Due to drivetrain placement and the overall shape of the car, there’s really only one place the shocks “want” to go, just ahead of the axles. The bottom of the shocks attach to the lower trailing link, but the proposed upper trailing links run right through where the shock body wants to be. While the shocks <em>can</em> go there they would have to lean inboard to miss the upper arms, getting too close to the drivetrain. Since it’s unknown what drivetrains will be used it’s best to provide as much room as possible. So, the upper links are returning to A-arms…
Mitchell’s uber-cool BumpSteer program made short work of determining the toe-links locations, resulting in 0.016 degrees of bumpsteer from 1″ of droop to 3″ of compression, or about 0.008″ toe-change measured across the face of the tire. (A big thanks to my suspension advisor buddy – it’s great to have someone to bounce numbers off of who’s been through this before!)
I hope this stuff is helpful, though perhaps a bit dull and boring; weeks of going round and round to get everything to work together. Sometimes this means violating engineering practices, such as the lower rear suspension arm having the trailing link intersect the lateral link part way along its span. The shocks will attach near the intersection, feeding a large bending load into the assembly. As mentioned, it’ll be dealt with by using large OD tubing and overlapping plates, not a perfect solution but a reasonable compromise. The realities of using existing suspension uprights and clearances with the drivetrain and body sometimes dictate a less than perfect solution. It’s all about compromise, which are all over cars, one-offs, and even production vehicles. As long as the reasons are understood and a safe solution arrived at, it’s okay, and allows moving on with the rest of the design. This reminds me of what someone said during the Kimini build, about “walking a fine line between doing it right and getting it done.”
FWIW, the long lateral links provide near-zero vertical roll-center migration in roll – within 0.001″ over +/-2 degrees.