Just picked up a 90 civic Si with 156k runs great and smooth around town but once I get on the freeway I get a shake in the front end. No shaking in the steering wheel, I feel it in my seat and feet mostly. Starts above 70mph. The more gas I give it or try to speed up the worse the shake gets. Put a set of hx wheels on and still does the same thing. Any ideas?

 

Had the wheels and tires rebalanced today but problem is still there. No one can help?

 

Axles, wheel bearings, brake rotors, it could be anything rotating.

 

Check for play in ball joints.

Also take your rotor off and clean the face of the hub. I fond once the rotor was not cleaned off when new and there was a film of the anti rusting oil from when they come new was that was not cleaned off. So it was causing vibrations and warping my rotors too.

Similar to you since the vibration happens after 60 mph when it's typically just a balance issue.

 

Could be mounts. They get old and the bushings crack.

 

Also could be a bent wheel.

 

Jack the front end up and place it on jackstands, leave the wheels on the car. Then grab a wheel and check for any play from <> like 9 and 3 oclock and also ^V 12 and 6 oclock.

Any play from 12 and 6 is most likely ball joints
Any play from 9 and 3 is most likely tie rod ends (or the rack end bushing if play is only on pass side)

No play, possibly a balance issue.
Play in all directions possibly wheel bearings.