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XJ-S Brake Modifications

// JagWeb // XJ-S Help // Contents //

 

XJ-S Brake Modifications

It is worth noting that when the XJ-S was raced competitively (a 4-speed manual was offered as a limited edition for this purpose), the rear brake rotors were always replaced with ventilated discs -- whenever it wasn't against the rules, of course. There are kits available to do this today (see XK's Unlimited, GT Jaguar, or Bob Greene Developments).

Most of us don't use the brakes that hard repeatedly (the stock brakes work fine once) but if you drive fast down mountain roads, take note.

Ventilated Rear Brakes

According to Chad Bolles, the rear brake rotors from a Maserati Quattroporte are ventilated disks and will fit the XJ-S -- and are even reasonably priced. It is then a relatively simple matter to fabricate a spacer to go between the halves of the stock calipers, and to revise the emergency brake setup a bit.

Jeffrey Gram installed ventilated rotors in his car. The interesting thing was his reason for doing so: to reduce the tendency for the brakes to cook the final drive seals. He expects that the use of ventilated disks, even if braking performance is insignificantly improved, will extend the life of his final drive unit.

Conversion to Ventilated Discs

A couple of items to note regarding this conversion. First, the lower swingarms may contact the wider discs at the bottom of their travel. This must be corrected, usually by some minor grinding on the arms themselves. This is described in the instructions with the kit, and doesn't seem to cause any problems -- this swingarm has plenty of strength to spare!

Second, the widening of the caliper to accept the wider disks results in the bleed screw being moved farther outward. It already tends to foul the exhaust pipe, and this mod may require considerable fiddling with the hangers to prevent banging. And, when done, it may make bleeding the brakes even more difficult than before. All of this is reportedly NOT mentioned in the instructions with the kit.

A note regarding the ventilated rear disk kit from GT Jaguar: At least one person who purchased this kit was far less than impressed. He had the following complaints:

  1. The rotors were two-piece, consisting of a cast iron ventilated disk bolted to an aluminum hub. As a result of the multiple mating surfaces involved, he ended up with a runout far beyond tolerances -- on a brand new installation. Much fiddling was required to get the disks true.
  2. The bolts holding the disk to the aluminum hub were too long and fouled the differential case, had to be ground off.
  3. The hub didn't fit the output shafts on the differential. Apparently the manufacturer knew this, and in the instructions suggested cutting on the output shafts to make the hubs fit!
  4. The kit didn't include a yoke for holding the emergency brake pads in place with the widened calipers, so the original yokes had to be cut and a section brazed in. Apparently it is possible to use the sectioned yoke without brazing it back together, but this owner didn't care for that idea.
  5. The general workmanship was unimpressive.

Judging from a picture in their catalog, the XK's Unlimited kit also appears to have a two-piece disk.

If you wish to install such rotors, perhaps the following procedure should be followed:

  1. Modify the aluminum hubs to fit the differential output shafts properly.
  2. Assemble the disks to the hubs using proper length bolts (or the original bolts properly shortened). Mark the disk and the hub so that it can always be disassembled and reassembled in the same position if necessary.
  3. Assemble the disk assemblies to the differential and measure the runout. If not within specs, remove the disk assemblies and have them turned without separating the disk from the hub.

An even better plan might be to perform steps 1 and 2, then reassemble the whole car. Drive it for a while to get everything settled in, and then drive it really hard a few times to get the brakes very hot. Then perform the runout measurements on the rotors, and if excessive disassemble and have them turned. This way, whatever distortion occurs within the first few times getting hot will also be corrected by the turning. It's a lot more work, though.

According to Jeffrey Gram, the ventilated disks offered by Bob Greene are one-piece castings.

Cooling Air Flow

The race cars also added an oil cooler for the differential. Taken together, this all means there is a lot of heat in that area. Perhaps the performance enthusiast would be well advised to improve the cooling air flow to this area. A scoop on the bottom of the car directing air up into this space might help, or perhaps some ducting is called for.

Brake Lines

Chad Bolles suggests that the brakes on the XJ-S or any other car can be made to feel firmer by replacing the flexible lines with steel-braided lines. The stock lines expand somewhat when pressure is applied, causing a spongy feel. The steel braid won't stretch, so the pedal feels more rigid.

 

On to Weight Reduction/Relocation

 


// JagWeb // XJ-S Help // Contents // 

 

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