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  1. #1
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    Disc brake mounts: IS or post - what's your preference and why?

    OK - today I had a dialouge going with a respected person on the manufacturing side of the business. The discussion was on the benefits/drawbacks of post-mount versus IS-mount for a disc brake caliper. I have a preference for the IS mount, where he was (after my initial "attack" on post-mounts) defending the migration to post-mount.

    My arguments in favor of IS mounts:
    1) You do not have to re-adjust or re-align the caliper to the disc with IS - you simply remove the adapter if you are doing work on a fork, etc... where you have a need to remove the brake from the mount. Re-attaching takes seconds.
    2) If a mounting bolt should get cross-threaded or stripped/sheared, at the worst case, a new adapter for an IS mount costs in the $10 - 20 range for most brakes. A post mount that is damaged from cross-threading, over-tightening or if the bolt shears below the top of the mount can potentially call for serious frame or fork lower repair/replacement.
    3) Badly damaged disc or caliper can be quickly moved to the "outside" of an IS mount to remove interference issues and maybe get a bike rolling without having to unbolt the disc from the hub (an option that is not that difficult, but takes longer - esp with the little torx wrenches you have on the average multi-tool).

    Arguments in favor of post-mount:
    1) Potentially lighter complete mounting system
    2) Potentially stiffer mount to help reduce vibration that could result in brake noises
    3) Lower overall cost of manufacturing as there is one less part to manufacture/stock and distribute.

    What I wonder is: How many paying customers were consulted on this change, and how much were their inputs considered? Racers would be all for the light weight, and pretty much everyone would like to have quiet, vibration-free brakes. But a post-mount is not a guarantee of that. Let's hear what everyone else's input is on this and see if I am just a retro-grouch or do I bring up enough valid points that perhaps the manufacturers should listen to the paying public and go with the desire of the people who are actually purchasing the frames and forks, and not getting them to race on by a sponsor...
    Last edited by ATBScott; 08-19-2009 at 09:46 AM.
    R.I.P. Corky 10/97-4/09
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  2. #2
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    IS PRO:
    1. The interface surface area is larger and generally better machined/formed allowing better caliper alignment.

    2. IS to IS mounting adjustment is done using shims. Adjustment results are repeatable if taken apart and offers waay better stablity during use (a small bump will not wreck your alignment).

    PM CONS:
    1. When PM calipers are bolted near the end of their slot for side to side adjustment, un-even pressure is applied to the fork lowers, causing deformation of the mounting surface. This leads to NG caliper alignment angle.

    I personally work in the automotive industry in QC, and wouldn't accept a weld jig built using slot adjustment. It's the same thing with brake calipers. In my opinion PM is a large step backwards. Whoever came up with PM should be shot.

  3. #3
    Live 2 Ride
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    Well it seems more and more fork makers are adopting the PM. I like my IS mounts thanks.
    My Bike: '96 Gary Fisher Aquila
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  4. #4
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    Are frames starting to go post mount too?

    Advantage #2 seems like the biggest reason to stay with IS:
    2) If a mounting bolt should get cross-threaded or stripped/sheared, at the worst case, a new adapter for an IS mount costs in the $10 - 20 range for most brakes. A post mount that is damaged from cross-threading, over-tightening or if the bolt shears below the top of the mount can potentiall call for serious frame or fork lower repair/replacement.

  5. #5
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    I really have no preference....

    I've used forks with both styles of mounts and it really isn't that big of a deal one way or the other in actual use.

    What I've found so far:

    1. I've never had an IS mount fork that I didn't have to face the tabs, i.e. actually remove some metal. I've never had to face the mounts on a PM fork. Just make sure that the surface is free of paint and your good.

    2. PM forks do require more care when mounting calipers, i.e. proper torque (torque wrench use is HIGHLY recommended), care in tightening the bolts alternately to avoid binding, etc.

    3. I've never had the caliper mounting bolts come loose on a PM fork. I've had both caliper mounting bolts and adapter mounting bolts loosen on IS mounts, even though they were torqued to proper spec and thread lock was used. I believe the longer bolts used in the PM allow for more thread contact and better torque retention. But I still check all the mounting fasteners at least once a week reguardless of what system I'm using. This is how I know that IS bolts often come loose more frequently than PM.

    The argument that a stripped PM is more expensive to fix than a stripped IS adapter isn't necessarily valid in most cases. Unless the post is sheared off or completely destroyed in some other way, there is an easy fix for even a sheared bolt. Easy outs are the solution for sheared bolts, and it works well. I've done this in the shop a couple of times with no problems and it takes about 10 minutes. Stripped out threads are also easy to fix with a helicoil. I've also done this a few times as well in the past couple of years and the forks that I've done it on are still going strong. When done properly a helicoil repair is both safe and permanent. And neither is more than a $20 to $30 repair. And to be honest I've replaced more stripped out IS adapters than I've had to repair stripped post mounts.

