Not that I own an RX-8, but I was wondering if any of you owners considered going to a smaller diameter wheel to increase the performance of the vehicle?

Even though the stock wheels are lightweight compared to most other 18" wheels, they still have a high amount of inertia and weight compared to, say, a 16 or 17" wheel. I think 17" wheels would look just as good on the '8 if chosen properly.

The RX-7 had amazing wheels- 16x8" and 15.4lbs. By calculating out the rotational inertia going from the stock RX-8 18" wheels to the stock RX-7 16" wheels, I figure you stand to gain about .15 second off the quarter mile and have a better ride too. For that caculation, I assumed 22lb 225/40/18 TS-1 tires for the RX-8 rims and 22.6lb 225/50/16 TS-1 tires for the RX-7 rims.

Because the large 18" wheels on the RX-8 put the wheel's mass so far from its center of gravity, spinning it takes more effort than the smaller 16" wheel. While each wheel/tire combo is only about 6lbs ligther, that 6lbs takes a lot more torque to spin up and slow down. The chassis equivelant of the weight savings would be somewhere in the neighborhood of 150lbs!!!

Of course, the RX-8 is all about the drive, not drag racing. Stepping down a wheel size means running higher profile tires, which is a questionable move. I'd hope that having less unsprung weight would help the suspension keep the tires glued to the pavement better- enough to make up for additional mushy tire sidewall. Whether or not handling deminishes with smaller wheels probably depends soley on the tire.

Speaking of tires- dropping a pound in tire weight is more helpful than dropping a pound in wheel weight for acceleration and braking for the same reasons. Toyos, Continentals, and Hoosiers are all very light.

Anyway, it's just food for thought. I think the 18's look great, but I'd argue that lightweight 17's would be more practical for the performance-oriented buyer. I only say that because, well, so many people here seem to be performance oriented :)

Here's a rotaional inertia calculator:
http://www.the-welters.com/racing/rotational.xls

Further reading:
http://www.daliracing.com/v666-5/info/article_read.cfm?articleID=401

Yeah, I've been thinking along the same lines (I don't have an 8 either). I would probably go with the lightest wheel with the smallest radius I could get for exactly the reasons you mention. I'm not really that into the looks of larger radius wheels anyways. I think you could find tires with stiffer sidewall to eliminate the mushiness you mention from having a higher profile.

The formulas for rotation inertia are actually very simple (for simple shapes). For a cylinder of mass m and radius r, the inertia is 0.5*m*r^2. This means that as you increase radius from 17 to 18 inches (even if you can preserve mass), you get about an 12% increase in inertia. Of course I'm assuming that the wheel+tire is a cylinder, but you get the idea.

This was discussed awhile back. Your points are valid the issue with a smaller wheel, like a 16', is that is won't fit over the rotors. If I remember correctly, a 17' wheel is manageable.

Yup, I agree completely - I've posted before that I think the 18s were spec'd by the Mazda marketing department rather than the engineering department! I'm dropping to a 17 lb alloy 17" wheel for my winter tires (picking them up today), and I will be comparing weights when I change them. I suspect I'll save at least 3 lbs per wheel.tire, so should see a ride improvement from the lower unsprung weight and lower rotational inertia.

Regards,
Gordon

Originally posted by Gord96BRG
I suspect I'll save at least 3 lbs per wheel.tire

Way off - the 17" Konig Holes wheel with 215/55-17 Toyo Garit HT winter tire weighs 35.2 lbs, the OEM 18" wheel with Bridgestone tire weighs 41.2 lbs!!! The 17" combo is 6 lbs lighter per corner - that's a huge reduction in unsprung weight.

Regards,
Gordon

Do 16's really not fit? I thought they were standard in some markets... or at least I swore I read that here. Maybe it was someone's typo, and it was intended to read 17.

I could see running 245/45/16 on a 16x8 rim (or something like that) as race tires... it'd be a slightly smaller diameter tire for lower gear ratios, but it'd be a super-quick combo. Rotational mass goes up exponentially as the diamter increases.... stepping down to a 16 from an 18, if it fit, would be a substantial difference (as would the slightly smaller diameter tires).

I can't wait to see RX-8's start dominating S2000's in SCCA B-stock, it's just neat to see a four-seater beat a little roadster, and I'm tired of watching M3's. Just the same, I'd love my largish Mazda6 to dominate the little Celicas it is up against, but so far no-can-do (but I'm a novice still).

If I ever got a good excuse to buy an RX-8, I'd probably really get a lot more serious into autocrossing, hence my interest here :) Actually, autocrossing is my excuse to buy an RX-8, but it telling the wife that would never work :/

Ha, nevermind... after seeing a picture with 17's on the RX-8, I believe there's no way 16's would fit.

Originally posted by StretchSJE
Do 16's really not fit?

Actually, 16" wheels are standard on the Base model RX-8 in the US and Japan, BUT - they have smaller brakes. On the Sport package and GT packages that come with the OEM 18" wheels, the front brakes and calipers are larger and the smallest wheel that will fit is 17".

Regards,
Gordon

Yeah, there might be a 16" rim that might fit, but most arent' going to.

Most 17"s should fit just fine, but get a fitment guarantee before ordering them ;)

I just got a spare set of stock 18's so I can mount some slicks on.

One stock rim is curbed and slightly bent, so the new ones will replace the stock rims on the car first.

So no one knows what the smallest rim is? I am getting killed by my friends wrx mid track and I am killing him at launch and the 60ft so I think a smaller wheel and slick tire combo would get me that extra oof to run a 14.3 or so. I can run 14.5's consistantly but that's it no 14.4's or anything lower than high 14.5's. I know the power is there it's just traction, weight, and gearing i need to work on for my 1/4 mile fun.

wow, well i'm glad you searched, but this thread is having its 3rd birthday in the fall ;)

wow, well i'm glad you searched, but this thread is having its 3rd birthday in the fall ;)


Wow, it seems like yesterday ;)

Wow, I did'nt realize the age. Any input?

See Sticky
http://www.rx8club.com/showthread.php?t=51865&page=1

I know what effect wheel size and weight has. I have been racing for years. i just need to know if anyone else has done the homework on exactly which wheels will fit. i would like to find a 16inch wheel that will fit over the rotors if that is possible.