In short. A cast wheel is made by pouring molten aluminium into a mould. A flow-formed wheel starts as a cast wheel; its barrel, the part that carries the tyre, is then heated, spun and stretched with rollers, while the centre stays cast. A forged wheel is shaped from solid aluminium under pressure, then machined to its final form. The alloy usually differs too: A356-T6 for casting and 6061-T6 for forging. On typical data sheets the forged alloy is not lighter for a given volume (2.7 against 2.6 g/cm³), but its yield strength is 35 per cent higher (270 against 200 MPa) and it stretches 10 per cent before it breaks against 6 per cent, so a forged wheel can use less metal for the same job. How much lighter or stronger a particular wheel is depends on its design, not on the process alone: no single percentage is true for all wheels. The standards that matter test the wheel, not the process, so cast and forged wheels are judged by the same trials. When you buy, compare the alloy, the process for the centre and for the barrel, the weight at your size and width, and the warranty.
The three processes
Cast. The molten metal fills a mould and solidifies; the alloy usually used for casting wheels belongs to the A356 family (aluminium, silicon, magnesium). The defect the process has to control is porosity: voids left as the metal shrinks while it solidifies, and gas bubbles, hydrogen being the main gas in aluminium castings (Wikipedia, casting defect). What you get is a catalogue design, usually at the lowest price of the three.
Flow-formed. The starting point is a cast wheel. Enkei, which uses the process, says it combines "one-piece cast wheel technology with a rim forming technology called spinning process" (Enkei). Yamaha describes its SpinForged wheels the same way: the wheel turns on a mandrel while a burner heats it and a roller shapes the rim, a process Yamaha compares to forging for what it does to the metal's grain structure (Yamaha). The centre, the spokes and the hub area, remains the cast part. Only the barrel is worked. Makers who use it say the barrel is stronger; we have found no comparable public values from one maker to the next, so we give none.
Forged. Forging shapes the metal in the solid state, for aluminium between about 350 and 550 degrees Celsius, and replaces the structure the metal has after solidifying with an oriented grain flow that improves the strength along the direction of the deformation (Wikipedia, forging). Forging generally does not give shapes as complex as moulding, so the design of a forged wheel is largely cut out afterwards, on a CNC machine. That is also why a forged wheel can be made to order: the offset, the PCD and the centre bore are cuts, not part of a mould. Forged wheels come in one piece (monoblock), two pieces (a forged centre with a barrel) or three (a centre between an outer and an inner barrel).
In British usage, "alloys" means any light-alloy wheel, cast or forged; "forged alloys" is the term to use for the second.
| Cast | Flow-formed | Forged | |
|---|---|---|---|
| How it is shaped | Liquid aluminium poured into a mould | Cast wheel; barrel heated, spun and stretched on a mandrel | Solid aluminium pressed, then machined |
| Usual alloy | A356-T6 (aluminium, silicon, magnesium) | That of the cast wheel | 6061-T6 (aluminium, magnesium, silicon) |
| What the process must control | Shrinkage and gas porosity | Those of the cast centre | Holding the narrow temperature window in which aluminium can be forged |
| Where the design comes from | The mould | The mould, for the centre | Mostly machining |
| What you usually get | A catalogue design | A catalogue design, sometimes with a machined offset or PCD | Depends on the seller: a catalogue design, or dimensions made to order |
What the alloy data say
These are typical values from a materials database, for the two alloys most often used. They describe the metal, not a wheel, and they are not guaranteed values (A356.0-T6, 6061-T6).
| Property | A356.0-T6 (cast) | 6061-T6 (forged) |
|---|---|---|
| Density | 2.6 g/cm³ | 2.7 g/cm³ |
| Tensile strength | 270 MPa | 310 MPa |
| Yield strength | 200 MPa | 270 MPa |
| Elongation at break | 6 per cent | 10 per cent |
| Fatigue strength | 90 MPa | 96 MPa |
What they say:
- Density does not make a wheel light. The forged alloy is 4 per cent heavier per cubic centimetre. A forged wheel weighs less when it contains less metal.
- Yield strength lets a designer use less metal. 270 against 200 MPa is 35 per cent more stress before a lasting deformation, so for the same load the sections can be thinner.
- Elongation tells how the part gives way. The forged alloy stretches further before it breaks. That is a property of the alloy, not a guarantee about a given wheel.
- Fatigue strengths are close, 96 against 90 MPa. A wheel turns millions of times, so between two wheels the difference comes mostly from the design, the thicknesses and the internal defects, such as the porosity of a casting.
Why there is no single weight percentage
Pages on this subject give "up to 20 per cent lighter", "30 to 35 per cent less material" or "3 to 5 lb per corner". None that we read says which wheels were compared. The difference depends on the diameter, the width and the design of the spokes, and a single figure would be wrong for most wheels. Wikipedia's article on forging says forgings generally have about a 20 per cent higher strength-to-weight ratio than castings or machined parts of the same material: that is about parts in general, not about wheels, and a strength-to-weight ratio is not a weight.
To compare, ask for the weight of one wheel at the same diameter and width, for each model you are choosing between, and weigh it if you can. A well-made cast wheel can weigh less than a poorly designed forged one.
What the standards test: the wheel, not the process
Three texts matter, and none of them sets different trials for cast and forged wheels.
