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Old 12-10-2009, 08:31 PM   #1
juanmedina
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Default Wheels & tires weight/size and the effects of performance in 1/4th of mile

I am little bored and I always wonder how it affects the car..

Car Weight 2815 lbs + 150 lb driver = 2965lbs
1/4th 12.14sec@115.54mph
1/8th 7.845sec@91.15mph
60' 1.845

I run those time with some heavy wheels 18x9.5
Wheel weight: 21.2 lbs
Tire weight: 25 lbs
Tire size: 245/40/18

I am thinking on switching to 17x7 wheels
Wheel weight: 14.4 lbs
Tire weight: 18 lbs
Tire size: 215/45/17

Total savings of weight about 58-60 lbs.

I want to know how much quicker and faster my car will be by switching the wheels on the 1/4th? Assuming the same conditions...

I have an idea but I want to see what you guys come out with

Thanks
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Last edited by juanmedina; 12-11-2009 at 12:25 PM.
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Old 12-10-2009, 08:51 PM   #2
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I know I'll get crap for this, but I'd guess ~1.5 to 2.5mph additional in trapspeed.
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Old 12-10-2009, 09:11 PM   #3
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I'd agree with those #s but it really depends. I can say that I did some rotating mass reduction in my car (lighter wheels, tires, driveshaft, front rotors) and the car felt noticably faster.
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Old 12-10-2009, 09:14 PM   #4
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I noticed a difference going from 06 wrx stockers to 04 STi bbs.
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Old 12-10-2009, 10:10 PM   #5
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aluminum rear driveshaft feels completely different when you loose 15lbs off rotating mass
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Old 12-10-2009, 10:20 PM   #6
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12.0@117
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Old 12-10-2009, 10:22 PM   #7
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Juan, just go to the track and drive your car. you have enough for 11.60's as is. Just go.
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Old 12-10-2009, 10:22 PM   #8
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Originally Posted by Junior2JZ View Post
12.0@117
12.0
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Old 12-10-2009, 10:24 PM   #9
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Originally Posted by STi Mikey View Post
12.0


I have a friend that always gets stuck @ the .00s . He went 10.0s @ 140/6mph for over 1yr.. Then went 9.0s for another year before going 8s..

hahahaha

Junior
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Old 12-11-2009, 12:33 AM   #10
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Quote:
Originally Posted by mgreen View Post
I know I'll get crap for this, but I'd guess ~1.5 to 2.5mph additional in trapspeed.
I agree with this but that is only for the weight. I think by adding the saving on size you can add another .1 and mile per hour

Quote:
Originally Posted by STi Mikey View Post
Juan, just go to the track and drive your car. you have enough for 11.60's as is. Just go.


Never mind the math is too complicated

I posted in another site and some guy came up with this:
Quote:
You have several factors going on by changing tires and wheels, but saving weight is always a good one.

1rst factor: Weight is directly related (inversely) to acceleration
Force = mass x acceleration
Your total weight = 2915
If you cut weight by 60 lbs that is (2915 - 60) / 2915 * 100 = 97.9% of your original weight

100 - 97.7 = 2.1% decrease in weight which directly relates to a increase of 2.1% in acceleration
This part alone should increase either end velocity by 2% (= 117.8 mph in 1/4mile) or decrease time by 2% (= 11.90s in 1/4 mile). One negative factor to increased speed is increased air resistance, so not all of that 2% can be applied.

factor 2:
Kinetic energy of flywheel = 1/2 mass[mass at a given radius] * Velocity squared

Your tire is in effect a spinning flywheel. It takes energy to spin a flywheel up to speed. By cutting down on tire weight (especially perimeter weight) you decrease the energy needed to spin the tire to the same RPM as a higher weight tire. This energy is quite significant, maybe even more than the loss of mass that increases acceleration directly, from factor 1. If you want tires for racing only, use the lightest tires possible and smallest radius.

factor 3:

