When to replace bike brake pads and rotors, and how often to bleed

When to replace disc brake pads

SRAM gives a wear limit you can measure: backing plate plus friction material, 3 mm or less in total, and the pads are done. Its help page says 3 mm or less and its AXS guide says under 3 mm, so our view is to replace them on reaching 3 mm. That limit is for SRAM pads. Another maker may set a different one, so check the figure from whoever made the brakes on your bike.

SRAM also gives two answers on how often to look. The help page says every month, or more often if you ride in sand or dirt, and the AXS guide says every few rides. Sand and dirt caught between the surfaces speed up wear, which makes a muddy mountain bike ride a good reason to check sooner.

This way of measuring is our own practical judgement, with no maker behind it:

1. Take off the wheel and remove the pads from the caliper, following the manual for your brakes.
2. Using a vernier gauge, measure each whole pad, backing plate plus material, at its thinnest point.
3. Repeat on the other wheel: front and rear need not wear at the same rate.

When to replace the rotor

SRAM's rotor wear limit is 0.3 mm of lost thickness, or the minimum engraved on the rotor. On its AXS road brakes that minimum is 1.55 mm. Those figures are for SRAM rotors: if yours comes from another maker, look for its own engraved minimum or its manual.

You can only check the 0.3 mm if you know the starting thickness. If you do not, go by the engraved minimum. Measure on the braking track, where the wear happens, and stay off the outer edge; that last part is practical judgement.

SRAM adds three rules for when you change parts:

• New rotor: fit new pads as well and bed them in.
• Different pad compound: it advises replacing the rotors too.
• Keep your hands off pads and rotors, and clean them only with isopropyl alcohol.

Shimano and other makers: what can be said

Shimano's maintenance guide gives no millimeters. Its limits sit in dealer manuals that could not be read, so this page has no Shimano figure for pads or rotors. What the guide does publish is a list of signs:

• Pads: look at the material showing past the metal clip. If you see little of it, replace them.
• Metal-on-metal squeal under braking: the pads are worn. Replace them straight away or you may damage the rotor.
• Discolored rotor: clean it or replace it. Some surface wear is normal; too much makes braking worse. Check that it is straight as well.
• Spongy lever, or one that reaches the bar: there may be air, contamination or a leak. Bleed the brake or take the bike to a shop.
• Frayed hoses, or oil on calipers and levers: get it repaired.

How often to bleed the brakes

SRAM asks for a bleed at least once a year on its road brakes and anything running DOT fluid, and at least every 2 years on DB8 mineral oil. Shimano publishes no interval in what was read.

Between dates, the lever has the last word. Shimano links a spongy lever, or one that reaches the bar, to air, contamination or a leak. Our view is that this calls for a bleed or a trip to the shop, whatever the calendar says.

Organic or sintered pads

This also comes from SRAM, from its AXS guide, and describes its own brakes and pads. Other makers may describe their compounds differently.

If you switch compound, remember SRAM's advice to replace the rotors as well. Nothing read gives a distance for either compound.

How many km brake pads last

No maker in what was read gives a distance. Figures from 500 to 2,500 km get passed around with no maker behind them, and a spread that wide shows that km alone cannot settle it. The guide to component wear explains why.

Pedalytics uses 1,500 km as a planning prompt, with low confidence. The number is the project's own, and its job is to remind you to measure. It can sit alongside your other parts in a maintenance schedule.

A made-up example: 150 km a week. 1,500 ÷ 150 = 10 weeks to the prompt. At the ends of that spread, 500 ÷ 150 is about 3 weeks and 2,500 ÷ 150 about 17. With SRAM's monthly check you would look twice in those 10 weeks, and if your pads lasted only 3 weeks a monthly check would come too late: one more reason to look more often in sand or dirt.

In Pedalytics

Pedalytics is not out yet: it is listed as «Coming soon». Each ride saves a snapshot of the bike's setup and adds the wear to the parts it had on. The reference is 1,500 km for pads and 12 months for brake fluid, which matches SRAM's yearly bleed with DOT. It warns you at 80 to 90% of the reference and treats the part as due on reaching it.

A part can start as new, used (with its km) or unknown; if it is unknown, the percentage is a minimum. Pedalytics does not measure wear. It gives an estimate and does not replace a gauge or a mechanic.