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Flat Mount Location

Old 12-13-20, 12:00 PM
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Flat Mount Location

Quick question...

Is the location of the mounting holes for flat mount disc brakes universal? Ie are the holes mounted in the same place regardless of the size of the rotor? Or are 140mm rotor mounting holes in a slightly different location than for 160mm?

thanks!
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Old 12-13-20, 03:40 PM
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Good question. I would hope it's standard and that the offset you need for a particular size of disk is built into the caliper.

But it's not like the bike industry to miss an opportunity for creating new and surprising sources of incompatibility to catch us all out so nothing would surprise me.
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Old 12-13-20, 04:11 PM
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My understanding is that there is only one location and there are adapters. Maybe that will change as companies unwisely adapt it to mountain bikes.

What size rotor do you want to use?
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Old 12-14-20, 05:18 AM
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The flat mount standard is for a 140mm rotor and an adapter is used for every other rotor size.
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Old 12-14-20, 09:10 AM
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I ask because I am having a frame built and got a call from the assembler telling me that he didn't know I ordered 160 rotors but they built the frame for 140 in the rear so they need to take the rear stays apart to reinstall the proper mounts....

I am using Campagnolo Record which have different calipers for 140 and 160 and have an adapter for going from 140 to 160.

Still confused by this.
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Old 12-14-20, 11:19 AM
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Doesn't sound like they know what they are doing.
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Old 12-14-20, 09:48 PM
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I wonder if the builder is actually referring to the rotor/stay clearances and not the caliper mounting points. But I agree with Eric in the suggestion that the builder isn't on top of the game. At the least he didn't get a final components being used spec hammered out PRIOR to cutting any metal. This isn't exactly surprising or a new problem. Poor business practices are as equally common as poorly fitted bikes are. Andy (who has only two disk bikes under his belt and was glad all the parts were in hand each time.)
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Old 12-15-20, 01:34 AM
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My understanding is that you need an adaptor to take your 140 mounts to 160 rotor. Does it make sense to go to 140 discs? A lot less painful than re-doing the rear end and more than 140 on the rear is generally excessive anyway for the road/skinny tires.
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Old 12-15-20, 07:04 PM
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the adapter does move the caliper up a little more than it moves it forward. It's wedge shaped though, with the thick part in front. I could see the caliper possibly hitting the seat stay if there was very little clearance.
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Old 12-16-20, 02:49 AM
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Originally Posted by Andrew R Stewart View Post
I wonder if the builder is actually referring to the rotor/stay clearances and not the caliper mounting points. But I agree with Eric in the suggestion that the builder isn't on top of the game. At the least he didn't get a final components being used spec hammered out PRIOR to cutting any metal. This isn't exactly surprising or a new problem. Poor business practices are as equally common as poorly fitted bikes are. Andy (who has only two disk bikes under his belt and was glad all the parts were in hand each time.)
Good point. Guy (who made one disk brake bike, had no components except the wheels and tyres which he insisted on, didn't even know rotors came in different sizes, but got away with it through blind luck).
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Old 12-16-20, 10:27 AM
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Originally Posted by Rocket-Sauce View Post
I ask because I am having a frame built and got a call from the assembler telling me that he didn't know I ordered 160 rotors but they built the frame for 140 in the rear so they need to take the rear stays apart to reinstall the proper mounts....

I am using Campagnolo Record which have different calipers for 140 and 160 and have an adapter for going from 140 to 160.

Still confused by this.
Seems like a lot of work- taking the rear left stay apart and reinstalling the flat mount. That involves a new chainstay(i guess the current could be used?), removing/a new seat stay, and possibly a new dropout(depending on what style is used). All thats needed is an adapter, which are extremely common and is not some shoddy workaround even on a custom bike.
Set the flatmount at 140mm, use the adapter, and move on. I wouldnt think twice at that.
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Old 12-16-20, 05:05 PM
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I know of no 160 flat mount dropout currently available and there is no fixture available to place mounts in a 160 position. The only possible issue is not enough clearance to the seatstay for the caliper or the rotor.
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Old 12-17-20, 09:04 AM
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From what I have seen, not everybody is using a fixture (that we know about anyway). So far my opinion of flat mount is that it's kind of evil and no production company seems to be able to get the rear positioned properly unless they use a dropout with it built in. Even the carbon builders have problems with it. I haven't seen any frames from the custom metal framebuilders, but experience says it's probably hit or miss from many of them too.

Last edited by unterhausen; 12-17-20 at 09:10 AM.
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Old 12-17-20, 10:29 AM
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I'm not a fan of the flat mount, but its where the industry is going. The rear calipers are a pain to mount and adjust, because the bolts go in from underneath the chainstay. The front is a better system and I'd like to try it on the rear. The brake could then be mounted with a pair of M5 bottle bosses.
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Old 12-17-20, 05:10 PM
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I think the rear needs to be redone. It just doesn't make sense to have two giant holes in a chain stay.

I agree about the front though, for a steel fork it seems ideal. But a lot of people like more reinforcement up there.
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Old 02-21-21, 01:33 PM
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Campagnolo has frame and fork location specifications for their disc brakes. They are essentially the same as the original Shimano(140mm) mounting locations. If your builder has followed these specifications then your locations will be correct.
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