All about wheels

I was going round and a round (pun!) for the last couple of months on what wheels to run.  I like to wrestle with ideas and this one entertained me for a while.  Seeing as there is not a wealth of knowledge out there on the Catfish and what works and what doesn’t it really came down to a theoretical problem of what is the best wheel and tire package for club racing or track day toys.  Oh and of course the aesthetic taste of the owner; me.

Caveats before I get on my soapbox: 1. I am learning as I go.  I am not an expert at the business of tires and how to maximize traction.  But I read as much as I can and ask a great many questions.  Most of what I will present below I will back up with links and or site my sources.  2. I limited my self to 8″ wide wheels.  In retrospect this may have been a false limitation.  When I increase power at some point down the road I will be forced to increase wheel width.  3. As a product of caveat no.2 most of what is written below is based on a 225 width tire.  If you want to run a wider tire 15″ become difficult to source.

Cord recommends 15-17″ wheel diameter with low offset and an outer tire diameter not to exceed 24.5″ at the absolute max.  You can get more specifics in his FAQ entry on the site: here.  Also the Miata uses a 4×100 bolt pattern.  So we’ll use these as the constraints of the problem.

Other factors to consider are: wheel weight, wheel cost, tire weight, tire selection, tire availability, and tire cost.  Well get into these variables more in a minute.

There is a lot of information out there written on wheels.  Some of it good; most of it is not.  I think a lot of what is written is propaganda.  My driving coach recommended I read some articles online when we first started discussing wheels and tires.  After reading the articles I had to question why he asked me to read them.  The article were written to answer the question of which wheel diameter was better; 15″, 16″, 17″, & 18″?  Yet all three tires tested were of different widths and aspect ratios!!  The widest tire with the least weight came out on top, imagine that.  If a tester truly wanted to compare diameters they should fix the width; aspect ratio and width is a little harder to control at the same time.  Its in the interest of the tire manufacturers to sell the bigger tires; there is more profit in it for them.

Wheel and tire weight; this might be the most important of our variables.  The lighter the rotating mass the less your engine needs to work at rotating it and more energy is available for your acceleration (same with the deceleration).   Remember the little Miata engine is not a torque machine; its sensitive to rotating mass.  Even after the motor swap or supercharger you still want to try and reduce un-sprung weight as much as possible.  My initial goal was to get the lightest wheel in the desired size; ultimately my desire to reduce weight dictated wheel size.

I used the NASA Spec wheel weights as a lower limit of acceptable wheel weight.  My thinking is NASA does not believe a wheel that weights less than this spec is safe (maybe or maybe its to keep cost in check) and this is the sweet spot for competition between manufactures.  This is were the rubber meets the road (pun!) for supply and demand.  You can try to get under these weights but its going to cost you.

Spec e30: Wheels shall be 14 by 6 inches, 14 by 6.5 inches, or 15 by 7 inches, and shall weigh a minimum of thirteen (13) pounds. Any brand, unless specified in these regulations, is allowed. – Spec Miata is the same.

Spec 968: The 17 x 7 inch wheel must weigh at least 18.5 lbs. and 17 x 8.5 inch wheel must weigh at least 20.0 lbs.

If you were paying attention that’s a 7lb difference between a 17″ and 15″ wheel!!  Tires in the width and aspect ratio that are relevant will add about 1-2lbs per wheel.  36lbs potential weight reduction, sorry, added lightness.

To complicate matters further try finding one of these competition 17″x8″ wheels with a 4×100 bolt pattern, ha!  Most tuner wheels make less effort to achieve lightness.

Tire selection, nothing worse than wanting to run the latest and greatest tire and finding out its not available in your wheel size.  Try finding tires for 13″ wheels!  This is not an issue for the Catfish but if you wanted to build a GTAm tribute car or an original Mini Cooper, good luck.  As a function of shear number of cars being used in competition 15″ is a very common size.  17″ is becoming/is a standard too as they are whats generally being offered by manufactures on late model sports cars; lots of choices here.  16″ wheels are less common than either of the other two.  Some popular brands have a decent selection but not all of them.  The BFGoodrich Rivals, which are the tires I want to run, for example only come in one 16″ size which happens to have too large of an outside diameter (24.9″) given our constraints.

Similarly tire availability can be an issue too.  If you have friends who are in a tire war in their class and need to run new tires every other race to keep that 1/10th of a second edge they can be a great source for tires.  Given 16″ and 17″ tires are spec’ed in so few classes your are less likely to be able to find deals on slightly used R-comps in these sizes.

And that brings us to tire cost.   The smaller the wheel diameter the less expensive (with a few exceptions).  I was hoping to run BFGoodrich Rivals as I have read some good propaganda about them.  Wear and communication are high on my list of tire traits.  So here are some prices for comparison (taken from the TireRack website): 225/45R15 $115.00 – 225/50R16 $136.00 – 225/45R17 $141.00.   Not a big difference but those are the numbers.

There are other considerations for final wheel choice; manufacture and manufacturing technique, offsets, style, finish, supplier.  Of these offset is the only one that is really worth talking about in the context of this post.  Offset is the difference between the wheel’s geometric center and the inside hub face.  Too large of an offset and the wheels will be tucked deep under the body and will look poorly.  In addition you will reduce your track width; there’s all sorts of goodness that comes with a wider track width.  Too little of an offset and you wheels stick outside of the body and you increase the loading on the hub.  Goodwin Racing has a good write up on offsets and Miata’s here.  Cord recommended between +25 and 0mm for body fitment.  I chose +25mm as a function of availability and the ability to adjust down to 0mm with spacers.

(Wheel hubs are designed with a given loading.  Exceed this loading too excessively and you risk hub failure.  Hubs are wear items.  If it is not on your list of preventative maintance items on your track car you should put them there.)

Okay so what did I decide?  Konig Wideopens, 15×8 with a +25mm offset that weigh 12.95lbs.  I will post pics when they arrive Friday.  Weight savings ended up dominating my decision process.  Had the lighter wheels cost more than the larger ones I might have had a reason to consider another size.

Additional resources:

For understanding wheel/tire size this site is invaluable; RimsnTires

For a great selection of wheels specific to Miata fitment and with wheel weights confirmed with pictures! Goodwin Racing I can’t stress “with pictures” enough.  There are a lot of bogus weights listed out there.

XXR wheels has a really good selection of wheels with 4×100 spacing and offsets that work well for the Catfish.  I was pretty close to picking up a set of their 530’s but in the end they did not have available the offset I wanted.

Here is a e30 forum thread that covers wheel diameters well; short and to the point.

There is a lot of garbage written out there on this particular topic.  Do your homework.