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Jaguar XJ Inboard Brake Upgrade - TWR

Jaguar XJ-S

TWR Vented Inboard Rear Brake Rotors

Alan Heartfield's 1985 TWR XJ-S came off the showroom floor with vented rear brakes utilizing a two piece rotor.  Since the custom hub is not a wear item, when the discs are worn all he needs to replace is the outer portion of each disc.  In 2000, Heartfield had to do such maintenance, and took some pictures of his brake assembly.  These two pictures show it during reassembly with new discs from Essex.

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 1/4" thicker than the baseline solid rotors.  The hub, which is called 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" (distance from the inboard friction surface to the flat outboard surface of the hub, or the height of the center when the rotor is set on a table) of the solid rotor is 1", the TWR arrangement effectively adds about 1/8" to each side of the solid rotor location.


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