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Old 12-17-2010, 09:59 PM   #1
contour se driver
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Default wheel weight and friction

i know we have all seen this...
http://www.caranddriver.com/features...sted-tech_dept


2mph and 2/10 of a second slower for bigger heavier wheels. this is something I don't know. what distance is that on a track?

the thing that is bugging me is mathematically I lost 1lb per corner IF tirerack weights are correct and my home scale weighing my old wheels are correct. so why does my car feel noticeable slower?

friction is all i can think of. i went from 215-45-17 re960 to 245-40-17 pilot sport a/s plus. everything about driving, cornering and braking has improved but i do feel a loss in acceleration.

with that being said...will lighter weight wheels be enough to counter the increased friction...again assuming my new wheels are in fact 1lb lighter than stock. when I get the chance they will be weighed.

yes, more power could be added easily but why ignore the potential? same as taking out the drive shafts for cf shafts. why make (put on) more power when one can release what is already there?

ive read several statements about people feeling the difference when they go back to stock from a bigger size. they know they increased weight i am sure i decreased but we both have the seem feelings.

now there is a hoopla about the new wrx being 3/10 slower to 60 and speculation is due to wheels. all i can say is with the ap and my gf in the car with me on stock wheels...i pulled a 2.2 60ft and pulled a 5.8. recently i did it on my own on some wet road with sand and spun the fronts a little and got a 2.4 60ft at 6.0 sec. it still is missing that "oomph" that it had since the wheel swap. i do have a dyno scheduled for wednesday and the only sure thing i can say is airing up my old wheel and swapping them at the dyno. may be too much work though.

has there been any certainties or tests showing that even though a wheel is lighter due to friction increase it feels heavier? im already considering lighter wheels in spring because i don't want to add parts for power when what lost it was wheels. i want to maximize the potential before adding parts.

what say you?
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Old 12-17-2010, 10:05 PM   #2
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Quote:
so why does my car feel noticeable slower?


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Old 12-17-2010, 10:09 PM   #3
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Originally Posted by Uncle Scotty View Post


laugh all you want. stock wheels was enough to outrun a is350 on the top end with just a relfash on a 5eat. i guess now it would be equal or a car link behind?

maybe yo umissed it where i said i lost a lb even with the new larger wheel combo.

the steering wheel is def much heavier as well.

i think part of the problem is that you still have that sour taste in your mouth when you thought i did in fact buy 17x7 wheels and i made you look like a fool.

Last edited by contour se driver; 12-17-2010 at 10:16 PM.
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Old 12-17-2010, 10:52 PM   #4
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laugh all you want. stock wheels was enough to outrun a is350 on the top end with just a relfash on a 5eat. i guess now it would be equal or a car link behind?

maybe yo umissed it where i said i lost a lb even with the new larger wheel combo.

the steering wheel is def much heavier as well.

i think part of the problem is that you still have that sour taste in your mouth when you thought i did in fact buy 17x7 wheels and i made you look like a fool.
at least I know why you slowed down


and screw the wheels....they are ugly anyway
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Old 12-17-2010, 10:56 PM   #5
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Spin in a chair with your arms out.......then bring your arms in. Same weight, but now you spin faster.

Someone needs to watch Mr. Wizard from back in the day?
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Old 12-17-2010, 11:34 PM   #6
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Spin in a chair with your arms out.......then bring your arms in. Same weight, but now you spin faster.

Someone needs to watch Mr. Wizard from back in the day?
except that my old tires were 25lbs and my new are 26. my overall diameter increased from 24.6 to 24.7.

those that go from 215-45-17 to 225-45-17 on the stock rim gain more weight than what I lost. 2-3lb increase just on tire alone which is much heavier than what i did.

both are 17" wheels the 17x8 are lighter than the 17x7 stocks.

anyways...the c&d car gained 14lbs and lost 2/10. i lost 1 lb and feels like i lost 3/10.

you will have to excuse me. arm chair calculations will not be enough to satisfy what i want to know. as already stated will a lighter wheel be enough to counteract whatever i did whether outside rotational forces or friction. you telling me 4th grade science doesnt answer anything.

i had hopes of actually throwing numbers down and crunching actual calculations. i assume you are not an engineer nor have aspirations of being one. this thread is not for you.
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Old 12-17-2010, 11:36 PM   #7
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Originally Posted by Uncle Scotty View Post
at least I know why you slowed down


and screw the wheels....they are ugly anyway
i fail to see why you exist. you clearly have yet in any post state anything of substance.

