The non-detailed guesstimate answer :
The wheel rotating (accelerating on the ground without spinning etc) has an "effective" inertia somewhere between 1.0 and 2.0 times its actual weight. This is true for any rotating mass. If all the mass is at the "center" of the wheel (unrealistic lower limit) the factor is 1.0, if all the mass is at the "edge" (unrealist higher limit) the factor is 2.0, however looking at the construction of most wheels, it appears most of the metal is near the edge.

However, this mistakenly neglects the effect of the tire! The "radius" to be used is NOT the radius of the wheel, it is the outer radius of the tire. The tire having a larger diameter reduces the effect of the lighter wheel in two ways. First off - the larger radius means the wheel is turning less to cover the same distance, and the mass is no longer located nearer to the edge. Secondly, the mass is located inboard of the tire - not near the edge of the tire. How much this lessens the factor requires some math, or guessing.

------------------------
Geek answer (and hope the equations dont get garbled):

Dave's right, kinetic energy is the way to figure this out, the equation is

0.5 * (M * V^2 + I * W^2) : I=moment of inertia W=angular velocity, V = speed of car
I is unknown, but is modeled as X * M * (Rw)^2 , with X varying from 0->1, Rw = radius of wheel. With some simple math W = V / Rt , Rt = radius of tire....

0.5 * (M * V^2 + X * M * (Rw)^2 * V^2/ (Rt)^2 ) is the equation for energy, simplifying,

0.5 * M * V^2 * (1 + X (Rw/Rt)^2)

A weight bolted to the car has kinetic enegy 0.5 * M * V^2, so you can see this (1 + X * (Rw/Rt)^2) is the above factor I mention in the and waving answer. Call this F = (1 + X * (Rw/Rt)^2)

An exmaple : 205/50/15 , Rw = 7.5 Rt ~=11.5(23inch tire guess), and the only unknown is X. Guessing X is dangerous - if ALL the mass of the wheel were at 90% of radius X=0.8 (seems conservative). For the 205/50/15 (X=0.8) F=1.34 every pound saved on the wheel is like taking 1.34 lbs out of the chassis. This is substantially different than the 2 that Dave calculated. For handling, the benefits in unsprung weight are not so easy to calculate, as energy isnt the correct analysis tool... how much faster a car is with X lbs removed is also hard to calculate (which is what John wanted to know). A full simulation is the only way to be accurate.

Saving weight on tires is far more effective, you dont pay the (Rw/Rt)^ 2 factor, and the tire is heavier to start with (at least for my car - not my kart!)

Kevin Steven's response to my direct acceleration question
> Question? - is front rotational mass saved more important than rear savings or should it be equal? Does shaving 40lbs off the front = the same savings in the rear?

It's all the same unless your wheels are off the ground (or spinning).
If you're hooked up, everything that turns when the engine crank does
represents rotational inertia.

> Last weekend a friend of mine lightened his rear tire/wheel combo from 53lbs to 33lbs and saw no reduction in ET (similar launches and other factors). Doing the same on the front's netted a .25sec improvement with repeatable results. (FWD car if it matters)

It's probably hidden in the wheelspin, if any.
KeS

 

I follow the logic......but what I don't understand is that I am only spinning my tires for a split second, so the gains shoud thier with the lighter rear wheels.

 

Sorry for digging up an old post, I was looking for something else and got intrested.

My head nearly exploded from all those numbers but bottom line is at the start ofthe 1/4 mile your wheels are at 0 RPM. At the end of the 1/4 mile they are spinning at (aproximatly) 1000 RPM and therefore contain energy. A heavier wheel spinning at the same RPM will contain more energy. The energy would have to come from the engine.

But why you did not notice any impovements with lighter tires

 

does anyone know of a link of how much tires weigh?

 

So for the best 1/4 mile time you should get really light 15's....
How much of an effect does the width of the tire have on the time?
what size tire do you run and how wide with your 15's?

and light 17x7.5's on the front 235/45/17 wouldn't give you similar numbers?

 

Let me ax you dis;

If (on two hypothetical sets of rear tires) the overall weight and width of the rear wheel/tire combo is equal, what impact does the radius (axle center to the ground) have on the equation? What effect does changing the radius (again, weight/width equal) have on the mechanical advantage and by extension, energy required to rotate them?

If you want to prove the theory (lighter combo=lower et) wouldn't you need to install a lighter wheel/tire combo that was the same width and diameter as the heavy ones? If you install a lighter combination of a smaller diameter, wouldn't that reduce the mechanical advantage offsetting some of the potential gain from the weight reduction?

 

http://www.miata.net/faq/tire_weights.html

 

If you read this whole thread you would know that I replaced my rear 17" wheels with 235/45 tires that weight over 50lbs with Neal's 17" 235/45 tires that weigh a bit over 30lbs and saw no improvement at the track. Plenty of before and after runs to know for sure.

 

Sorry, I read the whole thread, maybe I need to get the study guide too.

"The answer, it turns out, depends on how the weight is distributed on the wheel. An extra pound on the tread of a rolling tire has as much kinetic energy as 2 lbs on the floor of the car. As you move toward the center of the wheel, the rotational effect drops until, at the center, a pound is just a pound." - article quoted above

Was the lighter comb you tried light wheel/heavy tire?

 

The tire and the wheel I switched out were both significantly lighter.

 

The original wheel and tire combo weighs like 51lbs, the combo he replaced it with weighs 33lbs. 18lbs difference, on each side. Toyo T1-S that I have is the 2nd lightest radial tire on the market. I don't F around with heavy tires.

 

I've been reading this thread a little bit. There is one thing that 99% of drag racers overlook and that's weather conditions.

In my years of bracket racing, I have to follow weather conditions and figure up the air density for that particular day. If you compared days with the same air temperatures, your 1/4 mile time could still be as far off as 4 tenths. That is just due to barometric pressure and humidity being different.

I have found a great website that keeps a good record of weather history. Please give me the two dates that you are comparing your 1/4 mile time and I will look up the information. Please also give me the city and state that you raced in those days. Thanks.

*edit* I just reread the thread and it appears that you made these runs on the same day. But since you only got 2 runs in, I highly doubt the consistancy. I'm not saying your car is not consistant, but with only 2 runs, it's hard to determine anything. And after reading that SCC article, the math they used is not "for every 1 lbs unsprung, it's 8 lbs curb weight". It's 1lb unsprung is 2lbs curb weight. That's only 72 lbs of curb weight. Which is almost a tenth of a second in the 1/4 mile. It's VERY easy for a 5spd Maxima to run a tenth off at any given moment.

So with just the theory on rear wheel tire diameter and weight, I will continue to use a light rear tire setup just to make me feel happy with the couple hundredths of a second it might be shaving off.

 

Mike made like 10 runs that day. He only typed out the times of two runs. This has been stated a number of times.

 

Dude, I'm sorry. I re-reread the thread. Now I feel stupid.