In short. A forged magnesium wheel is lighter than a forged aluminium one, but by less than the metals suggest: Porsche publishes about 9 kg saved per set on the 911 GT3 and 10 kg on the 718 Spyder RS, roughly 2.25 to 2.5 kg a wheel. Magnesium corrodes faster than aluminium when its coating is broken and when it touches steel in salty water, so on a magnesium wheel the coating is the protection, and every chip matters. A solid wheel is hard to set alight: the hazard data we read say solid magnesium has to be heated above its melting point, 650 °C (1,202 °F) for the pure metal. That means a tyre or a car already on fire, or a wheel scraped along the road for a long way after a puncture, not braking. If it does burn, water makes it worse.
How much lighter: the figures car makers publish
Car makers are the only sources that state both wheels on the same car. Here is what Porsche writes in the three press kits we read.
| Car | Magnesium wheel | Compared with | Saving per set | Per wheel (our division) |
|---|---|---|---|---|
| 911 GT3 (current model) | Forged magnesium, 20 in front, 21 in rear, centre lock, with the Weissach or Lightweight Package | The standard forged wheels of the same size | 9 kg | 2.25 kg |
| 911 GT3 S/C | Forged magnesium from the 911 S/T, 20 and 21 in, centre lock | Comparable forged aluminium wheels | About 9.1 kg | About 2.3 kg |
| 718 Spyder RS | Forged magnesium, 20 in, with the Weissach Package | The standard forged aluminium wheels | 10 kg | 2.5 kg |
Sources: Porsche press kits for the 911 GT3, the 911 GT3 S/C and the 718 Spyder RS.
Two details matter. The GT3 S/C kit credits the saving to "lightweight magnesium and the back-milled spokes", so part of it is design, not metal. And these are savings against forged aluminium wheels that are already light: the GT3 kit says its new standard aluminium wheels cut more than 1.5 kg of unsprung mass against the previous ones. People search for the GT3 RS; we did not find its figure in the documents we read, so we give none; ask Porsche for it.
What 9 kg of unsprung, rotating mass changes in the way a car rides and turns is explained in lightweight wheels and unsprung weight. All three cars use centre-lock wheels; how those are tightened and looked after is in centre-lock wheels: torque, care and conversions.
Why the saving is smaller than the metals suggest
Magnesium has a density of 1.738 g/cm³ and aluminium 2.70 (PubChem, magnesium; PubChem, aluminum). The same shape in magnesium would weigh 64.4 per cent as much, 35.6 per cent less. But a wheel is not made to a volume; it is made to a stiffness and a strength.
Magnesium is also less stiff: a Young's modulus of 45 GPa against 70 for aluminium (Wikipedia, magnesium; aluminium). Divide stiffness by density and the two metals come out the same, 25.9 in both cases. Our arithmetic for a flat plate that must bend as little as an aluminium one (stiffness grows with E × thickness³):
| Aluminium | Magnesium | |
|---|---|---|
| Density, g/cm³ | 2.70 | 1.738 |
| Young's modulus, GPa | 70 | 45 |
| Stiffness per unit density | 25.9 | 25.9 |
| Thickness for the same bending stiffness | 1 | 1.159 |
| Mass for the same bending stiffness | 1 | 0.746 (25.4 % less) |
This is a simple plate model, not a wheel calculation, and a real design is also sized for fatigue and impact. It still shows the mechanism: the magnesium part has to be thicker, and it ends up about a quarter lighter rather than a third. If Porsche's wheels followed this ratio, a saving of 2.25 to 2.5 kg a wheel would mean aluminium wheels of about 9 to 10 kg each. Porsche does not publish their mass in the kits we read, so take that as arithmetic, not a measurement.
Among wrought magnesium alloys, AZ80 is the one named for the highest-strength forgings (Wikipedia, magnesium alloy); the press kits we read do not name the alloy of Porsche's wheels. How forging differs from casting and flow forming is in forged against cast against flow-formed wheels, and the forging steps themselves in how forged wheels are made.
