High Speed Shudder/Vibration

Hi all. I've had my 996 for just over a month, and notice that at interstate speeds (~70mph+) there seems to be a vibration/shudder coming from the rear. I'm thinking more than likely rear tires need balancing, but want to get anyone else's input on what to look for if this doesn't correct problem. Thanks!

Also, forgot to add had a PPI done before purchase and this was not a noted issue. So either within normal limits, they didn't take it to interstate speeds, or has developed since transport.

I had a shake that turned out to be a bent rim in the rear,also my alignment was toed in in the rear. I don't know if that helps but it's a starting point anyway. I'd start with the cheapest which would be checking the balance.

I get what i would describe as the exact same thing also right at 70. I took mine in to get a quality load force balancing done and they found a bent rim like Eric.

Be careful with shops telling you have a bent rim. I've had a couple shops tell me that, but when I took the rim into the shop to have it straightened, the expert wheel straightener who's been welding wheels for 30 years told me it was perfect, and saw the tire just had a little more excess rubber in one spot. A trip back to a shop with a road force balancer (the latest and greatest) cured the problem. Mounting, torque, tire wear, and tire pressure are also critical to a smooth ride!

I had a bad shake and vibration on the fronts at >80mph .... tried to rebalance the wheels at wheel works - but that didn't make much difference. Now I put on new tires and had them balanced at a race shop - car drives perfectly. Indie had noticed rims were "slightly bent" but the race shop told me no problem at all - he normally sees a lot of Porsche turbo twists that bend over time - but mine looked pretty good - so maybe time for new tires and make sure you get them balanced at a prof. shop - not just national tire chain ....

Logray is right on. I would only trust the bent wheel advice with a road force/load force balance where they can tell if there is a heavy spot on the tire. Also, there are many shoddy shops who don't give a crap about doing a proper balance with or without a load force balancer. This was the case for me when I put new tires on. To you Nor Cal posters in the thread, Custom Alignment in Mountain View is a great resource. Www.customalignment.com

Improper air pressure?

For NorCal ... Also recommend Borelli Motorsports in San Jose for tires, mount, balance ....

I've experienced the same thing. In my case there was a big blob of road tar on the inside of the wheel.

Make certain your (cold) tire pressure is correct...an underinflated tire particularly can at times result in very noticeable vibration. Once that's eliminated as the problem, you can find a shop nearby that is equipped with the latest road force machine: http://www.gsp9700.com/search/findgsp9700.cfm . If a tire tech finds a defective or bent wheel, ask to see it - many problems that produce your symptoms can be seen on the balancer as wheel hop or side to side run out with the naked eye. GL with Solution & keep us posted.

This can also be caused by square tires (flat-spotted) due to long term storage, I don't believe that this can be fixed other than replacing the tires, if that is the problem. George

Thanks everyone, I appreciate all of the feedback. Will definitely make sure to have it road force balanced, and see how that changes the shudder. The rear tires were replaced right before I bought it, and the fronts still have about 70% tread life on them. I'll double check the tire pressure, but it was good a month ago when I got it. I'll post back after the road force balancing.