My car was stock suspension with FM sways. Then I added Koni Str-T shocks w/ racing beat springs. I had the alignment done and it handled ok. I swapped out the racing beats for FM springs a few months ago, and lately when I hit like 80mph, my steering wheel shakes A LOT. What could cause this? Improper alignment? Unbalanced wheel?

 

Could be both. Wheel balance will be noticable at lower speeds, but not as severe. Usually gets worse as you go faster, but you'd certainly feel it at, say 60. Seeing as you swapped springs, I'm betting it's alignment, though.

Are you sure everything is tight? Also, do a search on the infamous Miata shimmy. Loads has been written on it.

 

Sounds like your suspension mods were a few months ago but the shaking started recently ("lately")? I.e., the shaking is not directly linked to a mod? Makes me think you have a tire going bad (separated tread). Or maybe you threw a wheel weight/bent rim.

 

I'll check my car in the morning, but I'm willing to bet I threw a wheel weight. I know my tires on the front are worn on the inside pretty badly, but not "will not pass inspection" bad.

 

How many miles on it? Check your wheel bearings for play also, but most likely just wheel balance issue and the suspension mods brought it out.

 

162,000 miles...I checked and I'm not missing any wheel weights. I'm thinking they still could be improperly balanced, and my alignment could still be off. Wheel bearings seem ok. I've owned my car since 103,000 miles and I have never put brake pads on it...and they still look like they did when I bought it.
The brake pads comment has nothing to do with my issue...but I just felt like sayin.

 

Maybe one of the cam bolts for alainment cam loose and shifted a bit? I've heard if they aren't torqued down they will move around with suspension movement. Maybe the shop didn't get them tight and they moved a little.

 

Align and balance the wheels again.

 

Could be alignment, could be wheel balance. If you get some loud popping when cornering, then that sound may be loose alignment bolts.

You didn't get alignment done after the most recent spring swap?

Shimmy sucks. I had it with new tires, re-balance made it better but it's still there. Hard to find a shop near me that's ---- enough to get it just right.