Rear tires are becoming cupped

Its a 2006 Jetta tdi, 255k miles. I got it at 190k, I doubt that any suspension parts have ever been replaced. The car is vibrating a lot above about 50 mph. I hear the sound of a tire not rolling evenly, but all 4 tires are good when checked on balancong machine that simulates actual road condition/ car load, etc. The tire store guy says the rear tires are cupping. He said every fourth groove or so is flattening or cupping. Rims are new and perfect. All four wheels were just balanced. So i think there is a suspension issue causing the rear cupping. There is no noise other than that tire slapping sound....no rattles, nothing. When i push down on either the front or the rear, there is only one bounce. Can it be caused solely by a bad rear shock, or would it be some other suspension part? If it has bad rear shocks, wouldnt it fail the "bounce test" i tried?

Lack of rotation, poor quality/wrong tires are generally the most common cause. The A5 is hard on the inside edge of the tires, so certain tires will be more prone to this than others, as well as certain sizes. Directional tires are especially awful. Some will be so bad you simply cannot rotate them enough... they'll all go square.

The original tires on my 2010 Jetta were Continentals, made in Portugal, 225/45R17. At about 20000 miles, all four began showing signs of cupping on the inside edge and started to make this really annoying woo-woo-woo noise at higher speeds. It made no difference that I rotated them religiously every 5000 miles. I had the alignment checked, everything was well within the norm, so I simply continued driving on those tires. Finally, at around 30000 miles, the noise got so bad that I decided to dump them even though they still had at least 50% of the thread left. I replaced them with a set of Bridgestone RFT DriveGuards, same size, made in Japan, and problem solved. I now have about 30000 miles on them, and there are absolutely no signs of any cupping anywhere, and I rotate them at the same intervals as I rotated the Contis. From the design standpoint, the only difference between the Contis and the Bridgestones is that the latter are "run flat" tires, meaning the side wall will probably be a lot stiffer than was the case with the Contis. So, yes, as oilhammer suggested, you may just be driving on the wrong tires, just as I did.

Every time I blame shocks for cupping, I'm never right. Get an alignment. Jettas needs lifetime alignment plans. I do mine every year or 2 or else I get cupping in the rear. In your case, better tires and an alignment wouldn't hurt.

The wheels front and back were just aligned. They found that the rear toe was slightly out of spec. All else front and back were within spec, didnt need adjustment. I got the car at 190k and it now has 255k. I have michelin defender on the car, except the left rear (with the current rotation)....its a yokahama. Today i spent a few hours with a tread depth gauge, checking all four tires for uneven wear around their circumference. I measured the inside of tread, and the outside. There was not the slightest variation on any of the tires! So how is it the "expert" who checked tires at Discount Tire Store is saying the tires are cupped? What i did find is that the left front Michelin measured 5 mm on the outside tread, but consistently only 4 mm on the inside! Something is wrong! I used an angle meter to verify that the rotor is at perfect 90 angle to horizontal, and the shop that did the alignment did not find the camber off, either! (I have them rotated and rebalanced every 5k miles, btw)

Run your fingers across the tread blocks on the inside. And compare the feeling to the middle of the tire. Excess rear toe will cause the inside rear tread blocks to wear unevenly. They should not be serrated or sawtooth feeling.

Rear toe check (for a pre-existing condition with tires in original position and not freshly rotated) can be done without a jack...……..without a tire gauge...…….with your eyes closed...……...on a rainy day...…...in a used car lot...…….on the bad side of town

Too much toe will make any car go straight. A "lazy" alignment shop will ignore camber and give you a "toe and go"

For better tire wear, ask for five minutes per side of rear toe. (GTI settings). Some models have factory specs calling for 10 minutes each side.