In praise of the obsolete: IS disc mounts
 

Full disclosure, I dislike flat mount brakes. Quite a lot. I know some of you like them, and that's cool. I even have a flat-mount bike myself, but I curse at it every time I have to work on the calipers.

It always makes me happy when a frame is built with the (very obsolete) IS standard. A couple of days ago I switched from 160mm to a 180mm rear rotor on my Neuhaus Hummingbird and it was so easy - remove the caliper, swap to a rear 180 post-mount adapter with only two bolts, take up a smidge of slack from the brake hose, and bolt the caliper back in place. The caliper easily aligned and bike was good to go. It was nice to be reminded how easy this is.

Post-mount is also nice, but IS is easier for metal frame builders to implement correctly, and retains more flexibility, since post-mount can only be adapted for larger rotors than intended, not smaller ones.

What is it that you dislike about flat mount?

It is true that by definition it only accommodates 140mm/160mm rotors, but some frames/forks are built so that its flat mount will accommodate 160mm/180mm rotors.

The swap you did from 160mm to 180mm would have been just as easy with flat mount - for the front, you'd swap the orientation of the mounting plate, and for the rear you'd either insert or remove the mounting adapter (2 bolts on each).

One pain in the butt with flat mount is that the mounting flats must be perfectly parallel with the axle, but that should be done by the frame manufacturer.

One thing I don't like about flat mounts from a user-perspective is the tight clearance between the rotor and chainstay mounts. I have to be really careful guiding in the rear wheel so the rotor doesn't hit the mount and chip the paint.

Love IS mounts. So easy. I don't see it as obsolete at all!

But have you ever installed an IS mount caliper onto an IS mount? That's a total pain in the ass, requiring a good supply of shim washers. It was worth it, though, for the sweet vintage Hope caliper on a Lightspeed I briefly owned.

To fully air the grievances:

* Adjusting calipers is a major pain because the caliper fixing bolts must be accessed from the bottom. This is just so, so much easier on the post-mount calipers that you use with IS mounts, since the caliper bolts are accessed from the top. This is my biggest gripe.
* As you say, most FM frames can only accommodate 140/160. FM180 is extremely rare. Conversely, all rotor sizes are a snap for IS mounts.
* Carbon manufacturing tolerances being what they are in the bike industry, facing FM brake mounts is sometimes required on carbon frames - failure to do this can result in an inability to align the caliper.
* Flat mount is more difficult to implement on metal frames than IS, adding to frame construction costs. IS mounts are easily implemented by metal frame builders for both frames and forks in a variety of styles.
* Perhaps most subjectively, I think the aesthetics of IS are preferable for metal frames. Flat mount was invented to make carbon frames look more "tukt" and that's what it does. Metal bikes with flat mount often end up with wonky looking dropouts.

As for the question about actual IS mount calipers, as opposed to IS to PM adapted calipers, yes, those are an enormous pain due to the shims. Specifically my love is for IS mounts with PM adapters.

But, but, but ...

We all know the front derailleur has been killed by Classified PowerShift 2-speed hub (that's what all the bike bloggers and bike e-zines are saying, so it must be true!). And the Classified system only works with bikes with Flat Mount calipers (the required torque bar mates with the bottom of the Flat Mount). Therefore, all road and gravel bikes MUST have Flat Mount disc brakes!

In general I don't understand the fascination with chain stay mounted brake calipers. I guess the caliper itself looks cleaner, and there is no need for a brake bridge, but full length housing down the DT and around the BB has never felt very elegant to me, at least on MTBs. TT → SS routing FTW.

Fully agreed.

Probably not a factor for most of us but my son's bike is XS size and has flat mount on the chain stay.

So does my current bike, which hasn't really been an issue. (Exact same brakes after I replaced his)

But on the XS size frame it's really really hard to get the wrenches in to align the caliper and then get it tight. I should probably buy some kind of adapter, as right now I have nothing that fits well enough, and I have nothing to get the torque wrench on it.

This of course is not helped by that bike having a QR. It comes out of adjustment more easily than a bike with a TA. Very slight perhaps but the upgraded brakes I put on it might make this worse too.

My old MTB was QR, IS mount, and had more powerful brakes than his, and yet did not have any trouble.

For road bikes, it appears that more and more bikes are going to internal hose routing, and internal hose routing is much easier through the bottom bracket. In addition, the left chainstay is already pretty beefy, as it has to handle a lot of forces, so you don't have to worry about beefing up the left seatstay.

For MTBs, a big concern is how the braking torque affects the suspension. For many suspension linkages, mounting the caliper on the chainstay has a smaller affect on suspension response.

Horst-link designs reduce or eliminate excessive anti-rise concerns, but by and large mountain bikes, still put the brake mount on the seat stay. The only current designs I'm aware of with chain stay-mounted brakes are the flex-stay single-pivot designs that have taken over in the cross-country world, and I think they put the brake mount on the chain stay because the flexing seat stay would interfere with its operation.

I.S. isn't as pretty as Flat Mount and if you use Campy or Hope Brakes - no spacers needed for 160mm Rotors on 140 FM as they both have unique 160MM calipers available.

Campy and Hope do Flat Mount right even at their small volumes. Plus both work really well in terms of braking.

I'm not sold on SRAM braking and have not tried the most current ShimaNo options.

IS with post brakes is definitely the easiest to fool around with, easy access, easy compatibility, easy everything.

It’s just kinda big and heavy.

Flat mount does require a well made frame which you can’t always get and the bolt access is a pain.

I always liked the look of a nice post mount frame with the caliper bolted directly on but you are locked in on disc size

Today I was on the floor on my back removing the rear FM caliper from my new 3t to try and align it(I did). Yes it could’ve been done in a stand perhaps squatting instead, but it is in the storage unit right now across the street…

The front has bolts all the way through the fork and offers close to no adjustment side to side. I can’t get it to stop rubbing so perhaps the new carbon fork needs to be faced…