What disc brake adapter do I need?
Subtract the size your frame or fork mount is built for from the rotor you want. A fork built for 160 mm rotors needs a +20 adapter for a 180 mm rotor, a +43 for a 203, and nothing for a 160. The adapter is named for the difference in rotor diameter, but it only has to move the caliper by half that, because the rotor grows by 10 mm on each side of the hub. A +20 lifts the caliper 10 mm outward. The calculator does the subtraction for post mount, IS and flat mount, and flags the combinations that don't exist or shouldn't be tried.
Measure between the two mounting bolts. Post mount: 74 mm apart, threaded holes facing the caliper; most mountain bikes. IS (International Standard): 51 mm apart, bolts running across the fork leg or frame tab; older bikes, and every modern caliper needs an IS-to-post bracket to fit. Flat mount: 70 mm on the fork, 34 mm on the frame, bolts coming up from underneath; road and gravel bikes. (Bolt spacings from SRAM's mounting specification.)
The adapter chart, checked against SRAM's
The chart above covers every mount against every common rotor size. It's computed from the subtraction rule, and we checked the rule against the chart SRAM publishes for its own brakes: every IS bracket SRAM lists is exactly rotor minus mount (a 140 IS mount to a 185 rotor is SRAM's "45 IS"), and its post-mount steps match too: 200 to 203 is a 3 mm spacer, 160 to 170 a 10 mm spacer, 203 to 220 a "17 P" bracket. Brands label brackets differently, and some make the small jumps with spacers under the caliper instead of a bracket, but the number they have to make up is always the same.
200 mm vs 203 mm rotors: can I mix them?
Not with the same adapter. SRAM makes 200 and 220 mm rotors; Shimano, TRP and Hayes make 203 and 223. The difference sounds like 3 mm, and forum answers routinely say the pads "overhang by 3 mm". But diameter splits across both sides of the hub, so the pad is off by 1.5 mm. On a 203 setup, a 200 rotor leaves the top 1.5 mm of the pad hanging past the rotor's edge, so the pad wears a step and bites on less of its face. A 203 rotor on a 200 setup sits 1.5 mm deeper in the caliper, toward the pad backing and the caliper body. Neither is right. Buy the adapter listed for your rotor's exact size, printed on the rotor itself.
Every adapter number is a diameter difference, and every movement at the caliper is half of it. SRAM's own chart shows it: its "3 S" spacer set for going from a 200 to a 203 mm rotor is two 1.5 mm spacers, not 3 mm ones. Stacking two +20s to make a +40 is wrong in a different way: the caliper ends up in the right place, but on six bolts and three joints where one bracket needs four and two.
Is a bigger rotor worth it?
For the same squeeze on the lever, braking torque rises with how far the pads sit from the hub. The pads sit just inside the rotor's edge, so the ratio of rotor diameters is a slight underestimate of the gain, a floor:
| From | To | More braking torque, at least |
|---|---|---|
| 140 mm | 160 mm | +14.3% |
| 160 mm | 180 mm | +12.5% |
| 180 mm | 200 mm | +11.1% |
| 180 mm | 203 mm | +12.8% |
| 160 mm | 203 mm | +26.9% |
| 200 mm | 220 mm | +10.0% |
Same caliper, pads and lever force. Bigger rotors also have more metal to soak up heat on long descents, which is often the bigger reason to size up.
A 20 mm step buys roughly an eighth more stopping power at the same lever effort, and a cooler rotor on a long descent. That's why heavier riders, e-bikes and steep terrain size up first. It costs a little weight and needs a mount rated for it.
What's the biggest rotor my fork or frame can take?
Whatever the maker says, and not a millimeter more. A post mount's native size is the smallest rotor it takes (the caliper can only be lifted, never lowered), and the maker's rating is the largest: forks and frames are tested for the braking force a given rotor puts through the mount. An adapter existing for a size doesn't mean your fork is rated for it. Road and gravel flat mounts are usually 140 or 160 mm; if you want 180, the frame or fork maker has to list it. SRAM also notes that its 220 mm rotor is designed for four-piston calipers.
Installing an adapter
Bolt the adapter to the frame or fork, then the caliper to the adapter with its bolts loose. Squeeze the lever to center the caliper on the rotor, and tighten the bolts alternately while holding the lever. Use the bolts that came with the adapter. SRAM specifies 9–13 mm of thread engagement for post-mount bolts (5–7 mm on flat mount), and a bolt from a different setup can fall short of that. Mounting bolts are 6–8 N·m on Shimano and 9.5 N·m on SRAM (see the torque chart), with threadlocker where the maker specifies it. Then check the pads: a new adapter with old, glazed pads won't show you what the bigger rotor can do. Pad and rotor wear limits are on the maintenance schedule.