99% of the wheels out there including the ones members buy i think are ugly. rota which i feel are cheap knockoffs. do I say anything? no.

why do you? why are you even on this thread? youve proven its well over your head. are you just here to spam threads and offer nothing of value?
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Old 12-17-2010, 11:47 PM   #8
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calculate the inertia of your tire/wheel system with combo #1 and combo #2 and figure out which one has more rotational inertia. that one will be the "slower" tire/wheel system

obviously the heavier steering feel is because of a larger contact patch creating more "drag" on the ground for you to resist with the steering wheel. Newtonian physics. You also need to take into account how the change in wheel offset between the wheels changes the scrub radius for this assessment.

the wider tire is going to have more rolling resistance than the skinnier tire. This is a drag force that might make a 1-5/10ths of a 2nd in a 0-60 test....
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Old 12-18-2010, 12:55 AM   #9
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Originally Posted by Dirty25RS View Post

calculate the inertia of your tire/wheel system with combo #1 and combo #2 and figure out which one has more rotational inertia. that one will be the "slower" tire/wheel system

obviously the heavier steering feel is because of a larger contact patch creating more "drag" on the ground for you to resist with the steering wheel. Newtonian physics. You also need to take into account how the change in wheel offset between the wheels changes the scrub radius for this assessment.

the wider tire is going to have more rolling resistance than the skinnier tire. This is a drag force that might make a 1-5/10ths of a 2nd in a 0-60 test....
thank you. the math is a bit over my head. it looks like calculus and I havent seen that in 6 years or so. remembering how to do it isnt going to happen. i will be looking into it though. I would love to have a type of chart that I could potentially use. actually, once i get the calcs figured out it could be programmed into my ti-83.

right. I understand fully about the increased contact patch and increased friction. it appears you are calling it rolling resistance.

my theory is that I COULD put on heavier tires...which i did by 1 lb...but use the wheel itself to help counter that. let say I started out with a 30 spoke wheel. if I took 15 out that should help with rotation even though the outside weight is still the same.

im my case buying new tires will not be happening. I am happy with them and I dont drive enough for summer tires. that leaves me with messing with wheel weights. IF I cant get enough weight loss then the only other option for me would be to add power. but I need to make sure first. i also want to make sure its not rolling resistance as the only factor with the feeling of slowing. i think it may be RR and outside weight but I also decreased wheel weight by about 2lbs.

i did find this...

http://www.tirerack.com/wheels/tech/...currentpage=91

but they used lighter tires and wheels. cant compare to what I am looking for.

thanks for the formula.
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Old 12-18-2010, 01:13 AM   #10
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the rolling resistance and the increased contact patch effects are actually two separate effects.

Rolling resistance comes from hysteresis. you bounce a ball on the ground and it doesn't return to the height you dropped it from. some energy is lost in heat and sound generation. that is what I am referring to when I say rolling resistance. If you have a more massive tire, you'll have more rolling resistance.

The point that i was trying to make with that formula, which many not have been clear, is that its not only the mass of the tire/wheel assembly that matters, but the mass distribution as ritky said in the spinning chair analogy.

Lets say you have one tire/wheel combo and another thats -1lb of wheel weight, but add +1 lb of tire weight. The tire is farther from the rotation axis (the axle or hub) than the wheel. the wheel's mass is distributed between a radius of 0-8.5in(for a 17in wheel), but the tires mass is distributed completely out past that point, 8.5 to maybe 11inches. So even though the net weight of the tire/wheel system is equal, the rotational inertia is not and the -1wheel+1tire combination will be 'slower'


CONCLUSION: a pound of wheel isn't worth a pound of tire.
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Old 12-18-2010, 01:23 AM   #11
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Originally Posted by Dirty25RS View Post


CONCLUSION: a pound of wheel isn't worth a pound of tire.
i understand that part. but there has to be a point where the reduction of wheel weight in itself can help with the outside rotation of the tire.

its not like a 25lb tire will spin the exact same on a 15lb wheel as it would on a 30lb wheel.

those are the calcs I am looking for. what wheel weight will equal or lessen the increased weight on the outside. with the friction etc added into that.

funny thing though. the re960 in stock size is heavier than the re92 that came on the wheel.
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Old 12-18-2010, 01:41 AM   #12
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RE92 i'm guessing was an OE fitment tire. It probably had cut skid to meet subarus performance requirements.

