vibration--still can't find it
 

OK, ongoing symptoms: when warm, a drone eminates from front end starting at about 65. Low pitch. Under light to moderate braking, a vibration (not shimmy) comes through wheel and seat. Drone and vibration cease with slowing.

It comes and goes, which makes it more difficult to pin down.

I've done discs all around, pads, lines, fluid, high speed balance. Tires (Dunlop Direzza's that have been tracked) inspect OK.

Where do I look next? Some odd bushing somewhere?

Front wheel bearing ?
Even though you've had your tires inspected, i'd find a buddy so you can swap out the front tires just to verify.

More data: seems to occur after taking fronts off and putting back on. When it occurs, and after slowing, stopping, parking car for a bit...and then trying to duplicate, it doesn't happen.

I am doing an AX this weekend with both a wheel change at the event and a 60 mile drive to and from. Hopefully will find some more data points.

I recently rebuilt the calipers as well. They don't drag when hot or cold near as I can tell (car rolls easily, and when wheels are off, the hubs spin with minimal drag both hot and cold)

bearings was my first thought now maybe warped rotors or bent rim.
if you have a lot of play in the steering wheel, replace the bushing in the steering column and check tie rod ends

Bearings or tires. I would lean toward the bearings.

+1

I would too but I have thrown away about 4 Direzza Star Specs that came apart in different ways on my wife's 335 BMW, all with over 1/2 life left. Bubbling, splitting, leaking POS. Never again.

^^^This. I was thinking "tires" reading your original post.

This. Or bent rim. The slightest bend in the wheel can cause these symptoms.

vibrations
 

You might want to check your front rims by having them rotated and measured accordingly.

Is one wheel noticeably warmer that the other?

I had a similar situation on a BMW, and after a lot of trial and error measures, I finally discovered it was a sticky caliper that would grab the rotor just enough to start to get the disk hot. Once that started happening it started warping the rotor etc.

That problem presented a lot like what you are describing.


Good luck!!

Take a look at this from over on the 930 Turbo Forum;

http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/780083-when-time-new-motor-mounts.html

If you can't pinpoint your problem to wheels/tires or other rotating components it gives you another place to check.

I had a local specialist wheel shop take the fronts off and spin them; no noticeable bends/warps etc. I've had the wheels for several years, no curb bashing under my watch. I've had two set's of tires on them with this problem not occuring.

The brake calipers were rebuilt when I did the rotors/fluids/lines a month or so ago. I did that job because at the track under heavy decel I got a HARD shuddering and the front rotors were at the limit and there was lots of drag on the rotors.
My capable tech thinks it's possibly still an old caliper problem, even with the rebuild, considering it is an inconsistent problem.

The wheel bearings are adjusted properly and have only 7500 miles on them and 4 years. I'll inspect, but my thinking is a wheel bearing would be a consistent issue or come with some wheel play, of which there isn't any

The wheels are not hot under normal driving, nor is there a heat differential.

I'm starting to lean tires.

wheel bearings will make noise. usually more noise when turning. (left or right)
if you have a vibration, look at the lower ball joint and tie rod ends since you have had everything balanced and checked for warpage.
what about alignment? not sure if bad toe setting would cause a vibration.

have you checked the rears? still sounds like a warped disc

Check your parking brake adjustment and shoes inside the rear rotors. If there is the slightest contact with the shoes the car will vibrate after a mile or so then get more pronounced while driving and when you stop and they cool down the vibration disappears. One last point, the vibration seems to come from the front when this happens.

on rebuilt caliper are the piston are 20 degre angle just thought.

oooh, I like the parking brake angle. That would also square with the "changing tires at AX with parking brake set and then getting vibration after 20 miles on way home" scenario I've been experiencing.

Putting it up on the rack this afternoon to pull wheels and take a squiz at bushings, etc.

Alignment: car tracks true, and turns in crisply when pointed. That being said, it is pushed at times. Worth a look.

Cheers

I had a similar problem with my truck. It turned out that although the services front callipers worked perfectly, the pistons of bores were beyond tolerance and the piston would be allowed to cock slightly sideways in the bore causing it to vibrate and moan when off the brakes and shimmy when on the brakes.

Just a thought.

OK, so being up on rack revealed no issues with tie rods, bushings, play, etc. Tried to avoid setting parking brake.
Drove to AX venue 60 miles away this AM at highway speeds. No issues. Swapped tires, drove event, but set parking brake once inadvertently during tire swap. (I have chocks, so it's unnecessary, but habits...) Drove event, no issues with braking, vibrations, etc.
Swap tires back. Drive a bit and put gas in car. Start up the highway, 20 minutes into it.. .AAAAAGGGHHHH here it comes again. Car is tracking straight, I try some wheel inputs to see if it changes anything, no dice, I apply a bit of parking brake while under foot braking vibration, I definitely feel more vibration but that feels like it's coming from behind me (a bit different then the shudder in the wheel when I touch the foot brake) I slow down to lesson the vibration, it fades....and disappears for the rest of the 30 mile trip at north of 70 MPH.
Man, do I have to a)buy new calipers and try that, and if that fails b) buy new front tires and if that fails c)completely rebuild the hub assembly? (This is of course after I take a harder look at the E brake situation.)
The rotors look completely even, and the car rolls easily. I guess it's possible that at speed one caliper is binding a bit.
Any other ideas other then the GREAT inputs I've already got, for which I thank you all?

Caster?