I feel like I'm driving a stage coach

[Tony Radford]

For some reason, my car is REALLY riding rough. It feels like it's in the front end. I checked my tire pressure and it was really low (20 PSI). I filled it up to a little over 30 and it's still vibrating. I haven't had anything done with the tires, alignment or suspension since I bought her with 1,500 miles on her. Now there's almost 20K on the odometer. This has really happened all of a sudden. It seems that it's coincidental with my installation of the steering resistor, but I don't think there's any connection. It smooths out when I'm accelerating and comes back when I back off the throttle. Chill suggested I take her over to J3 Motorsports for a precision alignment and suspension tune. At least she's looking good!

[Justin Upchurch]

A couple of things could have happened, but the first thing I would check is the tire balance.
Also, as far out there as this may sound, you may want to check your balancer on the front of your motor. I had one start to come apart several years ago, and I first thought it was a front end shake.

Justin

[Okiesnake]

I have not installed a steering resistor but if you think that there is a correlation between the problem suddenly occurring and the installation of the steering resistor, I think that I would at least inspect and recheck its installation.
As far a tire pressure, these cars typically only require around 18 to 20 psi and actually will run smoother at that psi. As I recall that is the psi the Unique recommend when I built my car. The psi rating on a tire is a max pressure for the tire but you should go with the manufacture's recommend psi. That is why your production car or truck will most likely have the mfg's recommend air pressure on a plate on the driver's door jam or at least in the owner's manual. Those same tires on your cobra could also be used on a Fairlane or GTO weighing almost twice as much.
If you lost a tire balance weight, that could account for the sudden change in ride.
To eliminate engine or trans vibration, you could put the car in neutral while at speed and the kill the motor and coast.
If an alignment shim came out or alignment bolt came loose, that could also account for a sudden change ride.
Every so often we all should go around and check that nothing has loosened up. THINGS WILL AND DO!
How many miles are on the tires? It may be time to re-shod the stage coach

[GREENHOUSE]

If Your Car Has Been Sitting For A Bit You Might Have Flat Spots. I Have One Car If It Sets For More Then 2 Weeks It Will Take About 20 Min Of Slow Driving To Get Them Round. When The Weather Is Cold Sometimes Longer.
Mike

[Tony Radford]

I drove about 50 miles on the Interstate yesterday and figured the tires were sufficiently warm. It's not drive train vibration as I hit the clutch at speed several times and there was no effect. I can accellerate hard at some speeds and it smooths out. It seems to have come on suddenly. The tires (BFG 255 50 16s) have lots of tread. I guess a precision balance and alignment is probably due.

[RJacobsen]

Tony, Sounds like a U-joint to me , Putting the clutch in at speed won't stop your driveshaft from spinning. Accelerating and decelerating will load the joints differently, a worn u-joint will go out of balance on the worn side. if you check them and can't find a loose one look for one that might be frozen up. Hope this helps.