You didn't give the actual outside perimeter of your tire dimensions. A smaller tire in diameter also amounts to less travel per revolution of tire. A tire 2 feet in diameter at crown amounts to perimeter of 6.28 feet of travel per revolution. A tire that is 1 foot 10 inches in diameter = 5.76 feet of travel per revolution.
So the smaller the tire diameter the more easily it is accelerated (giving in essence more starting torque or a lower gear ratio) bu the actual travel is smaller than the larger radius. The only way to make up for this is more rpm's.
For example , using tire above, a 2 foot tire turning at 2000 rpm = 12,560 feet per minute or 209 feet per second of travel.
A 1 foot 10 inch tire at 2000 rpm = 192 feet per second

If you use the smaller tire, you have to increase the rpm by this much:
2,000 rpm/ 192 ft/s = X rpm/ 209
x = 2,177 rpm
So to go the same distance in the same amount of time you have to run your motor 177 rpm faster. The good thing though, is it is easier for your engine to go faster, because a smaller radius tire also is a lower gear ratio effect to your engine so the load is much lower and torque and horsepower develop faster.

To sum it up...with the lower flywheel effect and less weight you could easily shave up to a 0.5seconds from your 1/4 mile time and add maybe 3 mph to your top end.
I think is too much but a .2-.3 sound about right including both weight and size advantages

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Old 12-11-2009, 05:55 AM   #11
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Damn your car is light, 2900lb with you in, I'm jealous

yes rotational weight is huge difference,

Im saving up for this wheel and tire combo that should be 32lb - 35lb rim and tire

basicly 12-15lb lighter each wheel from my current, I can't wait

Last edited by downsti; 12-11-2009 at 06:08 AM.
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Old 12-11-2009, 07:23 AM   #12
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Quote:
Originally Posted by juanmedina View Post

Never mind the math is too complicated

I posted in another site and some guy came up with this:
You think this math is complicated, but youre going back to school for an engineering degree?























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Old 12-11-2009, 07:57 AM   #13
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LOL, Im sure he just didnt want to waste his time.

But yeah, simple math. But sipmle math and physics is hard to apply to real life tracking applications.

The conditions can rob you of power, and before and after results may not even vary all that much.

My stock turbo ran 12.02 @ 113 with the wing and spare out in 60 degree weather. The car at that point was making 390whp (dynodynamics 1.2 cf)

The next 1st half of the season I couldnt break 12.2, gutted.

Then I went back with 30 less whp and ran 12.0 again.

All in 60-65 degree temps.

Can someone do the math there??




Sure rotational mass plays a huge role, but so do 392 other factors that can negate the weight savings.

And yes, if every variable was relative, the light rims and tires would have put that 12.1 in the 11.9 range. But a 1.5-1.6 60ft and faster shifting would have put the 12.1 at 11.6-11.7.

Best way is to dodge the physics and do your own real world comparo at the track.



BTW, your car with you in it weighs 85lbs less than my gutted car, with my 240lb fat ass out of it. I hate you

Last edited by STi Mikey; 12-11-2009 at 08:03 AM.
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Old 12-11-2009, 08:36 AM   #14
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Yeah, Subies kinda suck for consistency. . .

When you dyno, you may have a fan blowing on the IC.

When you road dyno, you're moving up until the point you're ready to make the pass down the back road.

At the track you sit in staging lanes; one time you're sitting there and let the IC heat up quite a bit. Next time you go to the track, you remember this and bring a bag of ice. All of a sudden the car that made 390hp on the airboy actually makes a pass that resembles a 390hp car.

BUT, the next pass you decide to slam the gears hard, and speed shift, next thing you know, even with a cold IC and 60* weather and 1000' DA, on a sticky track, you lost 5 mph trapspeed from the prior pass. Yes, the dreaded shift knock.

In my '98 LS1 Z28 6spd, I could just look at the weather info on Wunderground.com, go to http://wahiduddin.net/calc/calc_da.htm plug in the numbers, and I could predict what I was going to run within about .5mph trap speed (not as easy to predict the E.T.). There was no concern w/ icing down an intake, there was no shift knock (all retard was set to '0' for shifts. GM has it programmed in to save the tranny/rear end). I have 400+ passes at the track about 5 miles from my house, 99% of them in the LS1.

BTW, I bought 17x9 5spoke SS wheels w/ 275 BFGs for $1200 from SLP back then. I went to the track and ran 1.5mph slower consistently. I sold them the following week (for $1000) after I threw the 16x8 wheels with 245 RSAs back on the car and got my trap speed back.

