It just disappears! I am a not sure where is goes! I have had people say that to me. Most bikes that come to the shop with flat tires are caused by the owners not doing good pre-ride checks or the travesty of not riding them at all.
Osmosis of air through the tube or tire casing will lead to low pressure. The composition and threads per inch (TPI) of the tire and inner tube plays a significant role in osmosis. Also, this is the reason why tubeless tires loose air at a much slower rate. Most high-pressure tires will lose up to 15 pounds in just a few days while lower pressure and tubeless tires may last a week or more before being noticeable.
Slow leaking punctures in the tread are another cause of tires losing air. You might be surprised how a small staple or thorn can cause a slow leak and be very hard to find. The sealant that is designed to fill holes helps tubeless tires combat small leaks.
Leaking valve stems and cores are another cause of low tire pressure. Over-tightening valve cores or damage caused by normal riding can lead to loss of air.
Tires can lose air through impact or pinching. Running over large objects or running with low pressure can cause a quick flat and adds to the plethora of good reasons to carry a flat pack when you ride.
Do quality pre-ride checks
Check your air pressure. You will need a gauge or a pump that has a gauge on it. Be sure you have the correct head on the pump, unless you use a duel head pump.
If the valve on your tire looks like the one on your car that is a schrader valve. To inflate remove the cap and attach pump and inflate.
If the valve on you bike is “that other one”, that is a presta valve. To inflate, release the little wheel at the top of the valve stem by turning it to the left. Then tighten the wheel by turning back to the right when finished inflating. If you are using schrader adapter, the same procedure applies just put the adapter on to inflate them remove to tighten down the presta valve.
If you are not sure how much the pressure should be, it is stamped on the side of the tire and is represented as PSI. (example: inflate to 50-70 PSI)
Visually inspect the tire, look for gashes, cuts, and the worn indicators. Quality tires have a ware indicator that may appear in the form of divots, or pin holes on the tire. If you can not find it, it may be time for new tires. If you see any damage, cord showing or your tire seems to lose excessive air over a day or two. Change it before you ride, is a lot easier doing it now than alongside the road.
Strong language alert!
Do not under any circumstance put slime in your tube to fix a flat. This will result in an enormous mess. And when you do take your bike to the shop for repair the mechanics will say horrible things that cannot be repeated in the presence of children.
Tire problems can turn a great ride into a frustrating afternoon on the bike. If you are unsure of your tires condition, swing by the bike shop before you ride. Any quality shop will be glad to take a few moments to help you gain knowledge about your bike. If they won’t take the time to help you or charge you for knowledge, you may want to consider another shop. Just saying!