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10-23-2012, 07:26 AM | #1 |
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when your rotating mass increased by X, how much of a difference did it make?
reason i ask is that i'm looking at going from the stock 16x6.5 alloys with 205/55's to some 18x8.5 DPT's at 19lbs each with 255/35's. however i can't help but think that the extra weight, size etc will be counter-intuitive to some nice light 17x8's or something.
when you changed wheels/tyres what did you think?
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10-24-2012, 08:56 PM | #2 |
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10-24-2012, 09:05 PM | #3 |
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For a DD, it won't matter much. I doubt you could feel. If it is for the track, well you know weight matters.
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10-25-2012, 03:38 AM | #4 |
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well yes it will be for the track.
i'm of course aware that it's going to matter, but i'm a bit mystified as to how much. i can't help but think it's one of those things you need to feel (or be told by someone that felt it themselves). yes, that sounds dirty. |
10-26-2012, 09:52 AM | #5 | |
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Quote:
This is about the best mainstream test I've ever seen. It would be hard for an average Joe to provide results with similar levels of accuracy. I've never felt much difference due to weight changes. For years my auto-x setup has been 15x8" TE-37s with 225/50s R-compounds. The combined weight of the wheel/tire was under 30#, while my street setup is about 10# heavier. I didn't feel as though my car became noticeably quicker when I put the auto-x wheels on, nor did the car feel like a dog when I left the parking lot after the event with the street wheels back on. Some folks on this forum act like "the Princess and the Pea" when it comes to detecting changes in wheel/tire weight. Personally, I don't buy it. I bet most of them wouldn't be able to perceive the difference if you put them in the car and did a blind test of lighter and heavier wheel/tire combos. If I had the resources, I would love to do some track testing of light and heavy combos. Say, my just-sold 15x8" TE-37s with some 225/50-15 R-comps vs. some cheap, heavy ass 17x8" wheels with 225/40-17 R-comps. Hoosier R6s come in both sizes, so same tire, same diameter (thus no gearing effect), but you could get about a 15# weight difference if you find the right (wrong ) 17x8s. Maybe a third set, too, with some light 17x8s, so the weight difference is only 6 or 7#. Do blind testing, so the driver doesn't have any preconceived notion about how the car should behave, and time 5 or 10 laps on each setup. I'd love to see what the time differences were, differences in top speeds on the long straights, driver perceptions in bumpy corners, etc. Anyone got a few thousand bucks and some track time they can give me? Last edited by Patrick Olsen; 10-26-2012 at 10:03 AM. |
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10-26-2012, 09:10 PM | #6 | |
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Quote:
i went from 225/45 kumho extsa mx tires to 225/45 falken rt 215 tires....about #5 ea heavier....and i could tell AS SOON as i let the clutch pedal up the first time that it was 'heavier' i know MANY-MANY kidi0ts who have had to put their 'light' stock wheel/tire back on after they had kiddie steamroller wheels/tires on their car and their FIRST REACTION IS man...the car feels and drives so much better with these rimz.....what causes that??? do NOT try to tell me that rotating mass isnt EASILY felt and wont make a difference THAT is pure bull**** ask the guys who have got the cf driveshafts what kind of differenct they felt when the installed them go ahead....ill wait right here. go on, now....git |
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10-26-2012, 09:22 PM | #7 |
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Funny how the butt-dyno always justifies what the brains wants. .
Different tires can mean everything as well as far as feel. I think if you tested on a track you will see better times. I feel if you blindfolded a bunch of nasiocers for DD it would be very close. The only way I would believe it, is with a double blind placebo controlled study and that is not happening. But if your car feels better good for you. |
10-26-2012, 10:28 PM | #8 | |
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Quote:
http://forums.nasioc.com/forums/show...&postcount=183 this guy picked up 3whp and 18whtq by installing the cf driveshaft this is NOT rocket science....even YOU could feel 18whtq and it DOES make a difference really saving 20 pounds on wheels and tires is INSTANTLY felt....unless you are an idiot who really cant tell **** from shinola anyway |
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10-27-2012, 01:06 AM | #9 |
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this is the reason i asked the question. i went from a 16lb 17x7 215/45 to a 19.something 17x7.5 235/45 and the difference was quite significant.
i've got the option of some 18x8.5 19.2lb'ers and i fear they're going to be like boat anchors. |
10-27-2012, 01:22 AM | #10 |
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best bet for the $$$ is the rpf1 or the FN01R-C in 17x8 with a 235/40 on it for a wrx...the 595rsr's arent too heavy in that size
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10-27-2012, 02:51 PM | #11 |
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Modified Magazine and Grassroots Motorsports both did comparos with light and heavy rims, in both the results were marginal at best...
http://www.modified.com/tech/modp-09...t/viewall.html |
10-27-2012, 04:51 PM | #12 | |
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"feel" is nice, but cold, hard data is what matters. Bottom line - get what looks the best to you and enjoy the wheels. |
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10-27-2012, 06:22 PM | #13 |
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http://grassrootsmotorsports.com/articles/speed-holes/
I can't find the other article online. It is project STR (2010 miata) update 5 "High Five" printed in the June 2011 issue of GRM (pg 123). They compare the 17x9 TRMotorsports C3 (19.6lbs) to the 17x9 rpf1 (15.9lbs). 255/40 star specs on both and 3mm spacer on the TRM so offset was identical. On the track TRM = 53.56 sec and RPF1 = 53.65 sec. On the dyno TRM = 142.61hp, RPF1 = 141.72 hp. Then they compared 255/40/17 to 245/40/17. And the 245's "felt more precise and nimble" and the stopwatch backed it up with the 245's running a 53.13 vs 53.65 on the 255. |
10-27-2012, 06:29 PM | #14 |
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^^^ Keep in mind the farther the weight is from the center of the wheel the more affect it has. So a lighter tire could have more affect than a lighter wheel. Some tires are very heavy. Stock 04-07 STi RE070 are 27lbs each.
