I just upgraded to a new set of tubeless road tires that are slick (no grooves or tread). Why do they still have a directional arrow, and should I have chosen a tire with some sort of groove?
This is a pretty common question, and almost any modern tire has a specific direction that is recommended. This is because as tires evolve, so has the construction of the tire. The layers of the tire are assembled in specific directions to minimize rolling resistance. This is especially important, as the understanding of lower tire pressures offers higher performance results. So, mounting your tires in the correct direction may not be noticeable but will offer better results.
As far as grooves and tread, a slick tire offers the most amount of traction on a paved road. Grooves are a void in rubber contact with the ground, so in ideal conditions, a slick tire is best. A groove could help move water out from under the tire, but the surface area of a road tire is so small that a groove offers very minimal benefits even in wet conditions. In most cases, the grooves are applied as a marketing and psychological sales tactic since the average person relates them to “more traction.” In reality, think of Formula One racing—they will stay on slicks tires till the water is pooling on the track. Remember, this is a bit different when the riding surface is loose, and it’s the reason gravel tires have lugs/knobs.