Frames with International Standard disc mounts were everywhere a decade ago, and plenty of them are still in service on trail bikes, cyclocross rigs, and older mountain frames. The problem is that almost no current brake calipers use IS mounting anymore—the industry moved to post mount years ago. The Shimano SM-MA90 Disc Brake Adapter bridges this gap, letting you bolt a modern 74mm post mount caliper to those older IS tabs. This particular version fits rear frame mounts and positions the caliper for a 160mm rotor, which is the standard size for rear brakes on most cross-country and trail builds.

Shimano machines these adapters from aluminum alloy with material strategically removed from the center to reduce weight without sacrificing the rigidity needed for consistent brake feel. The curved edges and machined window distinguish the SM-MA90 series from the blockier adapters that came before it. Anodized black finish resists corrosion and blends with most caliper aesthetics. The adapter body includes alignment markings stamped into the surface so you can confirm proper orientation during installation—important because incorrect positioning affects pad contact and rotor tracking.

Mounting hardware comes in the box: stainless steel bolts sized for the IS tab interface and a safety wire to secure the fasteners. The wire loops through drilled holes in the bolt heads after you've torqued everything to spec, preventing the bolts from backing out under sustained vibration. It's a detail borrowed from aviation and motorsport fastening practices, and it matters on a component that sits directly in your braking system. While originally designed for the XTR M985 and M988 brake calipers, this adapter works with any Shimano brake that uses the 74mm post mount pattern, which includes everything from entry-level hydraulic brakes through current Dura-Ace road disc setups.