8 Sep 2008

The rear suspension is slowly taking shape. True A-arms will be behind the drivetrain because trailing links didn’t work for several reasons, the main issue being the use of Miata rear uprights. Between the wheel offset and upright design, they’d had to angle inboard 30 degrees, fouling chassis tubes and the drivetrain.

The shocks will be in the traditional outboard position, tucked nicely into otherwise-wasted space. The installation ratio was the driving factor, where the initial try resulted in a value of 0.6 – bad news. Compared to a shock which moves the same as the wheel, a 0.6 results in springs having to be 1/(0.6)^2, or 2.78 times stiffer. This would be a real problem because there’s roughly 350 lbs sprung weight at each rear corner. A typical soft suspension starting point is to have the springs equal the sprung corner weight which would be 350 * 2.78 = 972 lbs springs, rates that are impossible to source, never mind that springing the car for the track may double that value. I managed to get the installation ratio up to 0.75 but it’s still marginal.

The Miata rear upright has two pickup points at the bottom, inline with each other such that a long bolt passes through both. It’s natural to use rod-ends, the problem being that the spring force bends the rod-ends radially. A spherical bearing in a welded-in cup will be used, with the other being a rod-end to allow adjusting toe; the upper joint will adjust camber.

Oh, and just for fun I moved some tubes around in the rear area to create a diffuser, much like Kimini’s which made the car feel very stable. Of course Midlana being open top will be – to coin a currently-popular phrase – like putting lipstick on a pig – but why not.

The free drawing program Google SketchUp is looking better and better after discovering plugins that make it more than adequate. It’s still going to be a lot of work, but that’s true of any CAD system.