14 Aug 2008

Today the decision was made on where and how the rear shocks will mount.

There’s going to be rocker-arms at the rear. Not because it’s ubersexy, just that it’s the most appropriate solution. Why? The Miata rear upright has its upper suspension pickup cushioned in a big rubber bushing. Putting a screwdriver through the bushing and pulling it from side to side shows that it’s fairly soft. I was going to mount the bottom of the shock to it, but applying an offset load of ~400 lbs to it isn’t a good idea. A solid bushing could be substituted for the rubber, but one design goal is to avoid lathe-work. On top of that, the steeply-inclined shocks gave too low an installation ratio, requiring spring rates higher than what’s commonly available in 10-12″ springs. They were also too close to the engine and exhaust for comfort, and that’s on my drivetrain. Who knows how close they’d be to whatever drivetrains builders are going to install. Tilting the shocks to be more upright wasn’t an option, either, because they’d stick through the engine cover with nothing to attach to. I would have attached the shocks directly to the lower suspension pickup point on the upright, but they’d would have to hang off a bolt in single-sheer about an inch out… won’t go there.

The best solution appears to be to keep the shocks away from the drivetrain and keep them low. While putting them behind the drivetrain makes them accessible, the exhaust is back there, too, cooking anything close by. For these reasons they’re being placed vertically, about a foot forward of axle centerline.

Speaking of the rubber bushing, if a single rod-end is bolted to one end, it’ll twist the bushing. If a U-shaped bracket is used so a bolt passes all the way through the assembly (like a Miata) then it’ll both bind slightly when toe is adjusted, and move “some amount” during cornering. I’ll probably leave it as-is until I figure out how much of an issue it is.