Rust on brake rotors is a common issue that many vehicle owners face. A small amount of surface rust is normal and not necessarily a major concern. However, excessive rust buildup on rotors can lead to problems with braking performance. So how much rust is considered acceptable? Here is a look at when rotor rust becomes problematic and steps you can take to prevent excessive rusting.
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What Causes Rotors to Rust?
Brake rotors are made of iron, which easily corrodes when exposed to moisture. The friction surface of rotors is also gradually worn away through normal brake use, exposing new metal underneath. This fresh bare metal has not yet developed a protective coating, making it prone to rusting. Some common causes of rotor rust include:
- Driving in wet conditions
- Not driving the vehicle regularly
- Parking outdoors
- Corrosive road treatments during winter
Rust typically starts as a light surface corrosion on the rotor edge and friction surface. Over time, the rust can spread and cause pitting if left unchecked. Once rustpenetrates deep into the rotor surface, it cannot be easily removed.
Effects of Rotor Rust
A small amount of surface corrosion on rotors is not generally harmful. But excessive rust buildup can lead to the following problems:
- Squealing or grinding noises when braking due to uneven rotor friction surface
- Vibration in the brake pedal, steering wheel, or vehicle body
- Reduced braking power due to rotor thickness variation
- Brake pulsation due to thick rust deposits
- Rust contamination of brake pads, reducing friction
These issues will gradually get worse as the rust penetrates deeper. Thick rust scaling can also cause rotors to warp or crack. At an extreme, the rusting can eat entirely through the rotor surface leading to brake failure.
Inspecting Rotors for Rust
It’s a good idea to periodically inspect brake rotors for rust buildup. Here are some tips:
- Check both the inner and outer rotor surfaces
- Look closely at the rotor edge for flaking rust
- Run your fingers over the surface to feel for pitting
- Measure rotor thickness variation with a micrometer
- Check rotors after washing the wheels or after driving in wet weather
Also take note of any vibrations or noises that develop during braking. This may indicate a rust-related issue. If you see any evidence of deep or extensive rusting, have a mechanic inspect the rotors.
Acceptable vs. Excessive Rotor Rust
Light surface corrosion on rotors is normal as they wear and completely preventing it is difficult. Here are some general guidelines on acceptable vs. excessive rotor rust:
Acceptable Rotor Rust
- A lightly speckled rust patina on the rotor edge and surface
- No visible pitting, scaling, or grooves in the friction surface
- Thick rust buildup in the rotor slots only
- Less than 0.5mm variation in rotor thickness
- No pulsation, grinding or vibration during braking
This level of rust is mainly cosmetic and does not significantly impact brake performance. Simply having some visible rotor rust is not automatically a reason to replace the rotors.
Excessive Rotor Rust
- Heavy rust scaling or flaking on the rotor edge
- Visible pitting, grooves, or divots in the rotor surface
- Thick rust buildup extending beyond the rotor slots
- More than 0.5mm variation in rotor thickness
- Grinding, pulsation or vibration during braking
Excessive rust that penetrates and damages the rotor surface will degrade braking performance and needs to be addressed. At this point, the rotors will likely need to be resurfaced or replaced.
Rotor Rust Limits by Vehicle Make
Vehicle manufacturers often specify maximum allowable runout and thickness variation on worn rotors before they are considered out of spec. This can help determine when rotors with rust are still serviceable. Here are typical rotor rust limits by vehicle make:
| Vehicle Make | Maximum Rotor Runout | Maximum Thickness Variation |
|---|---|---|
| Toyota | 0.05 mm | 0.3 mm |
| Honda | 0.05 mm | 0.3 mm |
| Nissan | 0.05 mm | 0.5 mm |
| Ford | 0.1 mm | 0.7 mm |
| GM | 0.05 mm | 0.8 mm |
These specs help determine if a rotor with some rust is still usable or if the variation is excessive. Always check your vehicle’s specific service limits as well.
Preventing Excessive Rotor Rust
You can minimize rotor rust and extend the life of your brakes with proper maintenance:
- Frequently wash the wheels to remove salt and road grime
- Avoid parking in tall grass or dirt which holds moisture
- Store vehicles in a garage when possible
- Drive regularly to prevent sitting rust
- Replace cracked or damaged dust shields
- Lubricate brake caliper pins
- Resurface or replace rotors at recommended intervals
Bedding in or burnishing new brake pads can also help form an initial wear pattern on rotors to reduce rust. When servicing brakes, wipe rotors down with brake cleaner before reinstalling wheels to remove any embedded rust. Consider having rotors coated to help retard corrosion in wet climates.
When to Resurface or Replace Rotors
Here are some general guidelines on when to resurface rotors versus replacing them due to rust:
When to Resurface Rotors
- Surface corrosion only with no pitting
- Thickness variation under manufacturer spec
- Rotors still above minimum thickness
Resurfacing can remove light rust buildup and restore a smooth friction surface on otherwise sound rotors. This saves the cost of new rotor replacement.
When to Replace Rotors
- Deep rust pitting or groove damage
- Cracked or warped rotors
- Variation exceeds manufacturer spec
- Rotors worn below minimum thickness
Replacement is required once the rotor structure is compromised. Excessively worn or damaged rotors cannot be safely resurfaced and reused.
Conclusion
Some surface corrosion is unavoidable on iron brake rotors exposed to wet conditions. But deep rust penetration can impair braking performance. Regularly inspect rotors and look for signs of excess variation, pitting, vibration or noise to identify problem rust. Keeping on top of maintenance and cleaning can minimize rust. Resurface rotors when possible to remove light rusting. But replace rotors with excessive corrosion damage or wear. With periodic inspections and service, minor rotor rust should not compromise the safety and operation of your vehicle’s braking system.