Experience
in a Book
Brake
Modifications
REAR BRAKES: 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.
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.
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.
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.
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:
- 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.
- The bolts holding the disk to the aluminum hub were
too long and fouled the differential case, had to be
ground off.
- 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!
- 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.
- 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:
- Modify the aluminum hubs to fit the differential
output shafts properly.
- 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.
- 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.
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
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