Mr. Stock

I did my first track day in California 1 week ago at Buttonwillow in my M3 and I am enthused again about doing track days.

So I am thinking of making my 93 R1 a track only car(It's almost there)

Currently, I have 17 inch wheels with R compund tires for the car.

But I was thinking of "upgrading" to 18 inch rims.

What are the advantages/disadvantages of going to 18 inchers?

I would think that the disadvantage would be:
heavier weight
fewer tire choices
more expensive tires

The only possible advantage of going to 18's that I could think of was:
better handling due to shorter sidewalls of the tire? I think this is debatable.
Oh... the only other advantage would be bling bling cosmetic factor?

Should I stay with 17's?

Rx-7Addict

iTrader: (2)

If youre racing, the only reason to go with a bigger (not wider) wheel/tire is to clear brakes....

WorldPax

Agreed, wheels should only be big enough to clear the brakes. Indy cars run 15 inchers.

DamonB

And in F1 they are only 14"!!!!!

Mr. Stock

So I should keep the 17's?

Nobody has anything good to say about the 18 inch wheels with lower profile tires?

DamonB

The only thing I see the 18's doing is making the car slightly quicker to change direction since the lower profile tires will have less sidewall flex all else equal. I really don't see a true performance advantage in going to the 18.

People get too caught up in sidewall profile. The tires in F1 must be a 55 series tire!

F1 tire

Whizbang

18 inch wells have less rubber. Also, they are less forgiving on bumps, etc. Plus going to high will deengineer the handling.

ptrhahn

iTrader: (9)

The only advantage I see to 18's on the track is that many of the newer R-compound tires are only coming in 18" (read: Porsche) sizes... like the new A048's and Pilot sport cups.

macdaddy

I'm trying to make the same decision myself. I'm running 17" wheels on the street (to clear my brakes), and could use either 17 or 18" wheels for the track. My thoughts so far are:
17" - wheels and tires cheaper and lighter than 18"
18" - less sidewall to deflect = slightly quicker handling. Main advantage, however, is that I could use used race tires @ $75/tire instead of buying new R-compound tires @ $200/tire. Search and you will find a number of threads on used race tires. Availability is much better in 18" because that is what a lot of the series (Grand Am, Speed Challenge, etc.) run.

The other issue I'm cogitating is whether to go with the same size all around or go bigger in back - ability to rotate tires vs max rear traction. There are a number of threads on this issue as well.

ArcWelder

I made the decision to go with 18" for several reasons.

The main reason being fitment. I'm using 18x10 at all 4 corners. The 18x10 gives more suspension clearance (spring and trailing link) than the 17x10.

The tire size for the 18x10 is a 285/30/18. This is very close to the stock diameter. The 17x10 tire size is a 275/40/17. It has a larger diameter than stock, and unless you raise the ride height, you run the risk of rubbing the fender liner at the top of the front wheel wells.

Also, more race rubber available in the 18" size.

Mark

maxpesce

iirc F1 fronts are only 13" - The FIA uses wheel size to limit brake size (outboard brakes are required) in F1

CrispyRX7

iTrader: (139)

My thoughts,
Reasons to go with 18's for track duty:
1) Clearence issues - suspension, brakes, etc. You have to be running some serious hardware to use this as justification IMO
2) Tires "choices." Slicks are more available in 18" sizes but tregardless swithcing from a 17" to an 18" is going to seriously bite your wallet when it comes to tires
3) Better responsiveness. Note I said responsiveness NOT handling. A shorter sidewall results in less sidewall flex when the tire chenges direction so feedback to the driver is more immediate. Similarly the car will respond quicker to driver input. However, and it's a biggie, going to 18" will not necessarily improve handling. In fact if the rest of the suspension is not tuned appropriately it will result in **LESS** grip. A shorter tire sidewall is less able to deflect and conform to surface irregularities and therefore is less likely to stay in contact with the surface and thus will have less adhesion unless the surface is as smooth as glass (read: Never gonna happen). The suspension MUST be tuned to suit the tire. In F1 the tire acts as a part of the suspension deflecting as would the springs and shocks....they tune to whole system. Cars we see with 18" or larger wheels as stock (such as Porsche) tune the rest of the suspension with the use of a 30 series tire in mind. I've read many test in major auto mags where a car, with a 15" stock wheel, was run with identical tires (brand and width) but on 15" 16" 17" and 18" rims. In all the cases the 17" outperformed the 18"
Choosing to switch to an 18" over a 17" is a compromise unless done right so take care on why you want to switch. Apologies if'n I'm sounding too preachy but HTH.
Regards
Crispy
17" with 275/40 Hoosiers - Track, 18" with 285/30 Michelins - street

gfelber

One more point re: difference in weight. It's not as much as you think (and sometimes the reverse), particularly when factoring tire weight into the equation.

