If designing a multi-pot calliper for the aftermarket it may just be a consideration to ensure that the overall fluid displacement required is similar to that of the original larger cylinders, or perhaps just a tad more, thinking that a little bit more pedal travel might be acceptable... Other than that, selling multi-pot callipers to the home mechanic/enthusiast which also required the expense of upgrading to a matched master cylinder, could cause folk to think of reassessing the initial purchase decision, and risk reducing sales of the calliper..?
As ever just thinking in type.
I agree that by adopting larger total piston area of calipers compared to OE you will increase pedal travel. However this also means you are
increasing mechanical advantage. In so doing you have the double whammy of increased likelihood of locking brakes from both the mechanical advantage and likely increased pad area if upgrading from say two to four pot. A reduction in caliper piston area would be better, or a more than compensating increase in master cylinder piston size.
As an example many years ago I improved the braking on my Guzzi Spada touring bike by fitting P09 Brembo calipers up front, an increase from 38 to 42mm pistons and attendant larger pads. I also increased master cylinder size from 15 to 17mm though to compensate and leave similar lever movement.
I do wonder if this may explain the dissapointment others have felt after fitting expensive caliper/disc upgrades to their Classics. If the new calipers have smaller pistons the pedal will feel very hard but requiring a heavier foot than previously. Possibly not what they were expecting!