Corrosion: the coating is the wheel's protection
Two facts explain the reputation. Magnesium sits at the anodic end of the galvanic series, below zinc, aluminium and steel (galvanic series): in contact with steel in salty water, the magnesium is the metal that corrodes. And immersion in salt water is described as "problematic" for magnesium alloys, improved by purer alloys and by coatings (Wikipedia, magnesium alloy). Old magnesium wheels suffered from pitting, cracking and corrosion (Wikipedia, alloy wheel).
The rules take this on board. Japan's technical standard for light-alloy wheels, the text behind the JWL mark, says in section 3.5 that magnesium alloy wheels must have an anti-corrosion surface treatment, a requirement written specifically for magnesium (MLIT, in Japanese). UN Regulation No. 124, which covers replacement wheels in the EU and other countries that apply UN regulations, puts magnesium wheels through the same corrosion, rotating bending, rolling and impact tests as aluminium ones (6.5.3). Its corrosion test is telling: a coated wheel from production is first scratched crosswise and stone-chipped at the rim flange and inside, to "represent damaging situations during the normal usage of vehicle", then sprayed with salt for 384 hours, turned 90° every 48 hours. There must be no significant corrosion after 192 hours, and after 384 hours the fixings and the tyre bead seat must not be affected (UN R124, Annex 5). The same annex asks for the measures around the fixings, such as zinc or cadmium bushes and insulating covers, to be assessed.
What that means for you:
- Chips at the rim flange and stone chips inside the barrel are the places the test attacks, so they are the places to look at every tyre change.
- The fixings are part of the design. Use the nuts, bolts, washers and balance weights the wheel maker specifies. A steel part in direct contact with bare magnesium is the galvanic pair to avoid. Tighten them to the car maker's torque.
- Winter salt is the harder case. Rinse after salted roads and do not leave a chipped magnesium wheel through a winter before it is sealed.
Do magnesium wheels catch fire?
The answer depends on the form of the metal. The hazard data gathered by PubChem, the US National Library of Medicine's chemistry database, separate them clearly (PubChem, magnesium):
| Form | What the source says | Temperature |
|---|---|---|
| Fine powder | Ignited at air temperatures below 900 °F | Below 482 °C |
| Ribbon and fine shavings | Ignited at air temperatures of about 950 °F | About 510 °C |
| Turnings | Autoignition (NFPA, Fire Protection Guide) | 473 °C (883 °F) |
| Solid metal | "Solid form is difficult to ignite"; it must be heated above its melting point (Sax's Dangerous Properties of Industrial Materials) | Above 650 °C (1,202 °F) |
| Massive metal in carbon dioxide | Ignition under steady heating, lower if moist (Bretherick) | 780 to 900 °C |
So a wheel does not ignite from a spark or from normal use. For a solid wheel to burn, something has to bring it above about 650 °C, the figure for pure magnesium; an alloy's melting range differs, and the sources we read give no figure for wheel alloys. Wikipedia's page on alloy wheels names the two ways it happens: a burning tyre, or prolonged scraping of the wheel on the road after a puncture (alloy wheel). The second is the one a driver controls: on magnesium wheels, do not drive on a flat tyre; stop as soon as it is safe. Wikipedia's page on magnesium alloys says the same about the workshop: highly combustible as powder or fine chips, but in sheet, bar, extruded or cast form "no real fire hazard" (magnesium alloy).
The myth has a true core. Once burning, magnesium is hard to put out. The NFPA guide quoted by PubChem says to use Class D extinguishers or to smother it with dry sand, and not to use water, foam or carbon dioxide; the ILO encyclopaedia adds that those agents make the situation more severe. If your car is on fire, get everyone away, call the fire service and tell them the wheels are magnesium. Do not throw water on a burning wheel yourself.
You will read that the US Federal Aviation Administration "cleared" magnesium. What we can confirm is narrower: the FAA developed a laboratory flammability test for magnesium alloys used in aircraft seats (report DOT/FAA/TC-13/52, February 2014, cited in the Federal Register of 3 July 2019). The approach is a test that a given alloy has to pass, not a statement that magnesium does not burn. The FAA's report site did not respond when we checked on 16 October 2026, so we do not quote its findings.
