Can't Diagnose 65mph Shake

[Countermoon]

Hi all. In August, I had new rotors put on my '07 Cobalt LS. When I drove away from the shop and got up to 60mph, the car shook violently. I could barely hold the steering wheel and the front end felt like it was going to fall apart. I brought it back to the shop and they re-seated the rotors and balanced the tires. It was better but by no means fixed.

Here is what I've done since then in an attempt to fix this shake:

-Tire rotation and alignment;
-Balance (3 times);
-New front wheels as they were slightly dented;
-New front wheel bearings;
-New bushing in the lower control arm, the old one was blown out.

My primary mechanic checked a few days ago and saw both front tires hopping around at about 60mph. The two rears are fine. At speed the passenger seat is shaking visibly, the wheel vibrates, and the glovebox rattles. I can also feel the vibrations through the floor and the gas pedal.

I am at a complete loss here, as are the three different shops who have taken a look. My best guesses at this point are tires (unlikely, though, as they've been rotated and issue persists) or a suspension issue. But this came on suddenly without any prior indication of a problem, and it happened on the day I had new rotors installed. Is it possible that mis-sized rotors could cause this problem, even though the shake occurs when I'm not applying the brakes?

Thanks for your help guys, hoping someone has encountered this very problem...

 

[onedoughboy]

You must be using the wrong three shops if they can't figure out your shake. Find a new place to take it. Or take it back to the shop that fixed it when the shake first started and shame them into FIXING it correctly. Maybe even try a GM dealership. There's not much to these cars, sounds like you changed everything but the cv axles and the tires. Both could make the car shake. Anyone with half a brain can spot a bad tire, bad cv axle is a bit more tricky. They should have put your car on a hoist and ran it up to 60 mph, with the wheels off the ground, supported by the control arms. Any shake would be obvious. Tire run out would be obvious. I don't know why mechanics think they can charge so much, when they are so lazy.

 

[bluebaltz]

i'm thinking cv axles too! i've seen similar issues on other vehicles, both fwd & 4wds