UFO Brakes 101

["Paul C. Waterloo" <74543.407@compuserve.com>]

>From two previous posts, BTW, the're called UFO's because the steel hub
looks like a UFO saucer:

After looking at many cars with UFO's, I still could never understand what
they actually looked like. I could easily identify them with the large
metal discs in them, but could never figure it out.

Well.... had my car two days and had to take the wheel off to look at them,
here's my description:

I think they should of called them INSIDE DIAMETER CALIPERS vs. internal
calipers, it would make more sense to me.

The disk that you see attaches to the wheel hub just like most brake rotors
do, they use the wheel bolts holes to align/secure them to the hub, and
they have another small hole with an allen head bolt to secure them in
place when the wheel is not installed.

The disk then protrudes from the hub out to the outside diameter of the
rotor. You can see most of this when looking through the wheel, especially
on the BBS mesh style wheels.

The disk attaches to the rotor at the OUTSIDE DIAMETER of the rotor, where
the rotor has the vents between the two side plates of the rotor. It is
attached around the complete outside diameter of the rotor.

The rotor is actually only a donut plate that HAS NO PORTION OF THE INSIDE
DIAMETER OF THE ROTOR ATTACHED TO THE WHEEL HUB. The disk on the O.D.
provides this support.

The brake caliper is then set up on the INSIDE DIAMETER of the rotor,
versus the outside diameter, like most cars. In other words, the portion of
the rotor that the caliper stradles is on the ID of the rotor versus the OD
of the rotor, hence the weird set up of the rotor.

They do look like they are really pretty heavy duty, my car seems to have
awesome brakes. The car has 90 K on it, and I think it has the original
rotors on them. The discs have dates of 10/90 on them. The last owner
didn't change them.

IS ALL OF THIS CLEAR AS MUD NOW? Hope it confused you a little bit.

---------------------2nd post-----------------------------

After the recent discussions about UFO's, I looked them up in the Bently
for the first time. Couple interesting things to report:

Brake Caliper - the outline drawing says "cannot be repaired, replace if
necessary"...that seems kinda weird, you can't overhaul them? Anybody ever
try?

There is a section on "Steering Wheel Shimmy During Braking", it has three
things you can do to fix it (they are outlined in this order):

CHECK CONTROL ARM BUSHINGS
1. Remove wheels, disconnect brake pad sensors, remove front pads, remove
calipers, but do not disconnect.

2. If you have less than 40K on your car, inspect the inner and outer
control arm bushings for cracking, splitting and deterioration.

3. Replace as necessary.

4. If you have more than 40K on your car, replace both front inner and
outer front control arm bushings. For those who don't know what a control
arm bushing is, it's the bar that the front sway bar and lower ball joint
are attached to. The bushings that they are talking about are where the
sway bar goes through the control arm, and where the control arm attaches
to the subframe (I think subframe is the correct term) of the car.

TRUING OF FRONT BRAKE ROTORS

1. True front brake rotors with an on-the-car brake lathe.

2. Note: if an on-the-car brake lathe is not available, replace rotors.

3. FYI - brake disc thickness new 25 mm (0.750"), brake disc wear thickness
- 23mm (0.690")

WHEN NEW BRAKE ROTORS ARE REQUIRED

1. Thoroughly clean contact surfaces of brake rotor and hub.

2. Install using all lug bolts with suitable spacers.

3. Tighten lug bolts diagonally in stages to 29 lb-ft.

4. Attach dial indicator to measure brake rotor runout.

5. Rotate rotor and record runout.

6. Remove brake rotor and remount after rotating rotor to next lug bolt
hole. 

7. Check rotor runout.

8. Continue remounting rotor checking runout at each lug bolt position.
        ---- Maximum rotor runout 0.06mm (0.002")

9. Mount rotor in position where lowest runout was achieved.

10. Mark hub and rotor with center punch as mounted in lowest running
position.

MORALE OF THE STORY

>From what I can see, if you car has front end shimmy, I would first check
brake rotor runout and determine what it is, maximum new is 0.002", I
wouldn't want to go past 0.004" or maybe 0.006"?

If rotor runout is acceptable, go after your bushings. If you are going to
go in and do a brake job, why not replace the bushings as a matter of good
maintenance practices. I doubt they cost very much, especially when
comparing them to the price of rotors and pads.

This procedure will help you determine the root cause of the problem so you
can then fix it!

Paul

*********************************************
Paul Waterloo
Brookfield, IL

91 200q

EMAIL: 74543.407@compuserve.com
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