Motorcycle Mousse Sizing and Tire Compatibility: Start With the Sidewall Numbers

A motorcycle mousse is sized to a specific tire size, and that size is printed on the tire sidewall. In a marking such as 120/90-18, three values define the space the mousse must fill: the nominal width (120 mm), the aspect ratio (sidewall height is 90% of the width), and the rim diameter (18 inches). The rim diameter must match exactly. The width and aspect ratio must fall within the range the mousse manufacturer lists for that insert. Load index, speed rating and construction codes on the same sidewall describe the tire's performance and build. They are not mousse dimensions, but they still matter for safe use.

How do you read motorcycle tire size markings for mousse fitment?

Motorcycle tire size markings combine dimensions, a ratio, a construction code and performance ratings in one string. A typical off-road rear tire might read 120/90-18 65M TT. Each part answers a different question.

Marking	Type of value	Plain meaning	Relevant to mousse size?
120	Dimension (millimetres)	Nominal section width of the tire	Yes. It sets the width class of the insert.
90	Ratio (percent)	Sidewall height as a percentage of the width	Yes. It sets the tire profile and internal height.
- (hyphen)	Construction code	Bias (cross-ply) construction. An R would mean radial.	Indirectly. Carcass shape affects the actual internal space.
18	Dimension (inches)	Rim diameter the tire fits	Yes. It must match exactly.
65	Performance rating (index)	Load index. 65 corresponds to 290 kg per tire.	No. It is a tire capability, not a size.
M	Performance rating (letter)	Speed rating. M corresponds to 130 km/h (81 mph).	No. Mousse inserts have their own use limits.
TT / TL	Fitment type	Tube type or tubeless	No. It describes air sealing, not insert size.

Dimensions: width, aspect ratio and rim diameter

The width is the first number and is given in millimetres. It is a nominal section width: the widest point of the tire's cross-section when the tire is mounted on a specified measuring rim. The aspect ratio is the number after the slash. It is not a length. It is a percentage that turns into a sidewall height once you multiply it by the width. The rim diameter is the number after the hyphen or the R, and it is given in inches. It describes the bead seat diameter of the wheel, not the tire's outside diameter.

Some older or traditional off-road tires use inch-based markings such as 4.00-18. Here, 4.00 is a nominal width in inches (about 101.6 mm) and 18 is the rim diameter. No aspect ratio is printed. For these tires, rely on the mousse manufacturer's chart rather than converting the marking yourself.

Construction and fitment codes

A hyphen between the ratio and the rim diameter means bias construction. An R means radial construction. Most off-road tires used with mousse inserts are bias-ply. TT (tube type) and TL (tubeless) tell you how the tire is meant to hold air. A mousse replaces the inner tube or air chamber, so these codes do not define the mousse insert size. Off-road tires may also carry markings such as NHS (not for highway service), direction-of-rotation arrows and Front or Rear labels. Read these as fitment instructions, not dimensions.

Performance ratings: load index and speed rating

The load index and speed symbol state what the tire can carry and how fast it is rated to run under standard test conditions. They do not change the space inside the tire, so they do not determine which mousse fits. A mousse insert has its own heat and speed limitations set by its manufacturer. The lower of the tire rating and the mousse limit is the one that matters in practice.

How do width, aspect ratio and rim diameter affect tire geometry?

You can estimate a tire's sidewall height, overall diameter and circumference from the sidewall numbers with simple arithmetic. These calculated values are nominal. They let you compare sizes on paper, but they do not replace measured dimensions.

How to calculate nominal tire geometry

1. Sidewall height (mm) = width (mm) × aspect ratio ÷ 100. For 120/90: 120 × 0.90 = 108 mm.
2. Rim diameter (mm) = rim diameter (in) × 25.4. For 18 in: 18 × 25.4 = 457.2 mm.
3. Overall diameter (mm) = rim diameter (mm) + 2 × sidewall height. For 120/90-18: 457.2 + 216 = 673.2 mm.
4. Circumference (mm) = overall diameter × π (about 3.1416). For 120/90-18: 673.2 × 3.1416 ≈ 2,115 mm.

Calculated comparison of common off-road sizes

The table compares nominal geometry for front and rear sizes often seen on off-road motorcycles. A higher aspect ratio does not always mean a taller sidewall. Height depends on width and ratio together.

