This is a twin-disk clutch and lightweight flywheel combo from ACT for Chevy LS applications. The trick in making this a direct bolt-on application to replace the stock flywheel and clutch is the stack height. As the clutch is a twin-disk, it is naturally taller than a standard single disk clutch. So ACT basically recessed the clutch into the flywheel a bit to reduce the overall stack height. On the flywheel, you can see where material and weight was removed from the outermost portion. As rotational inertia is a function of distance from the centerline of rotation to the second power, removing weight from the farthest areas possible has the greatest effect on reducing rotational inertia.
Whiteline had this full 2015 WRX rear subframe and suspension setup on display with their goodies bolted on. As you can see, there is an entire assortment of bushings, endlinks, and sway bar to replace the factory squishy components designed for better NVH and compromising handling performance.
In this shot, you can see crossmember bushing insert KDT938 going between the rear subframe and the unibody chassis. Why do you want less compliance here? Well, the suspension is connected to the rear subframe. If the rear subframe is allowed to shift relative to the chassis, then the rear suspension also shifts relative to the chassis. Keep in mind the front suspension is connected to the front of the chassis. So in the rough and tumble driving, the rear suspension will be shifting relative to the front and chassis creating less predictable handling. Less squish in the suspension creates more predictable handling and Whiteline has you covered. Having spent some time recently under the rear subframe of Project S2000, the design of the WRX rear subframe and suspension arms caught my attention. On the S2000, the rear control arms are all cast. Here you can see on the WRX, they are stamped sheet metal. Pretty much the whole rear of the WRX looks to be made from stamped sheet metal which is then welded together. Stamping of the sheet metal allows for some complex shapes as you can see in the rear subframe.
Adding bends to sheet metal adds stiffness. On the lower arm composed basically of two pieces of sheet metal bolted together, you can see the 90 degree bend of metal at the tops of the plates to add stiffness. If you remember my folding a sheet of paper test to describe the CSF Radiator B-tube technology, the same applies here. If not, a flat sheet of paper is all flappy. Now fold it in half; the paper is now stiff and resistant to bending perpendicular to the direction of the fold. To go further, make a U-channel and it gets even stiffer. Notice many of the stamped parts of the subframe have basic U-channel shape to them and then welded to another U-channel shaped piece created a fully boxed and stuff part which is hollow and lightweight. Checking out the Whiteline parts, the endlink connecting the rear sway bar to the lower control arm looks to be adjustable to work in conjunction with the adjustable rear bar. The bearing used where the endlink bolts to the lower arm should make for stiction free movement. On the OEM engineering side, check out the bottom of the hub where the lower arm attaches; it appears to use a solid bushing to eliminate any squish. I haven’t driven the new WRX yet, but they say it’s the first WRX to rival the Evo in terms of handling and details like that solid bushing contributes to the improved handling.