A 100-406 inner tube is used with large-volume 20 inch fat bike tyres, usually marked 20x4.00. The ETRTO number means the tyre is about 100 mm wide and fits a 406 mm rim. A normal 20 inch tube for 1.75-2.125 inch tyres is not a suitable substitute for this volume.
Why 100-406 is different from normal 20 inch
Most BMX, folding and children's 20 inch tyres also use a 406 mm rim, but they are much narrower. The tube has to expand to fill the tyre casing. If it is too narrow, it stretches, heats, becomes more fragile and can fail early. For a 20x4 tyre, use a tube whose printed range covers 20x4.00, 20x4.00/4.90 or ETRTO 100-406.
Check the valve first
Fat bike tubes in this size are commonly supplied with Schrader valves. Before ordering, check the valve hole in the rim and the valve on the old tube. Do not drill a rim or force the wrong valve type unless you are deliberately rebuilding the wheel and understand the consequences.
Rim tape and repeat punctures
If a 20x4 tube keeps puncturing near the rim side, inspect the rim tape, spoke holes and valve area. A wide tyre can hide a poor rim-tape installation, but the tube still needs smooth protection from the rim. Replace damaged tape before fitting the new tube.
Fitting a 20x4 tube cleanly
Partially inflate the tube so it takes shape, place it evenly inside the tyre, then seat both beads carefully. Avoid pinching the tube under a tyre lever. Inflate in stages and check both sides of the bead before going to full riding pressure.
Should you add sealant or a liner?
For bikes used on rough paths, glass-strewn roads or thorny ground, a fat bike liner or suitable sealant product can reduce small puncture risk. It is not a cure for a worn tyre, incorrect pressure or a sharp object left inside the casing. Inspect the tyre before adding protection.
Shop 100-406 tubes from 1Bike
1Bike lists 20x4 fat bike tubes, 100-406 tyres and compatible protection products with product-page size details. Match the ETRTO first, then check valve type, tyre condition and rim tape before fitting.
