Bicycle Rim Cross-Section Anatomy Explained, Starting with the Numbers on Your Tire

A bicycle rim cross section is the shape you would see if you sliced through the rim at a right angle to its circumference. From the tire side inward, it shows the rim sidewalls (with bead hooks or a straight edge), the bead seat where the tire bead rests, the central rim bed (the tire well), and, on a double-wall rim, a hollow chamber that ends at the spoke bed. The numbers on the tire sidewall connect directly to this anatomy. In a marking such as 28-622, 622 mm is the rim's bead seat diameter and 28 mm is the tire's nominal width. The rim's internal width and depth are not printed on the tire, but they affect how that tire sits and measures.

What do the numbers on a bicycle tire sidewall mean?

A bicycle tire sidewall usually carries a dimension marking, sometimes in two or three sizing systems, plus pressure and construction ratings. Only the dimension marking tells you whether the tire matches a rim. The other numbers describe how the tire should be used or how it is built.

ETRTO / ISO marking: the reliable dimension

The ETRTO marking, which matches the ISO 5775 system, is written as width-diameter in millimetres, for example 25-622 or 57-584.

• First number (25, 57): the nominal tire width in millimetres. This is the width the manufacturer intends on a suitable rim. It is not a guaranteed measured width.
• Second number (622, 584): the bead seat diameter (BSD) in millimetres. This is the diameter of the rim's bead seat, the shelf the tire bead locks onto. A tire and rim must share the same BSD to fit.

The BSD is the most important compatibility number in the whole wheel. Different sizing systems can use different traditional names for the same diameter, but the BSD stays the same.

French and inch markings: traditional names, not measurements

• 700x25C: 700 is a historical nominal outside diameter in millimetres, and 25 is the nominal width in millimetres. The letter C was once a width class. Today, 700C simply identifies the 622 mm bead seat diameter.
• 29 x 2.25 or 27.5 x 2.4: the first number is a nominal wheel size in inches, and the second is the nominal width in inches. A 2.25-inch tire is about 57 mm wide (2.25 × 25.4).
• Fractional vs decimal inches: 26 x 1 3/8 and 26 x 1.375 look identical mathematically, but they are different sizes. The fractional size uses a 590 mm BSD. The decimal mountain-bike size uses a 559 mm BSD. They are not interchangeable, which is why the ETRTO number is the safer reference.

Ratings that are not dimensions

• Pressure range (for example 60–100 psi or 4–7 bar): a performance and safety rating for inflation. The rim may have its own maximum pressure. Use the lower of the two limits. Hookless rims are commonly limited to about 5 bar (roughly 72 psi) under current standards, so check the rim's marking.
• TPI (threads per inch): describes the density of the casing fabric. It indicates construction and ride feel, not size.
• Tubeless Ready / TLR / hookless-compatible: describes the bead and casing design. These labels tell you which rim types the tire is approved for, which matters for the rim's bead seat and sidewall shape.
• Arrows and rotation marks: show the mounting direction only.

Common bead seat diameters

Common name	ETRTO bead seat diameter	Typical use
700C / 29" / 28" (European)	622 mm	Road, gravel, hybrid, 29er mountain bikes
650B / 27.5"	584 mm	Mountain, gravel, some touring
650C	571 mm	Small road and triathlon bikes
26" mountain bike (decimal widths)	559 mm	Older mountain bikes, fat bikes, some touring
24" mountain bike	507 mm	Youth bikes
20" BMX / folding	406 mm	BMX, many folding and cargo bikes
16 x 1 3/8 (folding)	349 mm	Certain folding bikes

What are the parts of a bicycle rim cross section?

A bike rim cross section has five main parts. The sidewalls hold the tire in place. The bead seat sets the tire's diameter. The rim bed allows the tire to be mounted. The internal width controls the tire's shape. The spoke bed anchors the spokes. Rim depth is the overall height of this profile.

Sidewalls and bead hooks

The rim sidewalls are the two vertical walls that rise from the bead seat. On rim-brake wheels, their outer faces form the braking surface. On a hooked rim (marked "C" for crochet in rim sizing), the top edge of each sidewall curls inward to grip the tire bead. On a hookless or straight-sided rim, the walls are flat inside, and the tire is held mainly by the tight fit of the bead on the bead seat. Do not confuse rim sidewalls with tire sidewalls. The tire sidewall is the flexible casing between the bead and the tread, and it is where the size markings are printed.