The RE960 is a aftermarket tire. It has a full skid since the end user (you) cares more about treadwear than subaru does.

All else being equal, reduction in wheel weight ALWAYS improves acceleration. Its a reduction in vehicle mass, rotational inertia, & unsprung mass. Its a triple whammy.

When you go with a wider wheel you are going to increase mass, sacrifice accel for better handling. only gain accel from wider wheels if you are making enough torque to spin the wheels freely.

Considering tire OD approximately constant, when you go with a larger diameter wheel you are going to increase mass. more wheel, less tire sidewall, aluminum is heavier than rubber. The only two things you gain from a larger diameter wheel is styling and room for bigger brakes.

Basically a smaller diameter wheel is always going to be better from a mass & inertia standpoint, but if the sidewalls get too tall, say 60 series, now you are sacrificing maximum lateral g's and steering response quickness for mass reduction (acceleration).

Your image in your OP backs my statements.

There is no magic calc for this. Because the mass distribution is whats really important, not the numerical mass itself, you need to know the "mass moment of inertia" of the wheels and tires in question. You wont find that data because there are only two ways to get it.

1. measure it, mount tire and wheel apply torque via electric motor or something, measure angular acceleration, back calculate wheel/tire inertia. Use a control wheel/control tire so you know what contribution is from the tire and whats from the wheel.

2. model it with FEA, requires detailed knowledge of the dimensions of the wheel. and i mean detailed.


So I'm not exactly sure what you are trying to calculate/figure out. So i basically just rambled off everything i could think of related to handling/accel/decel related to tire/wheel size & mass. lol.
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Old 12-18-2010, 04:14 PM   #13
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wheel diameter is the same. i went from 17x7 with what i am sure weighs 23.5 lbs to a 17x8 with what tirerack says is 21.6lbs. already stated that I gained 1lb per tire. aka net loss of about 1 lb.

i keep pushing it but I am sure there is an equilibrium with the wheel weight and tire weight with the increased friction. even if it says I need 4lbs wheels to equalize what I did vs stock sizes. obviously 4lb isnt going to happen then I would add power.

technically i could squeeze a 215 stock size onto a 17x8. no tire weight loss/gain, no increased friction etc...

see what I am saying?
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Old 12-18-2010, 08:12 PM   #14
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Using these sites, I came up with some rough numbers for your comparison:
http://en.wikipedia.org/wiki/List_of_moments_of_inertia for inertial formula of a cylinder
http://www.miata.net/garage/tirecalc.html for final tire diameters

Your setup:
Old wheel weight: 23.5 lbs
New wheel weight: 21.6 lbs
Delta: -1.9 lbs @ 8.5 inches (least conservative since this says all the weight was dropped at the rim portion and the spoke weight stayed the same, yielding a greater drop in inertia than really happened)
Delta inertia: -137.275 lb*in^2

Old tire weight: 25 lbs
Old tire diameter: 24.6"
Old tire inertia: 3782.25 lb*in^2 (most of the tire weight is at the tread diameter and not in the sidewalls...again not completely accurate but an estimate based on experience)

New tire weight: 26 lbs
New tire diameter: 24.7"
New tire inertia: 3965.585 lb*in^2

Delta old to new combo: -137.275 - 3782.25 + 3965.585 = +46.06 lb*in^2
It's not much, but you'd roughly have to drop another 1 lb in wheel weight to make up for the tire weight increase.

------
By comparison, here's my setup change from stock:
Old wheel weight: 21.0 lbs (2007 WRX stock 17x7)
New wheel weight: 16.5 lbs (Advan RS 17x8.5)
Delta: -4.5 lbs @ 8.5 inches
Delta inertia: -325.125 lb*in^2

Old tire weight: 23 lbs (stock 215/45R17)
Old tire diameter: 24.6"
Old tire inertia: 3479.67 lb*in^2

New tire weight: 25 lbs (Dunlops, 235/45R17; Tire Rack says 27 but my mounted weight went up 25; I'm getting Continental DWs 245/40R17 @21 lbs next)
New tire diameter: 25.3"
New tire inertia: 4032.25 lb*in^2

Delta old to new combo: -325.125 - 3479.67 + 4032.25 = +227.455 lb*in^2
Delta old to new combo with Continentals: -325.125 - 3479.67 + 3177.09 = -627.705 lb*in^2

----
I hope this helps explain what's happening a little better.

