Does it come and go in cycles? Does it change or get better with time?

If it cycles in and out, 90% chance it's the wheels / tires. One wheel is going to be turning faster than the other, and as the defects go in and out of phase you get different types of vibrations. I tend to lock up my fronts in braking pretty bad at least once per season, which flat spots the tires, so I'm especially experienced with this phenomenon. ;)

I'll also second grabbing your front tire and trying to shake the heck out of it. There could be a not-apparent play in the strut housings, a loose bearing, a worn ball joint, a dying camber plate, etc. It might look totally fine static but loading it up could show otherwise.

I know you said you tried two sets of wheels, but it doesn't mean one of those was definitely good. As stated by someone above, jack up the car and spin a tire. What you're looking for is a "hop" or irregular movement. A wheel can balance out perfectly and still do this hop. Something about manufacturing tolerances and an unlucky alignment of the wheel and tire when they are mounted. I had one wheel / tire combo where the tire tech showed me this, said there was nothing he could do. Mid season I had the tire flipped on the rim - same wheel, same rim - and it was fine.

Another thing: after my car sits all week, some brands of tires develop sort of flat spots. I get a similar vibration / shake for the first several miles. Once the tire heats up a bit, it works itself out.

I'm a fan of methodically trying to figure out the issue at hand rather than blindly throwing money at a problem. Take your time, think about everything, eliminate one by one, and you'll solve it. Good luck.

How much movement is any? Does 1.5 mm equal enough movement to cause the type of vibration the OP mentions? I seem to be experiencing the same type of vibration.

My car came with curb rash on the right front wheel.

In the first photo the rim section is good, and there is no space between the wheel and the white 'pointer'.

In the second photo there is about 1.5 mm space where the rash is worst. The 'best' local tire/wheel/balance shop checked this and said that it should cause no issue.

No space.
http://forums.pelicanparts.com/uploa...1461251176.jpg

1.5 mm space.
http://forums.pelicanparts.com/uploa...1461251205.jpg

I am having the same issue. Sometimes the shake is there and sometimes not. Sometimes at 40 sometimes at 60 its very odd and frustrating.

I was about to step up to the turbo tie rods thinking that was the issue.

If I play with the tire pressures it feels better.

The simple and for sure way to figure it out is find another PCA member in your area with a SC or Carrera. Swap wheels for a short drive. If the problem goes away, you will know for sure that is the problem.

My 911 is super sensitive to the wheel shake. I finally found a shop that the owner will do the final road force balance on my wheels. There in zero shake at any speed. I just accepted some shake for years because the other shops just could not get it spot on. I have weights on the inside and the outside of the wheels as needed.

You may not know if it is the wheels, or the tires; you will only know that it involves one of them.

My .02, tires have flat spots. You say in first post that you replaced wheel bearings 500 miles / 3 years ago. Sounds like the tires are sitting on the same spot for extended periods resulting in flat spots.

good luck!

Jerome
no expert on how much is acceptable.
I did ask my wheel guy and he said there should be no movement,
my opinion on yours? I doubt you would feel that if that is the true amount it is out. perhaps not till around 100mph.
you really need a dial gauge or you have to find the hi spot and the low spot the way you are doing it.

my understanding is vibrations that come and go are either balance or bent wheels.

the inexpensive tires "we" buy these are really cheap and do flat spot easily.
I put Kumhos on my Mercedes and they would flat spot over night. 5 miles or so and you could not feel it anymore.

still fighting my BMW vibration.
now I feel in the seat.
going to check the rears tonight and if they are good will look into driveshaft.
I don't know what an out of balance drive shaft feels like.
my front is much much better now that I got that bent wheel off but it needs shocks.

the other thing to remember, the 911 has a LOT of input back into the steering.
my 930 has a slight vibration but well over 100.

Right, I didn't have a dial gauge at that moment to measure the wheel's irregularities, but I recall that it seemed pretty 'flat' except for the scuffed part being very slightly pushed in. Therefore, I agree that this small section of pushed in rim is not likely the culprit in this case. Actually, I did measure more wobble in the tire's tread when I set up a more accurate measuring method, and it wobbled in a different area than the location of the wheel scuff.

So, I'm going with a tire being the culprit. They need to be changed anyway. I'll be swapping out Yokohamas for Continentals. I hope to do that in the next couple of weeks.

Thanks for the input.

If you're going to put on new tires anyway, I'd have somebody check, true and maybe refinish your wheels while the tires are off.

Steering wheel shake
 

If it was my car, I would jack up the front end and check the ball joints by holding the tire at top and bottom and pulling and pushing to see if there is any movement. Then, I would do the same thing holding the front and back of the tire to see if the tie rod ends were loose. You can also check by moving the steering wheel a small amount while looking to see if the wheels/tires move. Another thing that can cause your problem could be the toe setting being incorrect. The first two you can check at home, the third you will need to take it to an alignment shop unless you have a set of alignment tools. Good luck!

My '82 SC does pretty much the same thing.

I've purchased new tires which were well balanced, had the whole car aligned and corner balanced, swapped wheels and tires with another car, repacked wheel bearings, installed a Leyland column bushing, checked ball joints & tie rods for play, etc. and the shakes still come and go from time to time when I'm between 50-70 mph. Sometimes when the tires are cold sometimes not. Not sure what my next move is but I'm a little skeptical of all these claims to how "sensitive" 911s are to balancing. I've owned a lot of cars and driven cars with seriously bent rims and thoroughly trashed suspensions that still didn't exhibit the type of shakes I'm experiencing on a 88K mile car that has been meticulously maintained. Seems a little primadonna to me, it's just a car with normal car parts so there must be a root cause.

One thing I will offer is when I push down on a front fender there's a little bit of a metallic "clunk" with one side being worse than the other. It sounds like it's higher up near the fender rather than down low in the rotating parts. I'm starting to suspect something with the strut inserts, people have mentioned a bushing in there that can go bad. The car also tends to pull to the right under braking, but not all the time. I've replaced hoses and checked the caliper for drag which there is none.

Anyway it's at least comforting to know others have similar issues!

cheers

The OP hasn't chimed in for a while, but I'm sure his issues are the same as what is being discussed now, since they seem quite common.

My car passes all of the pushing, and pulling, and shaking, and shoving up and down tests that have been recommended, and everything seems to be tight - with no other unexplained noises, either. It has to be in the wheel/tire.

For me, the problem with having a shop check, and true, and refinish my slightly irregular wheel is that there is no such shop within 100 miles (160 km) of here, if there is one that close. I'll try to find that kind of shop, but it won't be easy.

There is a safety feature in all of this. The car's speed is self-limiting due to the vibration, thereby forcing you to drive at a legal pace and avoid getting seriously killed. Boring, though.