An article from 2009 that is still relevant today can be read at: Avon tyre direction arrows
It mentions tread splice direction and the water displacement mentioned by Robert. As I said in an earlier post, some tyres are universal and can be fitted either way.
Not sure if I am being a bit slow on the uptake here, but when I read that article it mentions water dispersal as copied here:
There are many different tread patterns but there is one main reason to have any tread and that is to disperse water. (dust, dirt)
A tread pattern can be designed to disperse more water by making it rotate in only one direction. Thus, the need for directional arrows.
So how come when I look at the V pattern grooves in my AM26 Roadrider tyres on my 19" diameter Vincent and Commando wheels, plus the 21" & 19" combination on my Rudge race bike, all the tread patterns on the front tyres point in an opposite direction to those on all the rears?
My 19" tyres can be used on the front, or rear, wheels, but the 21" is a front tyre only. All the direction arrows are correct for the application.
This does not seem to be in line with the second sentence in blue above.
Can others confirm if the pattern on their tyres point in opposite directons when the direction arrows are correct on each wheel?