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Old 05-15-2003, 10:16 AM   #1
Heather
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Question curious of the difference between stock 16'' wheels and 17' wheels of equal weight

Hello,
I just bought Michelin Pilot Sport A/S tires for my stock 16" wheels, so I will not be upgrading to 17" wheels. I am very curious though as to what 17" wheels of the exact same weight (16 pounds) with the exact same tires will improve? I keep reading how the WRX in Japan and Europe has the 17" wheels and how our 16" wheels are substandard (SPD Tuning and MRT Racing, along with all the forums on the WRX) but no one says why! What will the 17" wheel of equal weight using the same tires improve specifically? My interest is in handling dynamics and not looks. It seems to me my purchase of good tires on stock wheels, 20mm rear swaybar and aluminum endlinks front and rear, was a smarter purchase handling wise then 17" wheels and I wanted to findout what those of you who upgraded their wheels to 17" feels is the difference.
Thanks,
Heather
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Old 05-15-2003, 10:43 AM   #2
kenchan
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there are many reasons. you should continue to search. many people have explained this over the years.
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Old 05-15-2003, 11:41 AM   #3
MorePower
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Ah heck, I can't resist. Typo in your title, I think there would be a huge difference "between stock 16'' wheels and 17' wheels".

Anyway, on a more constructive note, the main difference is going to be sidewall height and possibly tire weight. You see, you didn't specify tire width in your post, but you did say they would be the same in this comparison (although most 17" wheels seem to be 7" or 7.5" wide and I believe the WRX (USA) stock wheels are 6.5".) So, assuming the wheel weights were the same, and that the tire widths and diameters were the same, there would be a little less rubber in the 17"s and they *should* weigh a little less.

As to the effects of these changes on the handling of the car... well, I don't think I know enough to really speculate. From what I've read though, less weight and less sidewall flex are both good things. I have to admit that I've been wondering the same thing you're asking though. Would I really be "better" off with 17" wheels for my wagon? I totally agree with you that I'm more interested in performance than looks (not that I want it too look bad, but performance has priority.)

In the end I have decided that I will probably go with new 17" wheels/tires when I replace the stockers. My reasoning is that there is a benefit in handling because you see this option on the WRX from the factory (US STi and overseas.) I may not be able to explain exactly why, but Subaru and many others who actually race the car seem to agree. Of course, I mean to use the lightest wheels I can reasonably afford along with good tires.

The questions left unanswered are: Is this change significant, and is it cost effective? Since I'm not sure I have the driving skill to really detect the change and I have enough money to spend on the wheels, neither of these questions are really deal-breakers. Then, when I add on the fact that I do think they look a little better and make my car a little more personalized, I'm sold. I do believe that from what I've read light 17" wheels/tires will not hurt performance.
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Old 05-15-2003, 01:32 PM   #4
Heather
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Have a Nice Day? What I meant was...

O.K. I am asking the difference between our 16 pound stock wheels on stock tire size of 205/55/16 with Michelin Pilot Sport A/S tires compared to the optional 215/45/17 wheels of simular weight (within one pound either way) and the same tires. How the change in wheel size effects handling dynamics. I realize unsprung weight is critical so therefore I am assuming the same unsprung weight on the 16" wheels and the 17" wheels. The OZ wheels I like are 15 pounds each at PDM-RACING and have the correct Subaru offset- which I also know is critical. Some tuner websites say the 17" wheels are optimal for our cars and some say sticker tires on our 16" wheels is much preffered over larger wheels, that larger wheels make the car rougher, brake slower, and steer less effectively. But like I said so far, after extensive research, I have not read anyone give the physics- the engineering, behind why this is so. Handling issues fasinate me. If I had the 2 - 3 grand laying around, I'd buy the wheels I like and test it out for myself, but I do not, but that does not stop me from being curious and wanting to know what the handling differences are between these options!
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Old 05-15-2003, 03:49 PM   #5
MorePower
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I think you probably have as good or better an idea of what the differences are as I do, but I will try to break out some parts of your argument.