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10-27-2012, 07:32 PM | #15 |
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thanks guys. i should note that i don't care about looks in the slightest. function over form here.
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10-27-2012, 07:32 PM | #16 |
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The 245's are shorter as well, offering a gearing advantage
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10-27-2012, 07:34 PM | #17 |
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Hell going from my 275 to 285 I could tell a difference
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10-28-2012, 12:18 AM | #18 | ||
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Quote:
How many mattresses does it take until you can no longer feel the pea? Quote:
That's my take on it, too, but there are folks like Scott who will swear up and down they can tell a difference when a wheel weight falls off or a pebble gets stuck in the tread. |
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10-28-2012, 02:49 AM | #19 | |
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Quote:
it was THAT shocking that i hadda take a wheel off and figure out the weight difference and im NOT the only one who has reported similar results this is NOT fantasy. maybe some of you idiots arent nearly as connected to your car as you think or wish you were |
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10-28-2012, 07:36 AM | #20 | |||
Scooby Guru
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Quote:
Quote:
Quote:
Last edited by vicious_fishes; 10-28-2012 at 07:47 AM. |
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10-28-2012, 11:59 AM | #21 | |
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Quote:
I absolutely believe that lighter is better and the more you do to lighten up the car anywhere the better the power and feel. But I think there is a threshold you need to really feel it especially for DD. |
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10-28-2012, 12:41 PM | #22 |
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You can feel wheel weight pretty easily. It's hard to talk about end results on a track/dyno because we're measuring different things. Inertia has effects in different ways, but how that translates to sport use numbers vary. Lighter is generally better in most every regard though. It's hard to compare track numbers and dyno numbers because they will vary. The driver will not take the same line every time, will not brake at the same point every time, will not accelerate at the same point every time. A good driver will be relatively consistent, but there will always be variation. A dyno can vary a lot too by tire, tire pressure, how tight the car was strapped down, ambient temps, which way the fans are blowing, lots of things. A one, two, or three percent change is easily the margin of error. If you saw 10% plus, then yes there is something worth looking at. Otherwise you would need to take repeat tests and a large sample of data just to average out the numbers enough to get error sufficiently low to compare in a meaningful way.
Loss of rotating mass does not make HP, but it will require less energy turned into kinetic energy meaning more power ends up actually pushing the car forward instead of spinning up heavy masses. Dynos are unfortunately still operated through a time domain, so we still see HP changes even though there is zero gain. The only good dyno for this type of comparison is one you can hold at a specific rpm and measure HP. That way you cut out the time domain completely. A 20 lb wheel and a 2000 lb wheel will read the exact same HP when the rpm is kept constant. The instant you add time like spinning up rollers, now you are creating a change in rpm and a change in kinetic energy between the two different weights. Then you see loss in the higher mass product due to the need to impart more energy into that object to reach that new rpm. Since kinetic energy is dependent on rotational speed more than it is on mass, you will find greater gains from products that see a wider range of rotating speed, i.e. the flywheel. You can take a bone stock Subaru, put it in 1st gear, and go from idle to redline. How significant is rotating mass in this example. To accelerate the car from 0mph to 40mph, a 3000 lb. car, it will take just over 200,000 joules of energy. To spin up that stock flywheel from idle to redline, it takes around 24,000 joules. That's over 10% of what it took to move the 3000 lb. car from 0mph to 40mph. You used over 10% of what it takes to move the whole car just to spin up that stupid flywheel. Now this is due to the high rotating speed of the flywheel because we're going from 500 rpm to 6000 rpm or 1/2*mass*velocity^2, it's basically 500*500 and 6000*6000 in the equation. A wheel and tire is much less. The rotating speed is comparatively low, so the gains/losses are significantly lower in comparison. We can begin to see more when we look at a higher gear where we do a 3rd or 4th gear pull and impart a reasonable change in rotational speed due to gearing. |
10-28-2012, 02:08 PM | #23 |
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Your general concept is right (and dynos dont give you an indication of translational work) but your 10% claim is foul since what it takes to move the car forward is the sum of all the transnational and rotational work (including rotational work of the engine, axles, drive pulleys, and all wheels).
Here are some helpful equations based on real physics for those who are interested: Translational work (for a mass initially stationary) = 1/2*mass*velocity^2 Rotational work = (for a mass initially not rotating) = 1/2*I*w^2 I = rotational inertia, use parallel axis theorem (the more the mass that is farther away from the center of rotation the higher the inertia) w = angular speed in radians with some basic math, you can relate the angular speed to the translation velocity and see the difference your wheels might make. The toughest part of this whole exercise is determining the I of the wheel. |
10-28-2012, 06:18 PM | #24 |
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Too much math! Buy the lightest wheel you can afford and enjoy your new found awesomeness.
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