Water Velocity vs. Flow for Bathroom Drains

Tub or even Bathroom Sink....2" Trap on Tub / 1 1/2 Trap on Sink vs. 1 1/2 1 1/4.

Do you want more flow or same size trap for more velocity to reduce clogging ?

YOu cannot get "more flow" than the source can provide. I use a 2" trap on a bathtub to make cleaning the drain easier.
For lavatories, I use an ABS 1-1/2" trap if it is in a cabinet, or 1-1/4" chrome if it is exposed.

What I meant was, if I use the smaller trap - sized to the tailpiece - 1-1/2 or 1- 1/4 - tub / sink - will the water *scour* the trap more due to inherent volume reduce clogging versus having the larger trap ?

Stuff falls based on the strength of gravity, so speed would be the same, regardless.

Chef; I'm gonna say probably not. Like HJ said, we like a 2" trap on a tub because it's easier to clean and hair tends to not plug up the drain quite as often,........maybe but I have no real empirical proof of that either. A lot of plumbers like running a 2" trap on a kitchen sink for the same reason however the fixture and its drain are generally sized to the trap or the other way around depending on how you look at it. I can tell you that a tub will not drain any faster into a 2" trap than an 1 1/2 trap because it is limited by the size of the waste overflow.

The answer is yes, AND no. The volume of water is restricted by the drain's opening, so the pipe will seldom be "full", which means the "scouring action" will only occur on the bottom of a horizontal pipe. With a greater volume of water, the scouring WILL occur, which is why inspectors WILL often reject "oversized" drain lines, especially if they are greatly oversized.

From 1969 until now I use 2" waste line for bathtubs and showers and kitchen sinks and 11/2 for basins

Much easier to snake when the time comes which is usually decades