Tyre valve

The tyre valve is where you add air. On tubeless wheels it is a rubber snap-in stem (or a metal bolt-in stem) fitted in the rim. On tube wheels the valve is part of the inner tube. Inside each valve is a small spring-loaded core, and a cap screws on top.

What it does

The core lets air in and stops it coming out. The cap keeps out dirt and water and is a second seal. Rubber snap-in stems age, crack and can be bent by the wheel spinning at speed. Dirt on the core seat makes it leak slowly.

Signs it is worn or failing

• Bubbles at the valve when wet with soapy water: Air leaking from the tyre valve.
• Slow air loss: Tyre slowly loses air.
• Cracked rubber at the base of the stem, or the valve leans over.

When to replace it

Replace the core if it leaks after cleaning; it is cheap. Replace a rubber tubeless stem whenever a new tyre is fitted, and any stem that is cracked or bent. Always refit a cap.

What to match when buying

• Tubeless rims: the stem type and rim hole size (snap-in rubber or bolt-in metal). Some bikes use angled stems for better access.
• Cores are a standard car and motorcycle type in most markets, but check.
• Buy stems from tyre shops or known brands; cheap rubber cracks fast in sun.

Fitting notes

• A valve core tool removes and fits cores.
• Snap-in stems are pulled through the rim with a special tool, so the tyre bead must be off: workshop or tyre shop job.
• Line up the tyre's balance mark with the valve.
• See Check and replace a tyre valve.
• Put a little soapy water on the end of the stem each time the pressure is checked. Bubbles show a leaking core.
• A core that leaks in one stem may hold in another, so keep an old core that tested good. Tighten the valve nut on a tube only finger tight.

Reference (K-Light 125)