    And finally, post mounts are cheaper to manufacture, and many manufactures are setting up for them because of that. Even the adapters are cheaper to make, instead of four threaded bolt holes that must be concentic to within .005mm, you only have to machine two unthreaded holes to the same spec or a bit less on a PM adapter, and you only have to offer two of them for a 180 and a 203mm rotor, the 160 PM is a direct mount. Anytime you can remove parts requirements or lower spec tolerance a bit you save money. And reguardless of what we think or may want, money drives this industry. The guy that can deliver the goods with lower manufacturing costs wins!

    Oh and as a side note: There aren't allot of frames out there with post mounts yet. But there are some really clever ideas that are coming out in the area of replaceable post mounts for frames. Nothing in production yet as far as I know, but as soon as someone comes out with something viable and durable you can bet you'll see allot more post mount frames as well if they are more economical for the manufacturer.

    Bottom line is, PMs are cheaper to manufacuture for both the fork and brake manufacturers, they've proven safe, reliable, and durable. So get used to them, I don't forsee them going away anytime soon, reguardless of our opinions or desires.

    Just my 2 cents.

    Good Dirt
    "I do whatever my Rice Cripsies tell me to!"

  6. #6
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    Squash - great post and all very good points - but note that I prefaced my #2 case for stripped/sheared mounts or bolts with "at the worst case". Yes - Helicoils and Easy Outs are great saviors sometimes - but once in a while they cannot be used for one reason or another. I'd definitely like to see a replaceable PM system that was accepted as a standard if this will be the way the industry is heading... But that may offset the cost-savings of producing the forks and frames with the PM to begin with... Of course, an IS mount could theoretically be damaged in a crash to where your frame or fork was made useless also (I have seen a broken Fox Forx IS mount) but I am thinking of the more common issues where an inexperienced home mechanic, or resaonably likely crash might damage a bolt or component, etc... A shop that has a mechanic that cross threads and damages the part needs to suck it up and cover the cost of the mistake (and maybe replace an employee too). In 6+ years of running IS mount disc brakes, I have only had one adapter come loose and that was the first ride on a new build.
    R.I.P. Corky 10/97-4/09
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  7. #7
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    I can't comment on the manufacturing tolerances or economics, but I have an opinion as an end customer...my old Magura Marta brakes were direct bolt-on IS mount without adapters. Using the shims for initial setup was kind of a hassle, and you did need to make sure the disc tabs were clear of paint and faced. But..they should be anyway just like a headset or BB, and there is a shop tool for that. Once mounted...they stayed where they were...occasionally pushed a piston back to fix a drag, thats about it...care-free.

    Now with Post-mount on my front Fox fork I am stuck with an intervening adapter to run my preferred 180 rotor. I find that I periodically have to re-adjust the brake by loosening the bolts between the caliper body and adapter. The system is net-heavier due to the adapter. It does seem like a step back. This was actually Magura's product change, but I believe it was done to be in line with the industry trend towards PM and compete with Formula.

    I really, really hope NOT to see post mounts on frames. I am looking forward to a new rigid fork soon I will be back to IS front and rear. But hey..the industry moved away from the highly adjustable quill stem and threaded steerer...now you cut your fork too short you are screwed and it is permanent. Was that a good step for the end customer? Well..noone ever asked me. I think they were saving weight. Like those new integrated seatpost-masts that are like part of the frame. Ugh...anyway...

  8. #8
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    I would prefer to see PM's on the frame. My initial thought was to have something like a Turner PM with the replacable insert. Since problems with the fork PM's such as stripped or cross threaded holes are rare, then would it be safe to say that this would also hold true for the rear? I have a '09 Turner and the PM's on the rear work great, just like they do on the front. PRO's for me is that the finished product looks a lot cleaner without an adaptor, not having to get your IS disc tabs faced, no shims to fuss with and 2 less bolts to come loose. Con's, you still have to run an adaptor if you want anything bigger than a 160 rotor on the rear.

  9. #9
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    I have no preference. I can work with either

  10. #10
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    In my limited experience, my IS brakes (Hope Mono Minis and Mono M4s) took a while to set up correctly - forks definitely needed facing, but since then I have never had to make an adjustment. On the other hand my wife runs Avid Juicy 7's on her bike and they are forever requiring adjustment.

  11. #11
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    Good luck with that, mister 300-Mile Turn Radius.

