940 brake vibration

[Tony Stanley]

I changed the disks on my 1995 940 Turbo SE Estate front and back over a
period of a few months. They seemed OK at first but now I am noticing a
pronounced vibration under braking at lowish speeds with the steering wheel
oscillating back and forth.

Are these brakes suseptable to warpage and I need to select a better
supplier, or could I have assembled them poorly? I did clean up the
surfaces before assembly. Could it be something else?

I have modified the turbo for more power, and may have been driving the car
quite hard at times.

 

[Stewart Hargrave]

From a point at sea, to the circles of your mind, this is Tony
Stanley:

I changed the disks on my 1995 940 Turbo SE Estate front and back over a
period of a few months. They seemed OK at first but now I am noticing a
pronounced vibration under braking at lowish speeds with the steering wheel
oscillating back and forth.

Are these brakes suseptable to warpage and I need to select a better
supplier, or could I have assembled them poorly? I did clean up the
surfaces before assembly. Could it be something else?

I have modified the turbo for more power, and may have been driving the car
quite hard at times.

People here have suggested that over tightening the wheel nuts
promotes disc warpage - I've never had a problem on my 740. But warped
brake discs are more likely to be felt through the brake pedal than
the steering. I think I'd be having a look at wheel bearings or track
rod ends.

Jack the wheel off the ground and grasp it at 6 and 12 o'clock; try
rocking it. Ideally you shouldn't feel any play. Don't confuse it with
steering or suspension movement. If there is any play, suspect wheel
bearing.

Do the same at 3 and 9 o'clock. If you can feel movement, have someone
put the foot brake on and repeat. If the movement is still there,
suspect track rod end. If it disappears, suspect wheel bearing.


--

Stewart Hargrave

A lot faster than public transport


For email, replace 'SpamOnlyToHere' with my name

 

[Mike F]

I changed the disks on my 1995 940 Turbo SE Estate front and back over a
period of a few months. They seemed OK at first but now I am noticing a
pronounced vibration under braking at lowish speeds with the steering wheel
oscillating back and forth.

Are these brakes suseptable to warpage and I need to select a better
supplier, or could I have assembled them poorly? I did clean up the
surfaces before assembly. Could it be something else?

I have modified the turbo for more power, and may have been driving the car
quite hard at times.

While it's possible that the disks are indeed warped beyond specs, what
could be happening is the minor (below spec) warpage that is normal can
be amplified if the bushings at either end of the strut stay (aka
caster) rods are worn.

 

[Stuart Gray]

While it's possible that the disks are indeed warped beyond specs, what
could be happening is the minor (below spec) warpage that is normal can
be amplified if the bushings at either end of the strut stay (aka
caster) rods are worn.

I remember reading somewhere, probably here, that heating up the brakes a
lot, then sitting on the brake causes transference of material from pad to
disk. Apparently if you look at your disk (both sides) you may see an
outline of the pad left on the disk. Sort of like leaving an iron on a shirt
I suppose. This can cause snatching. I wish I could remember the website.

Stuart.

 

[Mike F]

I remember reading somewhere, probably here, that heating up the brakes a
lot, then sitting on the brake causes transference of material from pad to
disk. Apparently if you look at your disk (both sides) you may see an
outline of the pad left on the disk. Sort of like leaving an iron on a shirt
I suppose. This can cause snatching. I wish I could remember the website.

Stuart.

Most of the time what I see is this effect caused by rust. Where the
pads are sitting is protected, the remaining unprotected surface rusts.
This is not a problem on a car driven every day, or stored in a garage,
but can cause a really uneven surface on an infrequently driven car that
sits outside, especially in the rear.