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.

Tuesday, February 12, 2008

An essential addition to your library

Getting your locost library started is pretty easy. There are only three or four books necessary to a basic reference library. But which book should you get next? Without a doubt, you should get Kimini: How to design and build a mid-engine sports car - from scratch by Kurt Bilinski. In fact, I'm starting to think you should get it first.

At first, the Kimini doesn't seem at all like a locost. But despite it's more passive-aggressive styling, the car was designed, built and refined in the same manner as any other serious homebuilt. Kurt is fond of saying that this book is not a "cookie-cutter" set of plans on how to build, but instead is a detailed account of the process of building a car at home.

When I started looking into building a locost I was initially attracted to the "baseline" design. Heck, the car was done as far as I was concerned. As I've read more, I've found that the art and science of building a car at home is less about black magic and voodoo and more about carefully and purposefully thinking things through.

Reading about the Kimini build, I'm emboldened even more to research, analyze and detail every aspect of the car (within some kind of reason). After finishing the book, I started pouring through the standard locost books looking for the same level of engineering and design methodology as Kurt had put into his project. I didn't find it, and now I feel that there may be more left to improve in the design than I'd first thought.

Don't get me wrong. The Kimini book outlines years of research, development and building. Locost books make construction look easy enough to do. The Kimini book is much more realistic and transparent. If you still want to build a car after reading about Kurt's 10 year build, this is the hobby for you!

So I intend to roughly follow the design sequence Kurt laid out early on in his book. You can get a rough sense of the process from the table of contents. I'm sure there will be tons of references to his work in this blog in the years to come.

Buy this book. It's a good deal at twice the price.

And if you just want to cheat and skip the whole process, Kurt's Kimini is for sale...