Need help on 45-80 MPH shaky problem.....

[— kung fu jesus —]

Hmmmm...I am curious if the car was in an accident, enough to bend some steering geometry or suspension components.

Are the shocks blown?

[— Phatmiata —]

This was an existing issue (before the add-ons) ...............

I am wondering if maybe the chassis was tweaked from the prior owner being in an accident that may have damaged the frame?

With all those mods not getting rid of that shake there must be something else going on.

Did you ever run a carfax on it to see any accidents? Sometimes you can find info on there, sometimes you can't, if the car was taken to dealer for repair or insurance claim it will show, however if taken to a private shop those guys are not required to report damages.

[iRoadster]

The mention of checking the rear wheel bearings made me think of something that I've recently noticed on my car.

Maybe it's something similar, but I whacked my right rear wheel on a curb a few years ago. An alignment revealed that after a certain point of trying to reduce the camber made it toe really badly. It can still be evened out with the other side just fine, but I can only get down to -1.3 degrees of camber on the back because of it. Alignment guy suggested that maybe the knuckle is messed up. The rear bearings are overdue to be replaced, the right side one especially.

Just south of Olympia, I5 jumps up to a delightful 70. I drove down to Chehalis about a week ago and noticed a strong vibration around 70-80.

[— Agent☣Orange —]

Honestly, any shake in that great of range is a serious condition. First ensure the tires are balanced. I paid over $100 for a Hunter balance and it was a 4-hour ordeal rotating tires on the wheels 180 and even swapping a couple tires and wheels (225s on 9" wheels to be fair). That finally cured my 68-70 shimmy. A Miata can shake in the 30s and 70s but to shake the entire 40-80 range means something else is wrong IMO.

[BoBo]

OP, are you talking about the steering wheel vibrating, or the front end shaking? I never had any shimmy issue with my miata, but it's common for the pillars to shake without a hardtop. If the front-end of your car is shaking, you might want to check your rubber bushings if it's worn...But wait, you did mention you have polyurethane bushings, that's probably the culprit. Those things vibrate a lot, at-least in my experienced on my third MR2 Turbo. I had the polyurethane from suspension to engine mounts, everything vibrated even the dashboard LoL.

[BoBo]

Honestly, any shake in that great of range is a serious condition. First ensure the tires are balanced. I paid over $100 for a Hunter balance and it was a 4-hour ordeal rotating tires on the wheels 180 and even swapping a couple tires and wheels (225s on 9" wheels to be fair). That finally cured my 68-70 shimmy. A Miata can shake in the 30s and 70s but to shake the entire 40-80 range means something else is wrong IMO.

+1 on the Hunter wheel balancing. I think it was the Hunter 9700 that cured my old Bimmer from shimmies. I was gonna mention that but I have a feeling something else is wrong with OP's car.

[mx54life]

OP, are you talking about the steering wheel vibrating, or the front end shaking? I never had any shimmy issue with my miata, but it's common for the pillars to shake without a hardtop. If the front-end of your car is shaking, you might want to check your rubber bushings if it's worn...But wait, you did mention you have polyurethane bushings, that's probably the culprit. Those things vibrate a lot, at-least in my experienced on my third MR2 Turbo. I had the polyurethane from suspension to engine mounts, everything vibrated even the dashboard LoL.

Yeah just the steering wheel vibrating. I will rotate the tires and have them balanced with the "Hunter 9700". Thanks again guys.

[— Agent☣Orange —]

Well, fill us in later. It seems like your problem is more than a tire balance issue but if a precision balance cures it then good news.