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I want to use the XT 8120 4-piston caliper to my GRX 815 STIs to increase my braking power. After researching, I still haven't found a definite answer. I've also looked into the Hope RX4+ 4-piston calipers, however there are some users complaining with difficulty in fully bleeding the calipers due to the design.

Any one out there tried running STIs with 4-pot calipers?

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  • I run first-gen RX-4 and the bleed is pretty much the same as other MTB/road calipers.
    – Klaster_1 Нет войне
    Commented Oct 10, 2021 at 10:41
  • Just out of curiosity: my 2-piston 8100 brakes produce way more than enough stopping power. Under what conditions are yours not enough?
    – Burki
    Commented Sep 30, 2022 at 14:29

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Hm. Let’s think about this for a second. Info is specific to Shimano:

  • Flat mount MTB calipers are just relabeled road calipers. Therefore, MTB levers can properly actuate road calipers.
  • Accordingly, since road levers and MTB levers can both actuate a 2-piston caliper, one can then deduce that road levers can properly actuate MTB 2-piston calipers.
  • Since 2-piston and 4-piston MTB calipers use the same levers, one can therefore conclude than road levers can actuate 4-piston MTB calipers.

Of course, the logic is funky at best, but at least there’s no glaring reason for incompatibility.

  • The exception is Saint/Zee levers, which I believe use a smaller piston in the lever to generate more power, but that is irrelevant.
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  • This is my understanding as well. Stay out of the Saint/Zee/Dura-Ace/XTR zone and things should "just work"
    – Paul H
    Commented Sep 27, 2022 at 4:42

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