Tires are the most important – and most frustrating – decision during new car design. Nearly every suspension design parameter is related in some way to tire choice. I’m owing to stay away from hard-to-find 13″ street tires so it means going larger, to 15″ if not 16″ or even 17″. I’ve spent weeks going back and forth between brands, trying to settle on a size that everyone will be able to find, both now and over the life of the book. This very likely means going to 16″ or 17″ to guarantee some non-obsolescence. Yes, larger wheels and tires have a higher polar moment of inertia, weigh more, and cost more. On the other hand is the possibility of simply not being able to find what I specify, so the former outweighs the latter.
Before people jump up and down saying that it’s easy to find 13″ and 15″ tires, well yes, and no. Sure they’re out there, and they’ll continue to be for some years; the problem is the compounds. For as light a car as is being designed, high mileage tires are neither needed nor desired. This thing’s not going to driven in snow or rain (much) so all-weather tires aren’t wanted (besides, it’s a sports car). We need what tirerack.com calls “Ultra Performance Summer Tires”. This greatly cuts down the list of contenders, and even the list of high performance 15″ tires is getting thin, about how it was with 13″ tires when I designed Kimini. Note that this only applies to street tires, NOT race tires. For the track, getting 13″ and 15″ tires is easy, yet that’s not the starting point of this project – it’s going to be street-legal. 14″ tires aren’t mentioned because they never were a popular size and are fading fast, there’s no reason to even go there.
So in an effort to design in some staying power, the tires may be larger. For the moment, my sole concentration is on choosing tire diameter, a primary design concern. At this point, whether the tire’s made for 13″, or 15 – 17″ wheels is immaterial. However, when I read tire data and see that there’s a grand total of only one or two 15″ sizes, the writing’s on the wall.