And so it begins. Having decided on the front tire diameter now the real suspension design begins. Using Mitchell’s excellent WinGeo3 software, I’m working through many iterations, trying to get static FAPs (Force Application Points.) What’s frustratingly familiar is how I can get static FAPs, and roll centers for that matter, but I’m not happy with the camber gain curves yet. I’m not complaining and quite enjoy the iterative process. It’s a good feeling knowing that once this step is done, the car will have a stable, predictable nature to it, just like Kimini has. And like the Mini, it took weeks to gradually settle on those elusive points in space about which the suspension will pivot.
Regarding the bike shock, the spring rate is around 500lbs/inch. The problem is that it’s preloaded, and the high-pressure nitrogen adds to the overall rate. This gives a discontinuity, where nothing happens for several hundred pounds until it finally starts to compress. Until I plot points at higher force I won’t know what I have. This weekend I’ll make brackets to properly support it and run a force vs. compression test on Monday. Oh, and another impressive feature of the shock is the mount bushings. I pulled a bushing out and there are needle roller bearings inside – very cool! My expensive Konis don’t have that!
While a great value, I can see a potential problem using these shocks, which is a shock travel versus spring rate issue. That is, I can trade one for the other, which is fine if one’s not important. Unfortunately there’s a squared term in the installation ratio which makes things more interesting. Instead of getting all wound up over this I’ll remain calm until I get it all into the suspension design software to figure it out. Worst case I have to use different units at the back, or swap springs, which isn’t so bad. That’s part of the beauty of buying off Ebay. If you buy used stuff and end up not needing it, you can sell it for virtually the same as what you paid.
Several Locost builders I know have used these shocks so I know they can work. Of course most are using sportbike engines so the very low weight translates to lots of suspension travel. I’m going with a heavier but more “polite” street engine which will eat into the little shock travel that’s there. Guess I’ll be the first to know if it works or not.
On the tire front, I’m going to try to decide tire diameters this weekend.
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.