Before we can talk about directional arrows you must first understand a bit about tread patterns. 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. The arrow tells you which way to mount a tire for maximum water dispersal. Another, less apparent reason for directional arrows is the tread splice.
What is a tread splice? When a tire is manufactured the tread portion of the tire starts out as a long flat strip. This strip is wrapped around the tire and the two ends are cut on an angle so one end overlaps the other rather than having square cut ends.
This overlapping point or splice offers a bigger surface area to bond together, rather than the small surface area provided by square cut ends. (Imagine gluing your fingertips together, as opposed to gluing along the entire length of your fingers laid on top of each other. Like an angled splice, the overlapping fingers result in a much stronger bond).
To further ensure the strength of this bond along the tread splice the directional arrow will show you which way to mount the tire so that when the rider is “on the gas”; the acceleration force on the rear tire is pressing the splice together, rather than peeling it back.
As for braking, 80 % of the braking should take place in the front on most bikes. Therefore, the front tread splice is run in the opposite direction than that of the rear, so when the rider is on the brakes, he’s not peeling the tread splice back.
If you are using a tire that has a directional arrow for rear rotation only and for some reason you want to put it on the front, make sure it is rotating in the opposite direction so you don’t aggravate the tread splice. Avon Tyres.