Bead seat

The bead seat is the flat or slightly angled shelf just inside each sidewall where the tire bead sits when inflated. Its diameter is the BSD, such as 622 mm, which must match the second number of the tire's ETRTO marking. Tubeless rims often add a small raised hump or a tightly toleranced shelf here so the bead stays locked when pressure is low.

Rim bed (tire well)

The rim bed is the central channel between the two bead seats. It sits lower than the bead seats so the tire beads can drop into it during mounting, which effectively shortens the bead's path and gives enough slack to lift the last section over the sidewall. On tubeless rims, the rim bed has no open holes except the valve hole, or it is sealed with tubeless tape.

Internal rim width

Internal rim width is the distance in millimetres between the inside faces of the two sidewalls, measured at the bead seat (inside the hooks on a hooked rim). This is the width that shapes the tire. ETRTO rim markings combine it with the BSD. For example, 622x19C means a 622 mm bead seat with 19 mm internal width and hooked sidewalls. Tubeless rim markings may use TC (tubeless crochet, hooked) or TSS (tubeless straight side, hookless), as in 622x25TSS. External width is the outside measurement across the sidewalls. It matters for brake and frame clearance, but it does not set the tire shape.

Spoke bed

The spoke bed is the innermost wall of the rim, facing the hub, where the spoke holes are drilled and the nipples sit. On a double-wall rim, it is separate from the rim bed. On a single-wall rim, the rim bed and spoke bed are the same wall.

Rim depth

Rim depth is the radial height of the rim profile, from the top edge of the sidewalls to the outer surface of the spoke bed, in millimetres. Manufacturers do not all measure it identically, so treat small differences between brands with caution. Typical ranges are roughly 15–25 mm for traditional box-section rims, about 30–45 mm for mid-depth rims, and 50 mm or more for deep aero rims.

Single-wall vs double-wall rims: how the cross section differs

A single-wall rim has one horizontal wall that serves as both rim bed and spoke bed. A double-wall rim has two horizontal walls separated by a hollow chamber, which forms a box section. The double-wall design is stiffer and stronger for its weight.

Feature	Single-wall rim	Double-wall rim
Cross-section shape	Open U or channel	Closed box, V, or U-shaped hollow profile
Rim bed and spoke bed	Same wall	Separate walls with a chamber between
Spoke nipples	Pass through the tire-side surface	Seated in the spoke bed, below the rim bed
Rim tape	Essential to cover nipple ends	Still needed over access holes, or tubeless tape
Typical use	Budget, children's, and some utility bikes	Most modern road, gravel, and mountain wheels

Deep aero rims are almost always double-wall designs. Their depth comes from extending the distance between the rim bed and the spoke bed.

How tire size affects the bicycle wheel cross section and overall diameter

The overall diameter of a bicycle wheel is set by the rim's bead seat diameter plus twice the tire's section height. Rim depth does not change it. A deeper rim extends inward toward the hub, not outward, so a 25 mm-deep rim and a 60 mm-deep rim with the same tire have essentially the same outside diameter.

Is there an aspect ratio on bicycle tires?

Bicycle tires are not marked with an aspect ratio the way car tires are (such as the 45 in 225/45R17). A bicycle tire's cross section is close to round, so a practical starting assumption is that section height is roughly equal to the nominal width, a ratio near 100%. In practice, height is often slightly less than width, and the exact ratio varies with the casing design and the rim's internal width.

Simple calculation: diameter and circumference

1. Take the bead seat diameter from the ETRTO marking (for example, 622 mm).
2. Estimate the tire's section height as its nominal width (for example, 25 mm).
3. Calculate overall diameter: BSD + 2 × height = 622 + 50 = 672 mm.
4. Calculate circumference: diameter × π = 672 × 3.1416 ≈ 2111 mm.