Last edited by falconflyer912; 12-19-2010 at 01:22 AM. Reason: orig did with 225/45R17; updated for 245/40R17
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Old 12-18-2010, 09:56 PM   #15
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Originally Posted by falconflyer912 View Post
I hope this helps expain what's happening a little better.
yes it does. but not to be too picky. the new setup is 24.7 old is 24.6. everything is soooo close except the 1lb increase in tire weight. friction excluded.

according to your math i need to drop 2more lbs from the wheels to get back the feeling of what I had stock?

thats it?

am i reading that right? 146lb? does that have a direct relation to what kind of weight the car would feel like its carrying?

also...to my limited understanding...i did lose 1 lb in unsprung weight did I not? should I still have the advantages of the 1lb loss even though I am having a harder time accelerating?
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Old 12-19-2010, 01:17 AM   #16
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I fixed the values for you in the previous post. I saw 225/45R17 in your post half way down the page and grabbed that - my mistake. It's a little under 1 lb to call everything equal with inertia. As I said, I counted all the weight loss at the outermost portion of the wheel so that reduction in inertia is more than optimistic.

Keep in mind that these values are not equal to weight in a car. We normally think acceleration of the car is just going forward, but power is also used to spin up everything (crank, flywheel, driveshaft, wheels and tires, etc.) I'll try to work something up (or point you to a link that has it done already) to show how it relates in this case. I'm a bit rusty doing this since I haven't worried about this in a while.

You still have the advantage of a lower unsprung weight. The suspension will react quicker and keep the tires in contact with the ground better.
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Old 12-19-2010, 02:52 AM   #17
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Quote:
Originally Posted by falconflyer912 View Post
I fixed the values for you in the previous post. I saw 225/45R17 in your post half way down the page and grabbed that - my mistake. It's a little under 1 lb to call everything equal with inertia. As I said, I counted all the weight loss at the outermost portion of the wheel so that reduction in inertia is more than optimistic.

Keep in mind that these values are not equal to weight in a car. We normally think acceleration of the car is just going forward, but power is also used to spin up everything (crank, flywheel, driveshaft, wheels and tires, etc.) I'll try to work something up (or point you to a link that has it done already) to show how it relates in this case. I'm a bit rusty doing this since I haven't worried about this in a while.

You still have the advantage of a lower unsprung weight. The suspension will react quicker and keep the tires in contact with the ground better.

haha

thanks a lot man.

1lb. mann.

next question for myself is...how much is that 1lb or more to take off worth? im thinking stage 2 would put me back and then some with ease.

i have noticed the car doesnt jostle around as much anymore. car feels much better than it ever has. i do have the jdm rsb installed already. also i had no idea how much these wheels travel until these new wheels were put on. rolling the throttle from a stopped corner sent the wheel rubbing. this is on stock height. body roll was minimized and it feels perfect now. I prefer to have some to get an idea what the car is doing. everything drivability wise has been netter except for the loss in feeling of acceleration.

on wednesday I do have an appt for dyno. ill post back here what happened.
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Old 12-19-2010, 04:22 AM   #18
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Here we go with some results that may surprise you (and me too):

Some quick math to show how I got here and so people can check my work:
Force = mass * acceleration
F1=ma

Torque = Moment of Inertia * radial acceleration
T=Iα
Force = torque / tire radius
F2=T/r=Iα/r

Sum of forces (what your car engine produces)
F=F1 + F2
F=ma + Iα/r
---> α = a/r
F=ma + Ia/(r^2)
F=(m + I/(r^2)) * a

This shows that the tire and wheel combo moment of inertia divided by the square of the raidus of the tire gives a mass equivalent that you gain or lose from the overall weight of the car.

When I plug in your numbers for I/(r^2), I get 46.06/(12.35^2) = +.3 lb equivalent per corner
Combine that with the mass loss of 1 lb/corner you get an overall reduction of 2.8 lbs of mass and mass equivalent

Again, for comparison, my final setup yields -4.1 lbs mass eqivalent and -6.5 lbs actual mass per corner to total reduction of 42.4 lbs mass and mass equivalent.

