In short. A car wheel has two main parts: the rim, the outer ring the tyre sits on, and the centre (the "disc" of the standards), which carries the spokes and bolts to the car's hub. On the rim, working inwards from the edge: the flange, which holds the tyre sideways and whose outer edge is what people call the lip; the bead seat, where the tyre's bead rests; often a small ridge called the hump, which keeps the bead in place; and the well or drop centre, a deeper channel that lets the tyre be fitted. The whole cylinder of the rim is what the trade calls the barrel. In the centre: the spokes or face, the attachment face (mounting pad) that sits against the hub, the centre bore that fits over the hub's spigot, the bolt holes with their seats, and, on the rim, the valve hole. A one-piece (monoblock) wheel has rim and centre in one block; a two-piece wheel joins a centre to a one-piece barrel; a three-piece wheel clamps a centre between an inner and an outer barrel half, whose joint has to be sealed against air.
Two parts, three ways to join them
The UN rule for replacement wheels defines a wheel as "a rotating load-carrying member between the tyre and the axle", made usually of two major parts, the rim and the wheel disc, and adds that they "may be integral, permanently attached, or detachable" (UN Regulation No. 124, § 2.1). ISO 3911, the international vocabulary for wheels and rims, uses the same two parts (ISO 3911:2021, 3.1, 3.2.1, 3.2.2):
- the rim, "part of the wheel on which the tyre is mounted and supported";
- the disc, "part of the wheel which is the supporting member between the axle and the rim". On an alloy wheel the disc is the centre with its spokes; it has nothing to do with the brake disc.
Everything else on a wheel belongs to one of these two. If "rim" and "wheel" still feel like the same word, our guide to rim vs wheel vs tyre starts there.
The rim, from the outer edge to the inner edge
Take a wheel and cut it in half: the rim's profile is the same all the way round. Here it is, part by part, with the standard name, the name you will hear in a workshop, and what each one does:
| Part (standard name) | Also called | What it does | What damages it |
|---|---|---|---|
| Flange (ISO 3911, 3.7.1) | Lip, rim edge | Gives the tyre lateral support: air pressure pushes the bead against it | Kerbs: it is the part that gets scraped |
| Bead seat (3.7.2) | Seat, shoulder | Gives the tyre radial support; with the flange, it makes a tubeless tyre airtight | Corrosion between tyre and wheel, which can cause a slow leak |
| Hump (3.8.1) | Safety hump, H or H2 | A low ridge inside the bead seat that keeps the bead from sliding inwards | Rarely damaged; it can be marked by careless tyre fitting |
| Well (3.7.3) | Drop centre, rim well | A deeper channel in the middle of the rim, so the beads can be fitted and removed over the flange | Impacts on the inner side of the barrel |
| Valve aperture (3.7.4) | Valve hole | Takes the valve, or the valve of a pressure sensor | A wrong valve or sensor for the hole |
| Rim base, as a whole | Barrel | The cylinder that carries all of the above, between the two flanges | Potholes: a bent barrel shows as a wobble or a vibration |
Sources: ISO 3911:2021, 3.7.1 to 3.7.4 and 3.8.1; bead seat corrosion: General Motors bulletin 08-03-10-006I.
Flange and lip: the same edge, two vocabularies
The standards say flange; the trade says lip. On a one-piece wheel they are the same thing: the curled edge on each side of the rim, the outer one visible from the street and the inner one behind the spokes. On a three-piece wheel, "lip" often means more: the whole outer barrel half, whose depth is what makes a deep dish (deep dish and concave wheels explained).
The letter after the width in a rim size is the shape of the flange. In 8J x 18 it is J, which a German wheel maker's guide describes as the most common flange on passenger cars (composition of a wheel rim). The rim width, the 8, is taken between the flanges, so the wheel's overall width, lip to lip, is a little more.
Bead seat and hump: what keeps the tyre on
The bead seat is the narrow band on which the tyre's bead sits. The rim diameter printed on the wheel and on the tyre, the 18 in 8J x 18 and in 225/40 R18, is measured there, not at the lip: ISO 3911 marks the specified rim diameter on the bead seat in its drawing (Figure 9), and the same wheel maker's guide defines the rim diameter as the distance between the bead seats.
