The parts of a wheel, explained simply
- Module: Fundamentals
- Level: Beginner
- Reading time: 8 min
A wheel can be read as two zones. In the middle, the centre: the spokes that make the design, and the mounting area, with its centre bore and bolt holes. Around it, the hoop that carries the tyre: two flanges, two seats where the edges of the tyre rest, one or two small safety bumps (the humps) and a central well used for fitting. Each part has a precise role, and most of them carry a measurement.
In the previous lesson, you saw that the wheel is the metal heart of the wheel and tyre assembly. Now let us pick it up and go round it, from the outside towards the hub.
What are the parts of a wheel?
| Part | Where to see it | What it does | Terms used in English |
|---|---|---|---|
| Centre (wheel disc) | The middle, seen from the front | Joining the wheel to the car's hub and carrying the design | Wheel disc |
| Spokes (or arms) | The arms of the centre | Transmitting the forces, letting air through to the brakes | Spokes |
| Mounting area | The heart of the centre | Taking the bolts or the nuts | Centre, mounting pad |
| Bolt holes | Around the middle, on a circle | Letting the bolts or studs through; their seat takes the bolt head or the nut | Bolt holes, nut seats |
| Centre bore | The hole in the middle | Centring the wheel on the car's hub | Centre hole, centre bore |
| Mounting face | At the back, against the hub | Pressing the wheel flat against the car; the offset is measured from it | Attachment face |
| Flanges | The two edges of the hoop | Holding the tyre in at the sides | Flanges |
| Bead seats | Just inside the flanges | Carrying the edges of the tyre and sealing it | Bead seats |
| Humps | Small bumps next to the seats | Stopping the tyre from sliding into the well if it deflates | Humps (H, FH, H2…) |
| Well (or drop centre) | The hollow of the hoop | Making it possible to fit and remove the tyre | Well, drop centre |
| Barrel | The whole hoop, seen in profile | Forming the surface on which the tyre is fitted | Barrel |
| Valve hole | On the hoop | Inflating the tyre; it often takes the pressure sensor | Valve hole |
The names come from the ISO 3911 standard, which sets the vocabulary of wheels and rims, and from the recommendations of the ETRTO, the European body that standardises tyres and rims; barrel, lip and spokes are trade words. Let us go through the table part by part, with what each one changes for you.
The centre: the face of the wheel
The centre is what you see from the front: the middle part that joins the outer hoop to the car's hub. Technical texts call it the “wheel disc”, and for them the “wheel” brings together precisely these two elements: the disc and the rim proper, the hoop that carries the tyre. The previous lesson compares the two vocabularies.
The centre is not flat. It can bulge outwards, be straight, or be dished inwards: this last shape is what is called a concave wheel, and the lesson on concave profiles explains what makes it possible.
The spokes: the design and the structure
The spokes (also called arms) run from the middle to the hoop. They carry the design, but above all the forces: each spoke transmits a share of the weight, of the braking and of the loads in corners.
Their number and their shape are therefore chosen under constraint. Five wide spokes, ten slim spokes, split spokes: for the same strength, the design changes the mass, the flow of air to the brakes and how easy the wheel is to clean. Their profile on the inner side matters too: it is what leaves room, or not, for a large brake caliper.
The middle: centre bore, bolt holes, mounting face
At the heart of the centre is the mounting area. Three things can be read there:
- the bolt holes, spread on a circle whose diameter is the PCD (for example 5x112: five holes on a circle of 112 mm);
- the centre bore, the hole in the middle that fits over the car's hub and centres the wheel;
- the mounting face, at the back, the flat surface that presses against the hub.
These three elements decide whether the wheel fits a given car. The lesson PCD, centre bore and centring covers them in detail. At the front, a centre cap often closes the middle and carries the logo.
Flanges, bead seats, humps: where the tyre lives
Go round the hoop in cross-section, from the outer edge to the inner edge, and you meet, in order:
- the flange, which stands proud and holds the tyre's sidewall;
- the bead seat, the surface on which the edge of the tyre, called its bead, rests;
- the hump, a small bump that stops the bead from leaving its seat if the tyre loses pressure. The standard knows several shapes: H2 puts one on each side, FH means a flat hump, on one side only…;
- the well, then, depending on the wheel, a second hump, and again a seat and a flange, on the inner side.
One detail that avoids a common mistake: the bead belongs to the tyre, not to the wheel. It is the reinforced edge of the tyre, built around a metal wire. The wheel, for its part, gives it a seat. The tyre holds its air because the pressure presses its bead against this seat.
The well looks useless once the tyre is fitted. Yet it is essential: during fitting, part of the bead is pushed down into it so that it can pass over the flange.
The barrel and the valve hole
The barrel is the whole hoop seen in profile, from the outer flange to the inner flange. On a one-piece wheel, it is a single block with the centre. The valve hole goes through it: the valve is fitted there, and often the tyre pressure sensor.
Monoblock, 2-piece, 3-piece: what changes?
The same parts can be brought together in three ways:
- Monoblock: the centre and the barrel are a single part, cast or forged. It is the simplest construction: nothing to assemble.
- 2-piece: the centre and the barrel are two parts assembled together. Width and offset can then vary without redesigning the centre.
- 3-piece: the centre is held between an inner half-rim and an outer half-rim. The outer part, the visible one, forms the lip, the flat band between the flange and the centre.
The monoblock wheels, 2-piece and 3-piece pages show the three constructions.
Diameter, width, offset: where are they measured?
A wheel's measurements are taken on precise parts, and that is where most misunderstandings begin:
- The diameter (19 inches, for example) is measured at the bead seats, not at the outer edge. A 19-inch wheel is therefore larger than 19 inches overall, since the flanges stand proud.
- The width (8.5 inches, for example) is measured between the flanges, on the inside, where the tyre rests. It is seen in profile, not from the front.
- The offset is measured between the mounting face and the plane that cuts the wheel in half across its width.
The lesson Reading wheel and tyre sizes shows how these measurements are written on the wheel itself.
Words that are often confused
- Bead and flange. The bead belongs to the tyre, the flange to the wheel.
- Centre and rim. In technical language, the centre is the middle part; the rim is the hoop that carries the tyre. In everyday language, “wheel” (or “rim”) means both.
- Lip and flange. Every wheel has a flange; the lip is the flat, visible band between the flange and the centre, most marked on wheels made of several pieces.
- Centre bore and PCD. The centre bore is the hole in the middle; the PCD is the circle of the bolt holes.
- Well and drop centre. Two names for the same hollow. It is used to fit the tyre, not to hold it.
- Hub and middle of the wheel. The hub belongs to the car: it is the part the wheel centres on and rests against.
The Onyx approach — Every Onyx Forged wheel is designed part by part: the centre and its spokes for looks and strength, the mounting area from your car's measurements, the hoop from the chosen diameter and the calculated width. The Onyx Forged design office submits these measurements to you before anything is made.
Frequently asked questions
Which part of the wheel touches the tyre?
The bead seats and the flanges. The tyre rests on the seats, and the flanges hold it in at the sides.
What are the humps on a wheel for?
Small bumps next to the bead seats, on one side or both. If the tyre loses pressure, they stop it from sliding into the well of the wheel and coming off the rim.
Where are the sizes written on a wheel?
Most often at the back: on the rear of the spokes, in the mounting area, or inside the barrel.
In the next lesson: why the choice of wheels transforms a car, and not only its looks.
Lesson written by the Onyx Forged design office.
This lesson is part of the Onyx Academy, the free course on custom forged wheels.