Drivetrain squeak

[Gaz64]

Originally Posted by cycleguy55

The runout I'm referring to is normal, and at levels insufficient to be felt in the brake pedal. It's probably the extremely rare brake rotor that has zero runout at any given temperature, and even rarer for zero runout at all temperatures.

For example, here are excerpts from a Factory Service Manual for runout and rotor thickness variations:
Runout limit: 0.035 mm (0.0014 in) or less
Maximum uneven wear (measured at 8 positions): 0.015 mm (0.0006 in)

You'll note the maximum for uneven wear or runout isn't zero. IOW, some runout or rotor thickness variation is acceptable within limits.

While I've found references that would indicate each of us is correct, this would indicate it may be a bit of both: https://auto.howstuffworks.com/auto-...isc-brake2.htm

"the rubber piston seal and any wobble in the rotor may actually pull the pads a small distance away from the rotor"

Yes, that is all true, but note the max values in this case, about 1.5 thou, etc.

Watch a caliper grab a rotor, and then relax when the pedal is released.
That's the bore seals dragging the pistons back that fraction.
That's without turning a rotor.
I always aim for zero runout on car, and if indexing rotors doesn't achieve within tolerance, then an on-car disc machine is advised.
I've seen many cars come back after some low miles with brake shudder from a previous poor brake repair.

Now back to original topic.

Gary

[fintubi]

OK, here's some material new information:

1. As expected, the loose axle nut wasn't the cause. The squeak still appears now that it's fixed. But I'm darn glad I found it!

2. I'm realizing that time at speed alone doesn't precipitate the problem. It appears *after* going from high- to low-speed driving. That got me thinking brakes.

3. More testing confirms the issue is worst when "floating" between accel and decel, persists during light braking, and goes away during heavier braking.

4. Most important observation (in retrospect, this is a great test): when the squeak is being really pronounced at ~30mph after a recent run at higher speed, lightly applying the handbrake (just dragging it a little) makes it go away. Every time. Completely reproducible.

So here's my hypothesis: A braking event that puts sufficient energy into the rear rotors (e.g. slowing from highway speeds to a stop) causes them to grow just enough that one of the parking-brake pads is dragging intermittently with rotor runout. That also accounts for the issue fixing itself with continued driving - if the driving patterns allow the rotor to cool back down.

I'd originally rigged the parking brake with a little bit of cable slack, because the return mechanism at the rear isn't super powerful. But I need to double-check that it's doing everything it can to release the brake - perhaps some lube on the pivots.

I'm fairly certain at this point that there's nothing wrong with the diff, rear wheel bearings, or front end based on test (4) above.

Bill

[kevins2]

My car also has the Jag rear and, when the car was new, the emergency brake squeaked intermittently just as you described. If I pulled up the brake handle just a little the squeak would stop, so that confirmed it was the pads. I wanted to avoid tearing into the rear if I could so I tried making a number of stops using the emergency brake, not overdoing it so as to overheat or glaze the pads. That worked. The squeak did return after a while and the same process stopped it again. After a few cycles of this it never came back.

[patrickt]

Well this would be a perfect time to upgrade to the outboard braked rear with the nice big PBR calipers and an emergency brake that actually works.

[kevins2]

My emergency brake works well, thank you very much. It just went through a squeaky phase.... kinda like my kids did when they were young.

[DanEC]

Mine works well too on my Jag rear end. Pretty good mechanical leverage in the e-brake system.

[t walgamuth]

If there is a dust shield on that car it might be rubbing.