|January 9, 2009 - Here's a rough approximation of how the upper mount will be supported.|
A section of the same 2"x3" square tube that makes up the chassis rails will go up and out, and extend beyond the upper arm mounting to provide the upper shock mounting location. Some fore/aft bracing will be added later, and I'll run something from side to side at the top if the engine allows. The base of the bar will also move closer to the center of the frame rail, this is mostly just a convenient place to sit it right now.
I could mount the bar like this, with the upper arm mounting tube welded to the side of the bar. That gives a slightly better location for the shock mount.
Another option would be to angle the bar more and run the tube through the center of the bar. That would be stronger for the tube, but less ideal for the shock mount. Since the shock mount is going to get big loads (the vertical loads from the car go through it while the upper arm mostly has to deal with some side loads and a forward load on braking), I'm thinking the first option might work better.
I'm starting to wonder if I'm going the wrong way here. Should I have the frame rails kick upwards after the firewall, supporting the lower control arm mounting points with a crossmember that dips down under the engine? I'm worried about twisting loads coming from that upper shock mount located so far away from the rail. I could tie into the body, but there's not really much there. Then again, the Miata doesn't have a whole lot of metal in that area and there are no side loads on the shock due to the double wishbone configuration. Meanwhile, the lower control arms take the brunt of the cornering loads from what I understand. Hmm.
tags: planning, subframe, suspension