Tire (ETRTO)	Estimated overall diameter	Estimated circumference
25-622 (700x25C)	672 mm	≈ 2111 mm
28-622 (700x28C)	678 mm	≈ 2130 mm
32-622 (700x32C)	686 mm	≈ 2155 mm
66-584 (27.5 x 2.6)	716 mm	≈ 2249 mm
61-622 (29 x 2.4)	744 mm	≈ 2337 mm

What the differences mean: switching from 25-622 to 28-622 adds about 3 mm to the radius and about 19 mm to the circumference, roughly 0.9%. A bike computer still set for the 25 mm tire would under-read speed and distance by about that amount. The table also shows that a wide 27.5-inch tire can come close to the diameter of a narrower 29-inch tire, even though the rim diameters differ by 38 mm.

Calculated geometry vs actual tire dimensions

These figures are estimates. Real tires differ from the calculation for several reasons:

• Internal rim width: a wider rim spreads the beads further apart, so the same tire measures wider and its profile changes. A common rule of thumb is that measured width grows by around half a millimetre for each extra millimetre of internal width, but this varies by tire.
• Casing and tread: knob height, casing stretch, and the manufacturer's own sizing make actual width and height differ from the nominal numbers.
• Load and pressure: under a rider, the tire compresses, so rolling circumference measured by rolling the wheel one revolution is usually a little smaller than the calculated circumference.

For an accurate figure, measure the inflated tire's width with calipers on your rim, and measure rollout by marking the tire and rolling the loaded bike one full revolution.

How rim internal width and tire width work together

The internal rim width determines whether a given tire width is approved and how it will be shaped. A tire that is too narrow for the rim can sit with its sidewalls exposed and may be unsafe, especially on hookless rims. A tire that is too wide for the rim can become balloon-shaped and unstable when cornering. ETRTO publishes approved tire-width ranges for each internal rim width, and rim and tire makers publish their own compatibility charts, particularly for hookless rims. Check these charts rather than relying on the BSD alone, because a matching 622 mm diameter only confirms that the tire will go on the rim. It does not confirm that the width combination is approved.

What does rim depth change, and what does it not change?

Rim depth changes the rim's stiffness, weight, aerodynamic behaviour, crosswind sensitivity, the required spoke length, and the valve length you need. It does not change the bead seat diameter, the tire size you buy, or the wheel's overall diameter and circumference.

• Valve length: the valve must be longer than the rim depth and still leave enough exposed stem for the pump head. A valve that works on a 20 mm-deep rim will not reach through a 50 mm-deep rim.
• Spoke length: a deeper rim places the spoke bed closer to the hub, so spokes are shorter. Spoke length calculators use a rim value called effective rim diameter (ERD), which depends on depth.
• External width and profile: deep rims are often wider and more rounded, which affects how the tire and rim behave as one aerodynamic shape and how much frame and brake clearance is needed.

How to read your own rim and tire setup, step by step

1. Find the ETRTO number on the tire sidewall (for example, 28-622). Note the width and the bead seat diameter.
2. Find the rim marking, usually a sticker or print on the rim or in the specification (for example, 622x21TC or 622x25TSS). Confirm the BSD matches the tire.
3. Note the rim's internal width and whether it is hooked (C or TC) or hookless (TSS).
4. Check the tire and rim makers' compatibility charts for that internal width and rim type, and note the lower of the two maximum pressures.
5. Estimate overall diameter as BSD + 2 × tire width if you need to compare sizes, then measure the mounted tire if clearance is tight.
6. Choose a valve length that exceeds the rim depth with room for your pump head.

The limits of dimensional comparisons and fitment

A calculated diameter or circumference tells you how two tire sizes compare on paper. It does not tell you whether a tire will fit your bike. Frame and fork clearance depend on the tire's actual width and height at the chainstays, seatstays, and fork crown, and on knob shape and mud space. Rim brakes limit both tire width and rim width through caliper reach and opening. Mudguards reduce the space further. Wheels with different bead seat diameters, such as 584 mm and 622 mm, also change the bike's handling geometry and brake position, so they are only interchangeable on frames designed for both. Treat the sidewall numbers and the rim cross-section dimensions as the starting point for comparison, then confirm with the frame manufacturer's maximum tire size and a physical check of clearance.