Mike
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Old 12-11-2009, 09:15 AM   #15
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Usually you can feel the weight difference in rotating components. Its not just the physical weight though. Its about the diameter and the location of the mass. Rotational inertia is the key. The more inertia they have the more power you waste getting them to move.

The driveshaft inertia is I = (pi * mass * length *(outer radius^2 - inner radius^4)) /2

Due to packaging constraints you can't make it much larger. Due to torque output of the engine you can't make it smaller. The radius terms are going to be pretty constant, so any mass you save is going to be a pretty direct change.


A stock flywheel inertia is I = (pi * mass * thickness * outer radius^4) /2

When you change to a lightweight flywheel you can use the same equation as the driveshaft, but you need to break it into two or three parts and add the results to account for the fact that your new flywheel has windows cut in it.

A wheel can use the same equation as the driveshaft once again, but as with the lw flywheel you'd need to break it into separate sections and add the result. One section for the outer rim. One section for the spokes. One section for the hub area. Since the spokes are not solid you'd simply estimate what percentage of the total area is occupied by the spokes and multiply that through the section inertia.


So getting to your wheel and tire question and the point of the thread...

Reducing mass isn't necessarily the top concern. The top concern is reducing mass in high-inertia areas. You can have a 20lb wheel that has most of its mass at the hub and still have less inertia than a 15lb wheel with most of its mass in the spokes. You're better off going to a smaller diameter wheel so you get the sure benefit of a reduced inertia effect of the rim section.

Tires are a different concern. You tend to want more section width to get more traction. But a wider tire = more mass = more inertia. Your best option for a given tire width is to go with a little smaller sidewall. Instead of a 225/45r17 you can use a 225/40r17 and save yourself 1/2" on the inertial radius. That will take off a bit of weight and a good deal of inertia. You also get a gearing advantage from the smaller diameter. The engine needs to use less torque to get the tire/wheel moving. That means your reaction at the light and acceleration are going to improve. The downfall is top speed. You're going to lose speed per gear. If you were already close to topping out a gear you'll have to make an extra shift and that might not be worth the time gained by inertia and gearing. And you're going to throw your speedo off for daily driving...

This is a great tool for comparing tire sizes and check how much they'll throw off your speed/speedo...
http://www.miata.net/garage/tirecalc.html



Hi...I'm an enginerd

Last edited by Scooby921; 12-11-2009 at 09:24 AM.
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Old 12-11-2009, 10:41 AM   #16
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Quote:
Originally Posted by Scooby921 View Post
Usually you can feel the weight difference in rotating components. Its not just the physical weight though. Its about the diameter and the location of the mass. Rotational inertia is the key. The more inertia they have the more power you waste getting them to move.

The driveshaft inertia is I = (pi * mass * length *(outer radius^2 - inner radius^4)) /2

Due to packaging constraints you can't make it much larger. Due to torque output of the engine you can't make it smaller. The radius terms are going to be pretty constant, so any mass you save is going to be a pretty direct change.


A stock flywheel inertia is I = (pi * mass * thickness * outer radius^4) /2

When you change to a lightweight flywheel you can use the same equation as the driveshaft, but you need to break it into two or three parts and add the results to account for the fact that your new flywheel has windows cut in it.

A wheel can use the same equation as the driveshaft once again, but as with the lw flywheel you'd need to break it into separate sections and add the result. One section for the outer rim. One section for the spokes. One section for the hub area. Since the spokes are not solid you'd simply estimate what percentage of the total area is occupied by the spokes and multiply that through the section inertia.


So getting to your wheel and tire question and the point of the thread...

Reducing mass isn't necessarily the top concern. The top concern is reducing mass in high-inertia areas. You can have a 20lb wheel that has most of its mass at the hub and still have less inertia than a 15lb wheel with most of its mass in the spokes. You're better off going to a smaller diameter wheel so you get the sure benefit of a reduced inertia effect of the rim section.

