Folding-bike tyre sizes: 349, 406 and 451

Sources checked: 30 September 2026

A folding-bike tyre can introduce itself three times: perhaps as “16-inch”, “20 x 1.35” and “35-349”. Only one of those descriptions reliably settles whether its diameter matches the rim. The useful line is the ETRTO marking, usually moulded into the sidewall in the form width-diameter.

For example, 35-349 means a nominal width of 35 mm and an inner tyre diameter of 349 mm. That second number is the one to match exactly. A 406 tyre and a 451 tyre may both be sold under the broad “20-inch” heading, but they fit different rim diameters. They are neighbours in a catalogue, not substitutes in a workshop.

Read the second number first

Schwalbe’s current size guide explains that an ETRTO marking such as 37-622 gives the tyre’s nominal width first and inner diameter second. It describes the inch label as approximate and potentially unclear. The current international standard behind bicycle tyre designations is ISO 5775-1:2023, covering dimensions and markings for common pneumatic bicycle tyres.

For a folding bike, three numbers appear often enough to deserve their own introduction:

Example sidewall marking	What the numbers mean	Where the trap appears
35-349	About 35 mm wide; 349 mm inner diameter	Often described as 16-inch, although other incompatible diameters also carry 16-inch names.
54-406	About 54 mm wide; 406 mm inner diameter	Commonly called 20-inch. It does not fit a 451 rim.
37-451	About 37 mm wide; 451 mm inner diameter	Also appears under 20-inch descriptions, despite the larger rim diameter.

Schwalbe’s full tyre-size table shows the problem neatly: its 16-inch group includes several inner diameters, while 406 and 451 both appear among the 20-inch entries. The inch number remains handy in conversation. At the checkout, the ETRTO number is the calmer companion.

What current Brompton specifications show

The distinction now matters within Brompton’s own range. A current C Line specification names a 349 x 35C Schwalbe Marathon Racer, while the G Line specification lists a 54-406 Schwalbe G-One All Round. The manufacturer’s ordering on the C Line page differs from the usual width-diameter sidewall form, but 349 still identifies the diameter family.

That means “a Brompton tyre” is no longer enough information on its own. The bike line, existing sidewall and equipment documentation all matter. Our folding-bike specifications guide makes the same broader point: a familiar headline number becomes useful only after you know what was measured and how it affects the everyday decision.

The first number is useful, but not a promise

The first number describes nominal tyre width in millimetres. A 35-349 tyre is therefore narrower on paper than a 37-349 tyre, while both share the same inner diameter. Matching the diameter answers the first compatibility question; it does not answer all of them.

Actual fitted width can change with the rim, pressure and tyre construction. Schwalbe notes that a tyre may vary from its marked width and can expand after fitting. A wider option also needs suitable rim width and enough room at the frame, fork, brakes and mudguards. Folding clearances add another small puzzle: a tyre that spins freely still has to behave when the bicycle folds.

This is where the bicycle and rim maker’s compatibility information earns its keep. The Schwalbe rim-dimension guidance says the inner tyre diameter must match the rim shoulder diameter, and that tyre width must also suit the rim’s internal width. It separately flags tyre type and pressure limits for hookless or straight-sided rims. A matching final number is essential; it is not the entire fitting instruction.

Tubes and valves bring their own small print

An inner tube normally covers a range of widths, but its diameter still needs to match the tyre and rim family. The box may list several compatible ETRTO sizes; check that your complete marking falls inside that range rather than relying on “16-inch” or “20-inch” alone.

Then look at the valve. The rim must accept the valve type and the valve must be long enough for the rim. Even closely related bicycles can differ: Brompton’s puncture guidance notes that T Line uses a Presta-valve tube and that a Schrader tube will not work there, while the current C Line example specifies Schrader. This is a good moment for a photograph, because valve names have a habit of leaving the mind as soon as the shop page opens.

Pressure is separate again. Use the limits and guidance supplied for the actual tyre, rim and bicycle; the size code alone does not prescribe one pressure for every construction, load or model.

A two-minute sidewall note

Before ordering a replacement, save these details from the bicycle you have:

Check	What to record
Tyre	The complete ETRTO marking, such as 35-349, plus the model name if useful.
Rim and bicycle	The documented compatible diameter, width range and tyre type.
Clearance	Space at the frame, fork, brakes and mudguards, including during the fold.
Tube	A width range that includes the tyre, the exact diameter, and the correct valve type and length.
Pressure	The applicable limits from the tyre, rim and bicycle information.

Tom’s Schwalbe Marathon Racer review and Schwalbe One review record his experience of two tyres on his own bicycle. They are useful first-hand riding notes once the size question is settled; they do not replace the compatibility check for another bike.

The sidewall may look like it was typeset by a committee with a very small stamp, but its full ETRTO code is the quickest route through the confusion. Match the diameter exactly, then check width, clearance, tube, valve and pressure. “Twenty-inch” can begin the conversation. 406 or 451 is what stops it ending with a tyre that looks splendid beside the bicycle.