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.

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...

Why?

I've been talking about building this car for months now, and the first question everyone seems to ask is "Why?"

This question really should be 3 questions:

1. Why build a car?

I spend most of my life hacking away at a computer. Building a locost caters to my intense desire to make something with my hands (an obsession that drove my previous blog for 2 years). I want to learn to weld, to shape sheet metal, and to work on cars. I've been tooling around with my daily drivers for several years now, and I'd love to have another project to work on.

I've spent the entirety of my engineering career working under the burdens of schedule and budget. By necessity, I've had to stop work on several projects before I felt the work was as good as it could have been. With this project, I run the schedule and budget. Here's a chance to do engineering on my own terms. That's why we have hobbies, isn't it?

I don't have a budget for this car. I'm a natural cheapskate, so I think the car will cost somewhere between $5000 and $10,000. But if it costs more, then so be it.

One of the best chunks of wisdom I've read so far came from the book Kimini, Kurt Bilinski's build guide. He said this was a hobby, and as a result he didn't feel the need to create artificial deadlines for his hobby. Sage advice!

2. Why build a Lotus Seven clone?

This question starts when I was 12. I started getting into cars and reading Road&Track regularly. The articles on various new cars came and went, but the "Side Glances" columns written by Peter Egan stuck with me. He's been my biggest automotive influence, and his love for British cars wore off on me in a big, big way.

I owned a black 1977 MGB in high school, but what I always wanted was a Lotus Seven. This was a real sports car - light, agile and dripping with British charm. I didn't care that their tops were a joke or that it was based on an ancient car. It was elemental and pure. A racer for the road.

Fast forward to now. Original Sevens are far too rare, and Caterhams are WAY too expensive for someone of my means. I don't know how, but I found Kieth Tanner's web site about a year ago and I was hooked instantly! I read his entire build diary that day which is not easy when there are over 1400 entries.

Project started out as a cheap way to get a Seven, is now a way to get my dream car. It won't be perfect, but it will be uniquely mine.

3. Why use a Miata as a donor?

There's a saying in cooking - a meal is only as good as the quality of the ingredients that go into it. This project is exactly the same.

Despite its reputation as a hairdresser's car, the Miata is a true sports car. I drive one daily, and I think it's the most wonderful car I've ever owned.

As a donor, it's nearly a one-stop shop. It's got a great engine and all the requisite rear wheel drive bits. I can pick up a beaten and abused Miata and have nearly everything I need to build the car. The parts I won't use are also still pretty valuable, so hopefully I can recoup most of the cost of the donor by selling the bodywork and interior on Ebay.

There's also a huge aftermarket community providing performance parts for the brakes, engine and transmission. That means I'll be able to increase performance as much as I'd like down the road by bolting on parts. There's also an awesome community supporting the Miata that I can go to for help.

Here's another reason to build a Seven. Few cars are awesome enough to require sub-titles during their test drive.