Brake discs are one of the most important components on our bikes. They must withstand various conditions without failing. When we press the brake lever, the generated force pushes the brake pads against the brake disc. As we slow down, the mechanical energy is transformed into heat through friction on the brake pad and disc surface. Therefore, it is recommended to check your brake discs frequently. Below is a step-by-step guide on what to look out for when performing a brake disc check.
When working on your bike it is recommended to use a repair stand and nitrile gloves to avoid contaminating the brake pad and disc surface.
Safety notice: Do not work on your bicycle if you’re not feeling comfortable with it; leave it to trained personnel in a bike shop. The edges of a brake disc are as sharp as a knife! When working on the brake system, always wear protective gloves.
Is the disc bent or warped?
Check your disc for any discoloration. If the disc is purple, bluish, or black, it has been exposed to very high temperatures, increasing the likelihood that the disc is bent or warped.
To check the disc for bends or warping, simply rotate the wheel and watch if the disc rubs against the brake pads.
If the disc is bent or wobbling without touching the caliper, you can try to straighten it back into shape using a proprietary disc-straightening tool. Only minor bends can be corrected with this tool. If the disc is bent or warped too severely, it is time to replace the disc.
Skip this part if you don’t feel comfortable with straightening the disc.
For this step, you will need the disc-straightening tool and a Sharpie. Place your bike on a stand and rotate the wheel (front or rear). If you hear a rubbing sound, that means the disc is rubbing against one of the brake pads. Identify which pad is involved. Use the Sharpie to mark the beginning and end of the rubbing section. Attach the tool to the disc and gently push or pull in the direction opposite the bend. Do not apply excessive force—doing so may damage the disc further. Rotate the wheel again to check for rubbing. Repeat until the disc runs freely. If the disc still cannot be returned to shape, we recommend replacing it.
Is the disc grooved or scored?
Check the brake disc friction ring for grooves. Hard particles can become trapped between the disc surface and the brake pad during braking, creating grooves similar to a plowed field. These grooves can reduce braking performance and accelerate brake pad wear.
In some cases, hard particles can become embedded in the brake disc friction ring, forming a cold-welded joint that further damages the brake pad surface.
Another issue occurs when brake pads are not monitored and the metal backing plate begins to contact the disc. Metal-on-metal friction can cause permanent damage to the braking system. For when to change your brake pads, read our article Brake Pads: What Every Cyclist Should Know. If the grooves are deep or if embedded particles are present, it is recommended to replace both the disc and the brake pads.
Is the disc too thin?
Due to friction, the brake pads and disc wear down over time. It is recommended to regularly check the disc thickness. To measure the thickness of the disc, you can use a measuring gauge or, for greater accuracy, a vernier caliper or micrometer.
Every disc has its minimum thickness engraved on it, as specified by the manufacturer — on the disc below, on one of the spokes. If you cannot read the marking, look the figure up in the disc or brake manufacturer’s documentation.
When measuring disc thickness, first check for a visible transition edge between the friction ring and the disc spokes. If a transition edge is present, the friction ring is worn, and the disc rotor must be replaced. If no transition edge is visible, proceed with thickness measurements. Always measure the disc at multiple points, as wear may be uneven across the rotor.
If there is a significant difference in thickness measurements at various points on the friction ring, it is recommended to measure both the inner and outer edges to check for uneven wear. Over time, the friction ring can wear into shapes resembling the letters A, V, or I. Wear in the shape of the letter I is the most ideal.
When measuring the outer and inner edges, always record both measurements.
- If the outer and inner edge thicknesses are similar, the friction ring is worn in the shape of the letter I.
- If the outer edge thickness is smaller than the inner edge, the disc is worn in the shape of the letter A.
- If the inner edge thickness is smaller than the outer edge, the disc is worn in the shape of the letter V.
Repeat this measurement at a minimum of three points around the disc (more points provide better accuracy). If the thickness is at or below the manufacturer’s minimum specification, or if the friction ring exhibits a pronounced A or V shape, the disc must be replaced. Continuing to use a worn disc can result in failure and may cause serious injury or death.
Watch on YouTube: Checking your rotors
Replacing your brake pads as well? Find the Sinter pads for your brake system in our bike brake pads range, compare the compounds on our brake pad compounds page, and bed the new pads in with our bedding-in guide.