Most seem to forget this! Everyone quotes their wheel weights, but do you know how much your tires weigh?

For example, my 18x10 CCWs weigh 19.5 lbs using titanium fasteners. My old 17x10 CCWs were 18 lbs. However, Kumho V700s in 275-40-17 weigh 27 lbs versus 25 lbs for 285-30-18. Net "savings" with the 17 inch setup is -.5 lbs! Moreover, with the 17" wheel with a 275-40 results in a larger tire diameter (almost 3/4"). Inertial mass is located farther away from the center of the wheel (as most of the tire weight is in the belts).

Hoosier R3S03s are about 2 lbs lighter than the Kumho V700s and real slicks are lighter yet.

Gene

CrispyRX7

iTrader: (139)

Gene,
Would you also agree that the 18" wheel will have more inertial mass due to the rim halves being located further from the center than a similalry manufactured 17" rim? A 275/40 has more rubber further from the center of rotation than a 285/30 but an 18" wheel will have more metal further from the center than a 17" rim. The inertial mass argument is inconclusive. ;-)

The issue of weight is highly rim and tire dependent. Without the question of inertial mass I do think Gene has a valid point though in that rarely is tire weight included when determining sprung mass. A major oversight by many.

But for these reasons is why I didn't include it on my "general" list as there are too many combinations that could point to either 17" or 18" being the better choice if weight is the question.

BTW my 18" Kinesis rims with 285/30 street tires are boat anchors compared to my 17" Hoosier shod rims.

FWIW
Crispy

PS. Gene you have an update for me? :-)

Barwick

Pure weight measurements are ok, but where does that weight lie in relation to the center of the wheel? The farther out, the more energy it takes to get it going and to stop it.

gfelber

Exactly my point, hence the inertial mass statement.

gfelber

:P

It largely depends on the wheel. Purner's wheels have very little mass in the rim halves (In fact, when I disassembled mine, I was astounded at how flimsy they felt) and similarly, little mass at the perimeter of the mesh centers. A moot point, I guess since the 17" and 18" centers are identical. I'd say the mass of the larger diameter tire is a bigger factor.

Now, whether this actually matters is another story, particularly in light of your point regarding responsiveness/predictability between the two sizes. The 30 series tires can quickly get away from you- something I did not have a problem with using the 275-40s.

Agreed.

Yeah, but they freakin' rock. When you get tired of them, let me know. You can always trade them for you-know-what.

Hope to see something on the porch tonight...

redrotorR1

For DOT-R compounds, there are FAR more available sizes for 17's than for 18's ... especially with regard to big boy tire sizes. However, it is duly noted that the rear suspension geometry on an FD is more receptive to 18's than 17's.

Devilish

I know Hoosier used to run a slightly stickier compound for their 18's than their 17's. I have no idea if this is still the case.

I'd go with 17's mainly for cost and weight. Also, if you put some wheels in the dirt, you're less likely to bend a rim. And a Stoptech brake kit will still fit under the 17's.

2broke2race

i pretty much agree with what everyone is saying so far about the 17's vs. 18's.

just by curiousity though, what would be a decent size for the other model rx7s besides an fd? my friend ran 17's on his fc and they preformed great. the only other route he could have gone was a 16. basically the same idea of bigger rim vs. smaller rim that im seeing right here. i havent yet heard anything about the fb/sa though. and since im lookin for one im more than eager to hear what everyone has to say about that.

also... do you all take the power to weight ratio into consideration when deciding on what size tire? would there be a size difference between a 12a equipt rex vs. a more powerful 13b rex? more unsprung weight to push around right? just like the whole more weight equaling bad thing. just curious on what you guys think...

by the way... im a noob so please dont rip on me! thanx!

~k!m~

CrispyRX7

iTrader: (139)

Not a function of wheel size but of tire aspect ratio. On the FD one must use an aspect ratio of 30 or 35 for a 265 or 285 tire on an 18" rim. This is done to maintain the same overall diameter as the stock wheel/tire size. A 30 series aspect ratio is a very short sidewall. For a 275 series tire the aspect ratio is 45 on a 17" rim. This much taller sidewall allows more flex in the tire and allows it to follow road irregularities better = more grip on less than perfect surfaces. I'd suggest you consider the same argument when sizing wheels and tires for any car. A wheel with rubber bands for tires isn't going to grip very well on any surface less than a mirror.
FWIW
Crispy

Jims5543

iTrader: (2)

I have an FC so take this for what it is.