Tires are a different concern. You tend to want more section width to get more traction. But a wider tire = more mass = more inertia. Your best option for a given tire width is to go with a little smaller sidewall. Instead of a 225/45r17 you can use a 225/40r17 and save yourself 1/2" on the inertial radius. That will take off a bit of weight and a good deal of inertia. You also get a gearing advantage from the smaller diameter. The engine needs to use less torque to get the tire/wheel moving. That means your reaction at the light and acceleration are going to improve. The downfall is top speed. You're going to lose speed per gear. If you were already close to topping out a gear you'll have to make an extra shift and that might not be worth the time gained by inertia and gearing. And you're going to throw your speedo off for daily driving...

This is a great tool for comparing tire sizes and check how much they'll throw off your speed/speedo...
http://www.miata.net/garage/tirecalc.html



Hi...I'm an enginerd


You got that right out of your statics and dynamics book, didnt you

Its been three years and probably 75% of all my "complex" formula memorization has left my memory.

But, even as an engineer, though your equations and rotational mass talk is 100% right, its hard to factor in the other variables.
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Old 12-11-2009, 10:45 AM   #17
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Quote:
Originally Posted by Junior2JZ View Post


I have a friend that always gets stuck @ the .00s . He went 10.0s @ 140/6mph for over 1yr.. Then went 9.0s for another year before going 8s..

hahahaha

Junior
would this fellows name happen to be keith?
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Old 12-11-2009, 11:27 AM   #18
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Quote:
Originally Posted by juanmedina View Post

Car Weight 2815 lbs + 150 lb driver = 2915lbs
Sorry you guys were talking about the math being really hard and so I had to point this out. Good for a laugh.
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Old 12-11-2009, 11:33 AM   #19
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Quote:
Originally Posted by fourmicah View Post
Sorry you guys were talking about the math being really hard and so I had to point this out. Good for a laugh.


I absolutely FAIL. I can't believe I didn't catch that
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Old 12-11-2009, 11:35 AM   #20
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Juan Im just busting on you. Man, the weather must be perfect down there. Why dont you take a trip to the track man?
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Old 12-11-2009, 11:41 AM   #21
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Quote:
Originally Posted by STi Mikey View Post


You got that right out of your statics and dynamics book, didnt you

Its been three years and probably 75% of all my "complex" formula memorization has left my memory.

But, even as an engineer, though your equations and rotational mass talk is 100% right, its hard to factor in the other variables.
Unfortunately I spent 2 years designing crankshaft pulleys/dampers. I know those equations all to well when it comes to calculating the rotational inertia of the mass section of a damper. I also know too much about crankshaft vibrations and the effect a lightweight flywheel has on them. But we're not going to get into that.
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Old 12-11-2009, 12:26 PM   #22
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On my personal car, back in 2003, I used 18 inch wheels and was running 12 flat. I put the stock wheels and tires back on and ran 11.80s. No spray the car would run the same 2 full tenths faster with the stock wheels and tires.

C
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Old 12-11-2009, 01:52 PM   #23
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Quote:
Originally Posted by STi Mikey View Post
Juan Im just busting on you. Man, the weather must be perfect down there. Why dont you take a trip to the track man?
I can't

Quote:
Originally Posted by Clark Turner View Post
On my personal car, back in 2003, I used 18 inch wheels and was running 12 flat. I put the stock wheels and tires back on and ran 11.80s. No spray the car would run the same 2 full tenths faster with the stock wheels and tires.

C
That is awesome Clark
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Old 12-11-2009, 02:04 PM   #24
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most of the time you want a thicker sidewall for flexing (assuming the tire is a drag tire), going with smaller sidwalls generally means less traction and usually offsets any acceleration gains from traction losses.

in the real world expect to see gains from the reduction in weight only, the reduction in rotating mass is extremely minor (for drag racing). Although your butt dyno will probably lie and tell you different.

I'd expect a tenth at best under equal conditions and at best 1mph.

I stress equal conditions because if you dont swap the wheels / tires out on the same day, and also have 3 passes on each set with very consistant times it will be next to impossible to tell exactly the gain.
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Old 12-11-2009, 02:49 PM   #25
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The gains are alot more then you think for rotation. Where the weight is centered and the overal diameter. The dyno we used can measure this. Its a function for manufacturers and is not used in the performance industry. But it really makes a big difference.

C
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