JWL, the Japanese technical standard for light-alloy disc wheels for passenger cars, is a notice of the Japanese transport ministry. For a wheel of a given tyre load capacity F it sets three trials (MLIT, annex 2):
| Trial | What is done | Values in the text | To pass |
|---|---|---|---|
| Rotating bending fatigue | The wheel is fixed by its rim flange on a rotating plate; a shaft mounted like a hub applies a constant bending moment | M = 1.5 x F x (0.7 x r + d), where r is the loaded radius and d the offset; at least 100,000 revolutions | No crack found by dye penetrant, no significant deformation, no abnormal loosening of nuts or bolts |
| Radial fatigue | The wheel with its tyre is pressed against a rotating drum | Load = 2.25 x F; at least 500,000 revolutions; tyre at its maximum pressure or more | Same criteria |
| Impact at 13 degrees | The wheel with its tyre is tilted 13 degrees and a mass falls on the rim flange | Mass = 0.6 x W + 180 kg from 230 mm; tyre at 200 kPa; a new wheel for each point of impact | No through crack, no separation of rim and disc, no loss of the tyre's air within a minute |
The text applies to light-alloy disc wheels for passenger cars, and the trials are the same whatever the process: the only wording tied to a material is an option to use a higher bending factor (1.8 or 2.0) with 50,000 revolutions instead of 100,000, for certain alloys, among them cast-forged ones. The JWL mark is applied by the manufacturer after its own tests, on its own responsibility; the VIA mark is that of the Aluminium Alloy Wheel Test Council, shown after the Japan Vehicle Inspection Association, a third-party testing body, has confirmed the wheel and the wheel has been registered (NAPAC, in Japanese).
UN Regulation No. 124 covers replacement wheels for passenger cars in the countries that apply it. It lists the method of production (welded, forged, cast, and so on) among the characteristics that define a wheel type (§ 2.2.7), and then puts replacement wheels in one-piece aluminium alloy through the same corrosion, rotating bending, rolling and impact trials (§ 6.5.2.1; Regulation No. 124).
SAE J2530 is the US practice for aftermarket wheels for passenger cars and light trucks: it provides performance requirements, test procedures and marking requirements (SAE). The document itself is sold by SAE, so we do not reproduce its values.
The consequence is simple: a seller who says "forged" is describing a process, not a result. The result is what the trials measure, wheel by wheel. When Onyx Forged says its wheels are "designed to JWL/VIA standards", it means they are designed for the criteria of these trials; we do not claim the mark, nor a result for each wheel.
In Great Britain the MOT does not ask how a wheel was made. Its manual lists any fracture or welding defect on a wheel as a dangerous defect, and a badly distorted wheel as major (MOT manual, 5.2.2). Our article on new forged wheels and the MOT sets out the rest.
Which one for which use
Where the design of a catalogue wheel suits you and the use is on the road, a cast or flow-formed wheel from an established maker can do the job at a lower price. Check that its tyre size and load rating are listed for your car.
A forged wheel earns its price when you want exact dimensions (width, offset, PCD and centre bore made to the car), a particular design or finish, or less metal for the strength. The two-piece form widens the range of widths and offsets and lets the lip depth be set; the three-piece is made on quotation. Whatever the choice, the right question is what the particular wheel is, not what the process is called.
What to ask a seller
- The alloy, by grade: 6061-T6, A356, or "aluminium" with no grade.
- The process, for the centre and for the barrel separately. "Forged" on a flow-formed wheel describes its barrel at most.
- The weight of one wheel, at your diameter and width.
- The load rating, stamped on the wheel or written in the specification.
- The marks and what stands behind them: JWL, VIA, or a UN or SAE reference, and who applied it.
- The dimensions made for your car: width, offset, PCD and centre bore. If you need to know what they mean, see our guides to wheel offset and to PCD, bolt pattern and centre bore.
- The warranty, in writing, and what it excludes.
Frequently asked questions
Are forged wheels better than cast wheels?
They suit different needs. The forged alloy has a higher yield strength and elongation on typical data, which lets a designer use less metal. A well-designed cast wheel is a sound wheel for road use, and a poorly designed forged wheel is not a good one.
Are forged wheels lighter?
Often, at the same size and width, because they can use less metal. By how much depends on the two wheels you compare, which is why we give no percentage.
What is a flow-formed wheel?
A cast wheel whose barrel has been heated, spun and stretched with rollers. The centre remains cast.
Is "flow-forged" the same as forged?
No. A flow-formed wheel keeps a cast centre; a forged wheel is shaped from solid aluminium.
How can I tell a forged wheel from a cast one?
By sight, often not. Ask the seller for the process and the alloy in writing; the marks on the wheel and its documents are better evidence than its look.
Do forged wheels bend or crack?
Any wheel can be damaged by a hard enough impact. The forged alloy's higher elongation says it deforms further before it breaks; it does not make a wheel indestructible. Have a wheel checked after a heavy impact, whatever it is made of.
What do JWL and VIA mean?
JWL is a Japanese technical standard, applied by the manufacturer after its own trials; VIA is shown after confirmation by a third-party body. They test the wheel, whatever its process.
Are all forged wheels made to order?
No. Some sellers offer catalogue forged designs in set dimensions; others make the dimensions to order. Ask which.
Where to go next
Onyx Forged is a French brand of made-to-order forged wheels, designed to JWL/VIA standards, in monoblock, two-piece and three-piece construction: see Onyx Forged and the catalogue. For the tyre that goes on a wheel, read how to read a tyre size.
Sources
- MakeItFrom, A356.0-T6 and 6061-T6: typical properties.
- Wikipedia, casting defect and forging.
- Enkei, MAT Technology; Yamaha Motor, SpinForged.
- MLIT, annex 2 (in Japanese) and NAPAC (in Japanese); UN Regulation No. 124; SAE J2530.
- GOV.UK, MOT inspection manual, section 5.
All pages were read on 15 October 2026.
The Onyx Forged design office.