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Hi guys. I am in a bit of a quandry on the direction to take my car in and I am hoping the more experienced guys and the gods that understand wheel / tyre sizing and offsets can help.

I have a 1997 NA8 that is turbocharged. It has coilovers and has 10 pot brakes on the front that need a minimum of a 16 inch rim to clear them.I don't know how wide they are or what the offset is. It currently runs 205 / 50 / 16 tyres and these are neither bulging or stretched.

The car is used for trackdays and sprints and the like.

Now everyone I talk to seems to be running much smaller brakes and a 15 inch rim. I am trying to decide if it is going to be worth my while ditching the big brakes and running a smaller set up and smaller wheels / tyres.

The advantages I see to this are less weight and less cost in tyres and brake parts.

Is there going to be much of an advantage in gearing? I have no idea how to figure this out or what it will mean in the real world.

Should I just do something like run the lowest profile 16 I can find and stick with what I have? Lowest profile I have seen so far is something like a 225 / 45 or 245 / 45 but they are expensive and not common.

I am completely bamboozled by all this wheel and tyre stuff, I really have no idea what to look for.

More sidewall will give you more notice as the tyres start to let go.

Will factory 15s fit over the 10 pots?

I think Les runs 16s he might have some good tyre advice.

How heavy are the 16s?

No way a 15 will go over those brakes, you could barely get your finger betwixt wheel and calipers with a 16 on.

My rims are pretty weighty, with Bridgestone adrenalines they must be a good 12-15kg. I have a 2nd set of rims kicking around with ancients semis of the same size on them that are a little bit lighter.

I think my aim here is make buying parts and tyres easier / cheaper if possible and hopefully gain a bit of performance but I'm not sure if this is the right way to go about it.

Is there an ideal wheel size / width / offset and ideal tyre size to look for? I don't care about looks and I am not interested in running funky alignment to make them fit to the detriment of performance.

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There's some interesting advice on 15x7/7.5/8/9 on this thread: viewtopic.php?f=30&t=44727

I know it doesn't go into 16" wheels but thought it might be useful anyway.

15" tyres should be cheaper than 16" in almost all cases, so you'd definitely save some money. As you've pointed out as well the 16" tyres in the sizes you'd want are not common and not cheap. I think this is a big reason that most people stick to 15".

Seems like even some really, really fast track Miatas in the States are running pretty modest brakes (like stock-sized NA8 calipers/rotors with good pads, fluid, and some ducting). Even the fancy big brake packages like the Goodwin Wilwood package or the 949 Racing Corrado/Wilwood packages are runnning moderately-sized rotors and 4-piston calipers.

Without knowing too much about your current brake setup I'd hazard a guess that you could 'downsize' brakes and wheels and not lose any performance, plus have more choice in tyres and get them cheaper, too.

My brakes look like this:



So a 15 is not going over those!

Those 15X9's look the goods. I wonder what the offset is? The number does not mean much to me but it will let me look out for a set of rims to a similar spec.

Now I wonder who the heck would be wanting to buy a set of 10 pot brakes

Thanks for the advice guys , looks like I am barking up the correct tree.

Are you SOR? I have 2 different 15" wheels we could do a test fit on and know for sure.

Hehe yeah it would be a good idea to try some 15s before committing to anything else!

Yep I am SOR in Rockingham. Would be good to organise something! Thank you for the offer!

deviant wrote:
Those 15X9's look the goods. I wonder what the offset is?

+36.

I think I read on another thread that Rod with the V8 NA is waiting on a set of the 15x9 6ULs to arrive from the States. Maybe he'd let you do a test fit?

Seems between about +30 and +40 offset is the place to be for a track wheel on an NA. Lower offset will increase track but also increase bumpsteer so you don't want to go too low. I've got 15x7 +35 Enkeis and they seem to be just about perfect, but there's no way they'd fit over your brakes!

I have some Wilwood Dynalites and 11" 2-piece rotors that fit within the standard NB8A rims and most 15" wheels. They are lighter than the stock 01+ brakes too. Might be worth checking out something like this?

10 pot brakes on an MX-5? My 135i has "only" 8-pot brakes and it weighs ~500kg more! Seems like overkill to me . You'd be much better off with smaller/lighter brakes, smaller/lighter wheels and all the savings that go with them.

If it was me, I'd be dumping the 10 pot brakes (way overkill for a 1000kg MX5 and merely autosalon bling) and going back to 15s with suitable brakes that fit within them. Muuuch cheaper tyres and larger range of rims available..

Also from memory the RRP on the 10pot brakes is near $3000 so should be worth something to sell second hand..

pepejesus wrote:Lower offset will increase track but also increase bumpsteer so you don't want to go too low.

Lower offset rims would only make bump steer more noticeable by a very small amount. Approx 0.2 % for each extra 1mm of offset. (very rough calc)
The extra leverage effectively making your springs softer and gives a larger deflection for a given bump.

Increasing your spring rate by the same 0.2% percentage would eliminate any extra suspension travel and retain identical bumpsteer properties to original...

Thanks for all the advice guys. Decision has been made and I am going to head down this path. Just need to off load the brakes somewhere. I have not found an Aus supplier but overseas pricing puts them at $3000-$5000 + shipping

I am looking at the Wilwood 4 pot kits available from the states, they come in at well under AU$1000 for the calipers, front and rear lines, discs and a set of pads. They are designed to go under a set of 15 inch rims.

Yes 10 pot brakes are really really overkill and were fitted by a previous owner. Pads are dirt cheap though ($100 for road pads and $150 for track pads) and I don't think they will ever fade as the discs are so big.

Thanks again guys.

Hi Deviant

I think the idea of borrowing a set of wheels before you jump in and buy them is good advise. I run 16" wheels - heavy as, with Toyo R888 205 55 , these have heaps of side wall and are really predictable. I recently went down the road of thinking lighter is better and bought some 949 6UL 15 x 8. Put Toyo R888 on these as well. This decreased my rolling diameter by about 7%. Totally changed the dynamic of the car. Gearing isnt as tall as with the 16"s either. The big reduction in unsprung weight may also have contributed to the change and I could go down the road of fiddling with suspension set up for the track but I really liked the car the way it was and I'm feeling like "if aint broken dont fix it."

Just for info there is about $400 difference between a set of 15" and 16" R888's

Think Wilwoods is a good choice.

Good luck
Chris

Thanks for the advice Chris. It's good to hear from someone that has made that swap and felt the difference.

A forum member has expressed some interest in the brakes so I have been outside taking a few pics. There is no way a 15 inch rim is going on there, I could not even get a biro in between the caliper and wheel.

If the Wilwood 4 pots are good enough for the V8 MX5 that is destroying the competition here in Perth then they have to be good enough for me