215/45/17 vs. 205/55/16
  • Differences:
  • 10mm wider (approx. because tires vary even within a size)
  • Smaller sidewall (by 16mm if my calculation is correct)
  • Rotational inertia (Assuming all weights equal, more of that weight is on the outside of the wheel/tire on the 17" wheel because there is a bigger "hole" in the center. This works like a lever, the farther out from the center of rotation a given weight is, the harder it is to spin.)
  • Pro:
  • 10mm wider -- greater tire contact with the road means more traction
  • Smaller sidewall -- Less sidewall flex, more precise under hard cornering
  • Rotational inertia -- er... can't really think of anything good about this one
  • Con:
  • 10mm wider -- greater chance of hydroplaning
  • Smaller sidewall -- harsher ride, greater chance of wheel damage from pothole
  • Rotational inertia -- decreased acceleration

Now, things get considerably more complex once you factor in the importance of the pros and cons. Add to it that finding "identical" wheel/tire weights in two sizes is pretty much impossible along with the varying weights, widths, rubber compounds, treads, etc. from numerous manufacturers, and you have alot of combinations. In short, it is damn difficult to even come up with an objective test of any of this.

So, which one is better? It depends on what you want since it's all about compromise. Generally, the differences are minor. BUT, the general consenus from tuners/enthusiasts is that, for a non-straight road course the wider 17" would be better -- assuming the weights were equal. Now, if you could get an equally stiff sidewall on a 215 width 16" tire that weighed the same, I would say it was superior. The problem is that I don't think you could find it.

PS -- Whew! I don't mean to make this whole thing into a dissertation. I'm just rehashing what I've read and heard, but I'm not an expert in either driving or automotive engineering. I think what you're probably seeing in your own research is more of the same contradictions, liberally sprinkled with opinion and emotion.
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Old 05-15-2003, 04:41 PM   #6
Heather
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Exclamation It's those damn emotions!

Heck I agree with you. I guess it is the emotions that get in the way of the science and engineering behind the handling. I adore my new car! With my 1997 OBS that I had for 5 years, I did no mods to her whatsoever. I guess I am trying to make-up for that and I am excited by the increased power and fun the new stiffer chassis has over my old OBS. I had to drive Legacy's for the last two years and it is so nice to be back in an Impreza again! I want to spoil my wagon senseless. I found my wheels (when my new Michelin Pilot Sport A/S/s wear-out that is) at Tirerack. I want the OZ Superleggra 17 inch wheels. They look great and are only 15 pounds each. I would be getting the same tires on those wheels, probably three years from now. Until then I will stick with the stock wheels, larger rear swaybar, Whitleine aluminum endlinks and polyurithane bushings for the swaybars. I ordered the MRT WRX Training book ver. 2 and I hope that will give me some pointers. When I ordered a Whiteline swaybar for my GT last year, Renick Subaru accidently sent me that book instead but made me return it to them to get my swaybar. As far as I recall, it was an excellent book! In three years time I should be very familiar in how my car handles and I should be able to feel the differences between my stock wheels and the new 17" OZ wheels. It will be worth the wait and I will enjoy her plenty in the meantime! I do not care about springs, struts, or larger brakes (not yet anyway). This car is fanstastic stock and only needs to eliminate excessive body roll. Other then that, I have no complaints!
Heather
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Old 05-15-2003, 04:46 PM   #7
mch
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A couple of points.

A greater diameter rim moves more of the rotating mass to the perimeter. This increases rotational inertia which means a) more energy (i.e. torque) is needed to accelerate the wheel and b) more energy is released when stopping - i.e. brake heat.

Shorter sidewalls have less flexation. This translates to less lateral slop and less heat buildup at high speed. Highly desirable characteristics on a race track. They are less important on the highway (unless your goal is look like a racer and/or pretend to be one.)

FWIW - Wider tires DO NOT put more rubber on the road. The shape of the contact patch gets wider with a wider tire but it also becomes shorter. The overall area is the same for a given load and tire air pressure.

FWIW #2 - The stock 16" wheels can run a 225/50/16 which puts down the same contact patch shape as the 225/45/17.
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Old 05-16-2003, 02:19 PM   #8
Heather
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Post Info from Whiteline

I was reading through Whiteline's HOW TO GUIDES and I found an article from AutoSpeed magazine intitled, "Rex On Rails- How To Extract The Best Possible Handling Fro You Subaru WRX's Standard Suspension" A very good read by the way. Go to http://www.whiteline.com.au/ then Reviews and Articles then How To Guides. This is a 5 part article of 35 pages and tires and wheels are not even mentioned until the last page!

Anyway they say, " When it comes to rims the general rule is bigger is better. Large diameter rims necessitate low profile tires in order to maintain the correct rolling diameter and - again as a rule - low profile tires equate to reduced sidewall deflection: this improves handling and steering response." "The widest possible rims are also perferreable because there's further reduced tire squirm through corners."

