No Minimum.. but there is more torque windup, of the frame tube, and the posts spreading, in response to the braking forces,
of a smaller Diameter thin wall Tube.

then a Brake booster arch would help ( my Magura HS 33 Hydro rim Brakes include a Booster arch )

You also Have a Brazed on Pivot option with Center pull brakes .. New? Compass cycles or Pauls

May be Better with a small frame where you have heel strike potential ..




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