Vibration over 80

Hello all, im getting a pretty good vibration in the seat of my speed 6 whenever I go over 80, I had the rear tires balanced about a week ago hoping it's solve the issue, I tried feeling the hump in the floor pan to see if it was maybe the rear driveshaft but didn't feel like it. Any ideas? Thanks!

 

Always balance all 4 tires at the same time. Have them rebalance them again and before you go in check for weights in more than one location on the same side of the wheel. If they did then the wheels are counter balanced. Some people dont know how to read a balancer. They will do an initial spin and lets say it needs 1.0oz on the inner lip, then they respin and it shows 0.5oz on the same side but slightly further down. It means that the balancer wants the weights moved a little not just add the 0.5 to the next spot.

Check your wheels for improper weight installation and have them rebalanced, preferably have them Roadforced Balanced. Its a little more complicated but if you have an out of round tire or wheel issue it will show them where to mark the tire and wheel and have them rotate it on the wheel for a better balance.

 

post above me has some great point about balancing the other thing is an alignment will also help reduce vibration and identify suspension issues poor alignment can cause vibration as well.

 

If you rotate your tires, sometimes you are able to distinguish what corner a tire vibration is coming from. If there is no change, this will help you rule out tire vibrations.
If you arent able to find a solution after ruling out the tires or alignment, you might want to try the following.
When you have the vibration at 80mph, slightly increase and decrease pressure on the accelerator. If the vibration goes away you might have a worn inner CV joint. Pulling up on the e-brake 1 click briefly also can help diagnose the issue as it removes the power delivery to the rear wheels, unloading the rear axles.
I had the same problem on my 06 Speed6 a few years back. Left rear inner CV was the culprit. I think it was due to the Eibach lowering springs sagging in the rear. It was more pronounced when I loaded up the trunk and kids for travel. OEM rear coils and a Left rear shaft and I havent had any issues since.
Not very costly to check if you have the ability to do it on your own, however I would do as much checking on the cheaper and easier possible causes first if you have to pay a shop.
Goodluck!

 

Thanks for the info! I was just trying to be a cheap ass because I knew the rears were out of balance, one wheel didn't even have weights on it! I facepalmed really hard when they told me it was only $29, should've had them all done at the same time since it was that cheap. I'll definetly go back in and have them checked and the fronts balanced, thanks for the reply!

Thanks for the replying! I actually haven't rotated my tires yet so I might give it a try, who knows maybe it's one of the fronts and it's just not wiggling the steering wheel. It actually gets worse after after 85, around 90 the lid on the dash pops open. I took a quick glance at the rear suspension and cvs, they looked fine but who knows what going on behind the boots so I'll give the one click on the e brake a try. Thanks for the ideas!