Tire size	Nominal width (mm)	Sidewall height (mm)	Rim diameter (mm)	Overall diameter (mm)	Circumference (mm)
80/100-21	80	80	533.4	693.4	≈ 2,178
90/90-21	90	81	533.4	695.4	≈ 2,185
110/90-19	110	99	482.6	680.6	≈ 2,138
110/100-18	110	110	457.2	677.2	≈ 2,127
120/90-18	120	108	457.2	673.2	≈ 2,115
140/80-18	140	112	457.2	681.2	≈ 2,140

Two examples show what the numbers mean. First, 80/100-21 and 90/90-21 differ by 10 mm in nominal width, but their sidewall heights differ by only 1 mm. Their overall diameters are almost identical, while their cross-sections are not. Second, the three 18-inch rear sizes stay within about 8 mm of each other in calculated diameter. However, their widths range from 110 to 140 mm, so the volume inside each tire differs noticeably. For mousse sizing, this cross-section is usually more relevant than overall diameter.

How does a motorcycle mousse size chart work?

A motorcycle mousse size chart matches each mousse insert to one tire size or a group of tire sizes that share the same rim diameter and have similar cross-sections. Manufacturers usually label an insert by the tire sizes it is intended for, sometimes alongside their own model code.

Read a mousse size chart with these rules in mind:

• Rim diameter is fixed. A mousse made for 18-inch tires is formed as a ring for that diameter and is not interchangeable with a 19-inch or 21-inch tire.
• Width and profile are grouped. One insert may cover several neighbouring widths and aspect ratios, such as a few rear sizes on the same rim diameter. The listed group is what the manufacturer has approved, not a range you can extend by calculation.
• Front and rear are separate. Front and rear inserts differ in diameter and cross-section and are listed separately.
• Some charts name specific tire models. Manufacturer compatibility charts may list tested tire models or note tires to avoid, because carcass shape varies between brands.

What determines mousse and tire compatibility?

Mousse and tire compatibility depends on whether the insert fills the tire's actual internal space with the intended amount of compression, on a suitable rim, for the intended use. The size marking is the starting point, not the whole answer.

Internal volume and compression

A mousse is designed to sit compressed inside the mounted tire. That compression gives it the firmness that replaces air pressure. If the tire is larger inside than the insert was made for, the mousse is compressed less and the tire behaves softer. If the tire is smaller inside, the mousse is compressed more. The tire then rides firmer and is usually harder to mount.

Situation	Effect on the mousse	Typical result
Tire matches the chart size	Intended compression	Firmness as designed by the mousse manufacturer
Tire larger inside than intended	Under-compressed	Softer feel, more tire movement
Tire smaller inside than intended	Over-compressed	Firmer feel, more difficult fitting
Different rim diameter	Insert ring does not match the tire	Not compatible

Rim and wheel factors

The rim must match the tire's rim diameter. Its width should also suit the tire size, because rim width changes how the tire's cross-section is shaped once mounted. Off-road wheels used with mousse typically need rim locks to keep the tire from slipping on the rim, since there is no air pressure holding the bead.

Condition and use

A mousse can soften and lose volume with use and heat. A worn insert behaves as if it were a size too small, even in the correct tire. Mousse inserts are generally intended for off-road use within limits set by their manufacturer. The tire's speed rating does not override those limits.

Why calculated geometry differs from actual tire dimensions

Calculated sidewall height, diameter and circumference are nominal values derived from the size code. Actual tire dimensions can differ for several reasons:

• Measuring rim: nominal width assumes a specified rim width. A narrower or wider rim changes the mounted width and profile.
• Brand and model: two tires with the same marking can have different carcass shapes, tread depths and knob heights.
• Tread and knobs: on knobby off-road tires, the knobs add to the outside diameter, but they do not add to the internal space the mousse fills.
• Wear and load: a worn or loaded tire has a smaller effective diameter than its calculated value.

For these reasons, a calculated difference of a few millimetres between two sizes does not prove that one mousse fits both. Vehicle fitment, meaning clearance to the swingarm, fork and fender, is also a separate question. It depends on the motorcycle and on measured tire dimensions, not on the size code alone.

How to choose a mousse insert size from the tire sidewall

1. Read the full size marking on the tire sidewall: width, aspect ratio, construction code and rim diameter.
2. Confirm the rim diameter matches your wheel exactly.
3. Note whether the tire is a front or rear tire, and whether it uses a metric or inch-based marking.
4. Find that exact tire size in the mousse manufacturer's size chart. Check that the size appears in the insert's listed group.
5. Check any notes on specific tire models, rim width or rim locks in the manufacturer compatibility charts.
6. Check the mousse's stated use limits and compare them with the tire's load index and speed rating. Treat the stricter limit as the one that applies.

If your tire size is not listed, do not use calculated geometry to justify a fit. Treat the absence from the chart as a sign that compatibility is unconfirmed.