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My car was delivered with 14 inch rims. I suspect, since 15 inch rims and tires are legal, that everyone is running them, am I correct? Is the 14 inch a real disadvantage? Regarding tire cost, what is the cost of the 14 vs 15? Any real savings there? Anyone haver a spare set of round 15 inch rims they feel like selling?
-Scott
From what I’ve learned about Wheels and Tires over the years, typically larger wheels do offer a performance advantage over the gearing advantage of the smaller diameter 14 inch wheel.
There appear to be advantages with larger wheels in elongating the tread pattern on the road, as well as the basic advantage of having a wider wheel. (comparing 7x14 vs. 7x15…)
The 7x15 inch wheel is also an advantage in that the tires available are in massive supply what with SpecMiata also using the same size and brand.
Leo Franchi when running in IMSA Firehawk used a loophole in the rules to permit him to run 7x16’s before the rules loophole was changed. There was a performance advantage with the larger wheel despite the apparent disadvantage to gearing.
There were likely several influences on this, the larger wheel seems to place the tire on the road with more longitudinal bias, and improving feel. The larger wheel may also be distributing load and theoretically heat gain. The larger wheel also seems to allow the tire to run cooler - maybe allowing heat to escape more efficiently (surface area), maybe just from the efficient distribution of tire over the road…
Either way 7x15 inch wheels are everywhere.
The issue we’ll all need to keep in mind with understand which offset will work with our suspension package to make the car the fastest. In SpecMiata 30mm offset was figured to be the most beneficial offset for the car, the car was designed for a 45mm offset.
kgobey wrote:
I fell asleep during the rest of that stuff, but here’s the “close enough” advice for ya. There ain’t much difference in overall tire diameter between our two choices of 14" or 15" wheels. That theoretical stuff is moot.
14" wheels with legal tires (205/55) have a circumference 8/10 of 1% shorter than the 205/50/15s (72.5" vs 71.9" if my math is correct). In other words, it turns your 3.73 into a 3.76. Well shy of the preferred 4.10. 
The 14’s make for a softer combined spring rate due to the taller sidewall. The goober in the field approach is that 14s are good for rain when you want a softer spring, 15s are preferred for dry. In application, it probably doesn’t matter enough to NOT use up an old set of 14s.
The bigger issue in my mind is that the typical 14" wheel I see around is an old bottlecap that has been banging curbs for the last 20 years or so.
I feel better on new race wheels than old road wheels (except in the rain when side loads are negligible).
YMMV.
The circumfrence issue might be harder then that to figure out. The wheel width plays a role in tire dia. A narrow wheel creates a taller sidewall. The very common E30 14" bottlecaps are 6" wide. The fairly common E30 14" basketweaves are 6.5" wide. Our 15" wheels are 7" wide.
As Steve alluded to, a lot of folks put their rain tires on 14" wheels. That way they don’t have to buy a set of nice race wheels for their rains.
It really comes down to this tho…15" looks cooler.
Ranger wrote:
Not even theoretically. In practice, you can’t alter the circumference of the tire in the center of the contact patch (which is where the circumference that matters is measured) by moving the bead 1/2" either way. A radically narrower or wider wheel will affect the circumference at the shoulders by misshaping the tire, but not enough to matter one teeny tiny bit on 6" vs 7" wheels (in terms of the different tires’ effects on gearing, which was Kieran’s point about different wheel sizes).
What you eat for breakfast will have a bigger impact on your car’s performance than the rim size.
Steve D wrote:
[quote]
What you eat for breakfast will have a bigger impact on your car’s performance than the rim size.[/quote]
More fiber or less fiber? :laugh:
Fred42 wrote:
[quote]Steve D wrote:
[quote]
What you eat for breakfast will have a bigger impact on your car’s performance than the rim size.[/quote]
More fiber or less fiber? :laugh:[/quote]
Robinson? Quiche.
Jones? Captain Crunch with Muscle Milk.
The Rev? Flat beers.
M3Bill? Room service.
Travis? Marlboros.
Geegar? Wheaties (cuz he needs 'em)
cheat sheet:
Wider wheels = flatter tire on road - side walls are not “crowning” the tread. (better braking, better cornering)
Taller wheels = longer shape to the contact patch (better braking, slightly cooler running tire) - (note: there is actually more rubber longitudinally on a taller tire.)
Moe boring stuff
Taller wheel downside - if the tire/wheel package is taller than stock - will actually place more torque on the braking system - more torque - more heat - less efficiency
Taller wheel downside - poorer gearing
The benefits of gearing are nearly always vastly overstated IMO, we spend a lot of time on the throttle, but I have always felt that the gains of a taller wheel (comparing 15 to 14) are greater than the gains in acceleration. I guess if we ran at Grattan or Streets of Willow we might be better off on 14 inch wheels, but then we’re on the brakes more too at those tracks so the benefits are likely null and void.
I really feel the offset issue (unless it’s already been made a rule - maybe I should check) might be more important than any 15 vs. 14 inch issue.
(maybe we should try 20" DUBs :laugh: :woohoo: )
kgobey wrote:
[quote]cheat sheet:
Wider wheels = flatter tire on road - side walls are not “crowning” the tread. (better braking, better cornering)
Taller wheels = longer shape to the contact patch (better braking, slightly cooler running tire)
[/quote]
Dear God! Kieran… We are allowed 205/50/15 or 205/55/14. There is no “wider” between the two. Unless you want to argue the <1% difference in circumference, there is no “taller” between the two.
Steve D wrote:
[quote]kgobey wrote:
[quote]cheat sheet:
Wider wheels = flatter tire on road - side walls are not “crowning” the tread. (better braking, better cornering)
Taller wheels = longer shape to the contact patch (better braking, slightly cooler running tire)
[/quote]
Dear God! Kieran… We are allowed 205/50/15 or 205/55/14. There is no “wider” between the two. Unless you want to argue the <1% difference in circumference, there is no “taller” between the two.[/quote]I am talking wheel as opposed to tire. So 6inch vs. 7 inch - but yeah I am being too hypothetical and theoretical - sorry at work and minutia is what I do
To me the only real difference is a shorter and stiffer sidewall on the 15" versus the 14". Go with the 15".
So, 205/55/14 is a discontinued size for the RA-1’s. They are only making 225/50/14 now. I’m stuck between picking up a set of 14s (still available in limited quantities) and just going with 15s on a new set of rims ($400 more) and not to have to worry about it. Steve makes a good point about new vs old wheels too. I guess I can pickup a set of the konig wheels using the nasa discount.