ROTAX ul ?

It sounds like you have a wheel out of balance. 

Walt wrote:

It sounds like you have a wheel out of balance. 

Or just continuing to rotate after take off.

Try touching the brake when circuit height is achieved - if the vibration goes away it was likely the main wheels turning. On some aircraft this can occur at any time in flight. If the wheel bearings  should be  "snug" (just tight enough to remove side movement). Do not overtighten however if "snug" there may be just enough fiction/drag to prevent rotation in the air. If problem persist you may have to carry a little brake pressure to prevent the wheels turning (my last aircraft was like this).

Nose wheel vibration can usually be addressed by removing any play (looseness) depending on the configuration this could be a simple as tightening a lock/castellated nut or shimming may be required.

To check for side play, raise each wheel of the ground and see if you can shake/move the wheel from side to side - there should be no side movement. 

If the vibration is right at lift off then balance all you tires. I have never seen an aircraft tire in balance from the supplier. I always balance tires during installation and have cured many owner reported vibrations.

THANK  YOU GUYS FOR THE IMFOR.

I would question the need for such small diameter wheels to be balanced.

If very badly out of balance you may need to consider why and possibly replace tyre and or tube.