A tyre with a punctured sidewall or rim leakage can become difficult to seal. This can happen with car tyres, bicycle tyres, motorcycle tyres and farm machinery tyres. In some cases you can place an inner tube into a tubeless tyre. In other instances it would be dangerous, and isn't advised.
Tubeless bicycle rims might require modification if you want to install a tube into the tyre. If your rim is damaged or you can't get a good seal, installing a tube can make your tyre usable again. A tube can also make the tyre more durable in very rough riding conditions. You might have to drill out the valve hole in the rim to make a bicycle tube fit. Be sure to use a file on the edges of the hole so it doesn't cut into the valve stem.
Most tubeless motorcycle tyres will accept an inner tube, though you might have to drill out the hole for the valve stem for it to fit. When mounting the tube, be careful not to pinch it with the tyre iron you use to reseat the tyre. As you fill the tyre, bounce it around it to even out the tube and prevent it from being pinched or snagged between the rim and the tyre.
Do not install tubes into a radial car or truck tyre. Most tubeless rims have a drop centre. The drop centre prevents the tube from making good contact with the tyre. This can lead to damage to the rim or the sudden escape of air trapped beneath the tube and a drop in air pressure within the tube body. Tubeless tyres have thicker sidewalls than tube tyres. The inner tube helps support tube tyre sidewalls, and this helps maintain positive contact between the tyre and the inner tube. The stiffer radial sidewalls cause friction with the inner tube, and this can lead to failure of the tube or the sidewalls, and a sudden blowout while driving.
Lawnmowers and wheelbarrows
Adding an inner tube to a lawnmower tyre, garden trailer or wheelbarrow tyre is relatively simple and can extend the life of the tyre. Make sure not to pinch the tube with the tyre iron as you reseat the bead. Fill fully, and then let all the air out again to seat the tube evenly. Then fill the tube to the recommended pressure.