One of the most common mistakes that I see on motorcycles, more commonly with custom motorcycles, are front directional (tires with directional treads that often looks like V patterns) tires that are mounted the wrong direction. Intuitively with cars, all directional tires are facing the same direction. But that is not the case with our two wheeled vehicles. Most of the time there are arrows on directional tires for motorcycles to tell you which direction the rear, front, or both tire should be facing. The direction of the tread on directional tires should be facing the opposite direction between the rear and front tire. But why?
I tried to explain this on my own to some friends and started looking it up online and through forums. Everyone knows that the internet can become this wormhole or cesspool of wives' tales when searching for information on motorcycles. There are some great articles and videos that exist out there that are credible sources like Revzilla. However, I knew it had something to do with the traction or moving water away from the rear tire to avoid hydroplaning (which is partly why there is a front fender and why water splashing upward onto your face if you don't have one). I wanted to ask an expert in person to answer the questions that I may not have thought of while researching these things.
Forward time to a particular Thursday where I had a lucky chance to travel with Triumph to Florida to learn about their Street Triple RS and Bonneville Bobbers first hand with experts (and by riding them). The morning that we were at the Homestead-Miami Speedway to have a track experience on the Street Triple RS, they also had their Pirelli tire expert there to talk about the tires that come stock on the Street Triple RS. This was my chance to ask my question and have a professional engineer tell me exactly why does it matter what direction your rear and front tires should be mounted on motorcycles. And his answer could not have been a more perfect answer than I could have asked for.
To paraphrase him, the motorcycle's power comes from the rear and the motorcycle's (most) braking power comes from the front. The tires are engineered and made with certain compounds to be able to handle the power of these directions a certain way. Therefore the direction they are stretched and being used should be the direction the tires are facing. When front tires are mounted wrong and facing the same direction as the rear, they are being stretched the opposite direction than intended to from the braking. He likened it to being having a certain material in your hand that stretches only in one direction and not the other. Some may argue that they have had no problems with this, but the Pirelli expert made a good point that over time and hard abuse this may tear up the tread and become dangerous. He also said, "I sometimes see little kids who wears their shoes on the wrong feet and they can still walk... But it will hurt their feet and they won't be able to run."
So there you have it. Tires with directional tread helps the major function of your bike with the rear and power moving forward and the front moving water away and help with braking. These are how they are designed and intended to be used by companies who spend years and resources to create the most efficient tires for specific uses and purposes. So ultimately, I would follow the brand's suggestions, pay attention to arrows that tells you the direction to mount on the tire, and try to find set of tires that have rear and front tires. This is important stuff to know. Even if you don't work on your own bike, you don't want this mistake to come from a service shop.