Not my strong subject (is there one?) but it seems to me that to relieve its load the pinion gear tries to move backward (toward the D/shaft).
This pulls the tubes with it, so a back brace working in tension (stretched) would be useful.
I can't see what force the front-mount turnbuckles would counteract, since they work in tension and are nearly useless to resist bending forward.

An aside: unless there's space/package problem, the farther behind the axle the rear brace extends (like a "V") the stronger it is, which may allow less wall thickness or skeletonizing the walls to save weight.