On modern VWs, whenever you replace your brake pads, you should also replace your brake rotors. This is because brake rotors become scored from use, and a pad might not seat correctly on a worn rotor. Below we highlight a few key characteristics of brake rotors and show you what to look for when buying.
Most brake rotors are made of cast iron. Exact metal content varies based on a number of factors—where the rotor is made, manufacturing process, company—and information about what exactly your rotor is made of isn’t readily available. You’d have to take each rotor to a lab to get analyzed for that. Other materials (like ceramic) are available for extremely high-performing automobiles, such as Porsche and Mercedes race cars, but we’re not aware of a ceramic option for your everyday VW. In general, cast iron rotors are durable, cheap, and perform well—i.e., they have a reasonably high friction coefficient and dissipate heat at an effective rate. Most TDI owners are more interested in fuel economy vs. extremely high performance, so cast iron rotors work nicely.
Many rotors are advertised as having high carbon content. Their highlighted feature is that they produce less noise than regular rotors. All sorts of other advantages have been claimed, such as better heat dissipation, higher friction, improved damping, etc., but the main purpose of adding more carbon was to reduce the noise, and it’s unclear whether adding carbon actually does all these other things.
Many rotors nowadays are coated with a rust-resistant material, often where the rotor meets the wheel hub, a common place for moisture to gather. Zimmerman, a manufacturer of rotors for TDIs, describes their coated rotors like this:
- No formation of rust at the hub and herewith improved visual appearance
- Immediate functional capability without removal of the coating before mounting
- Constantly widened production range
- German production meeting the requirements of actual Quality Management Systems
- Well known, highest Otto-Zimmermann-Product-Quality
The vents you can see in the picture are between the two braking surfaces, and allow heat to escape through the middle of the rotor. Many rotors are vented now, and all front rotors for TDIs are vented.
SLOTTED & DRILLED
High performance rotors also commonly feature slots or drilled holes (or both). Many in the TDI community, for various reasons, don’t think that slotted or drilled rotors are necessary or effective. Adding to this point, it even says on the boxes of most slotted or drilled rotors that they’re not to be used for racing (rotor might not be durable enough), so caveat emptor. They do look nice, though.
Slotted rotors have slots cut into their sides, which help the rotors cool and help debris slide off the face of the pad. A moderate performance upgrade, the drawback to slotted rotors is that they wear on the pad a bit more. You also might notice some pedal flutter when braking from high speeds.
Drilled rotors have holes drilled onto the braking surface. Again, the holes reduce the weight of the rotor and dissipate heat. Originally, drilled rotors were also intended to solve another specific design problem. During intense braking, certain materials in the pad caused a gas to be created that acted as a lubricant on the rotor. The holes allowed the gas to escape. Nowadays, however, pad designs have improved and the material causing the gas is no longer created.
Drilled rotors are not without their disadvantages. As one would expect, the a rotor with holes drilled into it isn’t as durable as one without. Sometimes these rotors crack and last a shorter time period than normal rotors, and like slotted rotors they wear out pads quickly.
SLOTTED & DRILLED
You guessed it: these are rotors that are both slotted and drilled. Someone investing in these rotors would be looking for a serious performance improvement in their braking. Not really necessary for most TDI drivers.
BREAKING IN YOUR BRAKES
A final word: once you’ve replaced your brake rotors with a new pair, it’s important to properly break them in. Breaking in your pads/rotors transfers a thin film of resin from the pads to the rotor. If you drive erratically or brake heavily immediately after putting a new set of brakes on your car, it can cause the friction material to transfer unevenly, and this in turn will cause uneven braking and judder.
With common brake pads, all you need to do is drive the car for a couple hundred miles and the pads will bed-in correctly. For high-performance pads and rotors, the process usually involves slowing down from gradually increasing speeds and letting the brakes cool in between. Follow the procedures that come with your rotors/pads, and if none came, you can probably assume they just need to be used in order to be broken in.
Rotors have been known to “warp” from time to time. Rotor warping is when the surface of the rotor has changed shape and begun to affect the braking effeciency and smoothness. Warping can happen for a couple reasons. Sometimes, the rotor has been installed onto the wheel incorrectly, and that can cause it to warp. More often, a process occurs where under intense heat friction material from the pad deposits itself onto the rotor and makes its surface uneven. To say that the rotor is warped, in this case, is a bit of a misnomer, because the rotor itself hasn’t warped due to strain or anything like that, but rather material has been added to the rotor. Aside from extremely intense braking situations, such as those experienced while racing, you probably won’t experience warping.
Here are two links to some interesting articles about warping: