Drivetrain and brakes
Is It the Pads, the Rotor, or Both? Measure First
A lever that pulls closer to the bar can mean thin pads, a thin rotor, or a hydraulic problem. Measuring pad and rotor thickness against your brake's own published minimums tells you which, before you spend money on the wrong part.
The short answer
A lever that pulls closer to the handlebar than it used to can mean the pads are thin, the rotor is thin, air has gotten into the hydraulic line, or the caliper pistons need adjusting. Feel alone can't tell these apart. Pull the wheel, measure the pad thickness and the rotor thickness, and compare each number to the minimum your brake's own manufacturer publishes for that exact caliper and rotor. There is no single "disc brake" wear number. Shimano, SRAM and MAGURA each set their own limits, and a single manufacturer can set different limits for different rotors in its own lineup.
What "worn" means for your brake
The table below covers the published, checkable figures for three common brake systems. Every number is specific to the rotor or caliper named — none of it is a general rule for "disc brakes."
| Brake / rotor family | Pad: replace when pad + backing plate measures | Rotor: replace when thinner than | Rotor's new (nominal) thickness |
|---|---|---|---|
| MAGURA MT A2 (MDR-A / MDR-A CL rotor) | 2.5 mm, at any point | 1.6 mm, at any point | Not stated in the documents checked |
| SRAM CenterLine, CenterLine X, Paceline, Paceline X | 3 mm or less | 1.55 mm | 1.85 mm |
| SRAM HS2 | 3 mm or less | 1.7 mm | 2.0 mm |
| Shimano BR-R9270 / BR-R8170 / BR-R7170 (Dura-Ace/Ultegra/105 Di2 road disc) | 0.5 mm, at the most worn-down point | Not stated in this manual — check the wheel's own dealer's manual, or ask a Shimano dealer | Not stated in this manual |
A MAGURA MDR-A rotor is only rated for brakes whose own manual states the matching 1.6 mm limit; MAGURA's other rotor families use their own nominal thickness and their own minimum, stated separately for each brake they're paired with. If your brake isn't in this table, look for two things: a minimum thickness marked on the rotor itself — SRAM and MAGURA both put this on the rotor arm — and the pad limit in your own caliper's manual. Shimano states its pad figure as text in the dealer's manual for the specific caliper, not on the pad.
How to measure it, not guess at it
Use a caliper gauge — a basic digital or vernier caliper, not the brake caliper — on the pad and on the rotor. Check more than one spot. Every manufacturer's wording ("at any point," "the most worn-down part") means the thinnest point anywhere on the track or the pad, not an average, and both wear unevenly.
Some brakes build a pass/fail gauge into a tool you already have. MAGURA's transport device — the plastic spacer that also stops the pistons from firing when the wheel is out — has two openings cut into it: one checks whether the rotor is still above 1.6 mm, the other checks whether the pads still fit into it (if they do, they're worn out). SRAM's Maven brakes ship a similar black pad spacer, with a 1.55 mm slot, a 1.7 mm slot, and a pad slot — if the part slides in, it's done. A caliper gauge gives you the actual number if you want to see how much is left; these built-in gauges just tell you pass or fail.
Check both parts together
MAGURA's instructions are direct about this: when you replace the rotor, check the condition of the pads, and when you replace the pads, check the rotor. Worn pads against a new rotor, or a new rotor against worn pads, won't bed in properly and won't reach the braking force either part is rated for. If one measures below the minimum, check the other before you decide how much to order.
A few things catch people out when ordering the replacement part:
- Rotor family, not just diameter. The right diameter isn't the whole spec. MAGURA states that its MDR-A rotor is only for brakes whose own manual states the matching 1.6 mm limit — order by the rotor's stated compatibility, not size alone. If you also need to confirm Center Lock versus six-bolt for your hub, see Center Lock vs six-bolt rotors.
- Pad shape, by brake model. MAGURA lists its pad shapes by which brake they fit — MDP 10 for the MT A2, for example — so once you know your caliper model, the pad shape follows from it.
- New hardware, every time. MAGURA's instructions call for brand-new rotor-mounting bolts with fresh threadlocker on every installation, and a new split pin whenever the pads go back in, not the old ones cleaned up and reused.
- Pad compound and rotor, as a pair. SRAM notes that switching pad compound — sintered to organic, for example — while keeping the old rotor can introduce noise and cut braking performance, because the old rotor's surface carries material from the old compound.
If the measurements check out but the lever still pulls in
MAGURA's own pre-ride check treats "no oil leaking anywhere in the system" and "not worn past the minimum thickness" as two separate conditions, not one. If your pads and rotor both measure above the minimum and the lever still travels further than it used to, the wear check has done its job. The next thing to look at is air or a slow leak in the hydraulic line — that's a bleed, not a parts order, and it's a different job from the one this page covers.
MAGURA gives one more checkable number: with the wheel and pads fitted, the lever blade should still be at least 20 mm from the handlebar grip when you squeeze it to simulate braking. That figure is specific to MAGURA's own lever design — check your own lever's manual for the equivalent spec if you ride something else.
What to check on your bike right now
- Pull the wheel and find your caliper's brand and model — it's usually stamped on the caliper body.
- Measure the pad (with its backing plate) and the rotor with a caliper gauge, at more than one spot on each.