The hump is a ridge just inside the seat. That guide says it keeps the tyre firmly on its seat even in sharp corners; a US patent on hump rims describes the aim as minimising bead unseating, especially when the tyre is deflated (US 4,606,390). The US rule on rims for cars and light vehicles sets the requirement in plain terms: in a rapid loss of pressure at 97 km/h (60 mph) in a straight line, the rim must retain the deflated tyre until the car can be stopped with controlled braking (49 CFR 571.110, S4.4.1).
ISO 3911 lists five optional bead seat profiles: round hump (RH), special ledge (SL), flat hump (FH), combination hump (CH) and double hump (H2), a hump on each seat (3.8.1). The marking example in UN Regulation No. 124 reads 5½ J x 14 FH, a flat hump on one side only; the sample fitting table in the same regulation lists a 6Jx14H2 (Annex 3; Annex 10, § 1.5).
The well: why the middle of the rim is deeper
A tyre's bead is a hoop reinforced with steel wire (the US definition, 49 CFR 571.139): it does not stretch over the flange. The fitter pushes one side of the bead down into the well, where the diameter is smaller, and that slack lets the opposite side climb over the flange. ISO 3911 defines the well exactly that way, by its job: a part of the rim deep and wide enough for the beads to be mounted and demounted over the flange on the mounting side (3.7.3). A rim built like this is a "one-piece rim" or "drop-centre rim" (3.8.2). The rim size may say where the well is: an "A" if it is off-centre (asymmetric), an "S" if it is central (UN Regulation No. 124, § 5.1.2.1; both optional).
The valve hole
ISO 3911 calls it the valve aperture: the hole in the rim that takes the valve for inflation (3.7.4). Its diameter and position are part of the rim drawing (Figure 9, d and f). If your car reads a pressure sensor in each wheel, the sensor is mounted on the valve, so the hole has to suit it: our article on the MOT and pressure sensors with new wheels explains why that matters in Great Britain. At the MOT, a valve seriously damaged or misaligned and likely to cause sudden deflation is a dangerous defect (MOT manual, 5.2.3).
The centre: spokes, mounting face, bore and bolt holes
ISO 3911's drawing of a car wheel names the parts of the centre (Figure 1):
| Part (standard name) | Also called | What it does |
|---|---|---|
| Disc | Centre, face, spokes | Carries the load between hub and rim; its design is the wheel's look |
| Attachment face | Mounting face, mounting pad, hub face | The flat surface that sits against the hub; offset is measured from it |
| Centre hole | Centre bore, hub bore | Fits over the spigot, the raised ring on the hub, and centres the wheel |
| Bolt holes and nut seats | Lug holes, stud holes, seats | Take the wheel bolts or nuts; the seat shape must match theirs |
| Pitch circle of the bolt holes | PCD, bolt pattern | The circle the holes are on, such as 5x112 |
Spokes. Most alloy wheels replace the solid disc with spokes. Where they join the rim is a part the standards watch: the impact test of UN Regulation No. 124 is run at two positions, each on its own sample wheel: in the area where the spokes join the rim, and between two spokes, very close to the valve hole (Annex 8).
Attachment face. Offset (ET) is the distance from this face to the centre line of the rim (UN Regulation No. 124, § 2.6). Moving the face inside the wheel moves the whole wheel in the arch: see what is wheel offset and our wheel offset calculator.
Centre bore. The hub's raised ring is the spigot: UN Regulation No. 124 speaks of the "spigot mounting diameter" (§ 2.4.4). In Great Britain, the MOT lists wear between wheel and hub at the spigot mounting as a major defect (5.2.2). If the bore is larger than the spigot, a hub ring fills the gap: PCD, bolt pattern and centre bore.
Bolt holes and seats. The seat is the conical or rounded surface the bolt or nut presses on. A wheel and its fixings that are not compatible is a major defect at the MOT (5.2.2); seat types are in our wheel nut torque guide, and the tightening torque is always the figure the car maker gives for your car.
The inner contour. UN Regulation No. 124 gives a name to the shape the inside of a spinning wheel sweeps: the "wheel calliper" (§ 2.10). It is what has to clear the brake caliper. When the regulation assesses a replacement wheel, it asks as a rule for at least 3 mm to brake components and 4 mm to suspension and steering components (Annex 10, § 2.1). Two wheels of the same size and offset can differ here because their spokes are shaped differently on the inside.
