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I just bought a 2019 Giant Defy Advanced 2, with Giant's "Conduct SL Disc" brake system and it's the only thing about the bike I don't like, for many reasons. But my question is, are the rotor mount holes "universal" i.e. can I replace the Giant system with Shimano 105 mechanical brakes? Any help would be greatly appreciated.

  • I have never seen the Conduct Brake System in person, I am curious, what issues were you having? – Rider_X May 9 '19 at 3:48
  • Flat mount mechanical disc brake calipers should be straightforward to install. Routing brake cables through the frame will be non-trivial. You may need access to a kit like Park Tools IR 1.2 – Rider_X May 9 '19 at 3:57
  • The brake fluid pump is located in the middle of the handle bar, with cables sticking out. I can't mount my Wahoo, or light. The cables poke into my hands and the unit weighs a ton. I just want to make sure that the holes for the calipers will fit a flat mount Shimano caliper. – Lefty52 May 9 '19 at 5:20
  • Rider_X, thanks for the tip on the IR 1.2! – Lefty52 May 9 '19 at 5:24
  • Sounds like a job for TRP Hy/Rd. These will move the hydraulic conversion to the caliper. – gschenk May 9 '19 at 12:44
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There are two standards to mount disc brake calipers that are relevant to you:

  • flat mount

  • post mount

Have a look at Nathan Knudson's explanation to identify what your bike has. Its specs sheet might also help.

These are industry wide standards. A Shimano flat mount will fit, for example, on a bike that came with SRAM flat mount.

There are adapters for flat to post mount available. However, it is better to avoid these. In particular post mount calipers on flat mount frames are quite a bodge.

All rotors and calipers are cross compatible (at least all that are relevant to your case). It will be enough to align the new calipers for existing rotors. However, one needs to check brake pad compatibility. Some (mostly very cheap) rotors must not be used with sintered pads.

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  • Thank you, very helpful! – Lefty52 May 9 '19 at 14:18
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The Conduct Sl system allows Giant to use mechanical disc brake shifter/brake units with hydraulic calipers.

If you want to swap out to 'proper' 105 hydraulic you'll need new shifter/brake units, lines and calipers, a stem and maybe rotors. That's not inexpensive. If you wanted a fill 105 Hydro setup ideally your would have bought a slightly higher Defy model with 105 hydraulic already installed - but presumably you did not realize you didn't like the Conduct system until later. If you have done minimal miles on the bike you might want to return to the store and ask if you can upgrade - that might be cheaper than the new parts and installation.

Answering you compatibility questions: the rotors are standard 6-bolt mounting. you might need to swap them for rotors compatible with 105 calipers, I'm not sure about that. The Conduct calipers are standard flat-mount type so 105 flat mount calipers will bolt right on.

FYI I believe Giant sells a computer and light mounting adapter that works with the Conduct stem and head unit, if that helps.

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  • I was technically incorrect in my posting question, I meant to ask " are disc brake caliper holes universal?" You answered my question anyway, thanks for that! I will replace the Conduct system with Shimano 105 mechanical disc brake calipers, that way I can use the existing shifters. All I'll need are the calipers and cables. – Lefty52 May 9 '19 at 14:23
  • You may, but do not have to stick with Shimano mechanical calipers. TRP Spyre, TRP HY/Rd and Avid BB7 will also work with your levers. That's not to dissuade you from Shimano's CX77, only to let you know there's more choice. – gschenk May 9 '19 at 14:57
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    While the Conduct brake system might be heavy, it does offer excellent braking performance, better than anything other than complete hydraulic-from-the-lever setups (but at a much lower price). Every option listed in gschenk's comment will be a significant downgrade from your current setup. If you absolutely must take off the Conduct system, I would strongly recommend upgrading to 105 hydro levers and 105 hydro calipers. – Carbon side up May 10 '19 at 1:02
  • Also be aware of the difference between flat-mount and post-mount calipers if you still decide to switch. Your frame and current brake calipers are flat-mount. – Carbon side up May 10 '19 at 1:03
  • The calipers on the Giant are flat mount. I'm going with TRP Spyre, they have gotten excellent reviews and, as I'm not a racer, I believe the mechanical brakes will give me all the stoping power I need, for the kind of riding I do. Thanks to all for the great information and suggestions! – Lefty52 May 10 '19 at 15:19

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