Why Does the Disc Inside Car Wheels Rust?

[LeSarah]

Brake disc rusting is a very common issue. The brake discs of domestic vehicles are particularly prone to this - if they get wet and don't dry properly, they'll rust by the next day. Below are detailed explanations about car tire specifications: 1. Specification model representation: Tires are crucial automotive components with various specifications suitable for different vehicle types. Specification markings are typically found on the tire sidewall as alphanumeric codes indicating tread width, aspect ratio, type, rim size, load index, and speed rating. 2. Speed rating: Tires are classified into speed grades from A-Z based on maximum sustainable speed, with grade A further divided into eight subcategories (A1-A8). Generally, letters later in the alphabet indicate better tire performance.

[MaximusDella]

I often see this kind of rust at car washes. Actually, that metal disc inside the wheel hub is the brake disc. It's completely normal for it to rust, especially after rain or washing the car, when a layer of yellow rust quickly forms on the surface. This thing is mainly made of cast iron, just like the iron wok at home—it oxidizes when exposed to moisture. But don't worry, as long as the rust isn't deep, a few brake applications while driving will grind off the surface rust layer, and it won't affect braking performance at all. Cast iron components are like this—as long as they aren't cracked or deeply pitted from rust, there's no need to worry. Just be careful not to park for too long after driving through water, as parking in a garage tends to trap more moisture than leaving it outdoors.

[StephanieFitz]

After twenty years of repairing cars, it's quite common to see rusted discs when removing the wheels. That's called the brake disc, made of cast iron which naturally tends to rust. After driving in the rain, spiderweb-like red rust patterns may appear on the disc surface. Structurally, vented discs with cooling slots in the middle are more prone to rust accumulation. The aluminum alloy hubs of two-piece brake discs hardly rust, with only the friction ring area changing color. In fact, the depth of rust serves as a reference for wear. New discs come with an anti-rust coating; once worn off, rust starts to show. By the time it's worn to the limit, rust is no longer visible. Remember, the disc edge has a safety thickness mark—when rust reaches that point, it's time for a replacement.

[OMalia]

Coastal area car owners feel it the most: brake discs rust exceptionally fast. Last time I parked my car at the harbor for three days, the metal discs inside the rims were covered in orange-red patches. Cast iron brake discs lack surface rust-proof treatment, and seawater moisture corrodes them five times faster than fresh water. Interestingly, brake discs on rainy highway drives are cleaner than on sunny days, as rainwater wash and high-frequency braking act like automatic rust removal. But short urban trips are brutal, especially with frequent auto start-stop—the brake pads don’t get a chance to grind off the rust layer, and rust pits deepen instead. For this, a monthly highway sprint to fully engage the brakes helps.

[DelWilliam]

New car deliveries require the removal of the anti-rust oil film on brake discs, so rust after pickup is normal. In daily use, residual car wash foam, splashed de-icing agents, and adhered pollen or tree sap can accelerate rusting. Special attention should be paid: alkaline car wash solutions can corrode the disc surface, and parking immediately after washing is most damaging to the discs. Last time I forgot to dry the brake discs, and after three days, fish-scale-like rust patches formed on the surface. Light surface rust is harmless, but if parked for more than two weeks, it's advisable to check the rust layer before moving. If the rust has hardened, the wheels may make a 'clicking' sound when reversing, at which point professional rust removal is needed.