The little rubber and brass valve that holds the air in your tyre, and admits new air, is one of the world’s most successful inventions. William Schraeder designed its fundamentals nearly 120 years ago.
The little “springy thingy”, called the “valve core”, that screws into the brass valve really hasn’t altered all that much in that time, and all you need to remove it and let the air out, is a slotted valve cap. Or you can just depress the little button in the centre and you get the same effect, only slower.
Yet when you buy a new tyre, the fitter always replaces the valve. Why does he bother?
The modern tubeless snap-in valve is compressed into a hole in the rim to provide a seal. A brass stem is adhered to a rubber skin, with a domed shape on the inside of the wheel to prevent it being blown through the hole by the air pressure.
Over time, the degree of compression is lowered (it doesn’t fit as tightly). It may even crack around the groove in the rubber which lodges in the rim hole due to flexing.
The valve actually flexes as the wheel revolves, particularly if it is a long one designed to protrude past the wheel trim. Ultra-high-speed photographs have shown the valve actually touching the rim at right angles at very high speeds. Also the heat during service causes the bond of the brass to the rubber to deteriorate, and if this bond ruptures, the stem blows out, and the tyre goes down quickly.
So reliability is what it’s all about. It’s much better in the long run to replace it after one tyre life.
You can contribute by using dust caps or valve caps, and giving a blast of air around the valve before you clamp on the air chuck, which you should do monthly. If you suspect a leaking valve, a “dob of spittle” on the end of your finger into the brass stem is the tried and true method. If it bubbles, first check the valve core is tight. If it is, loosen it, let some air our, then retighten to dislodge any dirt that might be there.
If it still leaks, replace the core. Unscrewing the core right out will let all the air out, and coincidentally clean the seat that the “springy thingy” seals on. To do this, you need a slotted metal valve cap, or a valve tool, and a kindly service station operator to assist if needed.
The metal clamp-in valves are different. These are used in some alloy wheels, where the thickness of the metal around the hole is too great for a snap-in type. But they are even more desirable when high speeds are the norm. Unlike a snap-in, they do not flex, and they sandwich two air seal washers under compression to get the air seal. So even though they cost more, they last longer, because it is not as necessary to replace them after every tyre life.
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