Neither is a huge amount with our 3000+ lb cars.

So why the feel of slower acceleration? The only things I can think of are too low of tire pressure such that the tire is excessively deforming or a bad calibration of the butt dyno. I've been a victim of the second cause before so I'm guessing this may actually be the case if all that changed is the tire and wheel combo.

Last edited by falconflyer912; 12-19-2010 at 04:34 AM.
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Old 12-19-2010, 05:11 AM   #19
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Quote:
Originally Posted by falconflyer912 View Post
So why the feel of slower acceleration? The only things I can think of are too low of tire pressure such that the tire is excessively deforming or a bad calibration of the butt dyno. I've been a victim of the second cause before so I'm guessing this may actually be the case if all that changed is the tire and wheel combo.[/size][/size]
thats all that has changed. just wheel/tire combo. pressure at all 4 corners were 34lbs. had them there due to the crappy weather at the time. just took them to 35.5 15 minutes ago (averaged at 35.5 cause I used 4 gauges 2 said 34 the other 2 said 33. i went with the lower readers).

i did use the ap for a run after I picked them up and the ap said 2.4 60ft and 6.0 for 0-60. wet road with leftover sand.

i did it once before back in the day with my gf in the car 2.9 60ft and pulled a 6.2 0-60. a 2.3 = 5.8.

both were similar cold temps.

i could reset the ecu. its been driven softer lately due to stuck in traffic and i swear to gawd these ecu's are really touchy.

0-60 runs say its very close but it doesnt feel close. dunno man. i tested with the ap datalog before and knew my run was better than previous and it was only .2 sec off. 60-100 went from 7.9 to 7.7.

it just feels like it lost that snap. I am considering seeing if discount will let me return these and i will order the rpf1 in black. have had these for about 2 weeks.

Last edited by contour se driver; 12-19-2010 at 05:25 AM.
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Old 12-20-2010, 08:22 PM   #20
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Quote:
Originally Posted by contour se driver
you telling me 4th grade science doesnt answer anything.
Well apparently you didn't understand the concept of rotational mass of inertia so I tried to explain it in the simplest terms for you. As an engineer, I've learned to keep things simple in my explanations so others will understand.

Even with those online calculators you are making a lot of assumptions like the distribution of weight of the wheel and tires from the center axis. Gluck to you on finding what you are looking for.
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Old 12-21-2010, 07:56 PM   #21
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change in schedule.

decided to attempt to swap out for rpf1's in black. so far it appears i pay a 20% restock which is a lot cheaper than trying to resale.

dyno has been rescheduled accordingly.
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Old 12-21-2010, 07:58 PM   #22
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Quote:
Originally Posted by ritky View Post
Well apparently you didn't understand the concept of rotational mass of inertia so I tried to explain it in the simplest terms for you. As an engineer, I've learned to keep things simple in my explanations so others will understand.

Even with those online calculators you are making a lot of assumptions like the distribution of weight of the wheel and tires from the center axis. Gluck to you on finding what you are looking for.
understood man.

whats the saying? Making the Complicated Simple is Genius?

looking for the equalibrium. i think the rpf1 will give that and then some. not the best looking wheel around but it should do. hopefully all goes as expected. will find out tomorrow.
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Old 12-21-2010, 09:21 PM   #23
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I very much enjoyed the information that falconflyer912 and Dirty25RS have provided. Thanks guys.
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Old 12-28-2010, 12:05 AM   #24
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got a new scale. digital this time. it says my stock wheel with re960 is 47.3 lbs. my needle scale said 48.5.

i have to retract my initial calculations. i am not 1 lb lighter but .3lb heavier.

assuming all works out ok the rpf1's should bring that down a bit. hehe. should be here by the 1st is what discount said.
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Old 01-02-2011, 01:23 AM   #25
contour se driver
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Join Date: Sep 2002
Chapter/Region: NWIC
Location: Mukilteo
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07 Legacy GT
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update.

got the rpf1's up now and returned the edr9's.

pretty significant difference. car with a full tank of gas felt like it was at half. even with the 245's on there is still a good bit difference in feel.

rpf1's i think weigh just at or under 16lbs (have heard ~15.6). the edr9's were 21.6. stock wheels at or around 22.3 or so.

currently have a dyno scheduled for saturday.
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