Ok I just had four new tires put on Continental extreme contact DWS, with balance and alignment done. The ride feels great... UNTIL I get the car up to about 70-75 mph plus. Then the car feels like its bumping up and down. Not only can I feel it but you can hear the thumping too. I am getting pretty frustrated with the Tire shop that put them on since I have stronger suspicion now that they have faulty equipment or faulty help. (see other recent posts).

What would be the possibilites to cause this effect? Are the RX-8's that difficult to align and balance or am I just working with a shop that is being careless? What I find extremely interesting is I just had the rear tires put on a few days ago and the performance was messed up... the dealer had no explanation. When the matching front tires came in the performance problem went away but the bumpy ride appeared. Even more interesting was the car was out of alignment just 3 days after the initial alignment. Thoughts or comments?

Thanks in advance.

 

Now you got 4 tires - a start.

As be4 Car should be silky smooth to 120mph + beyond.
Either tires u got are junk - where did u buy the tires? Hopefully NOT the 'shop'.
Maybe ur shop is broken - date last calibration on balancer?
Make them show you. Best advice - never buy sushi at an Italian restaurant.

So go to normal busy tire store - not 'shop' - get rebalanced.
As be4 fav is Sears BION they are superb here
As be4 alignment does not = unbalanced tires...think. Common sense.

 

I think they did not put the rim on the car right, unbalance tires give you vibration, not bumpy ride. tell someone drive your car while you look at the wheels on both side from the back, you will see one wheel is warbling. That will give you error on alignment and bumpy ride.

 

Bumpy ride could be caused by an out of round tire. As well as a slipped belt internally in the tire. I would jack the car up and rotate/roll the tires one at a time and check that they are round.
Put a makeshift straight edge under the tread and evaluate.

 

Check your lug nuts and make sure they are tight.

 

^.......whoa......that would be the first thing I would check!

 

Check TPMS sensor if its not rattling around in one of the tires. Had it happen to me once I took the speed above 80 MPH, the ride would get unstable. turns out rattling at the left driver side at initial 1st gear pickup, jacked tire up hear rattling, took tire off, well whaddayaknow, theres the cause, lol