Originally Posted by WERK-I

Does the groaning occur only when cornering or does it occur under moderate to heavy braking? If it groans under braking, it is a good indicator that the wheel bearings are not adjusted properly, i.e. too loose in the bearing races, relatively speaking.

Originally Posted by Gary R.

Have you checked your wheel bearings Dave? Considering they are what keeps the rotor turning true it's something (relatively) easy to look at..

Originally Posted by WERK-I

Dave,
I know this a dumb question, but I gotta ask.

You say it's groaning when turning into the driveway or parking lot. Are you transitioning the car onto an incline?

Also, I suspect you only see the pulling to the right and right front lockup under heavy braking right? If you are coasting in neutral and apply light braking, there is no pulling, correct? The braking I refer to is light as not to cause to much weight transfer forward.

Originally Posted by PorschePhD

Shifting under braking could also be caused by a worn A-arm bushing, upper moutn bushing or Ball Joint. Lastly even the dist block for the brakes could cause it.

Originally Posted by Dave 86 930 Fl

Dave,

Yes, the car is transitioning onto an incline in my driveway.

Front right lockup is under heavy braking. I'm not sure if the car pulls to the right while in neutral under light braking ... I'll follow up on this question Saturday. You seem to know the symptoms, do you have a cause in mind?

Regards, Dave

Originally Posted by A930Rocket

How old are the tires? Older tires are more likely to lock up because they are harder.

If you have changed pads to a different type, rebuilt the calipers, replaced the brake lines, bleed the brakes, checked your hard lines for crimping and damage, checked the dist block, rotors, wheel bearings I've run out of ideas.

Originally Posted by Dave 86 930 Fl

I measured the rotors this morning, consistent thickness on inside and outside, overall thickness the same right to left.

Regards, Dave

Originally Posted by Rob S

Dave,

I'm interested in your problem.

Since you did all this work, have you confirmed that the right side is still generating more heat than the left? Is there any way you can try to quantify this for purposes of diagnosing the problem? (A pryrometer, maybe?) What I'm thinking is this: if you really are getting more braking power from the right side,it should show up as heat. If so, then there's still something wrong with the brakes. I suppose that if you had a bearing so far out of adjustment that the rotor was dragging on the pad, it could cause heating on that side. And if it were extreme, it could cause there to be a differential in the coefficient of friction (COF) from side to side, which would manifest as a uneven brake torque. Likewise, if you had a piston that wasn't retracting properly, it could drag a pad and heat one side to cause the side-to-side differential. Are you sure you've checked or eliminated these possibilities?

Right side is generating more heat, I've not tried to measure with pyrometer, but my hand saysit's hotter. Regarding the front bearings, they were replaced 5/6 years ago and they seem solid. WRT the front calipers, I rebuilt the original set and there was no improvement, so I replaced them with new units and no change.

I don't know what kind of pads you're using. Are they new? Are they the same pads on both sides? (Hope so, but need to ask!). You probably know that different pads have very different COFs, and could easily cause what you're describing (say, if you had race pads on one side and street pads on the other). Also, the temperature vs. COF characteristics could be such that a hotter pad has a higher COF or a lower one -- it depends on what kind of pad you're using and how hot it is.

Pads are Mintex (sp?), Porsche OEM and they are new all the way around.

It sounds like you confirmed the brake fluid pressure was the same side to side (I hope you did that with two gauges and two people - simultaneously). Did you check for even pressure side to side at all pressure levels? How high a pressure did you go to? I would check it from low pressures (50 psi) all the way to 1000 psi or more.

Yes, I checked the pressure with a set of gauges and they have memory indicators that indicate peak pressure at the caliper. I tested pressures from 0 - 1000 psi.

What you really want to measure (but is very difficult to do) is the brake torque on each side. There are "torque wheels" for this purpose which I have used for vehicle testing, but the ones I used were very custom instruments and specific to the vehicle being tested (which wasn't a Porsche). They consisted of a strain-gauged hub that sent its signal to conditioning equipment in the car through slip rings or telemetry. I'm not suggesting you try to do this -- but perhaps you can think about some clever way to determine whether you are truly getting more brake torque on one side than the other. Certain brake dynamometers will allow measurements like this, but the ones I've seen measure the torque independently for each front wheel, which means you need to monitor pedal force or fluid pressure in order to make sense of the data.

I have not measure the forces at each individual caliper piston, but there are devices available for this purpose ($179). I may pick up a set.

You might consider devising a way to measure the clamping force between pairs of pistons on each caliper. If you had a load cell and used your brake pressure gauge setup, you might discover something of interest.

But we're really reaching here. Exorcism may be a good thing to try next.

I think this may be a valid approach, although pushing the car off the causeway into the Intercoastal would be more fun.

I wish you luck getting this straightened out. Please keep up posted.

Rob

Originally Posted by nathanUK '81 930 G50

I read the the whole thread again. I am thinking wheel bearings. How evenly worn are the brake discs ? Usually when a bearing is worn the outside of the disc wears alot more than the inside (centre).

I have a bearing failed in my 915 trnamission. It looks perfect from the outside including the outer race. All the rollers are perfect too. However when turning the rollers on the inner race the fault is with the inner race. It took me ages to find it. Nothing visible at all.

Originally Posted by WERK-I

Dave,
Yes, I'm thinking that the noise and the pulling to the right under braking is symptomatic of collapsed A-Arm bushings. The groan would be the torsion bar rubbing against the housing. Under heavy braking, there's a huge weight transfer to the front and if one side of the front suspension is weaker than the other, you'll get unequal weight distribution over the front axle causing one rotor to do more work than the other.
It could be as severe as a broken front torsion bar, but it would be visually obvious by the way the car sits.