An inventor trying to build a better roller skate came up with what

breakthrough in transportation technology?

Steel ball Bearings

Explanation:

The Steel ball Bearings is a type of machine that feeds balls between two heavy cast iron and hardened steel plates, referred to as rill plates. SAE 52100 steel is the standard material used by the ball-bearing industry since 1920.

It is a very high-carbon chromium steel that also contains little quantities of manganese and silicon. This air-melted alloy is hard, dirt free, and also resistant to tear and wear.