Care: cleaning and inspection
- Mild cleaner, no acid. Magnesium reacts with dilute acids and gives off hydrogen (PubChem). An acidic wheel cleaner on a chipped edge attacks bare metal. Use a pH-neutral product and the method in how to clean alloy wheels safely.
- Rinse salt off, inside the barrel too, and dry the wheel before storing it.
- Look at the coating at the rim flange, around the nut seats and inside the barrel whenever the wheels are off.
- Tell your tyre fitter the wheel is magnesium. Tyre levers and clamping jaws that chip the flange break the coating the corrosion test assumes.
- Vintage magnesium wheels (often sold as "mag wheels") deserve a crack inspection before road use. The Japanese light-alloy standard looks for cracks with dye penetrant after its tests; a specialist can do the same on an old wheel.
Repair: what can and cannot be done
Magnesium does not repair like aluminium.
Straightening. Magnesium alloys harden rapidly when cold-worked; sharp bending has to be done at about 500 to 600 °F (260 to 316 °C) (Wikipedia, magnesium alloy). A bent magnesium wheel is a job for its maker or a specialist who knows the alloy, not a roller straightener.
Welding. British Columbia's regulation on wheel weld repair "is restricted to the repair of aluminum alloy wheels" (B.C. Reg. 5/97, section 2) (BC Laws): in our reading, it gives no framework for welding a magnesium wheel. In Great Britain the MOT treats "any fracture or welding defect on a wheel" as dangerous, and Queensland rejects a wheel that shows signs of welding as a repair (see the table below).
Refinishing. Stripping and sanding make fine magnesium dust, the form that does ignite. Tell the refinisher what the wheel is made of, and ask the wheel maker which surface treatment goes under the paint, because it is the corrosion protection.
A cracked wheel of any metal is not repaired: stop using it and have it checked before you drive on it again. The wider rules are in alloy wheel repair: what can be fixed and what cannot.
The rules, country by country
| Where | Text | What it says about magnesium wheels |
|---|---|---|
| EU and other countries applying UN regulations | UN Regulation No. 124, 6.5.3 and Annexes 4 to 6 | Replacement magnesium wheels take the same tests as aluminium: material, 384-hour salt spray on a damaged coating, rotating bending, rolling, impact. Same cycle counts for aluminium and magnesium in the bending test |
| Japan (the JWL mark) | MLIT technical standard for light-alloy disc wheels, I 3.5 | A magnesium alloy wheel must have an anti-corrosion surface treatment |
| United States | FMVSS No. 110, S4.4.1; SAE J2530 | The federal rule asks each rim to match a rim the tyre maker lists and to keep a deflated tyre on at 60 mph until a controlled stop; nothing on material. SAE J2530, the aftermarket wheel practice, applies only to the materials in its tables, which we could not read (paid standard) |
| Canada | Motor Vehicle Safety Regulations, section 110 | Every rim made for vehicles up to 4,536 kg must meet Technical Standards Document No. 110 (not read). British Columbia allows weld repair of aluminium alloy wheels only |
| Great Britain | MOT inspection manual, 5.2.2 | Fracture or welding defect: dangerous; badly distorted: major; wheel and fixings not compatible: major. No line on corrosion |
| Queensland, Australia | Light Vehicle Inspection Manual | Rejection if wheels are "cracked, corroded, bent, buckled", or show signs of welding as a repair |
Sources: UN R124, MLIT, 49 CFR 571.110, SAE J2530, Canada's regulations, MOT manual, Queensland manual. How to read the marks on the wheel itself is in how to read wheel markings.