Then in the Suspension Secrets - Part 2 article, also by AutoSpeed they say, "We generally suggest that people buy their tires first and their wheels second. Meaning there is more benefit to be had in a better quality tire then in a bigger wheel."

That is just two professional opinions out of thousands I am sure. I do recall in Consumer Reports review on the WRX when it first came out that they did not like the ride quality of the optional 17" BBS wheels at all. They thought it was way too harsh and suggested sticking with the 16" wheels.

I tend to agree with Whiteline that upgradeing the quality of the tires, playing with tire pressure (they recommend adding 5 pounds to the front and 2 pounds to the rear more then what the manufactuer says)and a more aggressive alignment first, then getting a larger rear swaybar before thinking of doing any further mods is the way to go. At that point once I have driven the car for a good year or two, I should know if I need larger wheels, better struts, and larger brakes. You would have to be rich to upgrade all these items at once anyway and we certainly are not!
Heather
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Old 05-16-2003, 02:45 PM   #9
rkkwan
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All the things the guys have said here are correct, and I believe you have a great grasp of the issues. Just one thing though:

Do not listen to Consumer Report about ride and handling. They have completely opposite priorities to what auto enthusiasts want. So, basically, if they tell you 16" is better than 17", go with 17".

-Ray
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Old 05-17-2003, 05:04 PM   #10
Heather
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Thumbs up An amazing book on handling

Hey if any of you are interested in the topic of how to make your car handle well there is an amazing book on the topic full of the engineering and physics behind all aspects of handling. It was written in 1981 and is a bit out of date, but the same theories apply today. I read it 14 months ago when I had a L.L. Bean Outback and was desperate to make it handle like the 1997 Impreza Outback Sport my husband encouraged me to trade-in for the Bean due to poor crash test data and our family. Now that I have a WRX (YIPPIE! ) I am reading it again. It is from Amazon.com
http://www.amazon.com/exec/obidos/tg...glance&s=books
It is called: How To Make Your Car Handle (incase this link does not work) and it is by Fred Puhn.
This book seems to indicate that the widest wheels that will fit are the best choice due to more rubber on the road since aluminun is much stronger then rubber. The book is extremely specific and detailed. I find myself reading the chapters over to let it sink in and the physics formulas are way above my head, but it does answer many of my questions on why certain parts work, center of gravity, roll center, wheel and tire size, etc. Everyone who cares at all about the handling of their cars should read this book. Just a tip from one Impreza lover to another.
Heather
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Old 07-01-2003, 02:28 AM   #11
Dave G
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I'll second the recommendation for the book: HOW TO MAKE YOUR CAR HANDLE.
I read that book in the mid-1980's and I've benefited from the info ever since. Great book.
There were other good books in that series, such as turbocharging, which was much less common back then.
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Old 07-01-2003, 02:43 AM   #12
TheMirror
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In a nutshell:

With 17's you will get,

Quicker reactions in the steering off of dead center, at all speeds.
Measurably higher cornering power.
More finite adjustments avavailable with the throttle when cornering at 9/10ths plus.
Fractionally worse gas mileage, due to a fractionally larger contact patch on the pavement.
A measurable amount of increased road noise.
Significantly more tossability and recovery potential.
Decreased low and medium speed understeer.
Measurably more uneven tire wear if you push it often.
Depending on tire choice, an almost certain degradation in ride, although many, many would disagree on this point here and elsewhere. Collect your own data on this one.

Pros and cons, pros and cons.l

My opinion?

The pros outweigh the cons in a heartbeat. Do it now, you really won't regret it. Just look forward to when you post about how wonderful it is.

Go. Go. Go.

-M
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Old 07-01-2003, 09:14 AM   #13
Heather
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Exclamation I know!

Yes I know it will be wonderful to have 17" wheels and one day I will. I want to really do research as to the weights and offsets first, and allow time for my current expensive Michelin Pilot Sport A/S tires to wearout. I will get the same tires for sure, but the wheels- either UK 17" at 17.5 pounds, but I do not want to add unsprung weight! The SSR Comp wheels at 12.7 pounds, but expensive! The OZ Supperleggra's good weight at 15 pounds., but incorrect offset! I hear you on the comprimises! After the anti-lift-kit she is much louder then before. I do not think I could bare to add even more volume to the cabin with 17" wheels. It's all about what I need, balance and comprimises. Now I do not track and will not for a good year (due to baby due in Sept.) but down the road I will be looking for a good performance driving school and possibly autocrossing. Then I will look seriously at the 17" wheel option I am sure. At least I've gathered a lot of knowledge and will make an informed choice when the time comes.
Heather
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