- Compare both numbers to that exact model's published minimum, not to a rule of thumb.
- If your brake has a built-in wear gauge, use it as a quick pass/fail check alongside the caliper gauge.
- Order the part that measures below spec, in the family and size your caliper's own manual names — and check the other part while the wheel is already off.
The same measure-before-you-buy approach applies to a stretched chain: see chain wear limit and cassette replacement if your drivetrain is next.
Check it on your own equipment
These take you to the manufacturers and standards bodies the page draws on, so you can look up your exact model.
- Manufacturer specification
SRAM's brake maintenance guide (opens in a new tab)SRAM's own pad (3 mm) and rotor (1.55 mm for 1.85 mm rotors, 1.7 mm for 2 mm rotors) replacement thickness, the Maven pad-spacer measuring trick, and a link to the service manual for your exact SRAM brake. — www.sram.com - Manufacturer specification
Shimano's dealer's manual: replacing the brake pads (opens in a new tab)Shimano's stated 0.5 mm pad replacement point for the Dura-Ace BR-R9270 / Ultegra BR-R8170 / 105 BR-R7170 road disc calipers, with the full pad-swap steps. — si.shimano.com - Manufacturer specification
MAGURA MT A2 user instructions (PDF) (opens in a new tab)MAGURA's 2.5 mm pad and 1.6 mm rotor wear limits, the built-in wear gauge on the transport device, and the 20 mm lever-to-bar minimum. — magura.com - Manufacturer specification
MAGURA disc brake pads by model (opens in a new tab)Which pad shape — MDP 10 for the MT A2, for example — MAGURA lists for each of its brake models. — magura.com
What this page does not tell you
- The Shimano figure in the table comes from the dealer's manual for Dura-Ace BR-R9270, Ultegra BR-R8170 and 105 BR-R7170 (road hydraulic disc, Di2); other Shimano calipers state their own pad limit in their own manual.
- No Shimano rotor minimum-thickness figure was found in any of Shimano's manuals for this caliper. Shimano's own rotor-installation instructions point to the wheel's dealer's manual instead, so that's what the page tells readers to check, along with asking a Shimano dealer.
- SRAM's figures cover its current CenterLine, CenterLine X, Paceline, Paceline X and HS2 rotors; older Avid-branded rotors are not covered by this guide.
- MAGURA's figures are specific to the MT A2 caliper and its MDR-A / MDR-A CL rotor; MAGURA states separate limits for its other calipers and rotor families.
Sources
6 documents, each with the facts on this page it supports. Manufacturers revise specifications, so every source is dated — open it before you buy.
- MAGURA MT A2 brake pads are considered worn when the thickness of the brake pad and backing plate is less than 2.5 mm at any point.
- The MAGURA MDR-A and MDR-A CL disc brake rotor is considered worn when its thickness is less than 1.6 mm at any point.
- MAGURA's transport device doubles as a wear gauge: one opening checks that the rotor is still above 1.6 mm, another checks whether the pads still fit into it (if they do, they're worn).
- MAGURA specifies that with the wheel and pads fitted, the lever blade must be at least 20 mm from the handlebar grip when depressed to simulate braking.
- MAGURA instructs that when replacing the brake rotor, the condition of the brake pads should always be checked, and vice versa.
- MAGURA's pre-ride checklist lists checking for oil leaks anywhere in the system and checking that pads/rotor aren't worn past the minimum thickness as two separate conditions.
- MAGURA's instructions call for brand-new rotor-mounting bolts coated with fresh medium-strength threadlocker on every installation, and a new split pin every time the brake pads are installed, rather than reusing the old ones.
- MAGURA states that the minimum thickness (wear limit) for its disc brake rotors is marked directly on the rotor itself and should be measured with a suitable caliper gauge; the wear limit depends on which rotor type is installed.
- MAGURA states that MDR A disc brake rotors may only be used with brakes whose own user manual explicitly states a wear limit of t=1.6 mm.
- MAGURA lists its MDP 10 pad shape for the MT A2 brake, organizing pad shapes and compounds by which brake model they fit.
- SRAM recommends replacing brake pads when the thickness of the backing plate and pad material is 3 mm or less.
- SRAM recommends changing rotors when thickness falls below the minimum stated on the rotor: 1.55 mm for its 1.85 mm rotors (CenterLine, CenterLine X, Paceline, Paceline X), and 1.7 mm for its 2 mm rotors (HS2).
- SRAM's Maven brakes ship with a pad spacer tool with slots for measuring wear directly: a 1.55 mm slot, a 1.7 mm slot, and a pad slot — if the part slides into its slot, it's worn out.
- SRAM notes that switching brake pad compound while keeping the same rotor (for example sintered to organic) can introduce noise and reduce braking performance, and recommends a new rotor when the compound changes.
- Shimano's dealer's manual for the Dura-Ace BR-R9270 / Ultegra BR-R8170 / 105 BR-R7170 hydraulic disc brakes states that pads should be replaced when the thickness at the most worn-down part of the pad is 0.5 mm.
- This Shimano dealer's manual's rotor-installation section does not give a numeric rotor minimum-thickness figure; it refers the reader to the wheel's own dealer's manual for installing/removing the rotor.