lobuxracer

iTrader: (2)

There's a center bearing in the driveshaft. If it were defective, it could cause serious vibrations, but nothing affecting control of the car. If the car is unstable at speed (and I know you've already been through more than one set of tires) I'd be looking at suspension bushings, torque on rear axle nuts, wheel bearings, torque on subframe to chassis bolts, etc.. Basically, I'd put the car on a lift (or on tall stands) and check every single bolt holding the suspension on the car and ensure every bushing is properly torqued and no bolts are bent, cross-threaded, or distorted. I'd also check all the hats on the top end of the suspension (where the shocks mount to the chassis) and ensure the alignment spots are positioned correctly (Supras have a blue alignment spot that must face inside, and I remember seeing something about this in the IS manual as well). There has to be something not tight (not exactly loose, but not as tight as it should be) allowing relative motion between two parts when there should be none. It's causing the chassis vibrations your are experiencing and they're manifesting themselves the vibration you feel in your seat and the instability in the car driving down the road. Too bad you don't live in Atlanta...

FiveOhNine

Good advice!

maddave

I think i am experiencing the same problem. It is super annoying. I had bought 4 new tires last month and decided to just put the front two on(wanted burn up the rears). Had no vibrations when i had low tread oem tires on and now i can feel vibrations when going 80+mph with new front tires..oh yea tires are sumitomo htzIII's. never thought that it could be loose bolts but i do drive my IS hard. Real hard. The thought of a bad motor mount crossed my mind. I'm gonna install the new rear tires and rebalance the fronts first to see if that helps.

FiveOhNine

It could be the htrzs. That's what I've got too.

Byprodrive

iTrader: (1)

If a tire is separating you can feel a thump once every revolution when that area hits the pavement. When it gets bad enough you can hear it also.