I prefer the 16" rims over 17 for 2 reasons.

1. Tires are cheaper

2. I lowered my cars center of gravity = good in my book.

I actually have to drive my car up onto 2x10's in order to clear my door when in on my car trailer with the 16's when I have the 17s its not necessary.

I am running 16x9.5 Circle aluminum rims with 16x10x22 Hoosier R-30 Slicks and my unsprung weight vs. my OZ superleggeras with Toyo Proxes dropped 12 lbs per tire.

If 16" rims would fit over your brakes that is the route I would go. Going to 18" in my book is a bling thing and totally not necessary. You car came stock with 16" rims, bigger isnt always better.

Jims5543

iTrader: (2)

If you are running an FC and you have a budget to stay under consider buying a set of Vert rims. They weigh 12-14 lbs each they are 15x7.5 and a 225/50/15 Spec miate tire fits them perfect.

Used spec Miata Toyo R-1 racing compund tires can be purchased for about $100 for a set of 4. They will last you an entire Autocross season as long as your good about rotating and flipping on the rim.

I just watched a kid with an 86 base and TII swap almost take the FTD at last months Autocross withthis rim/tire setup.

SPiN Racing

MOMENT OF INERTIA
MOMENT OF INERTIA
MOMENT OF INERTIA


Umm... MOI.

Basically you are spinning a Lot of metal with the weight focused at the outer edge.
Here is a link to the Quartermaster Clutch PDF. http://www.racingclutches.com/qmcatfinal-excl-cover.pdf

They have a nice section explaining the MOI for clutches on page 4.
It is very nice to see the difference on something soooo Light as a 2LB flywheel.

"MOI is read in lb-in2=1/2 x mass x radius2"
For example... a 13 1/4" Ford flexplate weighs 4.79 LBS and has a MOI of 133.25
a Similar weight Button flywheel 7.25" weighs 4.39 LBS and has a MOI of 34.99
Go down to a 4.5" button flywheel that weighs 2.06 LBS and the MOI is 7.33

The point of this slight off track is what energy does it take to spin that wheel. How much horsepower/torque are you loosing just making it spin?

IF you like... find a friend with a same model car.. and having your stock rims on your car.. make a few off the cuff rolling 5 MPH runs to 60.. or 100. And TIME THEM..
Then put on thier 18 or 19" rims.. and do it again. You may be really surprised.

There CAN be handling advantages.. but the things I am seeing above.. you are talking about tires that are not matching up on total diameter.. therefore you cant compare the 17 and 18 inch performance equally.
If you have different diameter.. the effective gearing is different.. the Ride height is different.. therefore your Center of gravity is different.
You are using a 30 and 40 profile.... well sidewall flex will be vastly different. Are they the same brand/model of tire? Each brand is different on what they consider the tread width. Some measure the contact patch.. some measure the sidewall to sidewall width.

My suggestion.
What do you want from your rims?
Looks?? Go 18.
Performance Go 17 or 16.
Shoot.. want to play hard ball? GO 15" with slicks.

If all you want is the cool swass bling look at me rims.. go 18. If you have money to burn there are some REAL NICE forged 18s out there that will be light as hell and strong. BUT.. you will pay for them. Will they be better than the smaller less pricey rims? Prolly not. But they will be close I am sure.

A battle I face is.. do I buy rims for 125 Each for the track that are 15x7 aluminum nice looking.
OR... do I buy at 35 bucks each ugly steel rims.. and have 3 or 4 sets with Slicks all mounted.. and get the GOOEY compound.. and stomp the person with only 1 or 2 sets of aluminum rims and harder tires cause they have to last longer.
Looks... performance.... cost.

I can think of things to spend cash on other than nice rims in my case. (But my car will not be a pretty track car. (It will be Uglah... but fast.)

MOI MOI MOI

ArcWelder

Yes yes true, MOI is important and what you say is true. All true but the basic premise of higher mass at the perimeter of an 18" wheel/tire than a 17". Read what Gene (gfelber) states above. The 18" wheel/tire combo actually has less mass at the perimeter than the 17".

This is due to two reasons:
One is the fitment for the 18" is a 285/30/18 which is very close to the stock circumference, the 17" is a 275/40/17 which has a 1" larger diameter than 285/30/18.
Secondly, the tire weight is more important than than the wheel weight for questions concerning MOI. Less tire at the perimeter, less weight. Also lighter tire less weight, less MOI.

MOI is just one concern in this choice. Taken to the extreme, if MOI was the main issue we'd all be riding on narrow, small diameter tires. Obviously there's more to consider.

Mark