Tubeless tyres: setup, sealant and roadside repair

Tubeless has quietly become the default. New road bikes arrive with tubeless-ready rims, most gravel riders have not seen a tube in years, and the thorn that used to end a ride now produces a two-second hiss and a green streak up your down tube.

It also fails in ways a tube never did, and almost nobody explains those before you commit. A tube goes flat; you fix it; you go home. A tubeless system goes flat in about six different ways, three of which are invisible and one of which happens in your garage at four in the morning.

Here is how the sealing works, how to set a tyre up so it stays up, and what to do at the roadside when it does not. One warning: this is among the least well-researched corners of cycling equipment, and where a number is workshop consensus rather than published evidence, this post says so.

Why a liquid seals a hole in a moving tyre

Sealant is a colloid. Latex particles — natural rubber, in most products — are suspended in a carrier liquid, held apart by a stabiliser, usually ammonia. Most sealants add solids too: glitter-like flakes, chopped fibres, or ground rubber, whose job is to bridge a hole too big for particles alone.

The suspension is deliberately unstable. Latex stays liquid only while its stabiliser is present and it stays wet; the colloid chemistry literature is full of ways to knock a latex out of suspension, including simple drying and exposure to volatile solvents. A puncture does several of those things at once.

When a hole opens, air rushes out and drags sealant with it. At the hole, three things happen in quick succession: the carrier liquid is blown out and evaporates, concentrating the latex; the ammonia flashes off, removing the stabiliser; and the particles are forced into a converging gap and jam against each other. Within a couple of wheel revolutions there is a rubbery bung wedged in the hole, held there by air pressure pushing it outward into the narrowing channel.

How big a hole can it close?

This is the least well-evidenced part of the post. There is essentially no peer-reviewed literature on bicycle sealant performance by hole size, so what follows is the consistent experience of mechanics rather than the same kind of claim as the physics above.

• Under about 1.5 mm — thorns, wire, small flints. Seals almost every time, usually without you stopping.
• 1.5 to 3 mm — seals on most gravel and mountain bike tyres. On a road tyre at high pressure it is marginal, because the escaping air is moving faster and keeps blowing the forming plug out. Letting the pressure drop first genuinely helps.
• Above about 3 mm on the road, 5 mm off-road — usually needs a plug. Sealant alone will keep going soft.
• Sidewall cuts of any length — poor odds. The sidewall is the thinnest, most-flexed part of the casing, so a plug there is worked loose every revolution.

The pressure effect is worth stating plainly because it is counterintuitive and it is the one piece of roadside technique that reliably rescues a puncture. If a road tyre will not seal, let it down to two bar and spin it. If you have not thought about what pressure you should be running in the first place, tyre pressure for cyclists is the place to start, and lower is usually the answer.

The setup sequence

1. Get the rim tape right. Most tubeless failures are tape failures. Clean the rim with alcohol, use tape matched to the internal width so it reaches the base of both bead walls, apply it under tension, and overlap two or three spoke holes past the start. Pierce the valve hole with an awl, not a knife.
2. Fit the valve snugly. Hand tight plus a nudge. Overtightening deforms the tape around the hole and creates a leak you will spend a weekend failing to find.
3. Mount the tyre dry. No sealant yet. Work both beads into the rim's central channel and put the last section on opposite the valve.
4. Seat the beads with a blast of air. A tubeless inflator, a compressor or a very determined track pump, with soapy water on the beads. Both beads should pop audibly and the moulded bead line should sit evenly all the way round — check it, because a bead seated at nine points out of ten will unseat later.
5. Add the sealant. Either unseat a small section of bead and pour it in, or remove the valve core and inject it. The second is cleaner; the first is more reliable with sealants containing large fillers.
6. Re-inflate, then drop to riding pressure. Take it briefly to the lower of the tyre's and rim's maximum, then let it down to where you actually ride.
7. Distribute it. Spin, shake, and lay the wheel flat on each side for a couple of minutes. You are coating the whole inner surface, which also seals the porosity of the casing.
8. Leave it overnight and check. A new tubeless tyre that loses more than roughly a third of its pressure overnight has a problem. Find it now rather than at kilometre 90.

Sealant life, and why the interval is shorter than you think

Sealant dries out. The ammonia goes first, then the carrier evaporates through the casing and the valve, and what is left is a skin on the inside of the tyre and eventually a lump. How fast depends on temperature, casing permeability, air volume and how often the wheel spins. A practical range is two to six months, at the short end in hot, dry weather and on a bike that sits unused.

The failure mode is quiet: the tyre keeps holding air perfectly well, so you have no reason to suspect anything until a thorn goes in and nothing happens. Check rather than guess — unseat a section of bead and look, or push a clean spoke through the valve with the core out. A slosh is sealant; a thump is a dead ball of rubber. Then set a calendar reminder, because nobody has ever remembered this on feel.

Accept too that a tubeless tyre is not airtight in the way a butyl tube is. The casing itself is permeable, so check pressure before every ride rather than every few weeks. A tyre that loses half a bar a day is a different tyre by hour six, which is worth thinking about when you are planning a 200 km ride.

At the roadside, in order

What to carry, given that list: two plugs and a plug tool, one tube, a valve core tool and a spare core, and a rag. The tube is the one people leave at home after a good year, and it is the one that gets you back.

One more note on sealant: dried latex does not redissolve, so topping up a tyre already full of skin and lumps gives you a tyre full of skin, lumps and some new liquid. After more than one sealant cycle, take the tyre off and clean it out.

The failure modes nobody mentions

When to stop trying

Some combinations do not want to work, and the sunk cost of an afternoon is not a reason to keep going. Fit a tube and move on if:

• The rim is not tubeless-ready. Bed shape and bead seat are what hold the tyre on, and testing across rim widths shows the rim is not a passive part of the system.
• The tyre will not hold overnight after two clean attempts with fresh tape. Something is wrong that more sealant will not fix.
• You have a sidewall cut longer than a few millimetres. Boot it to get home, then replace the tyre.
• It is your winter bike and you would rather spend the maintenance time on the drivetrain. A good tube in a 32 mm tyre at sensible pressure is an entirely respectable setup, and in wet weather the thing that punctures you is more often glass through the tread than anything sealant was going to save.

Set up properly, tubeless removes the most common reason a ride ends early, while letting you run the lower pressures that make rough roads bearable. Set up carelessly, it converts one predictable failure into several unpredictable ones. The difference is almost entirely rim tape, a genuinely seated bead, and a reminder in your calendar. For mixed terrain, gravel vs road covers the rest of that decision.