Disc Mounts
Used disc mount adapters bridge the gap between frame or fork mount standards and calliper standards. Flat mount is the road disc standard, but post mount callipers, IS mount frames and rotor size changes all require specific adapters. Shimano, SRAM and Hope all produce purpose-made adapters.
Flat mount, post mount or IS?
Flat mount is the current road disc standard, with callipers bolting directly to machined faces on the frame and fork. Post mount is the MTB standard, also found on older road disc frames, and uses two bolts perpendicular to the rotor face. IS (International Standard) is the oldest system and is rarely seen on road bikes. Each standard positions the calliper differently relative to the rotor, which is why adapters exist — a post-mount calliper on a flat-mount frame, or upsizing a rotor from 140mm to 160mm.
How do I fit a 160mm rotor to a 140mm frame?
Most road frames are designed for 140mm rotors as standard. To run a 160mm rotor for more braking power, you need a +20mm adapter between the calliper and the frame or fork. Front and rear adapters are different — don't mix them up. The common Shimano parts are SM-MA-F160P/S (flat mount, 160mm front) and SM-MA-R160P/S (flat mount, 160mm rear), and SRAM has equivalent adapters. Using the wrong adapter leaves the calliper sitting too high or too low relative to the rotor, and the brakes won't work properly.
What do I need to know before ordering?
Adapters are inexpensive, but getting the wrong one wastes time and money. Know three things first: your frame and fork mount standard, your calliper mount standard, and the rotor size you want to run. If you're changing calliper as well, check the mount standard on the brake set at the same time. Measure and check compatibility before ordering — the wrong adapter means misaligned brakes or unsafe braking.
Are used adapters worth buying?
Used adapters are fine as long as the mounting surfaces are clean and the bolt threads aren't stripped. Genuine Shimano and SRAM adapters are cheap and precisely machined, whereas aftermarket adapters occasionally have tolerance issues, so a used genuine part is often the better buy over a new budget one.
Do I need new calliper bolts too?
Include the correct-length calliper bolts with any adapter you buy — adapter thickness changes the bolt length needed. Check what a listing actually includes before you commit, and browse the wider road brakes listings if you still need hardware to finish the job.