15 or 16 inch wheels?

[QShip]

I am considering new wheels to use for autocross and would like opinions on whether to get 15 or 16 inch wheels.  I am interested from the point of cost of wheels, availability and cost of tires, etc.  I intend to get tires that approximate the rolling circumference of the stock 17 inch wheels so I do not need to worry about gearing, etc.

[KingSpeed]

I would say go with 15's if you want to autocross because of more tire selection. 

[muddy]

15x8s with 225/50/15s.  Brian Goodwin has a shipment of 15x8s coming in this week.  I am pretty sure the 225/50/15 is the same circumference as the stock 17s.   

[SilverMiataRacer]

15x8s with 225/50/15s.  Brian Goodwin has a shipment of 15x8s coming in this week.  I am pretty sure the 225/50/15 is the same circumference as the stock 17s.   

Close, they are a little taller.

When in doubt use this:  http://www.miata.net/garage/tirecalc.html

[QShip]

With 15X8 will I have to change the offset?  I presume the 15's will clear the brakes ok?

[muddy]

Yes and yes.  The ATS DTC has a 30 MM offset.

I was too lazy to check the wheel calculator again.   

[QShip]

I guess I am too ignorant to be able to ask the correct questions, but the antidote for ignorance is curiousity, so here goes:

I looked at the tire calculation page, but saw nothing dealing with offset.  Is it there but I just do not know what I am seeing?

Is offset built into the wheel itself, or will I need to use shims.  I understand there are both positive and negative offset, but do not know which is which.  I would guess positive extends the centerline of the wheel further outside the original plane of the standard wheel, but it is just a guess.  If one needs negative offset it must require that it be built into the wheel, since the concept of a negative shim hurts my brain.

Thanks for the help, you guys are a wonderful source of information for us neophytes

[SilverMiataRacer]

The tire calculation page is just to help you pick a tire size that is correct in diameter so you do not throw off your speedometer and rear gear ratio.  It has nothing to do with wheel offset.  You don't use shims you pick a wheel with the correct offset for the Miata.  This isn't a locked in stone thing.  The offsets can be slightly different from wheel to wheel and still be within the acceptable range.

As for what the offsets really are, that's a question for someone else because I'm the kind of person that has never bought a wheel that wasn't spec'd to fit a Miata so I've never worried about the offset.

[gtgray]

I guess I am too ignorant to be able to ask the correct questions, but the antidote for ignorance is curiousity, so here goes:

I looked at the tire calculation page, but saw nothing dealing with offset.  Is it there but I just do not know what I am seeing?

Is offset built into the wheel itself, or will I need to use shims.  I understand there are both positive and negative offset, but do not know which is which.  I would guess positive extends the centerline of the wheel further outside the original plane of the standard wheel, but it is just a guess.  If one needs negative offset it must require that it be built into the wheel, since the concept of a negative shim hurts my brain.

Thanks for the help, you guys are a wonderful source of information for us neophytes

Use caution as offset alone will not tell you the whole story. I have Rota GT3s in 15", they are very close to the stock offset but they just barely clear the brakes. If you are working with a 15" wheel it would be wise to know ahead of time if that particular wheel in that offset will clear the brake calipers. For example Good-Win Racing sells the Enkei RP-F1 in 15" which will not fit over the factory MSM brakes.

Mazda has varied the offset from 40-45 on Miatas over the years so you could call anything in that range a stock offset. Smaller number offsets which push the wheel out farther may cause larger tires to rub, and may increase some handling issues like bump steer. Having said that Spec Miata cars runs about 35 on the offset without issues but they are not running wide rims and extra wide tires. A lot of people like wheels with low number offsets because they like the looks and they slightly widen the track.

If you are working with some like Brian Goodwin he will tell what works and what to doesn't. The only knock on Brian is that he doesn't like Rota so you need to take his supposedly objective views on that with a huge dash of salt. Rota wheels get sold at bargain basement prices all over the internet and especially on eBay. If I was in Brian shoes I would be negative toward Rota as a supplier.. he says it is a quality problem.. Rota is by far the most popular brand of aftermarket wheel if you judge by the Tire Forum on Miata.net with almost universally good marks for quality. So if you don't let Brian snow you about Rota everything else he will tell you about wheels is pretty good advice.

[KingSpeed]

Pic of my MSM with 15x8 30mm offset ATS Complites wrapped in T1-R's.  Brian said theres was plenty but found out I was the last person to get a set of them.  You'll like em muddy great offset with extra width for your autocrossing.  Also in making more power these will definitly help.  Hopefully when i get back from my deployment I'm gonna get 6 more of those wheels. 2 drags and 4 race tires for autocrossing/track duty.

[gtgray]

Pic of my MSM with 15x8 30mm offset ATS Complites wrapped in T1-R's.  Brian said theres was plenty but found out I was the last person to get a set of them.  You'll like em muddy great offset with extra width for your autocrossing.  Also in making more power these will definitly help.  Hopefully when i get back from my deployment I'm gonna get 6 more of those wheels. 2 drags and 4 race tires for autocrossing/track duty.

And no rubbing, right? 

[muddy]

They look like they would rub, but I don't see any marks on the tires.  What is the ride height, Kingspeed?

[KingSpeed]

I still have the stock suspension for now so its at stock height and it best to keep it there.  I'm going to get the JIC's later on and lower the rear about half an inch and thats about it.  No rubbing.