Friday, February 15, 2008

Wheel and Tire Selection

Several sources I've read (the Kimini book, Competition Car Suspension by Allan Staniforth and others) recommend choosing tire sizes before ever embarking on suspension and chassis design. This makes sense, since nearly every book I've read so far starts out by stressing the critical role the tire plays in making a car go, stop and everything in between.

Tire Diameter

Since I've already decided on using the Miata as a donor vehicle, deciding on an overall wheel diameter is somewhat straightforward. The original NA Miata (1990-1997) was shod with 185/60R14 rubber, giving a wheel diameter of 22.74".

I'm going to add two additional requirements: I want to keep all 4 wheels the same, and I want to keep to relatively common sizes. These need to be in place for the simple reason that I don't want to have to spend a ton on odd tires or risk not having them available in the future. If I need larger rears that's something I'll have to address later.

So with a tire diameter set and a few requirements in place, I can start "shopping" for new shoes. My biggest priority for selecting wheels and tires is low weight (to reduce unsprung mass and rotational inertia). But at the same time I'd like to spec a wheel size that allows for reasonably short sidewalls and crisp handling.

Wheel Diameter

A stock Miata rides on tiny 14x6" wheels (45mm offset, approx. 11 lbs.). As has been mentioned many times, ther are not a lot of good wheel or tire options at this size, which is why most replace these little "daisies" with 15" wheels...which is exactly what the previous owner of my current Miata did. They replaced the stock wheels with the lightweight 15x6" wheels (40mm offset, approx. 13 lbs.) from an NB (1999-2004) Sport model.

Despite a relatively heated conversation on the LocostUSA forum, I've elected to stick to the Miata-plus-one 15" wheel diameter. While some may argue that 15" wheels are on their way out, I have faith that NA and NB Miatas will be in vogue for quite some time. Also, Spec Miata racing and autocrossing should ensure a long-lasting supply of tires for road and track.

Besides, that's what the front and rear uprights were designed for. Here's a great reference for stock and aftermarket Miata wheel sizes, offsets and weights.

Tire Width and Aspect Ratio

So given a wheel and tire diameter, it's time to pick rubber. Here's where things get broader. My current Miata rides on 195/50R15 tires (22.67" diameter). Staying in the ballpark of that size gives me 3 larger options. Here's the tire diameter calculator I used to get these figures.

There seems to be many more options at 205/50R15 than with my 195s, so I think that that's the best fit for my build. This gives me a tire diameter of 23.07" (+ 0.33").

Wheel Style

As far as the actual rims themselves are concerned, I've always loved the DTM look (image from the NY Times). I'd love to get a set of O.Z. Supertourismo WRC wheels (in white), but they're expensive and friggin 19 pounds!

I want to keep my wheels to less than 13 lbs (the weight of the upgraded 15" rims on my Miata). I also think that white painted wheels would really look awesome with my paint scheme. White rims will show every spec of brake dust and road grime, but I won't be driving my locost as much as my daily driver and it'll give me a lot of motivation to keep it clean!

These Kazera KZ-M 15x7" wheels (30mm offset) look awesome, come in white and weigh only 12.5 lbs. As an additional bonus, they were designed specifically for Spec Miata racing so they bolt right onto Miata hubs without any adapters. Finally, they're recommended by Flyin Miata which is all the validation I need. Did I mention they're also cheap?

Seems like a no-brainer. I just hope they aren't discontinued before I get around to needing them!

In Conclusion...

Whew. So long story short (too late), I'm looking to get a set of Kazera KZ-M 15x7" wheels shod with 205/50R15 rubber.

No comments: