To Post Mount or to ISO....
Can anyone tell me the advantages of running Post Mount Disk's over ISO? I'm running ISO's now but in looking at the new Magura, they seem to be moving in the Post direction. I'd like to know what I'd be getting myself into if I move that way. An adapter would seem to add weight and possibly another item which could cause issues.
Old man on a bike
Post mounts usually enable you to directly mount a caliper with a 160mm rotor without an adapter, so that's perhaps an advantage. Disadvantage perhaps would be now that the mounting bolts use threaded posts on the fork instead of an adapter...
"...the people get the government they deserve..."
The only advantage is that the brake can be altered to different sized disks with different adaptors. This means that manufacturers can produce a generic "post mount" (PM) caliper instead of multiple calipers.
With adaptors and bolts, PM weighs more. I have IS mount Hope Mono M4s that weigh less than the Special Edition (titanium and carbon) PM Mono M4s once you include the adaptor and extra bolts to mount a 200mm disk.
I distinctly dislike:
... inability to face the brake mounting surface to get perfect alignment
... slotted mounting holes for alignment instead of shims
Some people might prefer the slotted mounting holes if they are swapping different wheels on and off the bike, however shims will always provide accurate, repeatable alignment.
The only real advantage to....
the post mount caliper over the iso mount is it allows you to use any fork you choose. And with more manufacturers going to the post mount fork configuration and some frame manufacturers doing post mount rear (not too common yet, but there are a few), it's not a bad idea to consider post mount calipers for your next brakes.
Most adapters weigh so little that unless you are a total weight weenie it makes absolutely no noticeable difference.
The big disadvantage to post mount is the post mount itself. Both post and IS mounts are permanant casitings and non-replacaceable. The disadvantage is the post is threaded and those threads are an absolutely essential part of the mounting system. Strip them and you have a real problem, and an expensive part to repair or replace. Of course it isn't much different with a direct mount IS caliper, ala Magura, Shimano, and others, but it's a cheaper replacement part than fork lowers.
Anyway, I've used both IS and Post mounts and usually end up using an adapter because I tend to run larger rotors than 160s. I'm a big guy, 230lbs so a 160mm rotor just don't cut it. I've had no problems with getting perfect alignment from either system, nor have I had any problems with the adapters themselves, though the potential is there. I've had absolutely no problems with the sloted adjustment system, it's actaully less hassle to set up and get aligned than the shim system. A tad less accurate perhaps, but less fiddling. And you don't end up with extra shims sitting around. The only problem I've ever had with a PM system was the fork, not the brakes. I had an early first gen PM Manitou Black fork that I couldn't aling the caliper correctly on. Turns out that the lower post on the Black was out of alignment with the upper post by about 1mm to the outside of the fork. Other than that, nada. As far as facing goes, I've faced more IS tabs than I care to think about! I've never had the need to face a post mount. Other than that old Black, but the PM system doesn't allow for facing anyway so the point is moot.
Bottom line is, both systems work well, are strong and secure. Both have advantages and disadvantages. But IMHO neither comes out a strong winner. It's going to depend pretty much on what you are comfortable with and what features of one or the other are important to you.
Someday in the not to distant future, I forsee it as not being an issue, because you won't have a choice. Fox has gone post mount with all but their 40 RC2 fork, Manitou is all post mount, Rock Shox is post mount on all but their Revelation, Recon, Tora and Boxxer forks, Marzzochi has gone all post mount. You get the idea.
Would I run out and buy PM brakes if I had IS just because of this?, no. But I would make sure that my next new set of brakes are PM compatable because of it.
Your, choice, and just my 2 cents.
"I do whatever my Rice Cripsies tell me to!"