TWR Vented Inboard Rear Brake Rotors
Heartfield's final drive unit is a Dana, which is widely considered inferior equipment; most other XJ's have the better Salisbury/GKN unit. One difference is apparent in those bolts that hold the brake caliper to the differential housing; on this Dana unit, the bolts are installed from the outboard inward, requiring a hole in the disc for access to the bolt (the TWR doesn't have such a hole, though, so you must separate the outer disc from its hub to disassemble). On the Salisbury/GKN unit, the caliper mounting bolts are installed from the inboard side outward.
This second picture seems to show a "step" in the face of the outer disc, but Heartfield says this is an illusion; the outboard surface of the outer disc is totally flat, and that circle visible is merely a light polishing from turning the rotor around during fitting.
Heartfield's calipers are the standard item, except that they have been reassembled by TWR with a 1/4" spacer between the halves. This same method is used by most doing such a upgrade, since it simplifies the modifications and ensures that the brake effectiveness is not altered (which might screw up the front/rear balance).
Heartfield himself is responsible for the green paint.
This next picture was taken several thousand miles later.
Heartfield's vented rotor is 0.78" thick -- effectively 3/4", or
thicker than the baseline solid rotors. The hub, which is
a "hat" in the brake biz because that's what they look like on most
cars, is in fact a flat disc 3/8" thick. Since the "height"
from the inboard friction surface to the flat outboard surface of
or the height of the center when the rotor is set on a table) of
rotor is 1", the TWR arrangement effectively adds about 1/8" to
of the solid rotor location.
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