One-piece, two-piece, three-piece
The trade sorts wheels by how many parts make up rim and centre:
| Construction | What is assembled | Joint to seal against air | Marking of the rim |
|---|---|---|---|
| One-piece (monoblock) | Nothing: rim and centre are one block, cast or forged | None | "x", as in 8J x 18 |
| Two-piece | A centre joined to a one-piece barrel | None in the barrel; a US maker of two-piece forged wheels says its one-piece forged barrel "eliminates the air seal required of traditional three-piece wheels" | "-" for a multi-piece rim |
| Three-piece | A centre clamped between an inner and an outer barrel half by a ring of bolts | The joint between the barrel halves | "-" for a multi-piece rim |
Sources: UN Regulation No. 124, § 2.1 and § 5.1.2.1 ("x" for a one-piece rim, "-" for a multi-piece rim); a US maker's two-piece wheel page.
What each brings:
- One-piece. No joint, no hardware, nothing to reseal. The rim profile and the face are cut or cast together, so width and offset are tied to the design of the face. A concave face can still be machined into a forged monoblock.
- Two-piece. The barrel is a separate part, so the width and the offset can be set by the barrel and the position of the centre within it, and the bolts become part of the look. Joining methods vary by maker.
- Three-piece. The two barrel halves need not be the same width (ISO 3911 says as much of a divided wheel, 3.3.2), which is how very wide wheels and very deep lips are built. The price is a joint that must stay airtight and bolts that must stay tight; follow the maker's own procedure for both.
A wheel with more parts is not stronger or weaker by that fact alone: what counts is how each part is made and how the finished wheel performs in the standard tests described below. For the processes, read forged vs cast vs flow-formed.
What the tests add for a wheel with a detachable rim
UN Regulation No. 124 tests a one-piece aluminium alloy wheel four ways: corrosion, rotating bending, rolling and impact (§ 6.5.2.1). For an aluminium wheel with a demountable rim, which the regulation describes as a wheel whose rim is clamped to the wheel disc (§ 2.1.2), it lists the same four plus a fifth, an alternating torque test (§ 6.5.2.2). That test simulates the torque that braking and acceleration put through the wheel: the rim's flange is held rigidly and the torque is fed in, one way then the other, through the attachment face; no technical crack is accepted, and the wheel fixings may lose no more than 30 per cent of their initial torque (Annex 9).
Careful: "two-piece rim" means something else in the standards
ISO 3911 does use the words "two-piece rim" and "three-piece rim", but for a different object: rims whose flange is a separate ring that comes off, held in a groove of the rim base (the gutter) by its own spring or by a spring lock ring (3.7.5, 3.8.3, 3.8.4, Figures 12 and 13), a construction found mainly on trucks and other heavy vehicles. The MOT checks those retaining rings on the vehicles that have them (5.2.2). The closest ISO term for a car wheel made of two parts bolted together into a rim with two fixed flanges is the divided wheel (3.3.2). When you read "two-piece" or "three-piece" on a car wheel, it is the trade's sense: a multi-piece wheel, not a lock-ring rim.
Reading the parts in the marking
UN Regulation No. 124 requires the wheel to be permanently marked, where it can be seen with the tyre fitted, with the maker's name or trade mark, the rim designation, the inset (offset), the date of manufacture (at least month and year) and the part number (§ 5.1). Its example, read part by part (Annex 3):
| Marking | Part it describes |
|---|---|
| 5½ J | Rim width between the flanges (5½ inches) and flange shape (J) |
| x | One-piece rim ("-" for multi-piece) |
| 14 | Rim diameter, at the bead seats |
| (no letter) | Well off-centre; "S" would mean central, "A" asymmetric (optional) |
| FH | Flat hump on one bead seat (optional) |
| 36 | Inset (offset) of 36 mm, from the attachment face |
In the US, the federal rule on rims requires markings on rims for vehicles other than passenger cars (light trucks, vans and the like, up to 4,536 kg): the source of the rim dimensions, the rim size designation and, for a multi-piece rim, the rim type designation, plus the DOT symbol, the maker and the date (49 CFR 571.110, S2 and S4.4.2).
