How much piston is showing? (Should be around 3 inches) If the strut is over inflated, there is centering lug that prevents the nose wheel from turning. This lug is located on the underside of the upper torque link and mates with a flat on the rear of the nose gear. The only other thing that I can think of is the shimmy dampener could have a bent piston. Some folks have been known to service these units with engine oil to get rid of nose wheel shimmy, which makes steering difficult as well. BTW the unit is normally serviced with hydraulic fluid (MIL-H-5606).

There is an air valve at the top of the strut from where the strut is serviced. I highly recommend that you get a service manual and get guidance from a licensed mechanic. If you are unfamiliar with this it could hurt...

I use an engine hoist attached to the lift ring on the engine to take the weight off of the nose wheel.  I can then release the air in the strut without it spraying hydraulic fluid all over, and refill it to the recommended pressure, which I think is 45 psi on a C-172.  If you have access to comressed nitrogen, it works better that air.  This is usually a good time to top off the strut with hydraulic fluid as well, as you need to get the weight off of the strut to do that.  The procedure will be covered in your service manual.

too much air was the key for the hard to turn nose wheel.  i let some out and voila, strut went down about an inch or so.  i let it out really slow so as not to get fluid to come out.  turns like a dream now.  it looks like the strut was recently serviced, all greased up, etc. 

thanks for the hot tips.

kyle

nose wheel start vibrating after gaining takeoff speed.

As long as the fluid is 5606 it is OK. The strut should be filled with the strut in the collapsed position.