Magnesium against forged aluminium
| Forged aluminium | Forged magnesium | |
|---|---|---|
| Weight | Reference | 9 to 10 kg less per set on the Porsches above |
| Corrosion | Nobler than magnesium on the galvanic series; still oxidises where the finish is broken | Depends on an intact coating; galvanic contact with steel to avoid |
| Fire | Not a practical concern, in our reading | Solid metal hard to ignite; can burn above about 650 °C (burning tyre, long scrape on a flat); no water |
| Straightening and welding | Possible within limits and local rules | Hot work only, by a specialist; outside BC's weld-repair regulation, welding rejected in Queensland |
| Cleaning | pH-neutral products advised | pH-neutral, no acid on bare metal |
Which one suits a car depends on how it is used: a car that sees salt every winter asks more of a magnesium wheel's coating than a summer or track car does. Aluminium wheels corrode too where the finish is damaged; see removing oxidation from aluminium wheels. Carbon fibre is the other lightweight option, with its own repair limits: carbon fibre wheels for cars. Cost is the other side; how much forged wheels cost explains what drives it. Onyx Forged does not recommend a size or an offset: the dimensions are those in your car maker's documents.
Frequently asked questions
How much lighter are magnesium wheels than forged aluminium?
On the cars where the maker states both, about 9 to 10 kg per set: 9 kg on the 911 GT3, about 9.1 kg on the 911 GT3 S/C and 10 kg on the 718 Spyder RS, according to Porsche. That is about 2.25 to 2.5 kg a wheel.
Why are they not a third lighter, as the density suggests?
Because magnesium is also less stiff (45 GPa against 70), so the part has to be thicker. Our plate arithmetic gives about 25 per cent less mass for the same bending stiffness, not 36 per cent.
Can magnesium wheels catch fire?
Not from normal driving. The hazard data we read say solid magnesium is difficult to ignite and must be heated above its melting point, 650 °C for the pure metal. A burning tyre or a long scrape along the road on a flat tyre can get it there, so do not drive on a flat. Once alight, water makes a magnesium fire worse; get clear, call the fire service and tell them the wheels are magnesium.
Do magnesium wheels corrode?
Yes, faster than aluminium once the coating is broken, especially in salt and in contact with steel. That is why Japan's standard requires a corrosion treatment on magnesium wheels and why the UN test scratches the coating before 384 hours of salt spray.
Are magnesium wheels road legal?
Nothing in the texts we read bars them. UN Regulation No. 124 includes magnesium alloy wheels with the same tests as aluminium, Japan's light-alloy standard covers them, and in the US no federal rule names the wheel material. The MOT and Queensland inspections judge the wheel's condition, not its metal.
Can a magnesium wheel be repaired?
Cosmetic coating damage, yes, by someone who knows the alloy. Bends need hot work by a specialist. Welding is excluded from British Columbia's repair regulation and is a rejection in Queensland. A cracked wheel is replaced.
What should I clean magnesium wheels with?
A pH-neutral wheel cleaner, a soft brush and plenty of water. Avoid acidic products, because magnesium reacts with dilute acids.
Are old "mag wheels" made of magnesium?
Not always: "mag wheels" became a name for alloy wheels in general. A wheel maker's marking or a specialist can tell you; if it is vintage magnesium, have it checked for cracks before road use.
Where to go next
Onyx Forged is a French brand of made-to-order forged aluminium wheels, designed to JWL/VIA standards: see the catalogue.
Sources
- Porsche press kits: 911 GT3, 911 GT3 S/C, 718 Spyder RS.
- PubChem, magnesium (density, melting point, ignition, fire fighting, reactivity) and PubChem, aluminum (density).
- UN Regulation No. 124, 6.5.3 and Annexes 4 to 6; MLIT, technical standard for light-alloy disc wheels (in Japanese).
- 49 CFR 571.110; SAE J2530 summary; Canada, Motor Vehicle Safety Regulations; British Columbia, B.C. Reg. 5/97.
- GOV.UK, MOT inspection manual, section 5.2; Queensland Light Vehicle Inspection Manual.
- Federal Register, 3 July 2019, footnote 24 (FAA report DOT/FAA/TC-13/52).
- Wikipedia: magnesium, aluminium, magnesium alloy, galvanic series, alloy wheel.
All pages were read on 16 October 2026.
The Onyx Forged design office.