Place the wobbly chair on a plane surface and check if any leg is shorter than the others. If you see one leg shorter than the others, just adjust the furniture glide on the bottom of that leg.
If you find that the glides are not adjustable, you can do stabilize the chair by other method. You can place a small piece of heavy-duty cardboard or shim under the short leg and check if the chair becomes stable. Mark the shim with a pencil and set aside.
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Now place the chair sideways and remove the glide from the shorter leg. Place the shim at the bottom side of the leg and replace the glide on it. Now screw back the glide and make sure the shim is well placed.
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If you have a chair that does not have glides under the legs, you still have an option to fix it easily. In such case, you must not consider cutting down the longer legs to stable the chair. In fact, you can place a piece of cork or shim under the shorter leg and check if the chair stops wobbling.
Now cut the cork or shim to the size of the diameter of the chair. Make sure you do not cut larger piece, as it will look ugly at the bottom of the chair.
Take wood glue and rub it on the piece of cork or shim. Now join it on the bottom side of the shorter leg. To strengthen the joint, you can finish it by using an adjustable glide. In that case, also add the glides to the other three legs of the chair.
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