It's a tire balance problem. Seats mount might be part of the problem but it does not happen when you are at a stop light. The more out of balance your wheels/tire are the more it'll vibrate. Driver seat would probably vibrate as well but your weight is preventing that...

 

^^^ if its a tire balance problem my steering wheel would shake and vibrate...only the seat vibrates

 

yeah it's definitely not a tire balance problem...the seat does vibrate though...at least we got vibrating seats on the house!

 

With 20's mine did it badly. I had the rims balanced 3 times to try to correct the condition...but it didn't help. I put the stock 18's back on and it was much better. I even put each 20" rim on individually with 3 stock 18's to see if it was an individual wheel....but it wasn't anything specific as a single rim.
It's something our cars seem to do. Some days with my stock 18's are worse than others. It's a strange phenomema. My steering wheel is always rock steady.

 

vibrating passenger seat; OOooooo, is that a new premium pkg option?

 

LOL!

 

In theory an egg shaped tire might not make your steering wheel shake if your car tracks straight. My car's seat shakes to but only after installing 20” aftermarket wheels. My aftermarket 18’s did not have this problem. It's a wheel or tire problem. Most everyone seems to have this issue after switching wheels/tires. Put stock tires/wheels on and bam, fixed. At least in my case.

 

I took my car out today to specifically look for this problem. There is no vibration, smooth, realllllllllll smooooooooooooooooooooooooooooth, no problem here

 

Glad to hear you are vibration free, twice.

 

LOL, welcome

 

mine does it too... only after my mom adjusts the seat to where she likes it. if i move it back/lean the seat back, its usually fine. it seems to rattle against like the seatbelt or something.

 

my g35S with 600 miles hasn't seen seat vibration so far, hopefully I won't but the steering wheel vibrates at highway speeds, sucks

 

maybe this problem only exists with people who have after market wheels such as 20's...im planning on taking mine off and putting the stocks back on probably later this week.

 

I wish it were so, but I'm running with the stock sport wheels and have the same issue as well, of course only when I pay attention to the passenger seat for a short period of time while driving.

 

^^ +1000