13 Jan 2008

Tires:

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.

12 Jan 2008

Spent a few hours carefully measuring a front suspension upright to load the dimensions into WinGeo3. It’s time consuming to find the center of rotation of balljoints. Yes, in the interest of reliability, availability, and cost, I’m specifying stock balljoints at the outboard ends. That makes it easy for the builder to get them from any parts store.

Shocks: Those who read the Kimini book recall my angst over shock selection and pricing. Good shocks are very expensive; I recently saw a new astronomical price for one> shock… $2000. I say unless it comes with topless maidens who install it, I have to wonder what the price gets you. Maybe it includes having an F1 champion fly in and drive your car. Anyhow, I’m wondering once again how I’m going to deal with the expense. The shocks, by necessity, must have adjustable ride height and adjustable valving, preferably being double-adjustable.

In the back of my mind has been to use sportbike motorcycle shocks. It’s nothing new; people have been using them in Locosts for a while now, though many builders are clueless about spring rate versus installation rate. They change the springs to make it all work, then wonder why they can’t set the shock valving the way they want. Anyhow, new bike shocks are also expensive, too, BUT, many bike owner remove them when they upgrade the suspension. That’s excellent news for us because they end up very cheap on Ebay. As a comparison, the Koni shocks on Kimini are about $500 each, plus a $50 spring. The price of a used sportbike shock and spring is $40-$50. Yes, about 10 times cheaper. This is a huge reason to consider them even if they aren’t perfect. Heck, the saved money can easily pay for the entire drivetrain! Bike shocks can> work in a car; the secret is to use them as-is, without changing spring rates. That’s because the shock valving is made to work with a specific spring rate; so as long as it remains unchanged, it doesn’t care what it’s installed in.

This brings us to the next topic, whether to mount the shocks in the traditional outboard position, with the outer end on the lower A-arm,
or to use pushrod suspension. The traditional position is simple, easy, light, and easy to adjust. Using pushrods means more joints, a
pushrod, rocker-arm, and potentially burying the shock inside the body where it’s hard to access. Pushrod suspension came from Formula One where they had to get the shock and spring out of the 200mph air blasting over them. It allows putting them just about anywhere, which is important due to underbody airflow management. We have no such problems! However, many people think pushrod suspension is cool (and it is) but is the added complexity worth it? No… and yes. Technically it’s not, and completely silly, adding unnecessary complexity and making adjustment a┬ápotential pain. BUT, what overrides these objections is the $1800 we save. It means we have to compromise our “technical reasoning” on a cost basis.

This reminds me of something Dave Norton (Shrike designer) said about push-rod and rocker-arm suspension, “Just because you CAN design something doesn’t mean you Should design it.” He was referring to the added complexity versus what it does for performance. I have to agree.

Because of the complexity I had no intention of using pushrod suspension until I found how inexpensive motorcycle shocks are. Due solely to pricing, it’s probably the way to go. It does mean adding two rod-ends and a rocker-arm bearing. The one problem with designing in specific motorcycle shocks (and writing a book that uses them) is that over time, that particular unit will become hard to find. To offset this, I’ll choose one from a popular (therefore, common) which will hold off the obsolescence for a long time. Lastly, the ride height adjustment on the shock is unusual, using a ramp-type ride height adjuster instead a threaded collar. This could present a problem, BUT, push-rod suspension lends itself well to adjusting ride height elsewhere, by changing the length of the push-rod. So consider the decision made – probably.

11 Jan 2008

I realize I haven’t described what Midlana looks like. While the details are still squishy, it’s basically a Lotus Super Seven with the passenger compartment pushed forward enough to allow a transverse drivetrain behind the driver. It’s not original (what is?) as there’s one manufacturer in England (Sylva) that produces one now, the Riot. Will Midlana be a knockoff of that, no, though all these cars are a knockoff of something. No, it’ll be its own thing, but will use commonly available composite so the builder has little or no messy, stinky composite work to do. It’s going to be “buildable”, easy to maintain, and inexpensive.

Thanks for all the comments regarding seat placement, sorry I couldn’t respond to everyone separately. I think I’m going to stick to the traditional side-by-side placement. It’s a combination of reasons: routing the seatbelts, not getting undue attention from safety inspectors, motor vehicle registration people, and later, cops, and, being able to hear, say, a driving instructor. From a selfish point of view, it’ll be much easier to get it through registration in California if it looks like a “normal” car. Then there’s the “too different” aspect; I’d like this car to be something people <i>want</i> to build, and the center seating might be a bit much. I know I’ll never make everyone happy, but alienating a huge group right off the bat’s probably not a good idea!

Engine choice: The builder will have a decent-sized bay to install whatever they want (with limits). Everyone has access to different engines due to differing budgets and regional availability. Then there’s form factor. There are many transverse FWD drivetrains out there, all with similar layout; that’s the type that Midlana will accept. Of course there are other engines like the Subaru flat-four: low, light, with a “real” transaxle. The problem is that the transmission tail shaft sticks out quite far past the axle centerline. That’s of no consequence if the car’s designed for it, but it’s not. A transverse layout is shorter, front to back, which packages much nicer, and there’s the rub. There are far more transverse drivetrains out there than Subaru drivetrains, so I didn’t want to force builders to use only one engine brand. (Technically the engine bay could be made large enough to fit everything on earth, that it gets out of hand. This is the consequence of following a cookbook; you’re stuck with the designer’s vision).