Where wheels get damaged, part by part
| Part | Typical damage | What the MOT says (Great Britain) |
|---|---|---|
| Outer flange (lip) | Kerb scrapes | Not a defect in itself if the wheel is sound |
| Barrel | A bend after a pothole | A wheel badly distorted: major (5.2.2) |
| Bead seat | Corrosion, slow leak | A tyre incorrectly seated on the rim: major (5.2.3) |
| Spokes, rim, anywhere | A crack | Any fracture or welding defect on a wheel: dangerous (5.2.2) |
| Centre bore | Wear against the spigot | Wear between wheel and hub at the spigot: major (5.2.2) |
| Bolt holes and seats | Wrong bolts, damaged seats | Wheel and fixings not compatible: major (5.2.2) |
Source: MOT inspection manual, section 5. What can and cannot be repaired is in alloy wheel repair, and a leak at the seat in slow puncture: causes and fixes.
How it works at Onyx Forged
Onyx Forged makes forged wheels in one-piece (monoblock), two-piece and three-piece construction. You do not pick a barrel width, an offset or a bore: you tell us the car and how you want the wheels to look, and the Onyx Forged design office sets every part described above against the figures in the car maker's documents, from the rim width and offset to the centre bore and the bolt seats, and goes through them with you before anything is made. Onyx Forged does not recommend any wheel size or offset: we compare the options, and the car maker's documents remain the reference.
Frequently asked questions
What is the barrel of a wheel?
The cylinder of the rim between the two flanges, the part the tyre wraps around. On a multi-piece wheel it is a separate part, in one piece (two-piece wheel) or two halves (three-piece wheel).
What is the difference between the lip and the barrel?
The lip is the outer edge, the flange you see from the street. The barrel is the whole cylinder. On a three-piece wheel, "lip" often means the whole outer barrel half.
What is the bead seat?
The band on the rim, just inside each flange, where the tyre's bead sits. The rim diameter is measured there, and air-tightness depends on it.
What does H2 mean on a wheel?
Double hump: a small ridge on each bead seat that helps keep the tyre's beads in place, including when the pressure drops.
What is the centre bore?
The hole in the middle of the wheel that fits over the hub's spigot and centres the wheel. Its diameter is in millimetres.
What is the difference between a two-piece and a three-piece wheel?
A two-piece wheel joins a centre to a one-piece barrel. A three-piece wheel clamps the centre between two barrel halves, whose joint must be sealed against air.
How can I tell if a wheel is one-piece or multi-piece?
A ring of bolts around the face, near the rim, is the usual sign. The rim marking should confirm it: under UN Regulation No. 124, "x" between width and diameter means one piece, "-" means multi-piece.
Are the spokes part of the rim?
No. They belong to the centre, the "disc" of the standards, which joins the rim to the hub.
Where to go next
For the words people mix up most, read rim vs wheel vs tyre. For where the attachment face puts the wheel, wheel offset; for deep lips and concave faces, deep dish and concave wheels explained. Onyx Forged is a French brand of made-to-order forged wheels, designed to JWL/VIA standards: see Onyx Forged and the catalogue.
Sources
- ISO 3911:2021, wheels and rims for pneumatic tyres, vocabulary (free preview): 3.1 to 3.3.2, 3.7.1 to 3.7.4, 3.8.1 to 3.8.4, Figures 1, 9, 12 and 13.
- UN Regulation No. 124, replacement wheels: § 2.1 to 2.1.4, 2.4.4, 2.6, 2.10, 5.1, 6.5.2, Annexes 3, 8, 9 and 10.
- 49 CFR 571.110, S4.4.1 and S4.4.2; 49 CFR 571.139, S3 (US).
- GOV.UK, MOT inspection manual, section 5, 5.2.2 and 5.2.3.
- A German wheel maker's guide, composition of a wheel rim; US patent 4,606,390, rim with tire bead retaining humps; a US maker's two-piece forged wheel page.
- General Motors, bulletin 08-03-10-006I, aluminium wheel bead seat corrosion.
All pages were read on 15 October 2026.
The Onyx Forged design office.