  12. #12
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    Quote Originally Posted by Squash
    When done properly a helicoil repair is both safe and permanent. And neither is more than a $20 to $30 repair. And to be honest I've replaced more stripped out IS adapters than I've had to repair stripped post mounts.
    when you risk stripping your frame, you'll be more careful. An adaptor is $10-20 and you don't have to bring your bike anywhere. There's still an advantage

    Quote Originally Posted by Squash
    aren't allot of frames out there with post mounts yet. But there are some really clever ideas that are coming out in the area of replaceable post mounts for frames.
    Replaceable PMs? In the picture below, you will see a rear post mount that is replaceable. Actually, it's been around for awhile and is not even bike specific. All you need is a 51mm IS mount.


    Quote Originally Posted by Squash
    And finally, post mounts are cheaper to manufacture, and many manufactures are setting up for them because of that. Even the adapters are cheaper to make, instead of four threaded bolt holes that must be concentic to within .005mm, you only have to machine two unthreaded holes to the same spec or a bit less on a PM adapter, and you only have to offer two of them for a 180 and a 203mm rotor, the 160 PM is a direct mount. Anytime you can remove parts requirements or lower spec tolerance a bit you save money. And reguardless of what we think or may want, money drives this industry. The guy that can deliver the goods with lower manufacturing costs wins!

    Bottom line is, PMs are cheaper to manufacuture for both the fork and brake manufacturers, they've proven safe, reliable, and durable.
    So PM are cheaper b/c adaptors aren't threaded. Okay... and mounts are cheaper to make because they are threaded. You're making a lot of sense.

  13. #13
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    Is

    I prefer IS because once you set it you don't need to mess with it again, it can't move. After you do a few you get good at just eye-balling what shim thickness is needed to center the rotor in the caliper.

    PM are faster and easier to center but can move as you tighten the bolts or move like a floating caliper if enough force is applied. With PM you can easily go to bigger and smaller rotors with the use of an adapter.

  14. #14
    Stayin' Puft
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    One thing I meant to add...I have one rear dropout and one fork on different bikes where I have had to place shims between the IS mount tabs and the adapter of a post-mount style caliper. If it was a direct post mount, or a 160 PM adapted to a 180, there would be no right-left adjustment beyond the range of the slots in the caliper. Nowhere to shim. With the variance in the way different manufacturers comply to "standards"...this could cause certain combinations of frame/brake/hub to be unworkable.

  15. #15
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    How about who gives a sh!t

    They both work, I've used both with no issues, and most brakes work with both now days.

  16. #16
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    I'd say post is better. All of the braking force is directed into the mount - the bolts are there to just hold the caliper in line with the posts. With IS, all of the braking force is directed from the caliper into the adapter, and then into the IS mount through the bolts. The bolts are under a lot of shear force, especially for a rider like me at ~230lb.

    From an engineering and physics standpoint, post seems better.

  17. #17
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    Whilst I can see what you mean about the shear forces on IS bolts, the situation with a PM caliper is not straight forward either as the torque applied by the rotor via the pads is not derectly inline with the PM bolts so a twisting force will be applied to them.

  18. #18
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    Quote Originally Posted by UK-FLATLANDER
    Whilst I can see what you mean about the shear forces on IS bolts, the situation with a PM caliper is not straight forward either as the torque applied by the rotor via the pads is not derectly inline with the PM bolts so a twisting force will be applied to them.
    Right, but IS will have the shear forces on the bolts and the twisting, post just have the twist.

    All of that won't stop me from putting disc brakes on my IS mount fork and frame, but if I had a choice, I'd probably get post on both because of the physics and lack of hassle getting the proper adapters.

  19. #19
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    Agreed it wouldn't stop me buying forks or frame based on brake mount. What ever the mounting system I wish manufacturers would supply mounts accurately faced.

  20. #20
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    Quote Originally Posted by UK-FLATLANDER
    What ever the mounting system I wish manufacturers would supply mounts accurately faced.
    +100500

    But what frightens me about PM is exactly it being cheaper to produce. Manufacturers may be tempted, quite possibly, to further lower tolerances on PM mounts, and if they do so, then all buyers of that fork (and hopefully not a frame) are screwed.

    Manufacturers that are not capable of resisting that temptation should probably stay with IS, especially on lower end forks.

    I have not yet tried a full IS setup where you have to use shims. But I'm prepared for that, I do have the shims. Reproducibility and stability are great advantages. However, shim precision requirements (especially when several shims are stacked) and finite stacking options look like drawbacks.

    Right now, I find that the combination of IS mounts on fork, facing tool application, Hope IS to PM adapter, and a well made PM caliper works like a charm.
    26" faithful.

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