If all manufacturers and dealers were well equipped and did their jobs right, this wouldn't make a difference for consumers. A +20mm post mount adapter does the same thing on both.įacing a rear post mount on the chainstay, where they often are now on road/cross bikes, usually requires an offset facing tool to reach in and do it without bonking into the seatstay. Post mount does not have IS's issue where the geometry is different front and rear. I've done this repair a number of times, particularly on certain low-end suspension forks where the thread quality was poor to begin with. It's acceptable to helicoil a stripped post mount in most circumstances, so the actual risk of ruining a bike forever with one overzealous tightening is small. It has the disadvantage of putting the threads in the frame/fork, which introduces some risk of difficult or impossible to repair damage resulting from carelessness or low-quality parts. Post mount is mechanically simpler, cheaper, and arguably more elegant. Although it's seldom denoted this way by frame manufacturers, post mount is really a group of standards that you could call "140mm post mount", "160mm post mount", etc. downhill forks with 200mm minimum rotor sizes. The most common setup is for the minimum rotor size (the size you'd get the caliper positioned for if no adapter was used) to be 160 front/160 rear on most mountain bikes and 160 front/140 rear on most road and cross bikes, but all sorts of exceptions exist, i.e. Post mount places two threaded M6x1 holes 74mm apart on the frame or fork in the orientation that a caliper can be bolted to directly without any adapter, or with a post-mount adapter in between along with its supplied long bolts if a larger rotor is going to be used. With IS, outside of a weird custom fork, 160mm is the smallest size you can use in front, but all bikes with rear IS mounts can take a 140mm adapter. A adapter for 180mm in the front is an adapter for 160 used in the rear, for example. The disadvantages are that it's more mechanically complex and expensive since there's always an adapter, and the design of the standard is such that the same adapter doesn't adapt the same caliper to the same rotor size front and rear. IS has one clear upside: all the threads are in a relatively cheap and easy to replace adapter. Shimano, Hope, and Magura have all made brakes like this.) (There are a few caliper models that omit the adapter. In almost all cases the brake caliper is then bolted to an adapter that has one set of threads for the IS mounts and another 74mm apart for the caliper. IS mounts (also frequently called IS tabs, disc tabs, etc) are unthreaded eyelets 51mm apart that bolts run through parallel with the hub axle.
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